Cavity-enhanced absorption sensor for carbon monoxide in a rapid compression machine Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-12-14 Ehson F. Nasir, Aamir Farooq
A sensor based on cavity-enhanced absorption spectroscopy (CEAS) was implemented for the first time in a rapid compression machine (RCM) for carbon monoxide concentration measurements. The sensor consisted of a pulsed quantum cascade laser (QCL) coupled to a low-finesse cavity in the RCM using an off-axis alignment. The QCL was tuned near 4.89 µm to probe the P(23) ro-vibrational line of CO. The pulsed mode operation resulted in rapid frequency down-chirp (6.52 cm−1/µs) within the pulse as well as a high time resolution (10 µs). The combination of rapid frequency down-chirp and off-axis cavity alignment enabled a near complete suppression of the cavity coupling noise. A CEAS gain factor of 133 was demonstrated in experiments, resulting in a much lower noise-equivalent detection limit than a single-pass arrangement. The sensor thus presents many opportunities for measuring CO formation at low temperatures and for studying kinetics using dilute reactive environments; one such application is demonstrated in this work using dilute n-heptane/air mixtures in the RCM. The formation of CO during first-stage ignition of n-heptane was measured over 802–899 K at a nominal pressure of 10 bar. These conditions correspond to the NTC region of n-heptane and such results provide useful metrics to test and compare the predictions of low-temperature heat release by different kinetic models.
The impact of residence time on ignitability and time to ignition in a toroidal jet-stirred reactor Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-11-23 Robert D. Stachler, Joseph K. Lefkowitz, Joshua S. Heyne, Scott D. Stouffer, Timothy M. Ombrello, Joseph D. Miller
Understanding of ignition processes is central to design for reliable and safe aerospace combustor systems. Ignition is influenced by many factors including combustor geometry, flow conditions, fuel composition, turbulence intensity, ignition source, and energy deposition method. A toroidal jet-stirred reactor (TJSR) utilizes bulk fluid motion, presence of recirculation zones, a bulk residence time, and turbulence intensities which emulate characteristics relevant to cavity stabilized and swirl stabilized combustors. In this work, a TJSR was used to quantify ignitability and time-to-ignition of premixed ethylene and air. The effects of inlet temperature, residence time, and reactivity were studied on forced ignition processes. Experimental conditions ranged from residence times of 15–35 ms, mixture temperatures of 340–450 K, and equivalence ratios of 0.5–1 using capacitive spark-discharge ignition. The minimum equivalence ratio for ignition (MER), or the equivalence ratio at 50% probability, shows an inverse relationship with mixture temperature and residence time. Prior theory of real engine combustor performance for lean light off, proposed by Ballal and Lefebvre, was compared to the MER and displayed similar trends to the model. Spatially integrated OH* chemiluminescence was used to measure time to ignition within the reactor. Reduction in ignitibility was experienced as the time-to-ignition approached the residence time stressing the importance of device flow time scales in relation to kernel growth dynamics and ignition probability.
Mechanism of flame acceleration and detonation transition from the interaction of a supersonic turbulent flame with an obstruction: Experiments in low pressure propane–oxygen mixtures Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-11-22 Willstrong Rakotoarison, Brian Maxwell, Andrzej Pekalski, Matei I. Radulescu
The present paper seeks to determine the mechanism of flame acceleration and transition to detonation when a turbulent flame preceded by a shock interacts with a single obstruction in its path, taken as a cylindrical obstacle or a wall in the present study. The problem is addressed experimentally in a mixture of propane–oxygen at sub-atmospheric conditions. The turbulent flame was generated by passing a detonation wave through a perforated plate, yielding flames with turbulent burning velocities 10 to 20 larger than the laminar values and incident shock Mach numbers ranging between 2 and 2.5. Time resolved schlieren videos recorded at approximately 100 kHz and numerical reconstruction of the flow field permitted to determine the mechanism of flame acceleration and transition to detonation. It was found to be the enhancement of the turbulent burning rate of the flame through its interaction with the shock reflection on the obstacle. The amplification of the burning rate was found to drive the flame burning velocity close to the speed of sound with respect to the fresh gases, resulting in the amplification of a shock in front of the flame. The acceleration through this regime resulted in the strengthening of this shock. Detonation was observed in regions of non-planarity of this internal shock, inherited by the irregular shape of the turbulent flame itself. Auto-ignition at early times of this process was found to be negligibly slow compared with the flow evolution time scale in the problem investigated, suggesting that the relevant time scale is primarily associated with the increase in turbulent burning rate by the interaction with reflected shocks.
Experimental characterization of RDE combustor flowfield using linear channel Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-11-19 Jason R. Burr, Ken H. Yu
An experimental study was conducted to characterize fundamental behavior of detonation waves propagating across an array of reactant jets inside a narrow channel, which simulated an unwrapped rotating detonation engine (RDE) configuration. Several key flow features in an ethylene-oxygen combustor were explored by sending detonation waves across reactant jets entering into cold bounding gas as well as hot combustion products. In this setup, ethylene and oxygen were injected separately into each recessed injector tube, while a total of 15 injectors were used to establish a partially premixed reactant jet array. The results revealed various details of transient flowfield, including a complex detonation wave front leading a curved oblique shock wave, the unsteady production of transverse waves at the edge of the reactant jets, and the onset of suppressed reactant jets re-entering the combustor following a detonation wave passage. The visualization images showed a complex, multidimensional, and highly irregular detonation wave front. It appeared non-uniform mixing of reactant jets lead to dynamic transverse wave structure. The refreshed reactant jets evolving in the wake of the detonation wave were severely distorted, indicating the effect of dynamic flowfield and rapid pressure change. The results suggest that the mixing between the fuel and oxidizer, as well as the mixing between the fresh reactants and the background products, should affect the stability of the RDE combustor processes.
Impact of coolant temperature on piston wall-wetting and smoke generation in a stratified-charge DISI engine operated on E30 fuel Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-11-12 Xu He, Yankai Li, Magnus Sjöberg, David Vuilleumier, Carl-Philipp Ding, Fushui Liu, Xiangrong Li
A late-injection strategy is typically adopted in stratified-charge direct injection spark ignition (DISI) engines to improve combustion stability for lean operation, but this may induce wall wetting on the piston surface and result in high soot emissions. E30 fuel, i.e., gasoline with 30% ethanol, is a potential alternative fuel that can offer a high Research Octane Number. However, the relatively high ethanol content increases the heat of vaporization, potentially exacerbating wall-wetting issues in DISI engines. In this study, the Refractive Index Matching (RIM) technique is used to measure fuel wall films in the piston bowl. The RIM implementation uses a novel LED illumination, integrated in the piston assembly and providing side illumination of the piston-bowl window. This RIM diagnostics in combination with high-speed imaging was used to investigate the impact of coolant temperature on the characteristics of wall wetting and combustion in an optical DISI engine fueled with E30. The experiments reveal that the smoke emissions increase drastically from 0.068 FSN to 1.14 FSN when the coolant temperature is reduced from 90 °C to 45 °C. Consistent with this finding, natural flame luminosity imaging reveals elevated soot incandescence with a reduction of the coolant temperature, indicative of pool fires. The RIM diagnostics show that a lower coolant temperature also leads to increased fuel film thickness, area, and volume, explaining the onset of pool fires and smoke.
The influence of fuel type and partial premixing on the structure and behaviour of turbulent autoigniting flames Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-11-10 A.R.W. Macfarlane, M.J. Dunn, A.R. Masri
This paper explores turbulent autoignition and flame stabilisation for a range of fuels, utilising a jet in a hot coflow burner. Jet fuels including: hydrogen, dimethyl ether and hydrocarbons ranging from CH4 to C4H8 are investigated with the influence of partial premixing, dilution and hot coflow temperature. Simultaneous acoustic emission measurements and high-speed chemiluminescence imaging at 10 kHz are performed; investigating the flame lift-off dynamics and to study the initiation and evolution of autoignition kernels. For all fuels studied, a common trend is found for increasing coflow temperatures; where a transition from high lift-off flames exhibiting an autoignition kernel dominated flame stabilisation mechanism, to lower lift-off flames exhibiting a premixed flame propagation stabilisation mechanism. Three key findings are reported: (i) common to all fuels studied for the high lift-off flames, the lift-off height vs. time follows a sawtooth-like trend. The leading edge of the main flame body (flame base) drifts downstream with near constant velocity, whilst upstream of the flame base autoignition kernels form and grow rapidly merging with the flame base; thereby lowering the tip of the flame base. (ii) High amplitude acoustic emission events correlate well with auto-ignition kernel flame base merging events for high lift-off flames, for the fuels studied. The ethylene flames produced the highest sound levels for a given mean lift-off height. (iii) In the high lift-off height regime, the lift-off height for all fuels scales well with corresponding simple 0-D auto-ignition delay calculations. The good correlation of the lift-off height scaling with the computed autionition delay implies that chemical kinetics, rather than turbulent mixing controls the processes at the base of these flames for higher lift-off height flames, indicating that autoignition is the dominant stabilising mode.
Performance assessment of flamelet models in flame-resolved LES of a high Karlovitz methane/air stratified premixed jet flame Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-11-10 Flavio Cesar Cunha Galeazzo, Bruno Savard, Haiou Wang, Evatt R. Hawkes, Jacqueline H. Chen, Guenther Carlos Krieger Filho
Tabulated flamelets are commonly used in turbulent combustion modeling due to their relatively low computational cost, which is attractive in industrial applications. However, these models require assumptions of tabulated chemistry and subgrid-scale models for control variable distributions, both of which may contribute to modeling errors. In the present work, large-eddy simulation (LES) with tabulated flamelets is employed to study a laboratory-scale high Karlovitz number stratified premixed jet flame that was investigated recently using direct numerical simulation (DNS). Particularly, the LES resolves properly the transported control variables at a near DNS level, mitigating the errors from subgrid-scale modeling of control variable distributions. Five different flamelet tables are tested in the current work, including the conditional mean from the DNS, counterflow stratified premixed 1D flames with and without differential diffusion, freely propagating premixed 1D flames, and 0D autoigniting plug-flow reactors. The LES results show that although the flamelet tables perform differently for the instantaneous distributions of the progress variable source term, their mean distributions are similar. The mean and rms (root mean square) radial profiles for axial velocity and temperature from the LES with different flamelet tables are in good agreement with those from the DNS; more evident discrepancies are observed for the CH2O mass fraction radial profiles. Finally, the flame structures are examined in temperature space with the table from conditional means of the DNS having the best performance, as expected.
