Inorganic PM10 emission from the combustion of individual mallee components and whole-tree biomass Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-10-06 Xiangpeng Gao, Muhammad Usman Rahim, Xixia Chen, Hongwei Wu
This contribution reports the emission of inorganic particulate matter (PM) with an aerodynamic diameter <10 µm (PM10) from the combustion of both individual mallee components and whole-tree biomass. Three major components of a mallee tree, namely bark, leaf, and wood, were size-reduced to 75 – 150 µm and mixed at a dry mass ratio of 15% bark:35% leaf:50% wood, which is close to the real mallee's composition, to prepare a whole-tree biomass. The three individual mallee components and the whole-tree biomass were combusted in a laboratory-scale drop-tube furnace at 1400 °C in air to produce inorganic PM10 for further quantification and characterization. The results demonstrate that, whereas the particle size distributions of the PM10 from the combustion of the bark, leaf and wood components generally follow a bimodal distribution, the yields of PM0.1, PM0.1–1, PM1, PM1–10, PM2.5, and PM10 from the three mallee components are quite different. On the bases of dry biomass and useful energy input, the yields of the PM of various size fractions studied follow a sequence of the bark > the leaf > the wood, consistent with that of the ash contents in the three components. Oppositely, the ash-based yields of PM0.1, PM0.1–1, PM1, PM1–10, PM2.5, and PM10 from the wood are substantially higher than those from the bark and the leaf. No obvious synergetic effect among different mallee components in PM10 emission is observed during the whole-tree biomass combustion, enabling the prediction of the PM10 yield from the whole-tree biomass combustion based on that from the individual mallee components.
Auto-thermal reforming (ATR) of natural gas: An automated derivation of optimised reduced chemical schemes Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-10-06 Nicolas Jaouen, Luc Vervisch, Pascale Domingo
A fully automated strategy is discussed to construct reduced chemistry suitable for the numerical simulation of stationary combustion systems of large dimension, such as auto-thermal reforming (ATR) of natural gas for syngas production. Because of computing limitations in terms of space and time resolution, three-dimensional simulations of an ATR unit cannot be addressed with detailed chemistry. A procedure is proposed to automatically derive optimised and reduced chemical schemes under specific ATR operating conditions. A stochastic model problem is first designed to probe the dynamical response of a detailed chemical scheme, over a large range of chemical compositions of the mixture. Reference composition space trajectories are built, featuring turbulent micro-mixing and reactions. Following these trajectories, the chemical response is analysed using directed relation graphs with error propagation combined with quasi-steady state hypothesis, to reduce the number of species and elementary reactions. Then, the time evolution of the model problem is coupled with a genetic algorithm, to optimise on the fly the chemical rates of the reduced kinetics, according to the reference composition space trajectories. The accuracy of the reduced scheme is monitored with a fitness function and the results are tested against the reference detailed chemistry.
Oxy-fuel conversion of sub-bituminous coal particles in fluidized bed and pulverized combustors Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-10-10 Changsheng Bu, Alberto Gómez-Barea, Bo Leckner, Xiaoping Chen, David Pallarès, Daoyin Liu, Ping Lu
Oxy-fuel combustion in pulverized coal (PC) and fluidized bed (FB) boilers is being increasingly investigated due to its potential use for carbon dioxide capture. The combustion conditions in the two types of unit differ significantly because of fuel size, furnace temperature, and fluid dynamics. These differences affect the change of combustion characteristics from air (O2/N2) to oxy-fuel (O2/CO2) conditions in PC and FB in different ways. In this paper, the oxy-fuel combustion behavior of a single sub-bituminous coal particle in PC is compared with that in FB conditions. The FB data were measured in our bench-scale FB test rig, whereas the PC data were collected from literature. The FB tests were performed at 1088 K with 0%vol < O2 < 40%vol, using sub-bituminous coal with a diameter of 6 mm. An extensive list of parameters is compared, including the ignition-delay time, volatiles’ flame temperature, devolatilization time, burnout time and peak temperature of the coal particle. Results indicate that the impact of shifting from air-firing to oxy-firing affects the devolatilization, burnout times, and peak temperature in PC and FB differently: PC particles require higher concentration of oxygen than coarse particles for FB to attain similar results as in air-firing. However, the impact on the volatile flame temperature of shifting to oxy-fuel flame is similar in PC and FB. In general, the CO2 atmosphere delays ignition compared to air-firing, particularly for coarse particles at low O2 concentration.
Ash deposition propensity of coals/blends combustion in boilers: a modeling analysis based on multi-slagging routes Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-06-21 Xin Yang, Derek Ingham, Lin Ma, Nanda Srinivasan, Mohamed Pourkashanian
A method that is based on the initial slagging routes and the sintered/slagging route has been developed and used for predicting the ash deposition propensities of coal combustion in utility boilers supported by the data collected from power stations. Two types of initial slagging routes are considered, namely (i) pyrite-induced initial slagging on the furnace wall, and (ii) fouling caused by the alkaline/alkali components condensation in the convection section. In addition, the sintered/slagging route is considered by the liquids temperature, which represents the melting potential of the main ash composition and is calculated using the chemical equilibrium methods. The partial least square regression (PLSR) technique, coupled with a cross validation method, is employed to obtain the correlation for the ash deposition indice. The method has been successfully applied to coals/blends combustion in boilers, ranging from low rank coals to bituminous coal. The results obtained show that the developed indice yields a higher success rate in classifying the overall slagging/fouling potential in boilers than some of the typical slagging indices. In addition, only using the SiO2/Al2O3 ratio to predict the melting behaviors and slagging potential is inaccurate since the effect of the SiO2/Al2O3 ratio is dictated by both the original ash composition and the way in which the SiO2/Al2O3 ratio is changed. Finally, the influence of the acid components (SiO2 and Al2O3) on the ash deposition prediction is investigated for guiding the mineral additives. It is noticed that the predicted ash deposition potentials of the three easy slagging coals investigated decrease more rapidly by adding Al2O3 than by adding SiO2.
Performances and emission characteristics of NH3–air and NH3CH4–air combustion gas-turbine power generations Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-10-05 Osamu Kurata, Norihiko Iki, Takayuki Matsunuma, Takahiro Inoue, Taku Tsujimura, Hirohide Furutani, Hideaki Kobayashi, Akihiro Hayakawa
For the first time, NH3–air combustion power generation has been successfully realized using a 50 kW class micro gas turbine system at the National Institute of Advanced Industrial Science and Technology (AIST), Japan. Based on the global demand for carbon-free power generation as well as recent advances involving gas-turbine technologies, such as heat-regenerative cycles, rapid fuel mixing using strong swirling flows, and NOx reduction using selective catalytic reduction (SCR), allow us to realize NH3–air combustion gas-turbine system, which was abandoned in the 1960′s. In the present system, the combustor adopted gaseous NH3 fuel and diffusion combustion to enhance flame stability. The NH3 pre-cracking apparatus for combustion enhancement using generated H2 was not employed. The NH3–air combustion gas-turbine power generation system can be operated over a wide range of power and rotational speeds, i.e., 18.4 kW to 44.4 kW and 70,000 rpm to 80,000 rpm, respectively. The combustion efficiency of the NH3–air gas turbine ranged from 89% to 96% at 80,000 rpm. The emission of NO and unburnt NH3 depends on the combustor inlet temperature. Emission data indicates that there are NH3 fuel-rich and fuel-lean regions in the primary combustion zone. It is presumed that unburnt NH3 is released from the fuel-rich region, while NO is released from the fuel-lean region. When diluted air enters the secondary combustion zone, unburnt NH3 is expected to react with NO through selective non-catalytic reduction (SNCR). NH3CH4–air combustion operation tests were also performed and the results show that the increase of the NH3 fuel ratio significantly increases the NO emission, whereas it decreases the NO conversion ratio. To achieve low NOx combustion in NH3–air combustion gas turbines, it is suggested to burn large quantities of NH3 fuel and produce both rich and lean fuel mixtures in the primary combustion zone.
Small size burner combustion stabilization by means of strong cyclonic recirculation Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-06-21 M. de Joannon, P. Sabia, G. Sorrentino, P. Bozza, R. Ragucci
The exhausted gas recirculation inside the combustion chamber represents a challenging strategy to stabilize the oxidation process for novel combustion processes that aim at reducing pollutants emission, controlling the system working temperature by diluting the fresh incoming charge, and keep high process efficiency. The mass and sensible enthalpy ratio of recycled exhausted gas represents a key parameter to promote and stabilize the oxidation process. The chemical/thermodynamic features of the oxidation process were investigated by means of a numerical analysis. The process was schematized as a non-adiabatic constant-volume Continuous-flow Stirred-Tank Reactor (CSTR) where part of the exhausted gas was recirculated back to the reactor. The stability of the process was investigated as a function of the pre-heating temperature and of the dilution level of propane/oxygen/nitrogen mixtures for a fixed recirculation ratio. Following, experimental tests were realized in a small size burner characterized by a strong internal recirculation ratio, induced by a cyclonic fluid-dynamic pattern obtained by the geometrical configuration of the reactor and of the feeding system. The facility was designed to independently vary the mixture pre-heating temperatures and the mixture dilution levels. The experimental results suggest that the cyclonic configuration represents a challenging choice to stabilize the oxidation process in small-size applications. It contains the pollutants emission for a large range of preheating temperature – mixture dilution levels extending the burner operability conditions.
