Chemical-Looping Combustion (CLC) has emerged in recent years as a very promising combustion technology for power plants and industrial applications with inherent CO2 capture, which circumvent the energy penalty imposed on other competing technologies. The process is based on the use of a metal oxide to transport the oxygen needed for combustion in order to prevent direct contact between the fuel and air. CLC is performed in two interconnected reactors, and the CO2 separation inherent to the process practically eliminates the energy penalty associated with gas separation. The CLC process was initially developed for gaseous fuels, and its application was subsequently extended to solid fuels. The process has been demonstrated in units of different size, from bench scale to MW-scale pilot plants, burning natural gas, syngas, coal and biomass, and using ores and synthetic materials as oxygen-carriers. An overview of the status of the process, starting with the fundamentals and considering the main experimental results and characteristics of process performance, is made both for gaseous and solid fuels. Process modelling of the system for solid and gaseous fuels is also analysed. The main research needs and challenges both for gaseous and solid fuel are highlighted.

This paper reports the effect of water vapour on particulate matter (PM) during the separate combustion of in situ volatiles and char generated from chromated-copper-arsenate-treated (CCAT) wood at 1300 °C. Combustion of in situ volatiles produces only PM with aerodynamic diameter <1 µm (i.e., PM1), dominantly PM with aerodynamic diameter <0.1 µm (i.e., PM0.1). Water vapour could significantly enhance the nucleation, coagulation and condensation of fine particles and reduce the capture of Na and K by the alumina reactor tube via reduced formation of alkali aluminates, leading to increases in both yield and modal diameter of PM0.1. Water vapour could also enhance char fragmentation hence increase the yield of PM with aerodynamic diameter between 1 and 10 µm (i.e., PM1–10) during char combustion. For trace elements, during in situ volatiles combustion, volatile elements (As, Cr, Ni, Cu and Pb) are only presented in PM1 and water vapour alters the particle size distributions (PSDs) but has little effect on the yields of these trace elements. During char combustion, As, Cr, Cu and Ni are present in both PM1 and PM1–10 while the non-volatile Mn and Ti are only present in PM1–10. Increasing water vapour content increases the yields of As, Cr, Cu, Ni, Mn and Ti in PM1-10 due to enhanced char fragmentation. During char combustion, water vapour also originates less oxidising conditions locally for enhancing As release, promotes the generation of gaseous chromium oxyhydroxides and inhabits the production of NiCl2 (g), leading to increased yields of As and Cr and decreased yield of Ni in PM0.1.

To utilize sustainable biofuel, the current study proposes a novel combustion technique that directly burns liquid ethanol without a spray system. Two swirling air flows are induced by tangentially injected the gas from two concentric tubes at different stages. The liquid ethanol is fed by a liquid tank at the center. At the beginning methane flame assists in preheating the system to vaporize liquid ethanol and ignite the vapor. Thereafter methane is switched off, and liquid ethanol can be continuously vaporized through self-burning released heat. The heat and mass transfer processes are examined to illustrate such self-sustained burning–heating–evaporating system. The ethanol flow rate is gradually increased to provide different heat output. The flame structures, temperature distributions and pollutant emissions are carefully examined. The results show that the ethanol can be steadily burned to provide heat output between 0.7 and 2.5 kW. Generally a blue flame is obtained, and the NOx and CO concentrations are ultralow. By increasing ethanol flow rate to exceed 8 mL/min, an unsteady, sooting flame is observed owing to incomplete evaporation and poor mixing. A parametric study is conducted to evaluate the influences of liquid tank position, flow rate and tip structure on the combustion characteristics. Additionally, an optimal operation condition is proposed. The current study provides a promising method to burn low-boiling liquid fuel in a clean, efficient and compact way.

Radiation is the dominant mode of heat transfer near the burner of coal and biomass-fired boilers. Predicting and measuring heat transfer is critical to the design and operation of new boiler concepts. The individual contributions of gas and particle phases are dependent on gas and particle concentration, particle size, and gas and particle temperature which vary with location relative to the flame. A method for measuring the contributions of both gas and particle radiation capable of being applied in harsh high temperature and pressure environments has been demonstrated using emission from particles and water vapor using an optical fiber probe transmitting a signal to a Fourier Transform Infrared (FTIR) spectrometer. The method was demonstrated in four environments of varying gas and particle loading using natural gas and pulverized wood flames in a down-fired 130 kWth cylindrical reactor. The method generates a gas and particle temperature, gas concentrations (H2O and CO2), total gas and particle intensities, and gas and particle total effective emissivity from line-of-sight emission measurements. For the conditions measured, downstream of the luminous flame zone, water vapor and CO2 radiation were the dominant modes of heat transfer (effective emissivity 0.13–0.19) with particles making a minor contribution (effective emissivity 0.01–0.02). Within a lean natural gas flame, soot emission was low (effective emissivity 0.02) compared to gas (0.14) but within a luminous flame of burning wood particles (500 µm mean diameter) the particles (soot and burning wood) produced a higher effective emissivity (0.17) than the gas (0.12). The measurement technique was therefore found to be effective for several types of combustion environments.

Chemical looping combustion (CLC) is an advanced oxyfuel process that enables CO2 capture with low efficiency penalty. CLC of gaseous fuels has successfully been demonstrated in several pilots up to 150 kWth. Numerous oxygen carriers have been tested regarding fuel conversion performance and lifetime. This work is a scale-up study of gaseous fuel CLC to MWth scale. A Ca-Mn-based oxygen carrier has been developed and manufactured in ton-scale prior to the present test. Investigations were conducted in a 1 MWth CLC unit that was adapted to utilize natural gas as fuel. Stable CLC conditions were reached during tests with Ca-Mn-based material, and the transition to operation with ilmenite was studied. The fuel conversion was in the range of 80%. During operation, 99% of the unburned methane was converted in the post oxidation chamber. The solids circulation rate and the lifetime of solids were determined by means of solids samples from the process, which were investigated in terms of attrition and degree of oxidation. The solids circulation rate was 17 tons h−1 MW−1 which is higher than in former tests but lower compared to other units. The most important limiting factors of the fuel conversion are the low solids inventory of the fuel reactor and the oxygen carrier to fuel ratio that corresponds to the solids circulation.