Science and technology of ammonia combustion Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-11-09 Hideaki Kobayashi, Akihiro Hayakawa, K.D. Kunkuma A. Somarathne, Ekenechukwu C. Okafor
This paper focuses on the potential use of ammonia as a carbon-free fuel, and covers recent advances in the development of ammonia combustion technology and its underlying chemistry. Fulfilling the COP21 Paris Agreement requires the de-carbonization of energy generation, through utilization of carbon-neutral and overall carbon-free fuels produced from renewable sources. Hydrogen is one of such fuels, which is a potential energy carrier for reducing greenhouse-gas emissions. However, its shipment for long distances and storage for long times present challenges. Ammonia on the other hand, comprises 17.8% of hydrogen by mass and can be produced from renewable hydrogen and nitrogen separated from air. Furthermore, thermal properties of ammonia are similar to those of propane in terms of boiling temperature and condensation pressure, making it attractive as a hydrogen and energy carrier. Ammonia has been produced and utilized for the past 100 years as a fertilizer, chemical raw material, and refrigerant. Ammonia can be used as a fuel but there are several challenges in ammonia combustion, such as low flammability, high NOx emission, and low radiation intensity. Overcoming these challenges requires further research into ammonia flame dynamics and chemistry. This paper discusses recent successful applications of ammonia fuel, in gas turbines, co-fired with pulverize coal, and in industrial furnaces. These applications have been implemented under the Japanese ‘Cross-ministerial Strategic Innovation Promotion Program (SIP): Energy Carriers’. In addition, fundamental aspects of ammonia combustion are discussed including characteristics of laminar premixed flames, counterflow twin-flames, and turbulent premixed flames stabilized by a nozzle burner at high pressure. Furthermore, this paper discusses details of the chemistry of ammonia combustion related to NOx production, processes for reducing NOx, and validation of several ammonia oxidation kinetics models. Finally, LES results for a gas-turbine-like swirl-burner are presented, for the purpose of developing low-NOx single-fuelled ammonia gas turbine combustors.
Unsteady deflagration speed of an auto-ignitive dimethyl-ether (DME)/air mixture at stratified conditions Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-11-09 Swapnil Desai, Ramanan Sankaran, Hong G. Im
The propagation speed of an auto-ignitive dimethyl-ether (DME)/air mixture at elevated pressures and subjected to monochromatic temperature oscillations is numerically evaluated in a one-dimensional statistically stationary configuration using fully resolved numerical simulations with reduced kinetics and transport. Two sets of conditions with temperatures within and slightly above the negative temperature coefficient (NTC) regime are simulated to investigate the fundamental aspects of auto-ignition and flame propagation along with the transition from auto-ignitive deflagration to spontaneous propagation regimes under thermal stratification. Contrary to the standard laminar flame speed, the steady propagation speed of an auto-ignitive front is observed to scale proportionally to its level of upstream reactivity. It is shown that this interdependence is primarily influenced by the characteristic residence time and the homogeneous auto-ignition delay. Furthermore, the unsteady reaction front in either of the two cases responds distinctly to the imposed stratification. Specifically, the results in both cases show that the dynamic flame response depends on the mean temperature at the flame base Tb and the time-scale of thermal stratification. It is also found that, based on Tb and the propensity of the mixture to two-stage chemistry, the instantaneous peak propagation speed and the overall time taken to achieve that speed differs considerably. A displacement speed analysis is carried out to elucidate the underlying combustion modes that are responsible for such a variation in flame response.
The bottom boundary-layer structure of fire whirls Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-11-08 Shangpeng Li, Qiang Yao, Chung K. Law
The bottom boundary layer of a fire whirl over a fuel pool is coupled to the outer rotating flow, the vaporization process, and the subsequent diffusional burning in the gas. As such, it plays an essential role in the dynamics of the fire whirl, particularly the resulting burning rate and flame height. In this paper, the bottom boundary-layer structure of a typical laminar fire whirl, generated by a rotating screen, is investigated both numerically and theoretically. The associated local similarity solutions of the momentum and coupling functions are derived, leading to the determination of the flame configuration as well as the surface gasification rate. It is demonstrated that the bottom boundary layer becomes thinner and the flame base is closer to the pool surface as the swirl intensity increases. As a result, the applied circulation increases the burning rate and thereby the flame height of the fire whirl by enhancing the convective heat transfer and the evaporation of the fuel, especially in the outer region of the fuel pool. The theoretical results agree well with the simulation results over a wide range of the Ekman number Ek and the Grashof number Gr, both qualitatively and quantitatively.
Structure of turbulent nonpremixed syngas flames at high pressure Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-11-08 Wesley R. Boyette, Thibault F. Guiberti, Gaetano Magnotti, William L. Roberts
The effects of both pressure and Reynolds number on the structure of turbulent nonpremixed flames are investigated experimentally. Syngas (CO/H2/N2) flames with constant composition are examined at pressures up to 12 bar and at Reynolds numbers up to 66,800. The baseline atmospheric pressure flame is identical to the “chnA” flame of the turbulent nonpremixed flames (TNF) workshop. Low-speed OH-PLIF is used to reveal the flame structure in a small region 7 diameters downstream of the nozzle. The thickness of the OH layer decreases monotonically with pressure. Increasing pressure inhibits local extinction when comparing conditions at constant Reynolds number; an effect of changing exit strain rate. Neither changes in Reynolds number nor changes in pressure affect the mean flame front location. Corrugation of the flame front is highly sensitive to changes in Reynolds number but relatively insensitive to changes in pressure. Blow-off velocity limits the highest Reynolds number achievable by the TNF jet flames. By increasing the pressure, this ceases to be the limiting parameter, meaning that we can study the effect of Reynolds number on the structure of the Sandia-ETH syngas jet flame over a wider range of turbulence, approaching what is encountered in practical combustors.
A molecular dynamics study of fuel droplet evaporation in sub- and supercritical conditions Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-11-08 Guowei Xiao, Kai H. Luo, Xiao Ma, Shijin Shuai
Evaporation processes of a fuel droplet under sub- and supercritical ambient conditions have been studied using molecular dynamics (MD) simulations. Suspended n-dodecane droplets of various initial diameters evaporating into a nitrogen environment are considered. Both ambient pressure and temperature are varied from sub- to supercritical values, crossing the critical condition of the chosen fuel. Temporal variation in the droplet diameter is obtained and the droplet lifetime is recorded. The time at which supercritical transition happens is determined by calculating the temperature and concentration distributions of the system and comparing with the critical mixing point of the n-dodecane/nitrogen binary system. The dependence of evaporation characteristics on ambient conditions and droplet size is quantified. It is found that the droplet lifetime decreases with increasing ambient pressure and/or temperature. Supercritical transition time decreases with increasing ambient pressure and temperature as well. The droplet heat-up time as well as subcritical to supercritical transition time increases linearly with the initial droplet size d0, while the droplet lifetime increases linearly with d 0 2 . A regime diagram is obtained, which indicates the subcritical and supercritical regions as a function of ambient temperature and pressure as well as the initial droplet size.
The role of a split injection strategy in the mixture formation and combustion of diesel spray: A large-eddy simulation Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-11-06 Ahmad Hadadpour, Mehdi Jangi, Kar Mun Pang, Xue Song Bai
The role of a split injection in the mixture formation and combustion characteristics of a diesel spray in an engine-like condition is investigated. We use large-eddy simulations with finite rate chemistry in order to identify the main controlling mechanism that can potentially improve the mixture quality and reduces the combustion emissions. It is shown that the primary effect of the split injection is the reduction of the mass of the fuel-rich region where soot precursors can form. Furthermore, we investigate the interaction between different injections and explain the effects of the first injection on the mixing and combustion of the second injection. Results show that the penetration of the second injection is faster than that of the first injection. More importantly, it is shown that the ignition delay time of the second injection is much shorter than that of the first injection. This is due to the residual effects of the ignition of the first injection which increases the local temperature and maintains a certain level of combustion some intermediates or radical which in turn boosts the ignition of the second injection.
A parametric study of ignition dynamics at ECN Spray A thermochemical conditions using 2D DNS Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-11-03 Alex Krisman, Evatt R. Hawkes, Jacqueline H. Chen
The ignition process in diesel engines is highly complex and incompletely understood. In the present study, two-dimensional direct numerical simulations are performed to investigate the ignition dynamics and their sensitivity to thermochemical and mixing parameters. The thermochemical and mixing conditions are matched to the benchmark Spray A experiment from the Engine Combustion Network. The results reveal a complex ignition process with overlapping stages of: low-temperature ignition (cool flames), rich premixed ignition, and nonpremixed ignition, which are qualitatively consistent with prior experimental and numerical investigations, however, this is the first time that fully-resolved simulations have been reported at the actual Spray A thermochemical condition. Parametric variations are then performed for the Damköhler number Da, oxidiser temperature, oxygen concentration, and peak mixture fraction (a measure of premixedness), to study their effect on the ignition dynamics. It is observed that with both increasing oxidiser temperature and decreasing oxygen concentration, that the cool flame moves to richer mixtures, the overlap in the ignition stages decreases, and the (nondimensional) time taken to reach a fully burning state increases. With increasing Da, the cool-flame speed is decreased due to lower mean mixing rates, which causes a delayed onset of high-temperature ignition. With increasing peak mixture fraction, the onset of each stage of ignition is not affected, but the overall duration of the ignition increases leading to a longer burn duration. Overall, the results suggest that turbulence–chemistry interactions play a significant role in determining the timing and location in composition space of the entire ignition process.
Reference natural gas flames at nominally autoignitive engine-relevant conditions Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-11-02 Alex Krisman, Christine Mounaïm-Rousselle, Raghu Sivaramakrishnan, James A. Miller, Jacqueline H. Chen
Laminar natural gas flames are investigated at engine-relevant thermochemical conditions where the ignition delay time τ is short due to very high ambient temperatures and pressures. At these conditions, it is not possible to measure or calculate well-defined values for the laminar flame speed sl, laminar flame thickness δl, and laminar flame time scale τ l = δ l / s l due to the explosive thermochemical state. Here, the corresponding reference values, sR, δR, and τ R = δ R / s R , that account for the effects of autoignition, are numerically estimated to investigate the enhancement of flame propagation, and the competition with autoignition that arises under nominally autoignitive conditions (characterised here by the number τ/τR). Large values of τ/τR indicate that autoignition is unimportant, values near or below unity indicate that flame propagation is not possible, and intermediate values indicate that a combination of both flame propagation and autoignition may be important, depending upon factors such as device geometry, turbulence, stratification, et cetera. The reference quantities are presented for a wide range of temperatures, equivalence ratios, pressures, and hydrogen concentrations, which includes conditions relevant to stationary gas turbine reheat burners and boosted spark ignition engines. It is demonstrated that the transition from flame propagation to autoignition is only dependent on residence time, when the results are non-dimensionalised by the reference values. The temporal evolution of the reference values are also reported for a modelled boosted SI engine. It is shown that the nominally autoignitive conditions enhance flame propagation, which may be an ameliorating factor for the onset of engine knock. The calculations are performed using a recently-developed, detailed 177 species mechanism for C0–C3 chemistry that is derived from theoretical chemistry and is suitable for a wide range of thermochemical conditions as it is not tuned or optimised for a particular operating condition.