Highly resolved flamelet LES of a semi-industrial scale coal furnace Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-11-09 M. Rieth, F. Proch, A.G. Clements, M. Rabaçal, A.M. Kempf
A highly resolved large eddy simulation (LES) of the semi-industrial IFRF coal furnace [1,2] employing the steady flamelet model is presented. The flamelet table is based on mixture fractions of volatile and char off-gases as well as on enthalpy and scalar dissipation rate. Turbulence–chemistry interaction is treated with an assumed pdf approach, with the variance obtained from a transport equation. Radiation is computed by the discrete ordinates method and the grey weighted sum of grey gases model. The simulation is conducted with the massively parallel “PsiPhi” code on up to 1.7 billion cells and with 40 million particles. Results are processed and compared against the comprehensive set of experiments to (i) validate the new flamelet model and the simulation method and to (ii) gain further insight into the combustion process that is not available from the experiment. The simulation results show that the flamelet LES approach can successfully describe the flow field and combustion inside the furnace; major species and velocities are found in good agreement with the experiment. The results are further analyzed with a focus on the processes of particle heating, devolatilization, char combustion and flame stabilization in a highly turbulent environment. Additionally, the relative importance of scalar dissipation rate is highlighted, showing a large separation of mixing scales between volatile and char off-gas combustion due to the long residence time and generally much lower scalar dissipation rates than typical for lab-scale experiments.
Chemical looping with oxygen uncoupling of high-sulfur coal using copper ore as oxygen carrier Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-10-07 Xin Tian, Kun Wang, Haibo Zhao, Mingze Su
Chemical looping with oxygen uncoupling (CLOU) has been viewed as a promising candidate for solid fuel combustion with inherent CO2 separation at considerably low energy penalty. This work aimed to investigate the sulfur evolution behavior and performance of copper ore oxygen carrier (OC) within the CLOU process using a typical high-sulfur content coal in Sichuan (SC), China as fuel. Experiments at different temperatures and oxygen to fuel ratios were conducted in a laboratory-scale fluidized bed reactor and comparisons have been made between SC coal and Gaoping (GP) anthracite with low sulfur content. It was found that both the increase of temperature and decrease of oxygen to fuel ratio can enhance the generation of sulfurous gases. There were two SO2 peaks during the CLOU process of both SC and GP coals, due to the two relatively independent stages of coal combustion process: volatiles combustion and coal char combustion. The second peak was generally more sensitive to temperature, and a higher temperature would contribute to much more SO2 generation. Cyclic redox experimental results of SC coal in fluidized bed reactor showed stable sulfur evolution trend and moderate reactivity of copper ore OC. Moreover, calcium oxide (CaO) was introduced as desulfurizing agent to remove the sulfurous gases generated in the CLOU process of SC coal and the desulfurization efficiency was achieved as high as 98%. XRD and XPS analyses showed that no metallic sulfide was detected on the surface of the reduced OC samples and ESEM images indicated that no serious sintering problem occurred to the used particles.
Alternative fuels for internal combustion engines Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-10-13 Choongsik Bae, Jaeheun Kim
This review paper covers potential alternative fuels for automotive engine application for both spark ignition (SI) and compression ignition (CI) engines. It also includes applications of alternative fuels in advanced combustion research applications. The representative alternative fuels for SI engines include compressed natural gas (CNG), hydrogen (H2) liquefied petroleum gas (LPG), and alcohol fuels (methanol and ethanol); while for CI engines, they include biodiesel, di-methyl ether (DME), and jet propellent-8 (JP-8). Naphtha is introduced as an alternative fuel for advanced combustion in premixed charge compression ignition. The production, storage, and the supply chain of each alternative fuel are briefly summarized, and are followed by discussions on the main research motivations for such alternative fuels. Literature surveys are presented that investigate the relative advantages and disadvantages of these alternative fuels for application to engine combustion. The contents of engine combustion basically consist of the combustion process from spray development, air–fuel mixing characteristics, to the final combustion product formation process, which is analyzed for each alternative fuel. An overview is provided for alternative fuels together with summaries of engine combustion characteristics for each fuel, in addition to its current distribution status and future prospects.
Adaptation of a dynamic wrinkling model to an engine configuration Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-10-18 S. Mouriaux, O. Colin, D. Veynante
The dynamic model proposed by Charlette et al. represents an interesting alternative to the FSD balance equation to predict the transient growth of a flame kernel. The dynamic wrinkling model coupled to the algebraic Flame Surface Density (FSD) model of Boger et al. is here evaluated for the first time in an internal combustion engine configuration. Preliminary tests enable to evidence practical difficulties when applying the model to this type of complex configuration. Improvements are proposed to adapt the model to the engine configuration. Final simulations are performed on a spark ignition engine configuration, using both the adapted dynamic model and an equilibrium wrinkling formulation. The results are compared to the ones obtained by Robert et al. on the same configuration using the ECFM-LES model solving a FSD balance equation. The dynamic model proves to very well predict out-of-equilibrium values and to account for cycle-to-cycle variabilities, as the model parameter is calculated on the fly. On the contrary, the results obtained using the equilibrium formulation clearly demonstrate that the flame-turbulence equilibrium assumption is not adapted to such configurations.
Effects of equivalence ratio variations on turbulent flame speed in lean methane/air mixtures under lean-burn natural gas engine operating conditions Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-10-13 Zhiyan Wang, Emmanuel Motheau, John Abraham
Direct numerical simulations (DNS) of turbulent premixed methane/air flames are carried out to investigate the effects of equivalence ratio on the turbulent flame speed in lean mixtures. Turbulent flames are simulated as statistically stationary following a Lagrangian framework using an inflow–outflow configuration. The inflow velocity is dynamically adjusted at run-time to stabilize the flame brush location within the computational domain. Linear forcing is applied inside the unburned mixtures to maintain the turbulent intensities at desired levels. For the same turbulence properties, several equivalence ratios near the lean limit are selected and it is shown that the normalized turbulent flame speed is a function of the equivalence ratio. Velocity and length scales of the imposed turbulence are then selected in such a way that the Karlovitz and Damköhler numbers remain constant for flames of different equivalence ratios. Simulations are run for more than 80 eddy turnover times and the turbulent flame speed is derived by averaging the inflow velocity. The results show that equivalence ratio does not have an explicit effect on the normalized turbulent flame speed above the lean limit. Analysis of flame surface area shows that surface wrinkling generated by eddies of different scales is not affected by variation in equivalence ratios when the Karlovitz and Damköhler numbers are fixed. Furthermore, flame surface generated by large-scale eddies is independent of the Karlovitz and Damköhler numbers. Examining the flame surface statistics, it is shown that the flame surface normal is preferentially parallel to the most compressive strain rate direction for all equivalence ratios.
A study of laser induced ignition of methane–air mixtures inside a Rapid Compression Machine Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-06-14 Ciprian Dumitrache, Marc Baumgardner, Andrew Boissiere, Amir Maria, John Roucis, Anthony J. Marchese, Azer Yalin
Presented herein is a fundamental study of laser ignition of methane/air mixtures at temperatures and pressures representative of an internal combustion engine. An Nd:YAG laser operating at λ = 1064 nm was used to ignite methane/air mixtures at equivalence ratios of 0.4 ≤ Φ ≤ 1 in a Rapid Compression Machine (RCM). Experiments were conducted to study the lean limit, minimum spark energy (MSE), and minimum ignition energy (MIE). The results show that laser ignition exhibits a stochastic behavior which must be interpreted statistically. A 90% probability of occurrence was used to evaluate the MSE and MIE, which resulted in MSE90 =2.3 mJ and MIE90 =7.2 mJ at an equivalence ratio Φ = 0.4 at compressed pressure and temperature of Pcomp = 29 bar and Tcomp = 750 K, respectively. The lean limit was characterized based on the fraction of chemical energy converted into thermal energy, which was determined by calculating the apparent rate of heat release as derived from RCM high speed pressure data. A lean limit for 90% chemical energy conversion was found to correspond to an equivalence ratio of 0.47 (Tcomp = 782 K). Schlieren photography was employed as a diagnostics tool to visualize the flame initiation and propagation inside the RCM.