This paper reports a systematic study on the formation of particulate matter with diameter of <10 µm (i.e., PM10) during the combustion of two formulated water-soluble fractions (FWSFs) of bio-oil in a drop-tube-furnace (DTF) at 1400 °C under air or oxyfuel (30%O2/70%CO2) conditions. FWSF-1 was an organic-free calcium chloride solution with a calcium concentration similar to that in the bio-oil. FWSF-2 was formulated from the compositions of major organics in bio-oil WSF, doped with calcium chloride at the same concentration. The results suggest that similar to bio-oil combustion, the FWSF combustion produces mainly particulate matter with diameter of between 0.1 and 10 µm (i.e., PM0.1–10). Since there are no combustibles in the organic-free FWSF-1, the PM is produced via droplet evaporation followed by crystallization, fusion and further reactions to form CaO (in air or argon) or partially CaCO3 (under oxyfuel condition). With the addition of organics, FWSF-2 combustion produces PM10 shifting to smaller sizes due to extensive break up of droplets via microexplosion. Sprays with larger droplet size produce PM10 with increased sizes. The results show that upon cooling CaO produced during combustion in air can react with HCl gas to form CaCl2 in PM0.1. The predicted PSDs of PM10 based on the assumption that one droplet produces one PM particle is considerably larger than experimentally-measured PSDs of PM10 during the combustion of FWSFs, confirming that breakup of spray droplets takes place and such breakup is extensive for FWSF-2 when organics are present in the fuel.

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.

In chemical looping with oxygen uncoupling, oxygen carrier (OC) circulates between the fuel and air reactors to release and absorb O2 repeatedly. In order to assess the re-oxidation characteristic of Cu-based OC in the air reactor from the microscopic mechanism and macroscopic kinetics perspective, DFT calculations and isothermal oxidation experiments were conducted. In DFT calculations, Cu2O(111) surface was chosen as the objective surface to explore the oxygen uptake as well as the atomic transportation pathways, and to determine the rate-limiting steps basing on the energy barrier analyses. It was found that the energy barrier of the surface reaction step (0.96 eV) is smaller than that of the ions diffusion step (1.61 eV). Moreover, the Cu cations outward diffusion occurs more easily than O anions inward diffusion, which confirmed the epitaxial growth characteristic of Cu2O oxidation. The isothermal oxidation experiments were conducted in a thermogravimetric analyzer (TGA), and about 3.5 mg CuO@TiO2-Al2O3 particles within the diameter range of 75–110 µm were tested between 540 and 600 °C, where the internal and external gas diffusion effects were eliminated. Mixtures of 5.2-21.0 vol.% O2 in N2 were adopted as the gas agent for oxidation. Based on the understandings obtained from DFT calculations, a simple mathematical model with unknown parameters of the surface reaction process (mainly the activation energy, Ek) and ions diffusion process (mainly the activation energy, ED) was established to describe the overall oxidation process in TGA experiments. Eventually, these unknown parameters were determined as Ek= 50.5 kJ/mol and Ek= 79.2 kJ/mol via global optimization. With the attained parameters, simulations reproduced the experimental results very well, which demonstrated that this simplification model, where grain is converted almost layer by layer but different from the feature of the shrinking core model is able to accurately describe the overall oxidation process of Cu2O.

The effect of steam and sulphur dioxide on CO2 capture by limestone during calcium looping was studied in a novel lab-scale twin fluidised bed device (Twin Beds – TB). The apparatus consists of two interconnected batch fluidised bed reactors which are connected to each other by a duct permitting a rapid and complete pneumatic transport of the sorbent (limestone) between the reactors. Tests were carried out under typical calcium looping operating conditions with or without the presence of H2O and/or SO2 during the carbonation stage. Carbonation was carried out at 650°C in presence of 15% CO2, 10% steam (when present) and by investigating two SO2 levels, representative of either raw (1500 ppm) or pre-desulphurised (75 ppm) typical flue gas derived from coal combustion. The sorbent used was a reactive German limestone. Its performance was evaluated in terms of CO2 capture capacity, sulphur uptake, attrition and fragmentation. Results demonstrated the beneficial effect of H2O and the detrimental effect of SO2 on the CO2 capture capacity. When both species were simultaneously present in the gas, steam was still able to enhance the CO2 capture capacity even outweighing the negative effect of SO2 at low SO2 concentrations. A clear relationship between degrees of Ca carbonation and sulphation was observed. As regards the mechanical properties of the sorbent, both H2O and SO2 hardened the particle surface inducing a decrease of the measured attrition rate, that was indeed always very low. Conversely, the fragmentation tendency increased in presence of H2O and SO2 most likely due to the augmented internal stresses within the particles. Clear bimodal particle size distributions for in-bed sorbent fragments were observed. Microstructural scanning electron microscope and porosimetric characterisations aided in explaining the observed trends.

Conventional air incineration of plastic waste has been considered as one of important sources of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) through de novo synthesis and precursor conversion. Chemical looping combustion (CLC) is an attractive technology for the conversion of plastic wastes to energy with the potential to drastically suppress the formation of PCDD/Fs. In this paper, the iG-CLC (in-situ gasification CLC) experiments of plastic waste were implemented in a semi-continuously operated fluidized bed reactor, which actually simulates the fuel reactor of a continuously-operated interconnected fluidized bed reactor. A kind of low-cost material, natural iron ore without/with 5 wt% CaO adsorbent through the ultrasonic impregnation method, was used as oxygen carrier (OC). Firstly, some key performances of the reactor system, such as the relevance of the bed inventory to the flow rate of fluidizing agent as well as the relationship between the feeding rate and overflow rate of OC, were calibrated. Then, 90 min of single experiment was conducted for each experimental case and an accumulative operation of more than 10 h was attained. Typically, the combustion efficiency can reach at about 98%, and both the carbon conversion and CO2 yield can approach to 95% at 900 °C and input thermal power of 150 W with a mixture of 5 vol% H2O and 95 vol% N2 as the fluidizing agent (UFR/Umf = 3). Moreover, the results obtained in the semi-continuously operated fluidized bed reactor demonstrated that CaO decoration to iron ore is conductive to suppressing the formation of chlorobenzene (as a toxic matter and precursor/intermediate of PCDD/Fs) and does not obviously deteriorate the OC performance.