LES/CMC modelling of ignition and flame propagation in a non-premixed methane jet Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-11-02 Huangwei Zhang, Andrea Giusti, Epaminondas Mastorakos
The Large Eddy Simulation (LES) / Conditional Moment Closure (CMC) model with detailed chemistry is used for modelling spark ignition and flame propagation in a turbulent methane jet in ambient air. Two centerline and one off-axis ignition locations are simulated. We focus on predicting the flame kernel formation, flame edge propagation and stabilization. The current LES/CMC computations capture the three stages reasonably well compared to available experimental data. Regarding the formation of flame kernel, it is found that the convection dominates the propagation of its downstream edge. The simulated initial downstream and radial flame propagation compare well with OH-PLIF images from the experiment. Additionally, when the spark is deposited at off-centerline locations, the flame first propagates downstream and then back upstream from the other side of the stoichiometric iso-surface. At the leading edge location, the chemical source term is larger than others in magnitude, indicating its role in the flame propagation. The time evolution of flame edge position and the final lift-off height are compared with measurements and generally good agreement is observed. The conditional quantities at the stabilization point reflect a balance between chemistry and micro-mixing. This investigation, which focused on model validation for various stages of spark ignition of a turbulent lifted jet flame through comparison with measurements, demonstrates that turbulent edge flame propagation in non-premixed systems can be reasonably well captured by LES/CMC.
Structure and burning velocity of turbulent premixed methane/air jet flames in thin-reaction zone and distributed reaction zone regimes Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-11-02 Zhenkan Wang, Bo Zhou, Senbin Yu, Christian Brackmann, Zhongshan Li, Mattias Richter, Marcus Aldén, Xue-Song Bai
A series of turbulent premixed methane/air jet flames are studied using simultaneous planar laser diagnostic imaging of OH/CH/temperature and CH/OH/CH2O. The Karlovitz number of the flames ranges from 25 to 1500, and the turbulence intensity ranges from 16 to 200. These flames can be classified as highly turbulent flames in the thin reactions zone (TRZ) regime and distributed reaction zone (DRZ) regime. The aims of this study are to investigate the structural change of the preheat zone and the reaction zone as the Karlovitz number and turbulent intensity increase, to study the impact of the structural change of the flame on the propagation speed of the flame, and to evaluate the turbulent burning velocity computed in different layers in the preheat zone and reaction zone. It is found that for all investigated flames the preheat zone characterized with planar laser-induced fluorescence (PLIF) of CH2O is broadened by turbulent eddies. The thickness of the preheat zone increases with the turbulent intensity and it can be on the order of the turbulent integral length at high Karlovitz numbers. The reaction zone characterized using the overlapping layer of OH and CH2O PLIF signals is not significantly broadened by turbulence eddies; however, the CH PLIF layer is found to be broadened significantly by turbulence. The turbulent burning velocity is shown to monotonically increase with turbulent intensity and Karlovitz number. The increase in turbulent burning velocity is mainly due to the enhanced turbulent heat and mass transfer in various layers of the flame, while the contribution of flame front wrinkling to the turbulent burning velocity is rather minor.
Flame propagation speed and Markstein length of spherically expanding flames: Assessment of extrapolation and measurement techniques Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-11-02 J. Beeckmann, R. Hesse, J. Schaback, H. Pitsch, E. Varea, N. Chaumeix
Laminar burning velocities are of great importance in many combustion models as well as for validation and improvement of chemical kinetic schemes. Determining laminar burning velocities with high accuracy is quite challenging and different approaches exist. Hence, a comparison of existing methods measuring and evaluating laminar burning velocities is of interest. Here, two optical diagnostics, high speed tomography and Schlieren cinematography, are simultaneously set up to investigate methods for evaluating laminar flame speed in a spherical flame configuration. The hypothesis to obtain the same flame propagation radii over time with the two different techniques is addressed. Another important aspect is the estimation of flame properties, such as the unstretched flame propagation speed and Markstein length in the burnt gas phase and if these are estimated satisfactorily by common experimental approaches. Thorough evaluation of the data with several extrapolation techniques is undertaken. A systematic extrapolation approach is presented to give more confidence into results generated experimentally. The significance of the linear extrapolation routine is highlighted in this context. Measurements of spherically expanding flames are carried out in two high-pressure, high-temperature, constant-volume vessels at RWTH in Aachen, Germany and at ICARE in Orleans, France. For the discussion of the systematic extrapolation approach, flame speed measurements of methane / air mixtures with mixture Lewis numbers moderately away from unity are used. Conditions were varied from lean to rich mixtures, at temperatures of 298–373 K, and pressures of 1 atm and 5 bar.
Comparison of the Optical Connectivity Method to X-Ray spray measurements in the near field of a diesel injector Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-11-01 Karsten Gröger, Max Kaiser, Jin Wang, Friedrich Dinkelacker
For diesel sprays, the primary breakup processes are only rarely understood due to the high optical density and the resulting difficulties to measure them with extremely high spatial and sufficient temporal resolution. The Optical Connectivity Method (OCM) has been proposed in the last years to allow the determination of the breakup length of the connected liquid core, thus giving a measurement quantity of the primary breakup. In this work, an improved optical setup of the OCM is applied to a three-hole test injector nozzle where several measurement techniques are compared currently under well-defined conditions up to 100 MPa injection pressure. In this work, the direct comparison with X-Ray measurements done at the Advanced Photon Source of the Argonne National Laboratory will be described. This allows an evaluation of the OCM technique and a comparison of the different measurement quantities in the first 500 µm range of the spray. The structure of the spray is measured by X-Ray phase contrast imaging and the fuel mass distribution is measured by X-Ray absorption imaging. A detailed comparison of the two X-Ray techniques and the OCM technique has been possible for the first time. It is found that the measurement data of the spray near field are very congruent with all three methods. Due to this comparison, the measurement of the non-perturbed length, which describes the distance from the nozzle orifice up to the point where the formation of surface disturbances is starting, by the OCM is validated for the first time. Within this non-perturbed length of the spray, the OCM signal is weak before it starts to illuminate from the scattering of the perturbed surface. Thus, the OCM technique can deliver two characteristic length scales, the non-perturbed length and the breakup length, characterizing the primary spray breakup.
Experimental analysis of the pyrolysis of solids exposed to transient irradiation. Applications to ignition criteria Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-25 Simon Santamaria, Rory M. Hadden
Ignition and flame spread theory is fundamental for evaluating the risk posed by a material during the early stages of a fire. This paper presents an experimental investigation aimed at understanding the parameters which govern the ignition of solids exposed to transient irradiation. Emphasis is placed on the conditions at ignition, including an energy balance to describe surface phenomena and the link between gas phase and solid phase processes. Experiments were performed in a fire calorimetry apparatus and incorporated a gas analysis system to study gas phase composition. Samples of Polyamide 6 (PA6) measuring 85 × 85 × 20 mm were used. Experiments were carried out to independently measure temperature in the solid phase and mass loss rate (MLR) over time. The MLR at ignition was calculated to be between 2.0 and 6.0 g/(m2 s) for all but 3 experiments, were outliers presented values of 7.9, 10.7 and 14.4 g/(m2 s). Temperature was recorded through the thickness of the solid, at depths of 4, 8, 12 and 16 mm from the surface. A regression analysis was used to calculate the surface temperature at ignition for all experiments, and it was found to vary between 270 and 325 °C for all but one experiment, were a temperature of 402 °C was recorded. The temperature distribution in the solid phase was used to estimate the net absorbed heat flux at the surface by applying Fourier’s law; with values ranging between 2.0 and 9.8 kW/m2. From the gas analysis, it was possible to assess the identity, mass flux and concentration of three dominant species produced before ignition: carbon monoxide, methane and hexane. These results are of value for the physical modelling of ignition and flame spread phenomena, allowing for more accurate criteria to describe the onset of ignition under a range of heating conditions.
Kinetic modeling study of surrogate components for gasoline, jet and diesel fuels: C7-C11 methylated aromatics Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-24 Goutham Kukkadapu, Dongil Kang, Scott W. Wagnon, Kuiwen Zhang, Marco Mehl, M. Monge-Palacios, Heng Wang, S. Scott Goldsborough, Charles K. Westbrook, William J. Pitz
Kinetic mechanisms for aromatics are needed to successfully simulate the autoignition of transportation fuels using the surrogate fuel approach. An aromatic detailed kinetic mechanism that describes kinetics of C7-C11 methylated aromatics, including toluene, o-xylene, p-xylene, 1,2,4-trimethylbenzene, 1,3,5, trimethylbenzene and α−methylnaphthalene has been developed in the current study. The kinetic mechanism was built hierarchically using similar set of reaction pathways and reaction rate rules. In the mechanism developed, special emphasis is put on describing the detailed low-temperature ignition chemistry of o-xylene and 1,2,4-trimethylbenzene and, to our knowledge, this is the first attempt to do so in a detailed kinetic mechanism. In addition to kinetic modeling, new experimental data were acquired for toluene, o-xylene, and 1,2,4-trimethylbenzene using a rapid compression machine at low-to-intermediate temperatures and engine relevant pressures. In addition, the mechanism has been compared against data sets from the literature covering ignition delay times, flame speeds, and speciation profiles measured in a jet-stirred reactor and flow reactor. Good agreement is observed between the mechanism predictions and the experimental data. Kinetic analysis demonstrated the importance of including the low temperature chemistry of the benzylperoxy radicals to accurately capture the ignition propensity of o-xylene and 1,2,4-trimethylbenzene at low-to-intermediate temperatures and high pressures. The kinetic mechanism developed in the current study can be used for surrogate modeling of gasoline, jet and diesel fuels.