Flame kernel formation behaviors in close dual-point laser breakdown spark ignition for lean methane/air mixtures Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-10-10 Shinji Nakaya, Shingo Iseki, XiaoJing Gu, Yoshinari Kobayashi, Mitsuhiro Tsue
Ignition behaviors of close dual-point laser breakdown spark ignition were investigated experimentally for methane/air mixtures at 0.1 and 1.0 MPa in a constant volume combustion vessel. Absorbed energy was measured from the difference between incident and transmitted laser rays using two joule meters as the ignition energy, and the behaviors of the initial flame kernel were observed with Schlieren photography using a high-speed video camera. First, the ignition behaviors of a single-point laser breakdown spark ignition were investigated. The results indicated that the effect of the focusing lens on the minimum ignition energy (MIE) was limited in terms of the absorbed energy. Although the MIE at 1.0 MPa was lower than that at 0.1 MPa near the stoichiometric equivalence relationship was reversed near the lean limit. For lean mixtures, local quenching of the initial flame kernel was clearly observed including third lobe region especially at 1.0 MPa. In the case of the close dual-point sparks for an equivalence ratio of 0.6, formation of a third lobe was suppressed. When the dual spark gap was large, two flame kernels were formed as observed in the case of the single spark. An optimal gap in which the absorbed energy was minimal for successful ignition existed depending on the pressure, although the magnitude of the associated energy was not so different from that in the case of the single spark. However, the growth rate of the initial flame kernel formed by the close dual sparks was considerably higher than that formed by the single spark, especially at 1.0 MPa. Enhancement of the flame kernel development due to the close dual spark was clearly observed.
Spark discharge ignition process in a spark-ignition engine using a time series of spectra measurements Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-10-10 Nobuyuki Kawahara, Shota Hashimoto, Eiji Tomita
The spark discharge ignition process was investigated using simultaneous temperature measurements of the spark discharges and the initial flame kernel. We were able for the first time to measure a time series of emission spectra from the spark discharge and initial flame kernel inside a spark-ignition engine using a spectrometer coupled with a spark plug and optical fiber. The plasma vibrational temperature of the spark discharge can be measured using time series emission spectra from the electrically excited CN* violet band system. The gas rotational temperature of the initial flame kernel can also be measured using emission spectra from OH* radicals (P and R branches). Simultaneously, visualization of the spark discharge and a time series of emission spectra inside a spark-ignition engine were performed under homogeneous mixture conditions, to eliminate the effects of stratification of temperature and mixture concentrations around the spark plug. We discuss thermal energy transfer from the spark discharge to the combustible mixture. The main conclusions that can be drawn from this study are as follows. CN* emission can be detected from the spark discharge, visualized using a high-speed camera during the arc discharge phase. Our results confirmed that the plasma temperature of the spark discharge was nearly 6800 K and that thermal energy was transferred from the spark plasma channel to the combustible mixture. The gas temperature of the initial flame kernel approached that of the adiabatic flame temperature.
High-speed PIV, spray, combustion luminosity, and infrared fuel-vapor imaging for probing tumble-flow-induced asymmetry of gasoline distribution in a spray-guided stratified-charge DISI engine Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-10-10 Wei Zeng, Magnus Sjöberg, David L. Reuss, Zongjie Hu
In this study, the influence of intake-generated swirl and tumble flow on fuel–air mixing and combustion is investigated in a spray-guided stratified-charge direct-injection spark-ignited engine. Previously, it was demonstrated that the introduction of a combined swirl–tumble flow recovered combustion stability, which was otherwise lost when increasing the engine speed from 1000 to 2000 rpm. However, the improved combustion came at the expense of elevated engine-out soot emissions. Here, high-speed combustion luminosity and PIV measurements at 2000 rpm confirm that soot incandescence is more prevalent with high swirl and tumble. The application of high-speed infrared (IR) gasoline-vapor imaging introduced here provides unique insights, revealing that operation with a combination of swirl and tumble generates an asymmetric fuel distribution that spatially correlates with highly luminous sooting combustion. The IR fuel-vapor imaging technique collects line-of-sight mid-infrared thermal emission from the CH stretch band of the heated fuel near a wavelength of 3.4 µm. The IR images resolve the penetrating vapor plumes distinctly, demonstrating that the 3.4 µm band is suitable for quantitative measurements of vapor penetration during injection. After injection, the IR images provide a qualitative description of fuel-vapor spread without combustion. It is found that the no-swirl case has a symmetric fuel-vapor development during the latter part of the compression stroke. In contrast, for operation with strong swirl and tumble, vapor rotation and the development of an asymmetric and non-uniform fuel-vapor distribution is observed. PIV measurements reveal that the swirl flow dominates the vapor rotation, while the tumble flow appears to be a major reason for the asymmetric fuel-vapor distribution.
Assessment and application of tomographic PIV for the spray-induced flow in an IC engine Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-09-20 B. Peterson, E. Baum, C.-P. Ding, D. Michaelis, A. Dreizler, B. Böhm
Spray-induced turbulence proceeding late-injection is regarded to augment mixing, playing a primary role in controlling heat-release rates and pollutant formation in direct-injection engines. This work presents the first application of tomographic PIV (TPIV) to resolve the 3-dimensional, 3-component (3D3C) spray-induced turbulent flow in a spray-guided direct-injection spark-ignition (SG-DISI) engine. TPIV measurements were performed after a single-injection from a hollow-cone spray when particle distributions were suited for accurate particle reconstruction. High-speed PIV (HS-PIV) measurements (4.8 kHz) were combined with phase-locked TPIV measurements (3.3 Hz) to provide the time history of the 2D2C flow-field preceding TPIV. HS-PIV is also used to validate TPIV measurements within the z = 0 mm plane. TPIV uncertainties of 12% are assessed for non-injection operation. TPIV is used to spatially resolve spray-induced turbulent kinetic energy (TKE), shear (S), and vorticity (Ω) distributions. The added 3D3C velocity information is capable of resolving 3D shear layers that produce spatially-coherent 3D turbulent vortical structures, which are anticipated to augment fuel–air mixing. Measurements spatially quantify the increase of these parameters from injection and quantity distributions reveal significant differences to non-injection operation. The isosurface density ( ρ ¯ ), defined as the volume percentage for which a flow parameter exceeds a given value, is used to identify distributions of the largest TKE, S, and Ω magnitudes. Distributions quantify the increase of TKE, S, and Ω from injection and describe the decay of spray-induced turbulence with time. At ρ ¯ values below 10%, fuel injection increases TKE, S, and Ω magnitudes in excess of 400% compared to the tumble flow without injection. Magnitudes remained 2-times larger than non-injection operation 16 crank-angle degrees (CADs) after injection. This indicates that spray-induced turbulence enhancement can remain for a significant time after injection. Measurements and analyses provide insight into spray-induced turbulence phenomena and are anticipated to support predictive model development for engine sprays.
Influence of three-dimensional in-cylinder flows on cycle-to-cycle variations in a fired stratified DISI engine measured by time-resolved dual-plane PIV Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-10-05 J. Bode, J. Schorr, C. Krüger, A. Dreizler, B. Böhm
The objective of this study is an improved understanding of the three-dimensional in-cylinder flow and its influence on cycle-to-cycle variations in a fired, direct-injection, spray-guided, spark-ignition engine. The engine was operated at part-load. A triple injection scheme was applied late during compression to generate stratified fuel/air mixtures. These operating conditions are prone to cyclic variations. The large-scale tumble flow caused cycle-to-cycle variations in the spray shape. This was followed by cycle-to-cycle variations in combustion, which was characterized by the indicated mean effective pressure. Time-resolved particle image velocimetry was applied simultaneously in two parallel planes to account for the three-dimensional nature of transient in-cylinder flows. The focus was on the influence of the evolution of the large-scale tumble flow on combustion. The planes were 18 mm apart and located in the central tumble plane and one mid-valve plane. The central tumble plane is located below the fuel injector and spark plug; the mid-valve plane is below the center of one pair of intake and exhaust valves. Correlation analyses were performed to identify the flow features responsible for cycle-to-cycle variations in spray and combustion. By correlating single velocity vectors in the central tumble plane with indicated mean effective pressures, two flow regions important to combustion were identified: A flow above the piston, which had a significant impact on spray evolution, and a flow along the cylinder head, pointing from the injector toward the spark plug. By tracing in-cylinder flows backward in time, a clear correlation between flow features in the central tumble plane and the mid-valve plane was observed.
In-cylinder thermochemical fuel reforming (TFR) in a spark-ignition natural gas engine Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-07-25 Lei Zhu, Zhuoyao He, Zhen Xu, Xingcai Lu, Junhua Fang, Wugao Zhang, Zhen Huang
This experimental study demonstrates the potential to apply the thermochemical fuel reforming (TFR) concept to simultaneously reduce emissions and improve brake specific fuel consumption in a spark-ignition natural gas engine. CH4, H2 and CO are the major components of TFR exhaust gas over a range of rich equivalence ratios. A numerical analysis is conducted to illustrate the chemical reaction pathways for H2 and CO formation, which occurs in the cylinder during the TFR process. The main reaction pathways for H2 and CO formation under 3 modeling conditions (20%, 50% and 80% fuel consumed) are different from each other. According to the experimental analysis, thermochemical fuel reforming gas improves combustion performance and accelerates the burn rate in every phase of the natural gas engine. Combustion stability, brake thermal efficiency, brake specific fuel consumption (BSFC), brake specific hydrocarbon (BSHC) and brake specific carbon monoxide (BSCO) emissions can also be improved by TFR. The brake specific oxides of nitrogen (BSNOx) emissions for natural gas engines, combined with a TFR system, are still lower than those of an original natural gas engine in the same operation mode. Thermochemical fuel reforming has been shown to be effective in simultaneously reducing emissions and improving thermal efficiency for a spark-ignition natural gas engine. Furthermore, a 1.2 equivalence ratio for cylinder 4 (TFR cylinder) can be recommended in future research on TFR optimization, based on BSFC and combustion stability.