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.

Spinel CuFe2O4 is a promising oxygen carrier due to its synergistic enhanced performance. A fundamental understanding of the reaction mechanism between oxygen carrier and fuels is important for a rational design of highly efficient oxygen carrier. The reaction mechanism of spinel CuFe2O4 with CO during chemical-looping combustion (CLC) was studied based on thermogravimetric analyses (TGA) and density functional theory (DFT) calculations. Two distinct reaction stages were clearly observed. CuFe2O4 was mainly transformed into Cu and Fe3O4 with a rapid reaction rate in the initial stage, and then product Fe3O4 was slowly reduced to FeO or even to Fe. The reactivity of CuFe2O4 is much higher than that of Fe2O3, which is ascribed to the existence of Cu. The enhanced oxygen evolution activity of CuFe2O4 at low temperature is validated by both the experimental and theoretical methods. Three types of surface oxygen coordinated with different metal atoms show different reactivity. Two kinds of reaction pathways are involved in CO oxidation over CuFe2O4. In the one-step reaction pathway, CO directly reacts with the oxygen bonding to two octahedral Cu and one octahedral Fe atoms to form a CO2 molecule without an energy barrier, which corresponds to the surface oxygen consumption observed in TGA experiments. In the possible two-step reaction pathway, CO first adsorbs on the surface, and then reacts with the oxygen bound to one octahedral Cu and two octahedral Fe atoms to generate CO2 by surmounting an energy barrier of 10.84 kJ/mol, which is the most kinetically and thermodynamically favorable pathway.

Particle deposition on heat exchanger tubes is a serious concern in solid fuel combustion and gasification systems, such as power plants and syngas coolers. To predict deposition rates, several detailed computational fluid dynamic (CFD) models have been developed. However, these models are computationally expensive and cannot be used for quick determination of deposition rates and/or slagging tendencies. Particle impaction efficiency correlations, while not as accurate as detailed CFD models, are easier to use and are able to estimate the impaction rate of particles on the heat exchanger tubes. Nonetheless, since deposition and slagging are not just functions of particle impaction rates, but also sticking propensity, which is related to the particle temperature at impact, the impaction efficiency correlations fail to provide sufficient information. To address this shortcoming, similar correlations for particle temperature at impact have been developed in this work, based on a non-dimensional parameter that captures the flow and boundary conditions, as well as particle properties. When used alongside the impaction efficiency correlations, the new correlations developed can provide a reasonable estimate of the deposition and slagging tendencies, at negligible computational expense.

This paper aims to reveal the mechanisms governing the impaction and sticking dynamics of fly ash particles in pulverized coal combustion. The modeling work is of relevance to experiments in a 25 kW self-sustained down-fired furnace, which provides a sequence of real deposit shapes as varied boundary conditions for CFD simulations. Although the formed ash deposit has a comparable length scale with the probe, it has little effect on the global impaction efficiency of newly-coming particles. However, as the deposit builds up, incident particles impact the deposit and probe at generally larger impact angles and smaller normal velocities despite the almost invariant global impaction efficiency. It results in an enhanced local sticking probability in the center region of the probe, but a decreased one in the lateral regions. The incident kinetic energy of newly sticking particles to the deposit exhibits a converse correlation with their impact angle. The relationship of the averaged local sticking probability as a function of the azimuthal angle of probe is illustrated. Finally, the effect of Reynolds number on global particle impaction efficiency is examined. A universal formula is proposed, which is of importance to bridge lab-scale experiments and practical applications.

An online thermogravimetric measurement method of ash deposition was developed. Ash deposition and slag bubble in the reductive zone of pulverized coal staged combustion were investigated. Firstly, a steady pulverized coal staged combustion was achieved in an electrically heated down-fired furnace. Additionally, gas species, coal conversion, and particle size distribution were quantitatively measured. Secondly, real-time ash deposition rates at different temperatures (1100–1400 °C) were measured, and deposition samples were carefully collected with an N2 protection method. The morphologies of collected samples were investigated through a scanning electron microscope. It was found that the deposited ash transformed from a porous layer composed of loosely bound particles to a solid layer formed by molten slag. Different behaviors of the slag bubble were observed, and bubble sizes were significantly affected by the deposition temperature. A deposition and bubble formation mechanism was proposed and used for modeling. Results showed that the proposed model well predicted the observed ash deposition and bubble formation process.

The release of arsenic vapors (As3+) during high-arsenic coal combustion not only raises serious environmental concerns, but also causes catalyst deactivation in selective catalytic reduction (SCR) systems. To illuminate the mechanisms involved in the transformation of arsenic vapors towards less troublesome arsenates (As5+) during coal combustion, the accessory minerals in the high-arsenic coal were identified and the association relationship of these compounds with arsenic in fly ash was estimated. The results showed that Si/Al were the main inorganic elements in high-arsenic coal while the content of Ca was quite low. Ca was mostly transformed into sulfates during coal combustion and the effect of Ca on the arsenic transformation was limited. Al/Fe played a more significant role in arsenic speciation transformation and arsenic in the fly ash was predominantly bound with Al/Fe-oxides as arsenates. It was further confirmed that Al in kaolin/metakaolin showed good capacity on arsenic capture. In addition, few arsenic vapors were captured through the physical adsorption mechanism and the large fraction of As3+ in some fine particles was mostly attributed to the chemical reactions between arsenic vapors and Al-compounds. Meanwhile, a certain amount of arsenic vapors were converted into As2O5(s) under the influence of SCR catalyst and then carried by flue gas to participate in fly ash. Besides, part of arsenic distributed in the fly ash was through the stabilization of ash matrix in high temperature conditions. The transformation of arsenic from vapors towards particulate arsenic favored arsenic emission control by particulate matter control devices.