Role of inertial forces in flame-flow interaction during premixed swirl flame flashback Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-23 Rakesh Ranjan, Dominik F Ebi, Noel T Clemens
This study investigates the flame-flow interaction during a fully-premixed swirl flame flashback from flame-frame-of-reference. To capture the flame front movement during upstream propagation, high-speed chemiluminescence imaging and simultaneous three-component PIV measurements are taken at 4 kHz. The upstream propagation of the flame occurs along a helical path around the center-body. For low-turbulence and high-swirl conditions (Reh = 4000, Swirl number ∼ 0.9), the lab-frame speed of the flame structure remains nearly constant during the period of investigation. Simultaneously, the leading side of the flame tongue retains its topology during propagation. The steady-state propagation behavior of the flame structure and stationarity of the flame topology allows us to make a frozen-flame-surface assumption. Applying space-time equivalence, the three-dimensional flame surface and flow field are reconstructed by shifting and stacking the time-series of the planar flame front profiles and the three-component planar velocity data. Further, the steady flow in the flame frame-of-reference provides a powerful means of investigating the flame-flow interaction. Quasi-pathlines are constructed in the unburnt and burnt regions of the flow field. The motion of the approach flow along a quasi-pathline is analyzed to understand the role of centrifugal and Coriolis forces. It is shown that the tug-of-war situation between Coriolis and centrifugal forces gets disrupted by the dilatation-driven blockage effect from the flame surface. It leads to a radial deflection of the approach flow, which results in reduction in the flame-normal approach flow speed, thereby assisting in the flame propagation. In the burnt gas, the Coriolis Effect bends the pathlines towards the center-body. We show - for the first time - that the azimuthal motion of the flame assists in the upstream propagation of the flame structure. Error assessment shows that the approximations made to construct the flame-surface and the flow-field retains the physics of flame-flow interactions.
Emerging trends in numerical simulations of combustion systems Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-23 Venkat Raman, Malik Hassanaly
Numerical simulations have played a vital role in the design of modern combustion systems. Over the last two decades, the focus of research has been on the development of the large eddy simulation (LES) approach, which leveraged the vast increase in computing power to dramatically improve predictive accuracy. Even with the anticipated increase in supercomputing capabilities, the use of LES in design is limited by its high computational cost. Moreover, to aid decision making, such LES computations have to be augmented to estimate underlying uncertainties in simulation components. At the same time, other changes are happening across industries that build or use combustion devices. While efficiency and emissions reduction are still the primary design objectives, reducing cost of operation by optimizing maintenance and repair is becoming an important segment of the enterprise. This latter quest is aided by the digitization of combustors, which allows collection and storage of operational data from a host of sensors over a fleet of devices. Moreover, several levels of computing including low-power hardware present on board the combustion systems are becoming available. Such large data sets create unique opportunities for design and maintenance if appropriate numerical tools are made available. As LES revolutionized computing-guided design by leveraging supercomputing, a new generation of numerical approaches is needed to utilize this vast amount of data and the varied nature of computing hardware. In this article, a review of emerging computational approaches for this heterogeneous data-driven environment is provided. A case is made that new but unconventional opportunities for physics-based combustion modeling exist in this realm.
A computational investigation into the combustion byproducts of a liquid monopropellant Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-19 Mark E. Fuller, Richard H. West, C. Franklin Goldsmith
A detailed chemical kinetic mechanism is developed for the gas-phase combustion of a liquid monopropellant, which is a blend of propylene-glycol-dinitrate, dibutyl-sebacate, and 2-nitro-diphenylamine (Otto Fuel II). The combustor is modeled as a steady-state burner-stabilized flame. The simulations reveal that not all of the dibutyl-sebacate is consumed in the flame, with approximately 5% persisting in the post-flame region. A large class of combustion byproducts are formed that have boiling points above the post-flame temperature and thus would be expected to condense out along the length of the combustor. This post-flame, two-phase behavior is hypothesized to be the cause of empirically observed oily build-up within the engine. This work represents a novel advancement in predictive modeling for propellant design, as it provides mechanistic insight into the possible origins of engine fouling.
Large Eddy Simulation of MILD combustion using finite rate chemistry: Effect of combustion sub-grid closure Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-19 Zhiyi Li, Alberto Cuoci, Alessandro Parente
In this work, we present a detailed comparison between the conventional Partially Stirred Reactor (PaSR) combustion model and two implicit combustion models, named Quasi Laminar Finite Rate (QLFR) model and Laminar Finite Rate (LFR) model, respectively. Large Eddy Simulation (LES) is employed and the Adelaide Jet in Hot Co-flow (AJHC) burner is chosen as validation case. In the implicit combustion models, the filtered source term comes directly from the chemical term, without inclusion of turbulence effects. Results demonstrate that the two implicit models behave similarly to the conventional PaSR model, for the mean and root-mean-square of the temperature and species mass fractions, and that all models provide very satisfactory predictions, especially for the mean values. This justifies the use of implicit combustion models in low Damköhler number (Da ≤ 1.0) systems. The QLFR model allows to reduce the computational cost of about three times, compared to the LFR model. Moreover, the comparison between two 4-step global mechanisms and the KEE58 mechanism proves the importance of finite rate chemistry in MILD combustion.
Exploratory analysis of a sooting premixed flame via on-line high resolution (APi–TOF) mass spectrometry Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-19 Francesco Carbone, Manjula R. Canagaratna, Andrew T. Lambe, John T. Jayne, Douglas R. Worsnop, Alessandro Gomez
By taking advantage of recent advances in High-Resolution Atmospheric Pressure intake Time of Flight (APi–TOF) Mass Spectrometry (MS), the chemical analysis of naturally charged flame-generated soot nuclei and precursors is explored using a well-characterized dilution sampling approach. Measurements were performed for mass-to-charge ratio up to 2000 Thomson, bridging the gap between the gas phase and the particle phase. The flame products were sampled at several heights above the burner (HAB) in the soot inception zone of the flame, quickly diluted in nitrogen and directly transported to the APi–TOF inlet. The investigated sooting premixed flame has been the object of multiple studies over the years and the present results complement existing literature data. The analyses of flame products naturally carrying charge of either polarity revealed the chemical and polarity-dependent complexity of the nucleation and chemi-ionization processes. The measured high-resolution mass spectra include peaks attributed to (hydrocarbon) molecules/clusters containing oxygen and nitrogen atoms and suggest that collision charging of flame pyrolysis products likely involves protonation/deprotonation of neutral materials. Results clearly show the change of the overall composition of the charged flame products at different HABs. Patterns in the mass spectra under different conditions were investigated to discriminate between collision charging, chemical reaction and physical clustering (i.e., coagulation and condensation) growth mechanisms. A comparison of the results with those obtained with High-resolution Differential Mobility Analysis (HR-DMA) in a recent study allowed for a more quantitative determination of the ion number concentrations.
Enhancement of continuously rotating detonation in hydrogen and oxygen-enriched air Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-19 Qiaofeng Xie, Bing Wang, Haocheng Wen, Wei He, Piotr Wolanski
The enhancement of continuously rotating detonation in oxygen-enriched air was demonstrated in an annular rotating detonation combustor (RDC) under a diffusive supply of hydrogen and an oxidizer. Experimental tests were performed to reveal the effects of oxygen volume fraction, mass flow rate, and equivalent ratio on the propagation of continuously rotating detonation wave (CRDW). It is observed that an increase in the air mass flow rate from 25 g/s to 225 g/s causes an increase in the propagation velocity of the stable CRDW in the RDC. For an oxygen volume fraction up to 35%, the difference between the propagation velocity of detonation and the theoretical Chapman–Jouguet value is less than 5%. Under the chemical stoichiometric ratio condition for air, the CRDW is stabilized when the air mass flow rate reaches 185 g/s. However, stabilized CRDW is observed even when the air flow rate is only 45 g/s under the presence of 30% or 35% oxygen. Increase in the oxygen volume fraction leads to an extension of the rich/lean limit for generating a stable CRDW. This study aims to provide guidance for the modulation of continuously rotating detonation.
Infrared borescopic characterization of corona and conventional ignition for lean/dilute combustion in heavy-duty natural-gas engines Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-19 Ahmet Mazacioglu, Michael C. Gross, Justin Kern, Volker Sick
Natural gas (NG) is attractive for heavy-duty (HD) engines for reasons of cost stability, emissions, and fuel security. NG requires forced ignition, but conventional gasoline-engine ignition systems are not optimized for NG and are challenged to ignite mixtures that are lean or diluted with exhaust-gas recirculation (EGR). NG ignition is particularly difficult in large-bore engines, where it is more challenging to complete combustion in the time available. High-speed infrared (IR) in-cylinder imaging and image-derived quantitative metrics were used to compare two ignition systems in terms of the early flame-kernel development and cycle-to-cycle variability (CCV) in a heavy-duty, natural-gas-fueled engine that had been modified to enable exhaust-gas recirculation and to provide optical access via borescopes. Imaging in the near IR and short-wavelength IR yielded strong signals from the water emission lines, which acted as a proxy for flame front and burned-gas regions while obviating image intensification (which can reduce spatial resolution). The ignition systems studied were a conventional system and a high-frequency corona system. The air/fuel mixtures investigated included stoichiometric without dilution and lean with EGR. The corona system produced five separate elongated, irregularly shaped, nonequilibrium-plasma streamers, leading to immediate formation of five spatially distinct wrinkled flame kernels around each streamer. Compared to the conventional spark ignition, which produces a single flame kernel that exhibits an initial laminar growth regime before wrinkling, corona ignition's early achievement of higher flame surface areas significantly shortened the ignition delay, resulting in reduced overall combustion duration and CCV for each mixture. Additionally, although the lean, dilute mixture produced higher CCV than the stoichiometric, minimally diluted mixture with both igniters, the mixtures ignited by the corona system suffered less than those ignited by the conventional system. Image-based measurements of CCV agreed with those based on in-cylinder pressure.
Plasma enhanced auto-ignition in a sequential combustor Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-19 Yuan Xiong, Oliver Schulz, Claire Bourquard, Markus Weilenmann, Nicolas Noiray
To control the ignition and stabilization location of the second stage flame in a sequential combustor, nanosecond repetitively pulsed discharges (NRPD) were generated between three cylindrical electrodes. The NRPD were obtained by repetitively applying high voltage pulses to the central electrode. At operating conditions where the sequential flame was nearly quenched or weakly anchored, it was possible to re-ignite it and to control its location by adjusting the voltage (V) amplitude (5–10 kV) and repetition frequency (f) (1–100 kHz) of NRPD. This plasma enhanced auto-ignition was achieved with an acceptable increase of NOx emissions. Similar flame stabilization locations can be achieved by blending propane into the fuel stream, from which one could deduce that the auto-ignition delay was reduced by a factor of ten with NRPD. Direct images of the discharge indicated that applied NRPD corresponded to one the following modes depending on V and f: glow, transition and spark. With the electrodes placed downstream of the fuel injection, quenching effect of the cold fuel on NRPD generation was observed, while when the electrodes were positioned upstream of the fuel injection no ignition event could be observed. High speed imaging of OH* chemiluminescence revealed that spatially homogeneous and temporally continuous auto-ignition was much more efficiently triggered by NRPD in spark mode than NRPD in glow mode. High energy efficiency of NRPD was validated by measuring the plasma energy deposition. The result showed that NRPD with a power consumption about 100 W were sufficient to control a 50 kW sequential combustor.