Autoignition of pentane isomers in a spark-ignition engine Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-10-10 Song Cheng, Yi Yang, Michael J. Brear, Dongil Kang, Stanislav Bohac, André L. Boehman
This paper describes a study on the autoignition of three pentane isomers (n-, neo- and iso-pentane) in a Cooperative Fuel Research (CFR) engine operating at standard, ASTM knocking conditions. The Research Octane Numbers (RONs) of these three fuels are first measured and compared to historical data. Autoignition of pentane/air mixtures in the CFR engine are then simulated using a two-zone model with detailed chemical kinetics. Initial and boundary conditions for these kinetic simulations are systematically calibrated using engine simulation software. Two published, detailed kinetic mechanisms for these fuels are tested with a published NO sub-mechanism incorporated into them. Simulations using both of these mechanisms demonstrate autoignition in the engine for all three pentanes, and that residual NO promotes autoignition, as found in previous studies. Differences between these two mechanisms and the engine experiments are nonetheless observed, and these differences are consistent with those observed in simulations of published rapid compression machine (RCM) data. Comparison of the RCM and the CFR engine modelling also suggests the need for high accuracy experiments and high-fidelity models due to the significant impact that small differences in autoignition timing can potentially produce in real engines.
Understanding the effect of external-EGR on anti-knock characteristics of various ethanol reference fuel with RON 100 by using rapid compression machine Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-10-07 Jaeyoung Cho, Han Ho Song
In this study, the effect of external-exhaust-gas-recirculation (EGR) on anti-knock characteristics of ethanol reference fuels (ERFs) with research octane number (RON) 100 was analyzed by measuring the ignition delay of the simulated end gas from a representative spark ignition (SI) engine operation, i.e. ASTM RON test condition. An in-house SI engine model was used to derive temperature and pressure profiles of the end gas with and without external-EGR for various ERFs, and then the ignition delay was measured by using a rapid compression machine along the derived temperature-pressure paths. The effect of external-EGR on the ignition delay of the simulated end gas was divided into two effects affecting the auto-ignition behavior: composition effect and temperature effect, then each effect was evaluated separately. As a result, the composition effect by adding external-EGR was maximized when the fuel is ERF10. With a regression analysis, it was found that there is the correlation between the amount of composition effect and the amount of pre-heat release in the end gas during a flame propagation; therefore, it is understood that ERF10 shows the most sensitive composition effect due to its pre-heat release characteristic. On the other hand, ERF with higher ethanol content was more sensitive to temperature effect by external-EGR on the ignition delay. It is found out that the amount of temperature effect depends mainly on latent heat of fuel; therefore, high latent heat of ethanol in ERF leads to its being influenced more by temperature effect. Consequently, ERF10 has the highest external-EGR sensitivity in anti-knock behavior at RON test condition, and it is further discussed that the optimum ERF for external-EGR strategy could vary from ERF0 to ERF10 according to different engine operating conditions.
Antiknock quality and ignition kinetics of 2-phenylethanol, a novel lignocellulosic octane booster Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-06-28 Vijai Shankar Bhavani Shankar, Mohammed Al-Abbad, Mariam El-Rachidi, Samah Y. Mohamed, Eshan Singh, Zhandong Wang, Aamir Farooq, S. Mani Sarathy
High-octane quality fuels are important for increasing spark ignition engine efficiency, but their production comes at a substantial economic and environmental cost. The possibility of producing high anti-knock quality gasoline by blending high-octane bio-derived components with low octane naphtha streams is attractive. 2-phenyl ethanol (2-PE), is one such potential candidate that can be derived from lignin, a biomass component made of interconnected aromatic groups. We first ascertained the blending anti-knock quality of 2-PE by studying the effect of spark advancement on knock for various blends 2-PE, toluene, and ethanol with naphtha in a cooperative fuels research engine. The blending octane quality of 2-PE indicated an anti-knock behavior similar or slightly greater than that of toluene, and ethylbenzene, which could be attributed to either chemical kinetics or charge cooling effects. To isolate chemical kinetic effects, a model for 2-PE auto-ignition was developed and validated using ignition delay times measured in a high-pressure shock tube. Simulated ignition delay times of 2-PE were also compared to those of traditional high-octane gasoline blending components to show that the gas phase reactivity of 2-PE is lower than ethanol, and comparable to toluene, and ethylbenzene at RON, and MON relevant conditions. The gas-phase reactivity of 2-PE is largely controlled by its aromatic ring, while the effect of the hydroxyl group is minimal. The higher blending octane quality of 2-PE compared to toluene, and ethylbenzene can be attributed primarily to the effect of the hydroxyl group on increasing heat of vaporization.
Effect of initial temperature and fuel properties on knock characteristics in a rapid compression and expansion machine Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-10-10 Kimitoshi Tanoue, Taishu Jimoto, Takanori Kimura, Misato Yamamoto, Jun Hashimoto
In this study, the effect of initial temperature and fuel properties (such as ignition delay time variation with temperature) on knock characteristics was investigated for methane and propane dual fuels via a rapid compression and expansion machine, which can emulate one compression and expansion stroke in a real engine. Numerical calculations using CHEMKIN-PRO with AramcoMech1.3 were carried out to obtain fuel properties. The correlation between flame propagation velocity at the moment of autoignition and knock intensity was confirmed. The findings revealed that unburned mass fraction and initial temperature did not directly affect knock intensity. Additionally, experimental results were analyzed based on theories proposed by Zeldovich and Bradley. The results of the analysis indicated that smaller gradients of autoignition delay time and temperature caused the higher flame propagation velocity and the resultant higher knock intensity. It was concluded that initial temperature affected both the gradient of autoignition delay time and that of temperature, which in turn indirectly influenced the knock intensities.
Effects of flame propagation speed and chamber size on end-gas autoignition Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-10-05 Hao Yu, Chengken Qi, Zheng Chen
End-gas autoignition has direct relevance to engine knock and thereby has been extensively studied. However, in the literature there are still some contradictions on how different factors affect end-gas autoignition and knock intensity. Specifically, there is contradictory literature on (1) whether faster combustion may promote or inhibit end-gas autoignition and engine knock, and (2) whether knock intensity increases or decreases with burned mass fraction (BMF). To answer these two questions, one-dimensional flame propagation and end-gas autoignition in a closed cylindrical chamber are investigated and the effects of flame propagation speed and chamber size on end-gas autoignition are examined in this study. In the transient numerical simulation, two fuels, hydrogen and iso-octane, are studied; and detailed chemistry is considered. It is shown that if the flame propagation is fast enough or the chamber is small enough, end-gas autoignition and knock can be prevented; otherwise, the knock intensity may increase as the flame propagation speed increases or as the chamber size decreases. The maximum pressure is found to change non-monotonically with the BMF as well as the flame propagation speed and chamber size. This helps to explain why there is contradictory literature on those two questions mentioned above. The answers to these two questions depend on the amount of unburned mixture at the moment of end-gas autoignition: if there is enough unburned mixture before end-gas autoignition, the maximum pressure increases with the flame propagation speed and BMF; otherwise, the opposite trend occurs. Besides, comparison between the results for hydrogen and iso-octane indicates that end-gas chemical reaction and heat release occurring before autoignition can greatly reduce the maximum pressure.
Numerical investigation of the effect of pressure on heat release rate in iso-octane premixed turbulent flames under conditions relevant to SI engines Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-10-10 Bruno Savard, Simon Lapointe, Andrzej Teodorczyk
A series of direct numerical simulations (DNS) of iso-octane/air turbulent premixed flames in the thin reaction zones regime have been performed in order to investigate the effect of pressure on heat release rate under conditions relevant to spark-ignition (SI) engines (up to 20 bar and 800 K in the unburnt gas). Chemistry is represented by a reduced kinetics mechanism containing 74 species and 976 reactions (reduced from CaltechMech). The effect of pressure has been isolated by fixing the Karlovitz numbers, the Lewis numbers, and the ratio of integral length scale to laminar flame thickness. On one hand, the results suggest that pressure has very limited effect on the mean heat release rate, such that turbulent burning velocity is proportional to the turbulent surface area. In addition, while the chemical pathways are strongly affected by pressure in laminar flames, the global effect of turbulence on these pathways is negligible, independent of pressure. On the other hand, the local distribution of heat release rate was found to be significantly affected by pressure through differential diffusion-chemistry effects.