For stationary power sources such as utility boilers, it is useful to dispose of parametric models able to describe their behavior in a wide range of operating conditions, to predict some Quantities of Interest (QOIs) that need to be consistent with experimental observations. The development of predictive simulation tools for large scale systems cannot rely on full-order models, as the latter would lead to prohibitive costs when coupled to sampling techniques in the model parameter space. An alternative approach consists of using a Surrogate Model (SM). As the number of QOIs is often high and many SMs need to be trained, Principal Component Analysis (PCA) can be used to encode the set of QOIs in a much smaller set of scalars, called PCA scores. A SM is then built for each PCA score rather than for each QOI. The advantage of reducing the number of variables is twofold: computational costs are reduced (less SMs need to be trained) and information is preserved (correlation among the original variables). The strategy is applied to a CFD model simulating the Alstom 15 MWth oxy-pilot Boiler Simulation Facility (BSF). In practice, experiments cannot provide full coverage of the pulverized-coal utility boiler due to both practicality and costs. Values of the model’s parameters which guarantee consistency with the experimental data of this test facility for 121 QOIs are found, by training a SM based on the combination of Kriging and PCA, using only 5 latent variables.

O2/H2O combustion, as a new evolution of oxy-fuel combustion, has gradually gained more attention recently for carbon capture in a coal-fired power plant. The physical and chemical properties of steam e.g. reactivity, thermal capacity, diffusivity, can affect the coal combustion process. In this work, the ignition and volatile combustion characteristics of a single lignite particle were first investigated in a fluidized bed combustor under O2/H2O atmosphere. The flame and particle temperatures were measured by a calibrated two-color pyrometry and pre-buried thermocouple, respectively. Results indicated that the volatile flame became smaller and brighter as the oxygen concentration increased. The ignition delay time of particle in dense phase was shorter than that in dilute phase due to its higher heat transfer coefficient. Also, the volatile flame was completely separated from particles (defined as off-flame) in dense phase while the flame lay on the particle surface (defined as on-flame) in dilute phase. The self-heating of fuel particles by on-flame in dilute phase was more obvious than that in dense phase, leading to earlier char combustion. At low oxygen concentration, the flame in the H2O atmosphere was darker than that in the N2 atmosphere because the heat capacity of H2O is higher than that of N2. With the increase of oxygen concentration, the flame temperature in the O2/H2O atmosphere was dramatically enhanced rather than that in the O2/N2 atmosphere, where the diffusion rate of oxygen in O2/N2 atmosphere became the dominant factor.

Interactions between pyrite and silicates are very critical to ash slagging in coal-fired boilers. However, no work has been reported regarding the impacts on such interactions of CO2, the dominant component in the oxy–fuel combustion gas. This was investigated in the present work by using mixtures of pyrite and kaolinite, a prevailing silicate mineral in coal. Furthermore, the sintering strength of the generated products was also evaluated. The pyrite–kaolinite mixtures were treated on a fixed bed reactor in both N2 and CO2 for comparison. The treating temperature was 1050, 1150 and 1250 °C while the reaction time was 3, 6 and 12 min, respectively. The solid products were characterized by techniques such as X-Ray Fluorescence (XRF), X-Ray Diffraction (XRD) and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS). Sintering tests of these products were carried out as well. It was found that some interactions between pyrite and kaolinite took place in the N2 atmosphere. This was evidenced by the formation of iron aluminosilicate and attributed to the effects of water vapor released from kaolinite dehydroxylation. Nevertheless, pyrite–kaolinite interactions in N2 were limited and had insignificant effects on product sintering strength development. In CO2, pyrite–kaolinite interactions were significantly enhanced, compared with those in N2. Although the kaolinite-derived water vapor had some effects, CO2 was found to play a dominant role. Enhanced pyrite–kaolinite interactions resulted in an increase of eutectic phases. Consequently, the product sintering strength development was greatly elevated. It was further found that, under the conditions investigated, the interactions between pyrite and kaolinite were actually through reactions between FeO, rather than other intermediates, and aluminosilicate. This new finding enabled us to develop the mechanisms for pyrite–kaolinite interactions in N2 and CO2.

In an experimental study the effects of varied oxygen concentrations in the oxidizer gas on resulting flow fields, combustion products and general behavior of pulverized coal swirl flames under oxy-fuel conditions have been investigated. Experiments were carried out in a small scale down-fired cylindrical combustion chamber equipped with an annular swirl burner. Studied flames had a constant power output of 40 kWth and O2/CO2 oxidizer gas mixtures with O2 concentrations ranging from 23 to 33 vol%. Detailed two-dimensional flow field measurements are obtained from laser Doppler anemometry (LDA). Velocity profiles (Mean and RMS) have been obtained for all conditions investigated and serve as basis for identification of flow field characteristics. Velocity RMS values are provided as supplementary material. To complement flow field measurements, in-flame gas composition measurements were also conducted using a sampling probe combined with infrared gas absorption analysis via Fourier-transform infrared (FTIR) spectrometry. The results obtained show increased velocities, particularly along the main vortex for flames with increased oxygen contents, while lower velocities are found to occur inside the recirculation regions. The opposite occurs with lower O2 concentrations, showing significantly reduced velocities in the main vortex, but stronger recirculation than the high oxygen counterparts. This effect is attributed to a modification of the swirl level introduced by the expansion of product gases. Measured NO and CO in-flame concentrations showed significant variations under different O2 concentrations in the oxidizer.