Detonation onset in a thermally stratified constant volume reactor Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-17 Aliou Sow, Bok Jik Lee, Francisco E. Hernández Pérez, Hong G. Im
Understanding detonation development from a flame kernel initiated by a pre-ignition event is important for modern internal combustion (IC) engines operating at boosted conditions. To provide fundamental insights into the effects of bulk gas temperature stratification on the characteristics of detonation development, one-dimensional high fidelity simulations were conducted for a constant volume reactor filled with a thermally stratified reactive stoichiometric hydrogen/air mixture. A linear temperature variation in the upstream end-gas was introduced to represent the thermal stratification of the bulk mixture, and the evolution from the initial deflagration flame front to detonation development was examined. The results showed that the bulk-gas temperature gradient has a significant effect on the run-up time and intensity of the developing detonation. Detailed analyses further revealed that the mechanism of detonation development is qualitatively different for the positive and negative temperature gradient cases. In the former, the detonation development is initiated by the end-gas autoignition at the wall, while the latter exhibits detonation development following the process of the self-acceleration of the flame similar to the deflagration-to-detonation transition. This behavior is attributed to the longer residence time in the end-gas allowing the reinforcement by the interaction of incident and reflected pressure waves during the flame propagation, and results in the peak pressure even higher than the case with the same level of positive temperature gradient. Furthermore, yet another detonation development pattern was observed for the negative temperature gradient condition in the presence of a uniform temperature region just ahead of the flame. In this case, autoignition was found to start in the middle of the bulk end-gas, and subsequently leads to the transition to detonation. The results demonstrate the importance of the bulk gas conditions in predicting the detonation development, which corroborate the existing theoretical framework.
An experimental study into the effect of injector pressure loss on self-sustained combustion instabilities in a swirled spray burner Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-17 Guillaume Vignat, Daniel Durox, Kevin Prieur, Sébastien Candel
Combustion instabilities depend on a variety of parameters and operating conditions. It is known, especially in the field of liquid rocket propulsion, that the pressure loss of an injector has an effect on its dynamics and on the coupling between the combustion chamber and the fuel manifold. However, its influence is not well documented in the technical literature dealing with gas turbine combustion dynamics. Effects of changes in this key design parameter are investigated in the present article by testing different swirlers at constant thermal power on a broad range of injection velocities in a well controlled laboratory scale single injector swirled combustor using liquid fuel. The objective is to study the impact of injection pressure losses on the occurrence and level of combustion instabilities by making use of a set of injectors having nearly the same outlet velocity profiles, the same swirl number and that establish flames that are essentially identical in shape. It is found that combustion oscillations appear on a wider range of operating conditions for injectors with the highest pressure loss, but that the pressure fluctuations caused by thermoacoustic oscillations are greatest when the injector head loss is low. Four types of instabilities coupled by two modes may be distinguished: the first group features a lower frequency, arises when the injector pressure loss is low and corresponds to a weakly coupled chamber-plenum mode. The second group appears in the form of a constant amplitude limit cycle, or as bursts at a slightly higher frequency and is coupled by a chamber mode. Spontaneous switching between these two types of instabilities is also observed in a narrow domain.
Large Eddy Simulation of a supersonic lifted flame using the Eulerian stochastic fields method Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-16 Yuri Paixão de Almeida, Salvador Navarro-Martinez
Scramjet propulsion systems can be the key to deliver the next generation of hypersonic planes. The high costs and complexity of gathering experimental data is a limiting factor in the development of such engine. In this context, numerical simulation has become increasingly popular to investigate supersonic combustion phenomena that otherwise would be prohibitively expensive. Despite recent progress, the simulation of high-speed compressible and reactive flows is still very challenging and presents many associated challenges. The chemical source term is highly non-linear and most combustion models are designed to operate in low-Mach number conditions. The present work investigates the use of Probability Density Function (PDF) in the context of Large Eddy Simulation models under supersonic conditions. Two approaches are considered: an extension of the joint scalar-enthalpy PDF for high-speed flows and a novel joint velocity-scalar-energy PDF model. Both formulations use the Eulerian stochastic fields approach implemented in a fully compressible density-based CFD code. The performance of the models are investigated in a supersonic lifted flame, comparing the stochastic formulations with traditional models that neglect sub-grid fluctuations. The results show that sub-grid contributions are important at coarse meshes and the stochastic fields approach can reproduce the experimental data and the scatter observed. The simulations suggest that the scalar-enthalpy PDF is the most robust formulations and the sub-grid closures of the joint velocity-scalar PDF need further investigation.
Fragmentation of pulverized coal in a laminar drop tube reactor: Experiments and model Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-15 Osvalda Senneca, Sebastian Heuer, Piero Bareschino, Massimo Urciuolo, Francesco Pepe, Martin Schiemann, Riccardo Chirone, Viktor Scherer
Fragmentation during pulverized coal particles conversion shifts the particle size distribution of the fuel towards smaller particle sizes, affecting both conversion rates and heat release. After pyrolysis of a high volatiles Colombian coal in CO2 atmosphere in a drop tube reactor at 1573 K, solid carbonaceous particles of different size, from 100 µm of the particle feed down to the nanometric size, have been observed. A fragmentation model has been used to predict the fate of Colombian coal particles under the experimental conditions of the drop tube experiment and predict the particle size distribution (PSD). Model and experimental results are in very good agreement and indicate that in the DTR experiment the coal underwent almost complete pyrolysis and that fragmentation generated a 36 wt% population of particles with size close to 30 µm. The close match between the PSDs obtained from experiments and from the fragmentation model is an important novelty. It demonstrates that fragmentation occurs not only under fluidized bed conditions but also under the conditions of pulverized coal combustion. Experimentalists are warned against the fact that the fine particulate sampled at the outlet of laminar flow reactors and boilers is not always composed of soot only. Char fragments can be misidentified as soot. The implementation of fragmentation submodels in pulverized fuel combustion and gasification codes is highly recommended.
Pressure effects and transition in the stabilization mechanism of turbulent lifted flames Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-15 T.F. Guiberti, W.R. Boyette, W.L. Roberts, A.R. Masri
This study reports novel measurements on the effects of pressure on the lift-off behavior and the stabilization mechanism of turbulent non-premixed methane jet flames. A high-pressure combustion duct (HPCD) was operated within the range of pressure P = 1–7 bar using jet velocities of 4 m.s−1 ≤Uj≤30 m.s−1 and co-flow velocities of 0.23 m.s−1 ≤Uc≤0.60 m.s−1. Lift-off heights were measured from chemiluminescence pictures while joint images of hydroxyl and velocity, performed using joint PLIF-OH/PIV, were used to extract information about the stabilization mechanism. It is shown that while the lift-off height generally increases with pressure, the impact of pressure depends on the magnitude of the co-flow velocity. For Uc = 0.30 m.s−1, the flame's base remains near the nozzle over the entire pressure range and the measured flame speeds indicate that edge-flame stabilization is dominant. The slope of the lift-off height vs. jet velocity curves is positive. For Uc = 0.60 m.s−1 and P > 2 bar, the flame stabilizes further downstream and a transition to turbulent premixed flame propagation appears to have occurred. At these conditions, the slope of the lift-off height vs. jet velocity curves becomes negative. This reversal at high pressure is a new result for methane. More importantly, the transition in the stabilization mechanism with increasing Uc is consistent with results reported earlier for ethylene and appears to be independent of the fuel.
Auto-ignition of near-ambient temperature H2/air mixtures during flame-vortex interaction Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-14 Adam M. Steinberg, Ketana Teav, Sina Kheirkhah, Chaimae Bariki, Fabien Thiesset, Christian Chauveau, Fabien Halter
This paper demonstrates auto-ignition in reactants at approximately 350 K, upstream of curved H2/air flame surfaces during flame/vortex interaction. Temperature fields were measured using laser Rayleigh scattering during head-on interactions of toroidal-vortices with stagnation flames. Repeatable ignition occurred along the ring of the vortex – slightly towards the center – when it was approximately 1 mm upstream of the wrinkled flame surface. The resultant outwardly propagating toroidal flame led to approximately twice the volumetric heat release rate over the duration of the interaction. The ignition occurred in a region of low kinetic energy dissipation rate that was farther from the flame than the region of maximum vorticity. This region was upstream of positively curved flame segments. The advective time over which the vortex transports flame-generated products to the ignition site corresponded well to the time between the beginning of the flame/vortex interaction and ignition. It therefore is hypothesized that the vortex transports HO2 and H2O2 to the low-temperature, low-dissipation region wherein ignition is promoted by preferential diffusion of H due to the positively curved flame. Evidence of additional ignition pockets was found upstream of other flame wrinkles, preferentially near the highest magnitude flame curvatures. These results provide a novel test case for validating diffusion and low-temperature kinetic models, and also have potential implications for reaction rate closure models.
Large-eddy simulation of MILD combustion using partially stirred reactor approach Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-13 Hao Lu, Chun Zou, Shujing Shao, Hong Yao
Subgrid-scale (SGS) parameterization and method for calculating filtered reaction rate are critical components of an accurate large-eddy simulation (LES) of turbulent flames. In this study, we integrate gradient-type structural SGS models with a partially stirred reactor approach by using detailed chemical kinetics to simulate a turbulent methane/hydrogen jet flame under moderate or intense low-oxygen dilution (MILD) conditions. The study examines two oxygen dilution levels. The framework is assessed through a systematic and comprehensive comparison of temperature, and mass fractions of major and minor species with experimental data and other reference simulation results. Overall, the statistics of the combustion field show excellent agreement with measurements at different axial locations, and a significant improvement compared to some previous simulations. It suggests that the proposed nonlinear LES framework is able to accurately model MILD combustion with reasonable computational cost.
Modelling Chemical-Looping assisted by Oxygen Uncoupling (CLaOU): Assessment of natural gas combustion with calcium manganite as oxygen carrier Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-12 Alberto Abad, Pilar Gayán, Luis F. de Diego, Francisco García-Labiano, Juan Adánez
Chemical-Looping Combustion (CLC) is a promising technology for performing CO2 capture in combustion processes at low cost and with lower energy consumption. Fuel conversion modelling assists in optimizing and predicting the performance of the CLC process under different operating conditions. For this work, the combustion of natural gas was modelled using a CaMnO3-type perovskite as oxygen-carrier and taking into consideration the processes of fluid dynamics and reaction kinetics involved in fuel conversion. The CLC model was validated against experimental results obtained from the 120 kWth CLC unit at the Vienna University of Technology (TUV). Good agreement between experimental and model predictions of fuel conversion was found when the temperature, pressure drop, solids circulation rate and fuel flow were varied. Model predictions showed that oxygen transfer by means of the gas–solid reaction of the fuel with the oxygen-carrier was relevant throughout the entire fuel-reactor. However, complete combustion could be only achieved under operating conditions where the process of Chemical-Looping assisted by Oxygen Uncoupling (CLaOU) became dominant, i.e. a relevant fraction of the fuel was burnt with molecular oxygen (O2) released by the oxygen-carrier. This phenomenon was improved by the design configuration of the 120 kWth CLC unit at TUV, in which oxidized particles are recirculated to the upper part of the fuel-reactor. Thus, the validated model identified the conditions at which complete combustion can be achieved, demonstrating that it is a powerful tool for the simulation and optimization of the CLC process with the CaMnO3-type material.