Influence of turbulent fluctuations on radiation heat transfer, NO and soot formation under ECN Spray A conditions Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-11-09 Michele Bolla, M. Aqib Chishty, Evatt R. Hawkes, Qing N. Chan, Sanghoon Kook
This paper investigates the influence of unresolved turbulent fluctuations on radiation heat transfer and formation of NO and soot in an n-dodecane spray flame (known as Spray A) under diesel engine conditions. The transported probability density function (TPDF) model – including the effect of turbulent fluctuations in temperature and composition – has been compared with the well-mixed (WM) model – neglecting these fluctuations – to separate and quantify the relative influence of turbulence-chemistry interactions (TCI), radiation heat transfer and turbulence-radiation interaction (TRI). Radiation is solved with the discrete ordinate method (DOM) including grey radiative properties from soot and gaseous species in the Reynolds-averaged Navier–Stokes (RANS) framework. At Spray A conditions (60 bar), the contribution to the Planck mean absorption coefficient from gas-phase species (mainly CO2 and H2O) was found to be comparable to the one of soot, this plays an important role for the radiation reabsorption in the periphery of the jet. Radiation was found to reduce the flame temperature by 10–20 K with a consequent reduction of the total NO mass by approximately 5–10%, with these reductions being comparable for both combustion models. However, neglecting turbulent fluctuations results in an increase of the NO mass by a factor of two. The effect of TRI on the emitted radiation was found to be modest (10% at most). The effect of radiation on soot formation was seen to be minor. Overall, it is concluded that under Spray A conditions the effect of radiation heat transfer does influence the NO formation but the effect of the combustion model is more important and needs further attention.
Effect of jet–jet interactions on soot formation in a small-bore diesel engine Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-07-16 Minh K. Le, Yilong Zhang, Renlin Zhang, Lingzhe Rao, Sanghoon Kook, Qing Nian Chan, Evatt R. Hawkes
This study presents planar laser-induced fluorescence of fuel and hydroxyl (fuel- and OH-PLIF) and incandescence of soot (soot-PLII) together with morphology and nanostructure information of soot particles sampled via thermophoresis to clarify the in-cylinder soot processes under the influence of jet to jet interactions. The experiments were carried out in a single-cylinder, small-bore optical diesel engine fuelled by a low-sooting methyl decanoate fuel for diagnostic purposes. Two different nozzle configurations of one hole and two holes were used to simulate isolated single-jet and double-jet conditions, respectively. Results show that fuel-rich mixture formed in the jet–jet interaction region causes the faster initial growth of soot that persists for a longer period of time, compared to the soot formed in the wall-impingement region of the single jet. These soot particles impacted by the jet–jet interaction have larger aggregates composed of larger primaries, and the nanoscale internal structures show higher carbon fringe-to-fringe separations, both of which indicate higher particle reactivity and the formation stage of soot.
A direct numerical simulation of cool-flame affected autoignition in diesel engine-relevant conditions Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-11-11 Alex Krisman, Evatt R. Hawkes, Mohsen Talei, Ankit Bhagatwala, Jacqueline H. Chen
In diesel engines, combustion is initiated by a two-staged autoignition that includes both low- and high-temperature chemistry. The location and timing of both stages of autoignition are important parameters that influence the development and stabilisation of the flame. In this study, a two-dimensional direct numerical simulation (DNS) is conducted to provide a fully resolved description of ignition at diesel engine-relevant conditions. The DNS is performed at a pressure of 40 atmospheres and at an ambient temperature of 900 K using dimethyl ether (DME) as the fuel, with a 30 species reduced chemical mechanism. At these conditions, similar to diesel fuel, DME exhibits two-stage ignition. The focus of this study is on the behaviour of the low-temperature chemistry (LTC) and the way in which it influences the high-temperature ignition. The results show that the LTC develops as a “spotty” first-stage autoignition in lean regions which transitions to a diffusively supported cool-flame and then propagates up the local mixture fraction gradient towards richer regions. The cool-flame speed is much faster than can be attributed to spatial gradients in first-stage ignition delay time in homogeneous reactors. The cool-flame causes a shortening of the second-stage ignition delay times compared to a homogeneous reactor and the shortening becomes more pronounced at richer mixtures. Multiple high-temperature ignition kernels are observed over a range of rich mixtures that are much richer than the homogeneous most reactive mixture and most kernels form much earlier than suggested by the homogeneous ignition delay time of the corresponding local mixture. Overall, the results suggest that LTC can strongly influence both the timing and location in composition space of the high-temperature ignition.
Sparse-Lagrangian MMC simulations of an n-dodecane jet at engine-relevant conditions Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-09-29 F. Salehi, M.J. Cleary, A.R. Masri, Y. Ge, A.Y. Klimenko
Simulations are presented for the engine combustion network (ECN) n-dodecane spray A using the sparse-Lagrangian multiple mapping conditioning model for the reactive scalar field coupled with a large eddy simulation for the flow (MMC-LES). This is the first application of the MMC-LES to both transient and engine conditions and, in line with earlier simulations of the ECN flames, the fuel injection is modelled as a gas-jet. The model is evaluated initially for a baseline non-reacting case with ambient temperature of 900 K. Results for vapour penetration lengths and the radial profiles of mixture fraction show good agreement with experiments. The model is then applied to reacting cases, featuring transient ignition and combustion with a lifted flame base. A 106-species chemistry mechanism is employed. Simulations are performed at various ambient temperatures (800–1100 K) and various volume fractions of ambient oxygen (13–21%) that correspond to conditions in both conventional and advanced diesel engines. The trend towards decreasing ignition delay time and lift-off length with increasing ambient temperature and oxygen is predicted well. The quantitative agreement between the computed and experimental ignition delay times is satisfactory. The accuracy of lift-off length predictions is less accurate but comparable to the existing literature. Better results are obtained at the conditions with lower ambient temperatures and oxygen concentrations. This outcome is explored with a detailed analysis of heat release and scalar dissipation rates under various ambient temperature conditions. It is revealed that at high ambient temperatures, flame stabilisation occurs in a region with significant turbulence–chemistry interactions and close to the nozzle exit where the liquid phase would be expected to influence the flame.
Ignition of a lean PRF/air mixture under RCCI/SCCI conditions: Chemical aspects Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-10-06 Minh Bau Luong, Gwang Hyeon Yu, Suk Ho Chung, Chun Sang Yoo
Chemical aspects of the ignition of a primary reference fuel (PRF)/air mixture under reactivity controlled compression ignition (RCCI) and stratified charge compression ignition (SCCI) conditions are investigated by analyzing two-dimensional direct numerical simulation (DNS) data with chemical explosive mode (CEM) analysis. CEMA is adopted to provide fundamental insights into the ignition process by identifying controlling species and elementary reactions at different locations and times. It is found that at the first ignition delay, low-temperature chemistry (LTC) represented by the isomerization of alkylperoxy radical, chain branching reactions of keto-hydroperoxide, and H-atom abstraction of n-heptane is predominant for both RCCI and SCCI combustion. In addition, explosion index and participation index analyses together with conditional means on temperature verify that low-temperature heat release (LTHR) from local mixtures with relatively-high n-heptane concentration occurs more intensively in RCCI combustion than in SCCI combustion, which ultimately advances the overall RCCI combustion and distributes its heat release rate over time. It is also found that at the onset of the main combustion, high-temperature heat release (HTHR) occurs primarily in thin deflagrations where temperature, CO, and OH are found to be the most important species for the combustion. The conversion reaction of CO to CO2 and hydrogen chemistry are identified as important reactions for HTHR. The overall RCCI/SCCI combustion can be understood by mapping the variation of 2-D RCCI/SCCI combustion in temperature space onto the temporal evolution of 0-D ignition.
Autoignition studies of C5 isomers in a motored engine Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-10-17 Dongil Kang, Stanislav V. Bohac, André L. Boehman, Song Cheng, Yi Yang, Michael J. Brear
This study explores the autoignition characteristics of three C5 isomers, namely n-pentane, 2-methylbutane (iso-pentane) and 2,2-dimethylpropane (neo-pentane). These measurements are intended to enhance understanding of C5 autoignition chemistry, and provide experimental data to guide improvements to a general hydrocarbon oxidation mechanism. To that end, the autoignition behavior of these three C5 isomers was investigated in a modified CFR engine at an intake temperature of 120 °C and a fixed engine speed of 600 rpm to determine the critical compression ratio (CCR) at which hot ignition occurs. To find the critical compression ratio, the engine compression ratio (CR) was gradually increased to the point where CO in the engine exhaust rapidly decreased and significant high temperature heat release was observed, while holding equivalence ratio constant. Fundamental ignition behaviors such as the CCR and the calculated percentage of low temperature heat release (%LTHR) demonstrate the impact of chain length and methyl substitutions on ignition reactivity. The %LTHR shows a stronger two stage heat release for n-pentane than for neo-pentane observed at critical ignition conditions. In contrast, single stage heat release is observed for iso-pentane, leading to the weakest overall oxidation reactivity of the three isomers. Key reaction paths forming conjugate alkenes and C5 oxygenated species control the autoignition reactivity of n-pentane and iso-pentane within the low temperature and NTC regimes. However, neo-pentane forms no conjugate alkene due to its unique molecular structure, and instead produces iso-butene to retard its oxidation.