Transported probability density function (TPDF) simulation with sensitivity analysis has been conducted for turbulent non-premixed CH4/H2 flames of the jet-into-hot-coflow (JHC) burner, which is a typical model to emulate moderate or intense low oxygen dilution combustion (MILD). Specifically, two cases with different levels of oxygen in the coflow stream, namely HM1 and HM3, are simulated to reveal the differences between MILD and hot-temperature combustion. The TPDF simulation well predicts the temperature and species distributions including those of OH, CO and NO for both cases with a 25-species mechanism. The reduced reaction activity in HM1 as reflected in the peak OH concentration is well correlated to the reduced oxygen in the coflow stream. The particle-level local sensitivities with respect to mixing and chemical reaction further show dramatic differences in the flame characteristics. HM1 is less sensitive to mixing and reaction parameters than HM3 due to the suppressed combustion process. Specifically, for HM1 the sensitivities to mixing and chemical reactions have comparable magnitude, indicating that the combustion progress is controlled by both mixing and reaction in MILD combustion. For HM3, there is however a change in the combustion mode: during the flame initialization, the combustion progress is more sensitive to chemical reactions, indicating that finite-rate chemistry is the controlling process during the autoignition process for flame stabilization; at further downstream where the flame has established, the combustion progress is controlled by mixing, which is characteristic of nonpremixed flames. An examination of the particles with the largest sensitivities reveals the difference in the controlling mixtures for flame stabilization, namely, the stoichiometric mixtures are important for HM1, whereas, fuel-lean mixtures are controlling for HM3. The study demonstrates the potential of TPDF simulations with sensitivity analysis to investigate the effects of finite-rate chemistry on the flame characteristics and emissions, and reveal the controlling physio-chemical processes in MILD combustion.

Moderate or intense low oxygen dilution (MILD) combustion has been the focus of a range of fundamental experimental and numerical studies. Reasonable agreement between experimental and numerical investigations, however, requires finite-rate chemistry models and, often, ad hoc model adjustment. To remedy this, an adaptive eddy dissipation concept (EDC) combustion model has previously been developed to target conditions encountered in MILD combustion; however, this model relies on a simplified, pre-defined assumption about the combustion chemistry. The present paper reports a generalised version of the modified EDC model without the need for an assumed, single-step chemical reaction or ad hoc coefficient tuning. The results show good agreement with experimental measurements of two CH4/H2 flames in hot coflows, showing improvements over the standard EDC model as well as the previously published modified EDC model. The updated version of the EDC model also demonstrates the capacity to reproduce the downstream transition in flame structure of a MILD jet flame seen experimentally, but which has previously proven challenging to capture computationally. Analyses of the previously identified dominant heat-release reactions provide insight into the structural differences between a conventional autoignitive flame and a flame in the MILD combustion regime, whilst highlighting the requirement for a generalised EDC combustion model.

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.

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.

The present work shows an in-depth analysis about the role of mixing models on the simulation of MILD combustion using a finite-rate combustion model, the Partially Stirred Reactor approach (PaSR). Different approaches of increasing complexity are compared: a simple model based on a fraction of the integral time scale, a fractal-based mixing model and a dynamic mixing model based on the resolution of transport equations for scalar variance and dissipation rate. The approach is validated using detailed experimental data from flames stabilized on the Adelaide Jet-in-Hot Co-flow (JHC) burner at different fuel-jet Reynolds numbers (5k, 10k and 20k) and different co-flow oxygen dilution levels (3%, 6% and 9%). The results indicate the major role of mixing models to correctly handle turbulence/chemistry interactions and clearly indicate the superior performances of the dynamic mixing model over the other tested approaches.

The strategy to stabilize distributed combustion regimes adopting cyclonic flow fields has been proven to be challenging. In fact, the establishment of a toroidal flow field within a combustion chamber may ensure the recirculation of mass and sensible enthalpy required to simultaneously dilute the fresh reactants and increase the temperature above the autoignition one. The combination of reactants dilution and preheating may greatly increase system energy efficiency and lower pollutants production producing very peculiar combustion regime (named MILD Combustion). At the same time this strategy can be compromised if the sensible enthalpy is not high enough to promote the auto-ignition process of diluted mixtures. This may happen either because of an inefficient recirculation system or due to a too low calorific fuel. The paper aims at exploiting the performance of a small-size cyclonic burner for a conventional fuel (CH4) and a low calorific fuel (synthetic biogas) through the characterization of the process stabilization and pollutant emissions as a function of the mixture equivalence ratio and the nominal thermal power of the inlet mixture (from 2 to 10 kW), with the aim of identifying the optimal operating condition of the system. Results suggest that the system has to be exercised with mixtures with compositions slightly under the stoichiometric conditions and in a well identified temperature range to minimize both NOx and CO emission. The burner can be easily exercised also with low calorific fuels for higher thermal powers according to the low LHV. However, it results that an efficient recirculation of the exhausts produces a robust MILD combustion condition also when low calorific fuels are used.

The criterion used to define MILD combustion in non-premixed condition is analysed using Direct Numerical Simulation (DNS) of MILD combustion of methane-diluted air established with internal exhaust gas recirculation. The simulations reveal multiple interacting reaction zones in MILD combustion which are extremely different from conventional combustion. Furthermore, DNS deduced S-curves highlight the role of chemically active species. Specifically, the temperature rise is accompanied with an increase in the scalar dissipation rate of mixture fraction, which is quite contrasting to the classical S-curve from the classical flame theories. This observation is explained on a physical basis.

The need for more efficient power cycles has attracted interest in super-critical CO2 (sCO2) cycles. However, the effects of high CO2 dilution on auto-ignition at extremely high pressures has not been studied in depth. As part of the effort to understand oxy-fuel combustion with massive CO2 dilution, we have measured shock tube ignition delay times (IDT) for methane/O2/CO2 mixtures and hydrogen/O2/CO2 mixtures using sidewall pressure and OH* emission near 306 nm. Ignition delay time was measured in two different facilities behind reflected shock waves over a range of temperatures, 1045–1578 K, in different pressures and mixture regimes, i.e., CH4/O2/CO2 mixtures at 27–286 atm and H2/O2/CO2 mixtures at 37–311 atm. The measured data were compared with the predictions of two recent kinetics models. Fair agreement was found between model and experiment over most of the operating conditions studied. For those conditions where kinetic models fail, the current ignition delay time measurements provide useful target data for development and validation of the mechanisms.