Surrogate formulation for diesel and jet fuels using the minimalist functional group (MFG) approach Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-11 Abdul Gani Abdul Jameel, Nimal Naser, Abdul-Hamid Emwas, S. Mani Sarathy
Surrogate fuels aim to reproduce real fuel combustion characteristics in order to enable predictive simulations and fuel/engine design. In this work, surrogate mixtures were formulated for three diesel fuels (Coryton Euro and Coryton US-2D certification grade and Saudi pump grade) and two jet fuels (POSF 4658 and POSF 4734) using the minimalist functional group (MFG) approach, a method recently developed and tested for gasoline fuels. The diesel and jet fuel surrogates were formulated by matching five important functional groups, while minimizing the surrogate components to two species. Another molecular parameter, called as branching index (BI), which denotes the degree of branching was also used as a matching criterion. The present works aims to test the ability of the MFG surrogate methodology for high molecular weight fuels (e.g., jet and diesel). 1H Nuclear Magnetic Resonance (NMR) spectroscopy was used to analyze the composition of the groups in diesel fuels, and those in jet fuels were evaluated using the molecular data obtained from published literature. The MFG surrogates were experimentally evaluated in an ignition quality tester (IQT), wherein ignition delay times (IDT) and derived cetane number (DCN) were measured. Physical properties, namely, average molecular weight (AMW) and density, and thermochemical properties, namely, heat of combustion and H/C ratio were also compared. The results show that the MFG surrogates were able to reproduce the combustion properties of the above fuels, and we demonstrate that fewer species in surrogates can be as effective as more complex surrogates. We conclude that the MFG approach can radically simplify the surrogate formulation process, significantly reduce the cost and time associated with the development of chemical kinetic models, and facilitate surrogate testing.
Investigation of the structure of detonation waves in a non-premixed hydrogen–air rotating detonation engine using mid-infrared imaging Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-11 Brent A. Rankin, Joshua R. Codoni, Kevin Y. Cho, John L. Hoke, Frederick R. Schauer
The structure of detonation waves propagating through the annular channel of an optically accessible non-premixed rotating detonation engine (RDE) are investigated using mid-infrared imaging. The RDE is operated on hydrogen–air mixtures for a range of air mass flow rates and equivalence ratios. Instantaneous images of the radiation intensity from water vapor are acquired using a mid-infrared camera and a band-pass filter (2.890 ± 0.033 µm). The instantaneous mid-infrared images reveal the stochastic nature of the detonation wave structure, position and angle of oblique and reflected shock waves, presence of shear layer separating products from the previous and current cycles, and extent of mixing between the reactants and products in the reactant fill zone in front of the detonation wave. The images show negligible signal directly in front of the detonation waves suggesting that there is minimal mixing between the reactants and products from the previous cycle ahead of the detonation wave for most operating conditions. The mid-infrared images provide insights useful for improving fundamental understanding of the detonation structure in RDEs and benchmark data for evaluating modeling and simulation results of RDEs.
Effects of flash boiling injection on in-cylinder spray, mixing and combustion of a spark-ignition direct-injection engine Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-10 Xue Dong, Jie Yang, David L.S. Hung, Xuesong Li, Min Xu
Higher engine efficiency and ever stringent pollutant emission regulations are considered as the most important challenges for today's automotive industry. Fast evaporation and combustion technique has caused unprecedented attention due to its potential to solve both of the above challenges. Flash boiling, which features a two-phase flow that constantly generates vapor bubbles inside the liquid spray is ideal to achieve fast evaporation and combustion inside direct-injection (DI) gasoline engines. In this study, three spray conditions, including liquid, transitional flash boiling and flare flash boiling spray were studied for comparison under cold start condition in a spark-ignition direct-injection (SIDI) optical gasoline engine. Optical access into the combustion chamber includes a quartz linear and a quartz insert on the piston. In separate experiments, we recorded the crank angle resolved spray morphology using laser scattering technique, and distribution of fuel before ignition employing laser induced fluorescence technology, as well as time-resolved color images of flame with high-speed camera. The spray morphology during the intake stroke shows stronger plume-plume and plume-air interaction under flash boiling condition, as well as smaller penetration. Then around the end of compression (before ignition), the fuel distribution is also shown to be more homogeneous with less cyclic variation under flash boiling. Finally, from the color images of the flame, it was found that with the increase of superheat degree, the diffusion rate of blue flame (generated by excited molecules) is higher, which is considered to be related with the larger fractal dimension of the flame front. Also, the combustion is more complete with less yellow flame under flash boiling.
Effects of differential diffusion on nonpremixed-flame temperature Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-10 A. Almagro, O. Flores, M. Vera, A. Liñán, A.L. Sánchez, F.A. Williams
This numerical and analytical study investigates effects of differential diffusion on nonpremixed-flame temperatures. To focus more directly on transport effects the work considers a single irreversible reaction with an infinitely fast rate, with Schab–Zel’dovich coupling functions introduced to write the conservation equations of energy and reactants in a chemistry-free form accounting for non-unity values of the fuel Lewis number L F . Different flow configurations of increasing complexity are analyzed, beginning with canonical flamelet models that are reducible to ordinary differential equations, for which the variation of the flame temperature with fuel-feed dilution and L F is quantified, revealing larger departures from adiabatic values in dilute configurations with oxidizer-to-fuel stoichiometric ratios S of order unity. Marble’s problem of an unsteady flame wrapped by a line vortex is considered next, with specific attention given to large-Peclet-number solutions. Unexpected effects of differential diffusion are encountered for S < 1 near the vortex core, including superadiabatic/subadibatic flame temperatures occurring for values of L F larger/smaller than unity as well as temperature profiles peaking on the oxidizer side of the flame. Direct numerical simulations of diffusion flames in a temporal turbulent mixing layer are used to further investigate these unexpected differential–diffusion effects. The results, confirming and extending previous findings, underscore the nontrivial role of differential diffusion in nonpremixed–combustion systems.
The Effect of Curvature and Confinement on Gas-Phase Detonation Cellular Stability Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-09 Carlos Chiquete, Mark Short, James J. Quirk
We examine the evolution of cellular detonation patterns in a two-dimensional channel with yielding confinement on one side. It is shown that in a narrow channel of fixed width the number of cells first increase with decreasing level of confinement. Subsequently, with increasingly weaker confinement, the cells then grow in size and the total number of cells in the channel decreases. For sufficiently weak confinement, the flow becomes laminar with no detonation cells. We examine the relative importance of two fluid mechanisms underlying the observed evolution: global curvature of the detonation shock front due to induced flow divergence caused by the yielding confinement, and energy loss associated with transverse shock wave transmission to the confining material. In order to determine which effect is dominant, we compare two types of numerical calculations. One involves specialized calculations in which the explosive boundary, along which impermeable flow conditions are applied, is deflected through a range of specified angles upon detonation arrival. This set-up mimics the effect of yielding confinement in terms of induced flow divergence, but removes the transverse wave energy loss that would otherwise occur due to wave transmission into the confiner material. The second involves multi-material simulations which can account for transverse wave energy loss into the confining material. Shock polar theory is used to select confiner densities in the multi-material calculations that provide equivalent material interface deflection angles at the detonation shock to the angles imposed in the deflected solid wall calculations. We determine that the induced global curvature of the wave primarily drives both the evolution of the cellular pattern and eventual stabilization of the detonation front, characterized by laminar flow solutions. In wider channels, we show that the detonation front will likely remain unstable even for very weak confinement, as the mean curvature of the front only becomes significant near the edge of the explosive domain.
Analysis of supersonic combustion characteristics of ethylene/methane fuel mixture on high-speed measurements of CH* chemiluminescence Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-09 Shinji Nakaya, Ryosuke Kinoshita, Jeonghoon Lee, Hiromu Ishikawa, Mitsuhiro Tsue
Supersonic combustion behaviors in a Mach 2.0 scramjet model combustor were experimentally investigated at stagnation temperatures of 1700–2300 K. An ethylene/methane fuel mixture was used, and the mole fraction of ethylene and the equivalence ratio were varied. Six typical combustion modes were classified based on high-speed imaging of CH* chemiluminescence at 50,000 fps, shadowgraph imaging at 4000 fps and pressure distributions. Ram mode combustion was observed, without thermal choke at the nozzle throat. Mode maps indicated that an increase in the ethylene concentration improved the supersonic combustion performance. Proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) of sequential images of CH* chemiluminescence were also conducted. POD and DMD mode spectra showed large peaks in the frequency range of 100–500 Hz for cavity shear-layer stabilized combustion. Although the power of the spectra decreased, peaks were also observed in the same frequency range for jet-wake stabilized and ram combustion. In the case of the ram combustion mode, the peak heights decreased. The FFT of several primary POD modes and the power spectra of DMD modes showed peaks with similar frequencies. Combustion modes could be classified from these spectra. The fundamental combustion frequencies were captured by the modal decomposition of the high-speed images of CH* chemiluminescence.
Impact of pressure fluctuations on the dynamics of laminar premixed flames Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-09 Guillaume Beardsell, Guillaume Blanquart
Thermo-acoustic instabilities are problematic in the design of continuous-combustion propulsion systems such as gas turbine engines, rocket motors, jet engine afterburners, and ramjets. Conceptually, the coupling between acoustics and flame dynamics can be divided into two categories: flame area fluctuations and changes in the local flame speed. The latter can be caused by the thermodynamic fluctuations that accompany an acoustic wave. This coupling is the focus of the present work. In this paper, we are concerned with the dynamics of laminar premixed flames involving large hydrocarbon species. Through high-fidelity numerical simulations, we investigate the flame response for a wide range of fuels and acoustic frequencies. The combustion of hydrogen and methane is considered for verification purposes and as baseline cases for comparison with two large hydrocarbon fuels, n-heptane and n-dodecane. We extract the phase and gain of the unsteady heat release response, which are directly related to the Rayleigh criterion and thus the stability of the system. For all fuels, we observe a local peak in the heat release gain. At high frequencies, we find that the fluctuations of the different species mass fractions decrease with the inverse of the acoustic frequency, leading to chemistry being “frozen” in the high-frequency limit. This allows us to predict the flame behavior directly from the steady-state solution.