Ignition and formaldehyde formation in dimethyl ether (DME) reacting spray under various EGR levels Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-11-08 Khanh Cung, Ahmed Abdul Moiz, Xiucheng Zhu, Seong-Young Lee
As an alternative fuel to diesel in compression-ignition (CI) engines, dimethyl ether (DME) has gained interest in combustion research due to its high cetane number for fast ignition and ultra-low emission of particulate matter. In this study, ignition and important intermediate species including formaldehyde (CH2O) are experimentally investigated in a constant-volume combustion vessel facility that includes a fuel injection system for spray-like behavior of liquid DME. Experiments of different oxygen concentrations simulating various levels of exhaust-gas recirculation (EGR) were performed to study its corresponding effect on the flame structure and emissions from DME combustion. Results from the experiment were then used to validate a 3-D CFD simulation using a detailed chemistry solver. Different stages of ignition characterized by temperature profile, and certain species (e.g., CH2O for cool flame) after start of injection, are provided to conceptualize the DME combustion process under the effect of low-to-high oxygen ambient gas concentrations. Both simulations and experiments showed that there is a supporting link between CH2O formation and low-temperature combustion prior to diffusion-controlled flame uniquely for DME. By studying the temperature and equivalence ratio dependence of the reacting spray at different O2 levels, different stages of ignition along with the formation of CH2O suggested that the start of the depletion of CH2O can be used as an ignition indicator.
A versatile coupled progress variable/REDIM model for auto-ignition and combustion Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-10-06 Marc-Sebastian Benzinger, Robert Schießl, Ulrich Maas
In this work, we present a reduced model for treating chemical reactions in combustion simulations, with special attention on combustion in IC engines. The model which is based on low-dimensional manifolds in state space, is able to describe auto-ignition, burning in quasi homogeneous media as well as chemical reactions which are strongly coupled with molecular transport, like, e.g., in flame propagation. A coupling scheme is developed for existing concepts for reduced treatment of combustion, namely a progress variable model (PVM) and the reaction-diffusion manifold approach (REDIM). We discuss a simple, robust method for this coupling, based on an additional variable, namely the normalized strength of molecular transport. The implementation and behavior of the resulting coupled model are shown. To demonstrate the performance of the model, numerical simulations of representative combustion scenarios are performed, both with fully detailed calculations and with the reduced model. The comparison of results obtained with detailed and reduced computation shows that the strongly reduced model, which requires only five independent variables in total, still can accurately predict a wide range of combustion-relevant scenarios.
Ignition of a lean PRF/air mixture under RCCI/SCCI conditions: A comparative DNS study Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-10-07 Minh Bau Luong, Gwang Hyeon Yu, Suk Ho Chung, Chun Sang Yoo
The ignition characteristics of a lean primary reference fuel (PRF)/air mixture under reactivity controlled compression ignition (RCCI) and stratified charge compression ignition (SCCI) conditions are investigated using 2-D direct numerical simulations (DNSs) with a 116-species reduced mechanism of PRF oxidation. For RCCI combustion, n-heptane and iso-octane are used as two different reactivity fuels and the corresponding global PRF number is PRF50 which is also used as a single fuel for SCCI combustion. The 2-D DNSs of RCCI/SCCI combustion are performed by varying degree of fuel stratification, r, and turbulence intensity, u′, at different initial mean temperature, T0, with negatively-correlated T–r fields. It is found that in the low- and intermediate-temperature regimes, the overall combustion of RCCI cases occurs earlier and its mean heat release rate (HRR) is more distributed over time than those of the corresponding SCCI cases. This is because PRF number stratification, PRF′, plays a dominant role and T′ has a negligible effect on the overall combustion within the negative temperature coefficient (NTC) regime. In the high-temperature regime, however, the difference between RCCI and SCCI combustion becomes marginal because the ignition of the PRF/air mixture is highly-sensitive to T′ rather than PRF′ and ϕ′. The Damköhler number analysis verifies that the mean HRR is more distributed over time with increasing r because the portion of deflagration mode of combustion becomes larger with increasing fuel stratification. Finally, it is found that the overall combustion of both RCCI and SCCI cases becomes more like the 0-D ignition with increasing u′ due to the homogenization of initial mixture by turbulent mixing.
Different modes of reaction front propagation in n-heptane/air mixture with concentration non-uniformity Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-06-22 Chengken Qi, Peng Dai, Hao Yu, Zheng Chen
According to the reactivity gradient theory of Zel'dovich, the non-uniformity in temperature or concentration can lead to detonation development under certain conditions. In the literature, there are many studies on detonation development caused by temperature gradient or hot spot. However, the modes of supersonic reaction front propagation and detonation development regime caused by concentration non-uniformity have not been investigated previously. In this study, one-dimensional simulations were conducted to investigate the transient autoignition and reaction front propagation processes in n-heptane/air mixture with concentration non-uniformity. With the increase of equivalence ratio gradient, three modes (supersonic autoignitive reaction front, developing detonation and subsonic reaction front) of reaction front propagation induced by concentration non-uniformity were identified. The effects of heat conduction and mass diffusion on these three modes were examined and it was demonstrated that molecular diffusion has little influence on the first two modes. The detonation development regime caused by concentration non-uniformity was reported in this paper. This regime was found to be similar to the one caused by temperature gradient. A non-dimensional parameter was proposed to characterize the lower limit of the detonation regime. Furthermore, the effects of initial temperature on the detonation development regime were examined. It was found that the detonation development regime becomes wider as the initial temperature decreases. The initial temperature was shown to only have great impact on the upper limit of the detonation development regime while it has little influence on the lower limit. The influence of initial temperature was explained using the volumetric energy density of the mixture.
Effects of initial temperature on autoignition and detonation development in dimethyl ether/air mixtures with temperature gradient Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-09-30 Peng Dai, Chengken Qi, Zheng Chen
For large hydrocarbon fuels used in internal combustion engines, different low-temperature and high-temperature chemistries are involved in the autoignition processes under different initial temperatures. As one of the simplest fuels with low-temperature chemistry, dimethyl ether (DME) is considered in this study and one-dimensional autoignitive reaction front propagation induced by temperature gradient is simulated for stoichiometric DME/air mixtures considering detailed chemistry and transport. The emphasis is placed on assessing and interpreting the influence of initial temperature on the detonation development regime. Different initial temperatures below, within and above the negative-temperature coefficient (NTC) region are considered. For each initial temperature, four typical autoignition modes are identified: supersonic autoignitive reaction front (without detonation); detonation development; transonic reaction front; and subsonic reaction front. The detonation development regimes for two fuels, DME and n-heptane, at the same initial temperature and those for the same fuel, DME, at three different initial temperatures respectively below, within and above the NTC region are obtained. Based on these results, the influence of fuel type and initial temperature on detonation development regime are discussed. It is found that the detonation development regime becomes narrower at higher initial temperature. Moreover, the influence of initial temperature on reaction front propagation speed is investigated. The reaction front propagation speed is shown to be strongly affected by different chemistries involved in low and high temperature regions. When only the high-temperature chemistry is involved, the reaction front propagation speed is shown to be less dependent on the initial temperature.
Investigation of cetane number and octane number correlation under homogenous-charge compression-ignition engine operation Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-10-04 Dan Janecek, David Rothamer, Jaal Ghandhi
A novel fuel-substitution strategy was used to investigate the relationship between octane number and cetane number under representative low temperature combustion (LTC) thermodynamic conditions. The cetane number of the test fuels was known by using mixtures of the cetane number secondary reference fuels (SRFs) T-26 and U-19, which are full-blend certification fuels used in the place of 2,2,4,4,6,8,8-heptamethylnonane and n-hexadecane for engine cetane number testing. The octane number was derived from the fuel-substitution methodology in terms of the octane primary reference fuel (PRF, consisting of n-heptane and isooctane) mixture needed to maintain combustion phasing during homogeneous-charge compression-ignition (HCCI) engine operation as increasing amounts of the SRF mixture was substituted for the PRF mixture. A linear blending relationship was found to exist between the cetane and octane numbers. Results agree with other similar relationships from the literature acquired under substantially different operating conditions. The current study significantly expands the range of the cetane–octane correlation to a range of cetane numbers from 20 to 75 and validates the correlation at low equivalence ratios of interest to LTC operation.