An extensive experimental study is carried out to analyze scaling laws for the length of methane oxy-flames stabilized on a coaxial injector. The central methane fuel stream is diluted with N2, CO2 or He. The annular air stream is enriched with oxygen and can be impregnated with swirl. Former studies have shown that the stoichiometric mixing length of relatively short flames is controlled by the mixing process taking place in the vicinity of the injector outlet. This property has been used to derive scaling laws at large values of the stoichiometric mixture fraction. It is shown here that the same relation can be extended to methane oxy-flames characterized by small values of the stoichiometric mixture fraction. Flame lengths are determined with OH* chemiluminescence measurements over more than 1000 combinations of momentum ratio, annular swirl level and composition of the inner and outer streams of the coaxial injector. It is found that the lengths of all the flames investigated without swirl collapse on a single line, whose coefficients correspond to within 15% of flame lengths obtained for fuel and oxidizer streams at much larger stoichiometric mixture fractions. This relation is then extended to the case of swirling flames by including the contribution of the tangential velocity in the flow entrainment rate and is found to well reproduce the mixing degree of the two co-axial streams as long as the flow does not exhibit a vortex breakdown bubble. At higher swirl levels, when the flow features a central recirculation region, the flame length is found to also directly depend on the oxygen enrichment in the oxidizer stream.

Two key flame macrostructures in swirling flows have been observed in experiments of oxy-combustion (as well as air-combustion); as the equivalence ratio is raised, the flame moves from being stabilized on just the inner shear layer (Flame III) to getting stabilized on both the inner and outer shear layers (Flame IV). We report results of an LES investigation of two different inlet oxy-fuel mixtures, in a turbulent swirling flow at Re=20,000, that capture these two macrostructures. Previous work on the effects of heat loss have mostly focused on its impact on macro-scale observations. In this paper, we examine how heat loss impacts the flame microstructures as well for these two macrostructures. For both flames, the flamelet structure, as represented by a scatter plot of the normalized fuel concentration against the normalized temperature, depends on whether the combustor walls are adiabatic or non-adiabatic. For the adiabatic case, the flamelets of both macrostructures behave like strained flames. When wall heat transfer is included, Flame III microstructure is more bimodal. Since this flame extends farther downstream and part of it propagates along the walls, heat transfer has a greater impact on it’s microstructure. These results show that heat loss impacts not just the macro properties of the flame such as its shape or interactions with the wall, but also fundamentally changes its internal structure. Scatter plots of the turbulent flames are constructed and compared to different 1D laminar flame profiles (e.g., strained or with heat loss), and comparisons suggest the important role of the wall thermal boundary conditions in the accurate simulations of combustion dynamics and interpretations of experimental data, including data reduction and scaling.

Many proposed oxy-combustion concepts for carbon capture incorporate the recycling of flue gas which is used as a dilution gas to aid in the control of temperature and heat flux. Improvements in efficiency may be realized by significantly reducing the recycle flue gas (RFG), however, in application, care must be taken to avoid excessive radiant heat flux and gas temperature. One of the features oxy-combustion, unlike air-fired combustion, is that the oxygen and dilution gases are initially separated. RFG can, for example, be strategically blended with either the fuel stream, or oxidizer stream, or both, which affects the stoichiometric mixture fraction, Zst. In this work, the effects of the amount of dilution, or RFG, and Zst on soot fraction are experimentally investigated in a laminar coflow flame. Carbon dioxide is employed as the dilution gas to simulate the recycling of dry flue gas. Soot fraction and temperature are quantitatively determined by a flame image processing technique. In addition, the visible and near-IR emission spectra are given. When dilution, or RFG, is reduced, while holding Zst constant, soot formation and thermal radiation increase due to higher temperature. However, high temperature flames with reduced or zero soot are achieved by increasing Zst via the combination of fuel dilution and oxygen enrichment. This study highlights the inherent flexibility of oxy-fuel combustion, which offers the opportunity to control flame temperature and gas volume while independently controlling soot formation and radiant heat transfer.

Low-NOx NH3-air combustion power generation technology was developed by using a 50-kWe class micro gas-turbine system at the National Institute of Advanced Industrial Science and Technology (AIST), Japan, for the first time. 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 two-stage combustion with equivalence ratio control, we developed a low-NOx NH3-air non-premixed combustor for the gas-turbine system. Considering a previously performed numerical analysis, which proved that the NO reduction level depends on the equivalence ratio of the primary combustion zone in a NH3-air swirl burner, an experimental study using a combustor test rig was carried out. Results showed that eliminating air flow through primary dilution holes moves the point of the lowest NO emissions to the lesser fuel flow rate. Based on findings derived by using a test rig, a rich-lean low NOx combustor was newly manufactured for actual gas-turbine operations. As a result, the NH3 single fueled low-NOx combustion gas-turbine power generation using the rich-lean combustion concept succeeded over a wide range of power and rotational speeds, i.e., below 10–40 kWe and 75,000–80,000 rpm, respectively. The NO emissions were reduced to 337 ppm (16% O2), which was about one-third of that of the base system. Simultaneously, unburnt NH3 was reduced significantly, especially at the low electrical power output, which was indicative of the wider operating range with high combustion efficiency. In addition, N2O emissions, which have a large Global Warming Potential (GWP) of 298, were reduced significantly, thus demonstrating the potential of NH3 gas-turbine power generation with low environmental impacts.

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.

Detailed radiation modeling in piston engines has received relatively little attention to date. Recently, it is being revisited in light of current trends towards higher operating pressures and higher levels of exhaust-gas recirculation (EGR), both of which enhance molecular gas radiation. Advanced high-efficiency engines also are expected to function closer to the limits of stable operation, where even small perturbations to the energy balance can have a large influence on system behavior. Detailed radiation modeling using sophisticated tools like photon Monte Carlo/line-by-line (PMC/LBL) is computationally expensive. Here, guided by results from PMC/LBL, a simplified stepwise-gray spectral model in combination with a first-order spherical harmonics (P1 method) radiative transfer equation (RTE) solver is proposed and tested for engine-relevant conditions. Radiative emission, reabsorption and radiation reaching the walls are computed for a heavy-duty compression-ignition engine at part-load and full-load operating conditions with different levels of EGR and soot. The results are compared with those from PMC/LBL, P1/FSK (P1 with a full-spectrum k-distribution spectral model) and P1/Gray radiation models to assess the proposed model’s accuracy and computational cost. The results show that the proposed P1/StepwiseGray model can calculate reabsorption locally and globally with less than 10% error (with respect to PMC/LBL) at a small fraction of the computational cost of PMC/LBL (a factor of 30) and P1/FSK (a factor of 15). In contrast, error in computed reabsorption by the P1/Gray model is as high as 60%. It is expected that the simplified model should be broadly applicable to high-pressure hydrocarbon–air combustion systems, with or without soot.