Experimental and computational investigation of autoignition of jet fuels and surrogates in nonpremixed flows at elevated pressures Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-05 Gerald Mairinger, Alessio Frassoldati, Alberto Cuoci, Matteo Pelucchi, Ernst Pucher, Kalyanasundaram Seshadri
Experimental and computational investigations are carried out to elucidate the fundamental mechanisms of autoignition of surrogates of jet-fuels at elevated pressures up to 6 bar. The jet-fuels tested are JP-8, Jet-A, and JP-5, and the surrogates tested are the Aachen Surrogate made up of 80 % n-decane and 20 % 1,3,5-trimethylbenzene by mass, Surrogate C made up of 60 % n-dodecane, 20 % methylcyclohexane and 20 % o-xylene by volume, and the 2nd generation Princeton Surrogate made up of 40.4 % n-dodecane, 29.5 % 2,2,4-trimethylpentane, 7.3 % 1,3,5-trimethylbenzene and 22.8 % n-propylbenzene by mole. Using the counterflow configuration, an axisymmetric flow of a gaseous oxidizer stream, made up of a mixture of oxygen and nitrogen, is directed over the surface of an evaporating pool of a liquid fuel. The experiments are conducted at a fixed value of mass fraction of oxygen in the oxidizer stream and at a fixed value of the strain rate. The temperature of the oxidizer stream at autoignition, Tig, is measured as a function of pressure, p. Experimental results show that the critical conditions, of autoignition of the surrogates are close to that of the jet-fuels. Overall the critical conditions of autoignition of Surrogate C agree best with those of the jet-fuels. Computations were performed using skeletal mechanisms constructed from a detailed mechanism. Predictions of the critical conditions of autoignition of the surrogates are found to agree well with measurements. Computations show that low-temperature chemistry plays a significant role in promoting autoignition for all surrogates. The low-temperature chemistry, of the component of the surrogate with the greatest volatility, was found to have the most influence on the critical conditions of autoignition.
Three stage cool flame droplet burning behavior of n-alkane droplets at elevated pressure conditions: Hot, warm and cool flame Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-05 T.I. Farouk, D. Dietrich, F.L. Dryer
Transient, isolated n-alkane droplet combustion is simulated at elevated pressure for helium-diluent substituted-air mixtures. We report the presence of unique quasi-steady, three-stage burning behavior of large sphero-symmetric n-alkane droplets at these elevated pressures and helium substituted ambient fractions. Upon initiation of reaction, hot-flame diffusive burning of large droplets is initiated that radiatively extinguishes to establish cool flame burning conditions in nitrogen/oxygen “air” at atmospheric and elevated pressures. However, at elevated pressure and moderate helium substitution for nitrogen (XHe > 20%), the initiated cool flame burning proceeds through two distinct, quasi-steady-state, cool flame burning conditions. The classical “Hot flame” (∼1500 K) radiatively extinguishes into a “Warm flame” burning mode at a moderate maximum reaction zone temperature (∼ 970 K), followed by a transition to a lower temperature (∼765 K), quasi-steady “Cool flame” burning condition. The reaction zone (“flame”) temperatures are associated with distinctly different yields in intermediate reaction products within the reaction zones and surrounding near-field, and the flame-standoff ratios characterizing each burning mode progressively decrease. The presence of all three stages first appears with helium substitution near 20%, and the duration of each stage is observed to be strongly dependent on helium substitutions level between 20–60%. For helium substitution greater than 60%, the hot flame extinction is followed by only the lower temperature cool flame burning mode. In addition to the strong coupling between the diffusive loss of both energy and species and the slowly evolving degenerate branching in the low and negative temperature coefficient (NTC) kinetic regimes, the competition between the low-temperature chain branching and intermediate-temperature chain termination reactions control the “Warm” and “Cool” flame quasi-steady conditions and transitioning dynamics. Experiments onboard the International Space Station with n-dodecane droplets confirm the existence of these combustion characteristics and predictions agree favorably with these observations.
Direct observation of aliphatic structures in soot particles produced in low-pressure premixed ethylene flames via online Raman spectroscopy Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-04 Kim Cuong Le, Christophe Lefumeux, Per-Erik Bengtsson, Thomas Pino
Raman spectra of soot particles were monitored in the gaseous flow extracted from the burning regions of two low-pressure premixed ethylene flames, for the first time. The flame conditions were chosen to explore the diversity of soot nanostructure in slightly sooting flames. Evaluation of the Raman spectral parameters revealed that the soot particles exhibited a strongly disordered structure and a large proportion of sp hybridization of the carbon. The appearance of sp carbon chains composing up to 30% of the total carbon content as well as an olefinic component may indicate their important role in soot nucleation and growth in low pressure ethylene flames. Hence, Raman spectroscopy of soot particles in the aerosol phase revealed that accretion and cyclization of the aliphatic carbon including sp carbon chains could thus be of importance for the initial soot growth and require more investigation.
Large-eddy simulation of ash deposition in a large-scale laboratory furnace Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-04 Min-min Zhou, John C. Parra-Álvarez, Philip J. Smith, Benjamin J. Isaac, Jeremy N. Thornock, Yueming Wang, Sean T. Smith
A computational fluid dynamics model is presented that allows for the investigation of the ash deposition and provides an economical approach for studying design changes in new boilers and retrofit options for existing units. This study proposes a detailed description of ash deposition integrating three separate particle-sticking criteria: melt fraction, viscosity, and energy conservation upon collision. Also, a detailed model for predicting the thermal properties of existing deposit layers (thermal conductivity and emissivity) is implemented into a one-dimensional wall heat-transfer model. The coupled ash-deposition and wall heat-transfer model is implemented into a large-eddy simulation (LES) framework to predict the heat-flux profile, deposition rates, slagging and fouling for industrial boilers. The results of this approach are validated with experimental data from the University of Utah’s 100 kW down-fired, oxy-fuel combustion (OFC) furnace. Two OFC cases with different geometries are studied for their coal combustion and dynamic ash-deposit growth in this large-scale laboratory furnace. Comparisons of the deposition rates and gas temperature agree within 4.82% and 17.58%, respectively, of the measured data.
Characterization of dynamic behavior of combustion noise and detection of blowout in a laboratory-scale gas-turbine model combustor Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-04 Shogo Murayama, Kentaro Kaku, Motoi Funatsu, Hiroshi Gotoda
We present an experimental study on characterizing the dynamic behavior of flow velocity field during combustion noise from the viewpoints of statistical complexity and complex-network theory, involving the detection of a precursor of blowout. The multiscale complexity-entropy causality plane clearly shows the possible presence of two dynamics, (1) stochastic dynamics in the injector rim region and (2) noisy chaos in the shear layer between a vortex breakdown bubble in a wake of a centerbody and the outer recirculation region in a dump plate. The turbulence network is used to study the vortical interactions during combustion noise. The weighted permutation entropy incorporating the amplitude information of pressure fluctuations is useful for capturing a precursor of blowout.
Application of conditioned structure functions to exploring influence of premixed combustion on two-point turbulence statistics Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-04 Vladimir A. Sabelnikov, Andrei N. Lipatnikov, Shinnosuke Nishiki, Tatsuya Hasegawa
In order to investigate the influence of combustion-induced thermal expansion on turbulent flow within a premixed flame brush, a new method is introduced. The method consists in analyzing structure functions of the velocity field, which characterize velocity difference in two points A and B, with the structure functions being conditioned to various events; (i) unburned reactants in both points, (ii) combustion products in both points, (iii) intermediate states of the mixture in both points, (iv) the reactants in one point and the products in another point, (v) the reactants in one point and an intermediate state in another point, and (vi) the products in one point and an intermediate state in another point. Such structure functions and relevant probabilities are defined in the paper. Subsequently, the structure functions and the probabilities are extracted from Direct Numerical Simulation (DNS) data obtained from two statistically 1D, planar, fully-developed, weakly turbulent, premixed flames characterized by two significantly different (7.53 and 2.50) density ratios, with all other things being approximately equal. Obtained results indicate that (i) the conditioned structure functions differ significantly from the mean structure functions and (ii) the newly introduced approach could convey information important for understanding fundamentals of flame–turbulence interaction and finding issues that require further research. In particular, application of the approach to the aforementioned DNS data shows that the combustion-induced thermal expansion substantially affects small-scale two-point velocity statistics in the incoming constant-density turbulent flow of unburned reactants within a premixed flame brush.
Change of evaporation rate of single monocomponent droplet with temperature using time-resolved phase rainbow refractometry Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-03 Yingchun Wu, Haipeng Li, Xuecheng Wu, Gérard Gréhan, Lutz Mädler, Cyril Crua
Droplet evaporation characterization, although of great significance, is still challenging. The recently developed phase rainbow refractometry (PRR) is proposed as an approach to measuring the droplet temperature, size as well as evaporation rate simultaneously, and is applied to a single flowing n-heptane droplet produced by a droplet-on-demand generator. The changes of droplet temperature and evaporation rate after a transient spark heating are reflected in the time-resolved PRR image. Results show that droplet evaporation rate increases with temperature, from −1.28 × 10 − 8 m2/s at atmospheric 293 K to a range of (−1.5, −8) × 10 − 8 m2/s when heated to (294, 315) K, agreeing well with the Maxwell and Stefan–Fuchs model predictions. Uncertainty analysis suggests that the main source is the indeterminate gradient inside droplet, resulting in an underestimation of droplet temperature and evaporation rate. With the demonstration on simultaneous measurements of droplet refractive index as well as droplet transient and local evaporation rate in this work, PRR is a promising tool to investigate single droplet evaporation in real engine conditions.
Molecular-beam mass spectrometry study of oxy-combustion in a novel coal-plate experiment Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-03 Daniel Felsmann, Martina Baroncelli, Joachim Beeckmann, Heinz Pitsch
Oxy-fuel coal combustion could play a significant role in the foreseeable future for its application in carbon capture and storage (CSS) technologies. Therefore, detailed knowledge about the ongoing chemical kinetics in the combustion process is necessary. Here, we present an explorative approach to study volatile species gas phase kinetics in a novel coal-plate experiment probed with molecular-beam mass spectrometry. This coupling allows for time-resolved quantitative measurements of the gas-phase directly above the surface of solid fuels, which aid gaining more insight into the gas phase chemistry during coal combustion by detailed speciation information. Two coal samples, a rhenish lignite and a coal manufactured from hydrothermal carbonization, were chosen for this investigation due to their similar classification but different molecular structures. For both samples, our measurements show a two-stage devolatilization phase separated by a char-oxidation phase which can be attributed to the interaction of oxygen consumption in the gas phase and on the surface and the release of volatiles from deeper layers of the plate. Furthermore, detailed speciation data of light, oxygenated, and tar species allowed to identify fuel structure-specific decomposition patterns of the two different coal materials, thus providing comprehensive data that can be used for future model validation purposes.