Influence of injection parameters, ozone seeding and residual NO on a Gasoline Compression Ignition (GCI) engine at low load Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-06-23 P.M. Pinazzi, F. Foucher
The main objective of this work was to evaluate the potential of ozone to overcome the low load limitations of Direct-Injection Gasoline Compression Ignition (D-I GCI) engines. Experiments were performed in a single-cylinder diesel engine fuelled with directly-injected 95 RON gasoline at 2 bar IMEP. Engine speed was set to 1500 rpm. Intake pressure was set to 1 bar in order to investigate typical low load operating conditions. Chemical computation revealed that the O atoms coming from the O3 molecules and responsible for promoting fuel autoignition exhibited a maximum concentration near 30° before TDC. Experiments showed that a narrow umbrella angle extends the early injections range up to -60 CAD, better matching with ozone decomposition. Injection timing can be employed to better exploit the oxidizing effect of ozone, which proved to be dependent on local conditions of temperature, equivalence ratio, and concentration of O atoms and of the residual NO trapped after each combustion cycle. Results showed that the impact of ozone is generally higher at early injection, but that interaction between O3 molecules and NO contained in the trapped gas can drastically reduce or even make the promoting effect of ozone ineffective. Finally, seeding the intake of the engine with ozone is shown to be an effective way to improve the autoignition propensity of high octane fuel and to overcome the low load limitation of D-I GCI engines.
A skeletal gasoline flame ionization mechanism for combustion timing prediction on HCCI engines Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-06-25 Guangyu Dong, Yulin Chen, Liguang Li, Zhijun Wu, Robert Dibble
Ion current sensing technology has the potential to be a low cost and real time combustion phasing solution for HCCI or HCCI-like engine control. Based on a primary reference fuel oxidation mechanism and a C1–C4 hydrocarbon flame ionization mechanism, a skeletal mechanism for gasoline flame ionization process prediction on HCCI engines was developed in this paper. Since the ion concentrations significantly affect the aroused ion current signals, the mechanism is targeted on accurately predicting both the ion production concentrations and other key combustion characteristics. Through the comparison with the results from the detailed gasoline flame ionization mechanism and experimental results, the predicted maximum hydronium (H3O+) ion concentration and the concentration variation tendency are validated. Additionally, the auto-ignition delay time (tign) accurately predicted under HCCI engine conditions. Through coupling with a 3D-CFD engine model, the skeletal mechanism was applied to predict the important information of in-cylinder ion species, which are validated by the experimental ion current amplitudes and phases. The results show that the ion current phase (Ion50) matches well with the positions where the predicted ion concentration reaches its maximum, and the ion current amplitudes are well predicted under the conditions of different equivalence ratios (Φ) and fuel injection ratios (Injratio).
Doubly conditional moment closure modelling for HCCI with temperature inhomogeneities Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-06-16 Fatemeh Salehi, Mohsen Talei, Evatt R. Hawkes, Ankit Bhagatwala, Jacqueline H. Chen, Chun Sang Yoo, Sanghoon Kook
This paper presents a doubly conditional moment closure (DCMC) as an a posteriori predictive modelling tool for ignition of mixtures with large thermal stratification in homogeneous charge compression ignition (HCCI) conditions. Double conditioning is applied on enthalpy and its dissipation rate. The performance of the DCMC model is evaluated using a number of previously reported direct numerical simulations (DNSs) with various fuels. The DNSs modelled ignition of various lean homogeneous mixtures with a high level of temperature inhomogeneities. The selected cases exhibit a prevalence of deflagration mode of combustion as opposed to a spontaneous ignition-front mode, which has proven challenging for previous singly CMC. In all simulations, DCMC solver is run in a stand-alone mode with certain terms, such as the probability density functions of enthalpy and dissipation rate, being provided using the DNS input. The DCMC results are in a very good agreement with the DNS data, and are significantly improved compared with a singly conditional moment closure. A set of a posteriori DNS-DCMC tests is also performed to demonstrate importance of various terms in the doubly CMC equations. These tests first reveal that the effects of the cross dissipation and sources of enthalpy and dissipation rate (which lead to convective terms in conditional space) are insignificant and these terms can be safely neglected from the DCMC equations. The significance of this result is that the main unclosed models that would be needed for satisfactory results in a practical simulation of an engine would be the joint probably density function of enthalpy and its dissipation rate and the dissipation rate of dissipation rate.
Low temperature autoignition of conventional jet fuels and surrogate jet fuels with targeted properties in a rapid compression machine Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-10-05 Daniel J. Valco, Kyungwook Min, Anna Oldani, Tim Edwards, Tonghun Lee
The autoignition characteristics of conventional jet fuels (category A) and alternative fuels with targeted properties (category C) are investigated using a rapid compression machine and the direct test chamber charge preparation approach. The category C fuels were purposefully built to anticipate special property variations that generally occur in alternative fuels. Ignition delay measurements were made to examine the effects of these unique fuels at low compressed temperatures (625 K ≤ Tc ≤ 735 K), a compressed pressure of Pc = 20 bar and equivalence ratios of ϕ = 0.25, 0.5 and 1.0 in synthetic dry air. Chemical makeup of the fuel shows insight into the effect of the amount of branching in isoalkanes and aromatic influences on autoignition. The results show noteworthy variability in the ignition properties at these low temperature and lean conditions. This variability may impact combustion performance when the engine is running outside the normal operational map or for new engine architectures in the future.
Ignition delay times of Jet A-1 fuel: Measurements in a high-pressure shock tube and a rapid compression machine Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-07-21 A.R. De Toni, M. Werler, R.M. Hartmann, L.R. Cancino, R. Schießl, M. Fikri, C. Schulz, A.A.M. Oliveira, E.J. Oliveira, M.I. Rocha
Ignition delay time (IDT) measurements for Jet A-1 fuel samples have been performed with a rapid compression machine (RCM) and a high-pressure shock tube (ST). The IDT measurements span a pressure range from 7 to 30 bar, a temperature range from 670 K to 1200 K, and fuel/air equivalence ratios ϕ from 0.3 to 1.3. Expressions fitting the experimental data sets were obtained, with fitting parameters being provided. The combined RCM/ST data aimed at providing information on the two-stage ignition behavior and on the transition from NTC chemistry to high-temperature radical chain-branching, which are important and hard to meet targets in the development of chemical surrogates.
Computational characterization of ignition regimes in a syngas/air mixture with temperature fluctuations Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-07-25 Pinaki Pal, Mauro Valorani, Paul G. Arias, Hong G. Im, Margaret S. Wooldridge, Pietro P. Ciottoli, Riccardo M. Galassi
Auto-ignition characteristics of compositionally homogeneous reactant mixtures in the presence of thermal non-uniformities and turbulent velocity fluctuations were computationally investigated. The main objectives were to quantify the observed ignition characteristics and numerically validate the theory of the turbulent ignition regime diagram recently proposed by Im et al. 2015  that provides a framework to predict ignition behavior a priori based on the thermo-chemical properties of the reactant mixture and initial flow and scalar field conditions. Ignition regimes were classified into three categories: weak (where deflagration is the dominant mode of fuel consumption), reaction-dominant strong, and mixing-dominant strong (where volumetric ignition is the dominant mode of fuel consumption). Two-dimensional (2D) direct numerical simulations (DNS) of auto-ignition in a lean syngas/air mixture with uniform mixture composition at high-pressure, low-temperature conditions were performed in a fixed volume. The initial conditions considered two-dimensional isotropic velocity spectrums, temperature fluctuations and localized thermal hot spots. A number of parametric test cases, by varying the characteristic turbulent Damköhler and Reynolds numbers, were investigated. The evolution of the auto-ignition phenomena, pressure rise, and heat release rate were analyzed. In addition, combustion mode analysis based on front propagation speed and computational singular perturbation (CSP) was applied to characterize the auto-ignition phenomena. All results supported that the observed ignition behaviors were consistent with the expected ignition regimes predicted by the theory of the regime diagram. This work provides new high-fidelity data on syngas ignition characteristics over a broad range of conditions and demonstrates that the regime diagram serves as a predictive guidance in the understanding of various physical and chemical mechanisms controlling auto-ignition in thermally inhomogeneous and compositionally homogeneous turbulent reacting flows.
Ignition dynamics in an annular combustor for liquid spray and premixed gaseous injection Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-10-13 K. Prieur, D. Durox, J. Beaunier, T. Schuller, S. Candel
Ignition is of importance in many combustion applications and raises fundamental and practical issues. The light-round process corresponding to the flame spreading phase in the ignition of annular combustors is examined in this article by performing experiments in a model scale configuration “MICCA-Spray”. This system features 16 swirling injectors each comprising a hollow cone pressurized injector. Experiments are carried out with premixed gases as well as n-heptane and dodecane sprays. The flow, spray and flame are first characterized in a single injector configuration. Propagation from the initial kernel created by a spark plug is then observed using high speed light emission imaging. This provides flame structures at various times during the process and gives access to the time delays for flame merging. With n-heptane and dodecane fuel injection, it is found that the light-round process is similar to the one observed under fully premixed propane/air experiments but the duration of the process is augmented especially for the less volatile fuel. It is also confirmed that the delay is notably influenced by thermal conditions prevailing in the chamber at the moment of ignition, injection process and fuel composition. Making use of a flamelet like model of the combustion process, the relative changes in light-round time delay are found to be, to the first order, proportional to the relative changes in laminar burning velocity induced by the fuel spray in the air flow.