Recent studies have demonstrated stable generation of power from pure ammonia combustion in a micro gas turbine (MGT) with a high combustion efficiency, thus overcoming some of the challenges that discouraged such applications of ammonia in the past. However, achievement of low NOx emission from ammonia combustors remains an important challenge. In this study, combustion techniques and combustor design for efficient combustion and low NOx emission from an ammonia MGT swirl combustor are proposed. The effects of fuel injection angle, combustor inlet temperature, equivalence ratio, and ambient pressure on flame stabilization and emissions were investigated in a laboratory high pressure combustion chamber. An FTIR gas analyser was employed in analysing the exhaust gases. Numerical modeling using OpenFOAM was done to better understand the dependence of NO emissions on the equivalence ratio. The result show that inclined fuel injection as opposed to vertical injection along the combustor central axis resulted to improved flame stability, and lower NH3 and NOx emissions. Numerical and experimental results showed that a control of the equivalence ratio upstream of the combustor is critical for low NOx emission in a rich-lean ammonia combustor. NO emission had a minimum value at an upstream equivalence ratio of 1.10 in the experiments. Furthermore, NO emission was found to decrease with ambient pressure, especially for premixed combustion. For the rich-lean combustion strategy employed in this study, lower NOx emission was recorded in premixed combustion than in non-premixed combustion indicating the importance of mixture uniformity for low NOx emission from ammonia combustion. A prototype liner developed to enhance the control and uniformity of the equivalence ratio upstream of the combustor further improved ammonia combustion. With the proposed liner design, NOx emission of 42 ppmv and ammonia combustion efficiency of 99.5% were achieved at 0.3 MPa for fuel input power of 31.44 kW.

Pilot-ignited dual fuel combustion involves a complex transition between the pilot fuel autoignition and the premixed-like phase of combustion, which is challenging for experimental measurement and numerical modelling, and not sufficiently explored. To further understand the fundamentals of the dual fuel ignition processes, the transient ignition and subsequent flame development in a turbulent dimethyl ether (DME)/methane-air mixing layer under diesel engine-relevant conditions are studied by direct numerical simulations (DNS). Results indicate that combustion is initiated by a two-stage autoignition that involves both low-temperature and high-temperature chemistry. The first stage autoignition is initiated at the stoichiometric mixture, and then the ignition front propagates against the mixture fraction gradient into rich mixtures and eventually forms a diffusively-supported cool flame. The second stage ignition kernels are spatially distributed around the most reactive mixture fraction with a low scalar dissipation rate. Multiple triple flames are established and propagate along the stoichiometric mixture, which is proven to play an essential role in the flame developing process. The edge flames gradually get close to each other with their branches eventually connected. It is the leading lean premixed branch that initiates the steady propagating methane-air flame. The time required for the initiation of steady flame is substantially shorter than the autoignition delay time of the methane-air mixture under the same thermochemical condition. Temporal evolution of the displacement speed at the flame front is also investigated to clarify the propagation characteristics of the combustion waves. Cool flame and propagation of triple flames are also identified in this study, which are novel features of the pilot-ignited dual fuel combustion.

Furan and its derivatives have been receiving attention as next generation alternative fuels, related to advanced bio-oil production. However, the ignition quality of furans allows their use only as an additive to diesel fuel in CI engines, which potentially requires the continued use of a fossil-derived base fuel. This study first adopts tri-propylene glycol mono-methyl ether (TPGME) as a substitute for diesel fuel with addition of furan and furan derivatives, including 2-methylfuran, 2,5-dimethylfuran, and furfural, thereby removing fossil-derived fuels from the mixture. With this motivation, gas-phase ignition characteristics of furans were investigated in a modified CFR motored engine, displaying an absence of low temperature heat release (LTHR), while n-heptane as a reference fuel shows a strong two-stage ignition characteristic under the same condition. The structural impact of furans is represented as global oxidation reactivities that are as follows: furan < 2-methylfuran < 2,5-dimethylfuran < furfural < n-heptane. The ranking of individual furans is supported by bond dissociation energies of each fuel's functional group substituent on the furan-ring. Ignition characteristics of TPGME display a strong low-temperature oxidation reactivity; however, its reactivity rapidly diminishes with increasing amounts of furan, shutting down low-temperature oxidation paths. The structural impact of furan and methyl-substituted furans on reactivity is significantly muted when blended with TPGME, as observed in a motored CFR engine and a constant volume spray combustion chamber.

Derived Cetane Numbers (DCNs) of engine lubricating oil/multicomponent 95 Research Octane Number (RON) gasoline surrogate mixtures were measured in an Ignition Quality Tester (IQT). Measurements separately assess the effects of calcium- and magnesium-based detergent additive fraction, oil viscosity, oil degradation, and base oil classification on mixture ignition propensity at conditions with relevance to low speed pre-ignition (LSPI) in gasoline engines. Testing of 0–25% (by mass) oil blended into a six-component surrogate mixture representing an unleaded “average” European gasoline blend is used to determine sensitivity of DCN responses to variations in the properties. With one exception, mixture DCNs were found to increase with lubricating oil content. Despite variation in calcium and magnesium concentrations, DCN responses for all oil blends indicate no statistically significant effect of either calcium or magnesium. Similarly, neither aging of nor peroxide addition to the oil yields significant DCN changes compared to untreated oils. However, a distinct response is found for variations in the base lubricant chemical structural properties. At 25% oil blending with gasoline surrogate, the measured DCNs (RONs) of different group base oils range from 19.6 (95.7) to 42.1 (46.2). The DCN increases with increasing base oil API Group Number (I through IV); however, mixture DCN was found to decrease for a 25% blend of Group V-B with the gasoline surrogate. Using quantitative 1H NMR, the Group Number trend is interpreted to be a consequence of linear vs. branched character of the paraffinic base oil composition. Taken together, the present results indicate that at ASTM D6890 DCN test conditions, there is no significant ignition effect attributable to reasonable variations in the lubricant's calcium or magnesium content, viscosity, or degree of degradation. Instead, the isomeric character of the paraffinic base oil appears to be most significant in controlling lubricant autoignition properties relative to those of gasolines.