Interaction between self-excited oscillations and fuel–air mixing in a dual swirl combustor Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-02 Zhi X. Chen, Nedunchezhian Swaminathan, Michael Stöhr, Wolfgang Meier
A partially premixed gas turbine model combustor close to an industrial design is investigated using Large Eddy Simulation (LES). Two flames, one stable and another unstable with self-excited oscillations are computed. In particular, this study addresses the previously unexplained transition of flame shape in the experiments, from V-shaped to flat when the flame becomes acoustically unstable, suggesting a notable change of the important convective delay in the thermoacoustic feedback loop. The LES results show good agreement with the measured velocities, temperature and mass fractions. The acoustic power spectral density (PSD) obtained from the LES of the unstable flame also agrees well with the measured amplitudes in the air plenum and combustion chamber, and reasonably captures the frequency with a slight under-prediction. A comparison of the stable and unstable cases shows different mixing and reaction behaviours despite similar mean velocity fields. Further detailed analysis shows that the different mixing behaviour is driven by the significantly varying air mass split between the two air passages during a thermoacoustic oscillation cycle. This variation is due to the different impedances experienced by the pressure oscillations propagating through the two swirling injector passages with different internal geometries. This causes a periodic variation of the radial momentum of the fuel jets injected between the two swirling air flows. The resulting flapping of the fuel jets creates an enhanced radial fuel–air mixing that leads to a flattened flame in the unstable case. This provides a new physical explanation for the transitions of flame shape observed in the experiments.
Three-dimensional numerical thrust performance analysis of hydrogen fuel mixture rotating detonation engine with aerospike nozzle Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-02 Nicolas Jourdaine, Nobuyuki Tsuboi, Kohei Ozawa, Takayuki Kojima, A. Koichi Hayashi
The propulsive performance for an H2/O2 and H2/Air rotating detonation engine (RDE) with conic aerospike nozzle has been estimated using three-dimensional numerical simulation with detailed chemical reaction model. The present paper provides the evaluation of the specific impulse (Isp), pressure gain and the thrust coefficient for different micro-nozzle stagnation pressures and for two configurations of conic aerospike nozzle, open and choked aerospike. The simulations show that regardless of the nozzle, increase the micro-nozzles stagnation pressure increases the mass flow rate, the pre-detonation gases pressure and consequently the post-detonation pressure. This gain of pressure in the combustion chamber leads to a higher pressure thrust through the nozzle, improving the Isp. It was also found that the choked nozzle increases the chamber time-averaged static pressure by 50–60% compared with the open nozzle, inducing higher performance for the same reason explained before.
A computational investigation into the kinetics of NO + CH2CCH and its effect on NO reduction Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-10-02 Aaron D. Danilack, C. Franklin Goldsmith
A computational investigation into the kinetics of the NO + CH2CCH reaction is presented. The stationary points on the C3H3N1O1 potential energy surface are analyzed using the compound method ANL0, with key regions of the potential energy surface computed using multi-reference methods. The temperature- and pressure-dependent rate constants are computed using the RRKM/Master Equation. The dominant bimolecular products are HCN + CH2CO, CH2CNH + CO, and CH3CN + CO. Additional calculations for the thermal decomposition of an unimolecular intermediate, isoxazole, are in excellent agreement with the available experimental data. The new rate constants are implemented in a detailed chemical kinetic mechanism for the oxidation of C2H4 by O2 + NO. Analysis of a constant temperature, constant pressure batch reaction suggests that NO + CH2CCH could be an important pathway for both NO reduction and CH2CCH oxidation in reburn chemistry.
Development of a technique for establishing a pseudo tunnel length Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-29 Futoshi Tanaka, Kazuki Fukaya, Khalid A.M. Moinuddin
In the case of a tunnel fire, it is likely that the evacuation path of some of the tunnel users will be obstructed by the cloud of smoke that falls to the road surface of the tunnel. It is important to be able to predict the falling point of the ceiling jet smoke resulting from the fire for improving evacuation strategy. However, a considerably long tunnel is required for examining the distance traveled by the smoke generated in full and model scales fire experiments. It is often difficult to satisfy this requirement while performing fire experiments at laboratory scale. The objective of this study was to develop a new technique for examining the smoke falling phenomenon by using a model scale tunnel with insufficient length. In the new technique, a cooling apparatus was introduced to simulate heat transfer from smoke to tunnel walls. If the amount of convective heat absorbed by the cooling apparatus with length Lc is equivalent to the amount of convective heat lost by the heat transfer to the tunnel walls while the smoke flowed through the distance Ls under a tunnel ceiling, the cooling apparatus with length Lc can be equivalent of a tunnel length Ls. We denote the tunnel length simulated by the cooling apparatus by a pseudo tunnel length. A series of fire experiments were conducted using a 1:10 scale model tunnel with a length of 12 m. In this study, we assessed the effectiveness of the technique for simulating a pseudo tunnel. Experimental results showed that a tunnel with a length of 18.6 m can be simulated by a 12 m tunnel using the new technique proposed.
Auto-ignition study of FACE gasoline and its surrogates at advanced IC engine conditions Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-29 Dongil Kang, Aleksandr Fridlyand, S. Scott Goldsborough, Scott W. Wagnon, Marco Mehl, William J. Pitz, Matthew J. McNenly
Robust surrogate formulation for gasoline fuels is challenging, especially in mimicking auto-ignition behavior observed under advanced combustion strategies including boosted spark-ignition and advanced compression ignition. This work experimentally quantifies the auto-ignition behavior of bi- and multi-component surrogates formulated to represent a mid-octane (Anti-Knock Index 91.5), full boiling-range, research grade gasoline (Fuels for Advanced Combustion Engines, FACE-F). A twin-piston rapid compression machine is used to achieve temperature and pressure conditions representative of in-cylinder engine operation. Changes in low- and intermediate-temperature behavior, including first-stage and main ignition times, are quantified for the surrogates and compared to the gasoline. This study identifies significant discrepancies in the first-stage ignition behavior, the influence of pressure for the bi- to ternary blends, and highlights that better agreement is achieved with multi-component surrogates, particularly at lower temperature regimes. A recently-updated detailed kinetic model for gasoline surrogates is also used to simulate the measurements. Sensitivity analysis is employed to interpret the kinetic pathways responsible for reactivity trends in each gasoline surrogate.
Transient and steady-state behavior of auto-igniting propane and dimethyl ether fuel jets in high-temperature vitiated coflows Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-29 Rajat Saksena, Jeffrey A. Sutton
In the current study, the auto-ignition dynamics of cold fuel jets issuing into a high-temperature, vitiated environments is investigated. Due to the short time scale of these events, high-speed measurements are used to resolve the coupled spatio-temporal behavior. The present study uses high-speed (20-kHz) OH* chemiluminescence imaging to identify the location and timing of the formation of the initial ignition kernels, providing visualization of the ignition dynamics and a detailed statistical evaluation of ignition heights and ignition delay times across a broad parameter space which includes variations in fuel type, dilution levels, coflow temperature, and coflow oxidizer content. The auto-ignition location and ignition delay times show a strong sensitivity to coflow temperature with increased sensitivities at lower coflow temperatures. Comparisons between kernel formation location for the transient jet and the fluctuating flame base of the subsequent, steady-state flame is presented, highlighting the role of flame propagation on flame stabilization. Results indicate that at lower temperatures the flame stabilization mechanism is dominated by auto-ignition, but at higher coflow temperatures, flame propagation plays a key role. The effects of variations in the hot, coflow oxidizer content on ignition properties were found to be noticeable, but still significantly less than variations in the temperature.
Experimental investigation on the flame front resistance of gas channel growth with melt formation in iron ore sinter beds Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-26 Hao Zhou, Mingxi Zhou, Pengnan Ma, Ming Cheng
The resistance of the flame front within the solid bed constitutes a fundamental and crucial area in porous bed combustion as the flame front propagation is highly related to the productivity and product quality. This paper focuses on the iron ore sintering, a thermal agglomeration process in steel mills. The results from a detailed experimental study of the pilot-scale pot tests under the conditions of a wide range of fuel rate are presented. The primary objective is to provide better understanding of the growth of gas channels relating to melt formation in the flame front and its resistance to flow. The sintering bed was divided into several zones based on the temperature profile and component distribution. Even though there is a continuous one-to-one replacement of humidified zone with porous sintered zone, a constant air flow rate during sintering could be obtained, indicating the ∼100 mm high-temperature zone has a controlling effect on sintering bed permeability. The specific pressure drop value in high-temperature zone increases from ∼3 kPa in upper bed to ∼7 kPa in bottom bed, which varies with the bed temperature and structure properties. Both the green bed and sintered bed were scanned by X-ray computed tomography, the reconstruction and image analysis showed that the sintered bed has large gas channels and many more closed pores due to solid-melt-gas coalescence. More melt is generated when the heat is accumulated along the bed or input higher coke content, showing a propensity to suppress the gas channel growth and amplify the mismatch of gas transportation along the bed. Higher coke rate leads to a higher resistance in flame front, resulting in a slower flame front speed. These results are aimed to provide quantitative validation for improvements of a numerical sintering model in a future work.
Spherical turbulent flame propagation of pulverized coal particle clouds in an O2/N2 atmosphere Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-25 Khalid Hadi, Ryo Ichimura, Nozomu Hashimoto, Osamu Fujita
The present study aims to clarify the effects of turbulence intensity and coal concentration on the spherical turbulent flame propagation of a pulverized coal particle cloud. A unique experimental apparatus was developed in which coal particles can be dispersed homogeneously in a turbulent flow field generated by two fans. Experiments on spherical turbulent flame propagation of pulverized coal particle clouds in a constant volume spherical chamber in various turbulence intensities and coal concentrations were conducted. A common bituminous coal was used in the present study. The flame propagation velocity was obtained from an analysis of flame propagation images taken using a high-speed camera. It was found that the flame propagation velocity increased with increasing flame radius. The flame propagation velocity increases as the turbulence intensity increases. Similar trends were observed in spherical flames using gaseous fuel. The coal concentration has a weak effect on the flame propagation velocity, which is unique to pulverized coal combustions in a turbulent field. These are the first reports of experimental results for the spherical turbulent flame propagation behavior of pulverized coal particle clouds. The results obtained in the present study are obviously different from those of previous pulverized coal combustion studies and any other results of gaseous fuel combustion research.
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