An analytical model for the impulse response of laminar premixed flames to equivalence ratio perturbations Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-06-20 A. Albayrak, R.S. Blumenthal, A. Ulhaq, W. Polifke
The dynamic response of conical laminar premixed flames to fluctuations of equivalence ratio is analyzed in the time domain, making use of a level set method (“G-Equation”). Perturbations of equivalence ratio imposed at the flame base are convected towards the flame front, where they cause modulations of flame speed, heat of reaction and flame shape. The resulting fluctuations of heat release rate are represented in closed form in terms of respective impulse response functions. The time scales corresponding to these mechanisms are identified, their contributions to the overall flame impulse response are discussed. If the impulse response functions are Laplace transformed to the frequency domain, agreement with previous results for the flame frequency response is observed. An extension of the model that accounts for dispersion of equivalence ratio fluctuations due to molecular diffusion is proposed. The dispersive model reveals the sensitivity of the premixed flame dynamics to the distance between the flame and the fuel injector. The model results are compared against numerical simulation of a laminar premixed flame.
Localization of unsteady heat source in a tube from pressure measurements with the inverse method Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-09-28 Di Zhong, Suhui Li, Fanglong Weng, Min Zhu
This paper presents work to identify the position of an unsteady heat source in a one-dimensional tube from acoustic pressure measurements with the inverse method. The relationship between the oscillation heat release rate and the pressure can be represented as a Volterra integral equation of the first kind. The discretization method was applied to transform the integral equation into matrix form. To stabilize the solution of the matrix equation, the Tikhonov regularization method was proposed. Experiments were performed to validate the inverse method. A semi-infinite probe system was used to measure the pressure perturbations in the tube, to avoid the high temperature damaging the microphone. Before the pressure measurements were taken, calibration was performed for the semi-infinite probe system to obtain accurate pressure data in the tube. The experiments were performed in three steps. First, the localization of a pure sound source in the tube at ambient temperature was studied. Second, localization of a pure sound source at hot conditions was considered. Third, the pure sound source was replaced with an unsteady heat source, and pressure data were used to determine the position of the unsteady heat source. Results show that calibration and the regularization are both necessary for the determination of the sound source position in the tube. Meanwhile, at the hot and heat release rate conditions, with the consideration of the temperature distribution in the thermoacoustic model, the position of the sound source and the unsteady heat release source can be determined successfully.
Effect of equivalence ratio on the modal dynamics of azimuthal combustion instabilities Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-07-07 Nicholas A. Worth, James R. Dawson
The present paper investigates the effect of equivalence ratio on the modal dynamics of self-excited azimuthal combustion instabilities in a laboratory-scale annular combustor. It is shown that operating at different equivalence ratios not only affects the mode of oscillation (whether the instabilities are mixed, or predominantly spinning or standing modes), but also the way in which mode switching between these states occurs. Using pressure time-series data obtained at multiple locations around the annulus the phenomenon of mode switching is investigated through the spin ratio, the orientation of the nodal lines and the envelope of the pressure oscillations. These three parameters all suggest that mode switching events occur almost periodically, and over azimuthal convective time scales. Moreover, analysis of the spin ratio and orientation of the nodal lines show that these parameters are correlated, and that their mean rates of change or trajectories also have a preferred direction. Therefore, these quantities were found to oscillate backwards and forwards in-phase with each other, as opposed to the mode simply rotating in one direction, providing new insight into the nature of modal dynamics of self-excited azimuthal modes.
Effects of convection time on the high harmonic combustion instability in a partially premixed combustor Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-07-12 Jisu Yoon, Seongpil Joo, Jeongjin Kim, Min Chul Lee, Jong Guen Lee, Youngbin Yoon
The fuel composition effects of H2/CH4 syngas in a partially premixed model combustor (PP-MC) were examined for the unique phenomenon of combustion instability (CI) frequency/mode shifting (FMS), which is a transition of mode as well as frequency. The increase in the H2 composition of the fuel altered FMS from a longitudinal fundamental mode (≈250 Hz) to a 7th harmonic mode (≈1750 Hz). The cause and characteristics of this FMS were investigated using OH planar laser-induced fluorescence (OH-PLIF) measured at 10 Hz, particle-image velocimetry (PIV), and the flame transfer function (FTF). The convection time (τconv) was assumed to be the key parameter of the FMS. Thus, tests were conducted to determine the air flow rate ( V ˙ air ) and equivalence ratio (φ) variation, which are vital parameters of the τconv in terms of the flame length and mixing time. The φ variation caused obvious changes in the flame length and instability frequency/mode, while the V ˙ air variation did not. The τconv was analyzed by calculating the global convection time (τconv_global) and the real convection time (τconv_real) from the length of the OH-PLIF-based unburned mixture length divided by the averaged mixture nozzle exit velocity. The τconv_real was calculated from the integral of the real velocity determined from PIV. Both calculations showed an inverse correlation between τconv and CI frequency, which particularly signifies that the FMS is controllable and a specific mode of CI can be generated by adjusting the τconv. The FTF was measured to determine the intrinsic characteristics of the flame. The FTF phase was normalized by the Strouhal number (St) and identified a direct relationship between FTF gain and τconv variation. In conclusion, the τconv is the main reason for the FMS. The importance of τconv in understanding the CI characteristics was confirmed in a PP-MC using high H2 fuels.
Flame stabilization analysis of a premixed reacting jet in vitiated crossflow Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-07-22 Jason A. Wagner, Stephen W. Grib, James W. Dayton, Michael W. Renfro, Baki M. Cetegen
The flame stabilization behavior of a premixed ethylene–air jet injected normal to a hot vitiated crossflow (JICF) was studied experimentally using simultaneous hydroxyl (OH) planar laser induced fluorescence (PLIF), formaldehyde (CH2O) PLIF, and particle image velocimetry (PIV). Pixel-by-pixel multiplication of OH and CH2O fluorescence signals was conducted to estimate the reacting JICF flame front. The simultaneous PLIF-PIV measurements allowed for an in-depth study of the interaction between the flame and the flowfield. The flame structure was divided into two branches, a windward and leeward flame branch. The unsteady windward flame exhibited both attached and lifted flame behavior, while the leeward flame branch remained consistently attached at the jet exit. In some cases, formaldehyde signal was observed upstream of the windward flame base, suggesting the build-up of a radical pool due to mixing between the jet reactants and hot crossflow. Both flame branches were anchored in the jet shear layer, but with increasing distance from the jet exit the flames moved inside the shear layers. Small scale vortices caused local wrinkling of the flame front. The windward flame was observed to wrap around the large-scale vortices that formed along the jet shear layer. The large-scale structures distorted the flame front but the associated strain-rate was typically lower than that imparted by the small-scale structures. The leeward flame edge aligned with regions of high principal extensive strain-rate and high dilatation. On the other hand, the windward flame edge was located in regions where principal extensive and principal compressive strain rate magnitudes were high and dilatation was low. The results suggest that auto-ignition is the dominant flame stabilization mechanism for the unsteady windward flame and premixed flame propagation is the more dominant stabilization mechanism for the leeward flame branch.
Influence of self-sustained jet oscillation on a confined turbulent flame near lean blow-out Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-09-26 Zhiyao Yin, Isaac Boxx, Wolfgang Meier
Premixed methane–air turbulent flame is generated in a single-nozzle jet-stabilized combustor designed based on the FLOX® concept1. Confinement-induced, self-sustained jet oscillation is observed. Its influence on combustion stability near lean blow-out (LBO) is investigated using simultaneous particle imaging velocimetry (PIV), planar laser-induced fluorescence of OH radicals (OH PLIF), and OH chemiluminescence imaging at 5-kHz repetition rate. Via proper orthogonal decomposition (POD) of the velocity field and extended POD of the scalar fields, pronounced variations in the flame shape are observed during a cycle of jet oscillation. In extreme cases, flame is partially blown out in the combustor due to jet impingement on the wall during the first half of its oscillation cycle. In the subsequent half cycle following jet detachment, flame is restabilized after robust flashback and re-light. Statistical analysis shows that such pattern is by far the most prevalent mechanism for blow-out and restabilization to take place at the operating condition. Additionally, these events are found with much higher probability during slow-paced jet oscillations.
Azimuthally forced flames in an annular combustor Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-06-30 Nicholas A. Worth, James R. Dawson, Jenni AM Sidey, Epaminondas Mastorakos
The application of azimuthal acoustic forcing to flames in a laboratory scale annular combustor is demonstrated in the present paper, which not only allows large amplitude azimuthal modes to be excited, but permits control to be exerted over their frequency, amplitude, and their modal dynamics, including parameters such as the spin ratio and orientation. The effect of forcing frequency is investigated, and it is found that the excitation can be well controlled into a standing wave oscillation over a range of amplitudes for selected frequencies. However, close to the natural frequency of the system, the modal dynamics cannot be well controlled, and instead these tend towards their self-excited state, resulting in a mixed mode. Comparing the phase-averaged structure of the forced and self-excited responses for similar operating conditions, illustrate similar flame dynamics, making this novel forcing approach extremely useful for the study of these instabilities in annular systems.
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