Rich premixed turbulent n-dodecane/air flames at diesel engine conditions are analyzed using direct numerical simulations. The conditions correspond to a parametric variation of the Engine Combustion Network Spray A (pressure 60 atm; oxidizer oxygen level and temperature 21% and 900 K, respectively; fuel temperature 363 K). Three simulations with equivalence ratios of 3, 5, and 7 are performed with a Karlovitz number (Ka, based on flame time) of order 100 to match the estimated Ka of the rich premixed combustion region in Spray A. At these conditions, the reference laminar flames exhibit a complex structure which involves both low-temperature chemistry (LTC) and high-temperature chemistry over a wide range of length scales. In the presence of turbulence, the flame structure is strongly affected in physical space and the reaction zone exhibits a very complex structure in which broken, distributed, and thin regions co-exist, especially for the leanest case. However, the contribution of the LTC pathway is only weakly affected by turbulence. In progress variable space, the mean flame structure, including the chemical source terms, is found to match remarkably well that of the corresponding unity Lewis number laminar flame, particularly for the ϕ= 3 and 5 cases. This behavior is attributed to the strong turbulent mixing occurring throughout the flames/reaction zones, which suppresses differential diffusion effects. Nevertheless, large conditional fluctuations around the mean chemical source terms are identified. These are found to correlate very well with radical species mass fractions such as OH. In addition, a similar functional dependence is obtained from counterflow laminar flames. As such, it appears from these results that laminar flame models have a potential to be used to represent the thermochemical state of rich premixed turbulent flames under diesel engine conditions.

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.

Usually premixed flame propagation and laminar burning velocity are studied for mixtures at normal or elevated temperatures and pressures, under which the ignition delay time of the premixture is much larger than the flame resistance time. However, in spark-ignition engines and spark-assisted compression ignition engines, the end-gas in the front of premixed flame is at the state that autoignition might happen before the mixture is consumed by the premixed flame. In this study, laminar premixed flames propagating into an autoigniting dimethyl ether/air mixture are simulated considering detailed chemistry and transport. The emphasis is on the laminar burning velocity of autoigniting mixtures under engine-relevant conditions. Two types of premixed flames are considered: one is the premixed planar flame propagating into an autoigniting DME/air without confinement; and the other is premixed spherical flame propagating inside a closed chamber, for which four stages are identified. Due to the confinement, the unburned mixture is compressed to high temperature and pressure close to or under engine-relevant conditions. The laminar burning velocity is determined from the constant-volume propagating spherical flame method as well as PREMIX. The laminar burning velocities of autoigniting DME/air mixture at different temperatures, pressures, and autoignition progresses are obtained. It is shown that the first-stage and second-stage autoignition can significantly accelerate the flame propagation and thereby greatly increase the laminar burning velocity. When the first-stage autoignition occurs in the unburned mixture, the isentropic compression assumption does not hold and thereby the traditional method cannot be used to calculate the laminar burning velocity. A modified method without using the isentropic compression assumption is proposed. It is shown to work well for autoigniting mixtures. Besides, a power law correlation is obtained based on all the laminar burning velocity data. It works well for mixtures before autoignition while improvement is still needed for mixtures after autoignition.

Within the cluster of excellence “Tailor-Made Fuels from Biomass” diethoxymethane (DEM) was identified as a promising fuel candidate from a production perspective. Synthesized by combining a bio-based feedstock and CO2 as carbon source together with “green hydrogen” from water electrolysis DEM is defined as “bio-hybrid fuel” . To determine the molecules general applicability to a combustion system and to develop up combustion models a rapid screening of the ignition characteristics is performed in a rapid compression machine and a shock tube. Those suggest DEM being a potential fuel for gasoline controlled autoignition (GCAI) because of a relatively wide range of temperature independent ignition delay, a good autoignition behavior compared to conventional gasoline fuel and a multi-stage ignition behavior. To test the suitability of those molecules as a fuel and determine possible improvements to the production side, DEM was used in a single cylinder research engine operated in GCAI combustion mode. Compared to GCAI combustion with conventional RON95 E10 fuel, DME shows a significantly decreased ignition delay. As a consequence, the internal residual gas fraction, whose enthalpy is used to initiate autoignition, can be reduced and combustion stability is increased. Starting from similar combustion phasing using external exhaust gas recirculation to align the ignition behavior of DEM and RON95 E10, a variation of the intake temperature reveals that DEM has the potential to reduce the sensitivity of the combustion system.

Alkylbenzenes are major aromatic constituents of real transportation fuels and important surrogate components. In this study, the structural impact of nine alkylbenzenes on their ignition characteristics is experimentally and computationally investigated with particular emphasis on the blending effect with significantly more reactive normal alkanes. Experimental comparisons of mono-alkylbenzenes (toluene, ethylbenzene, n-propylbenzene, iso-propylbenzene) from a modified CFR engine showed that the difference in pure alkylbenzene reactivity significantly diminished when blended with n-heptane, as the strength of the radical scavenging effect of all three alkylbenzenes is similar. Among C8H10 isomers, the reactivity of pure ethylbenzene and o-xylene and their blends with n-heptane showed a complex competing effect between the difference in CH bond energy and the existence of intermediate/low-temperature chemistry caused by adjacent methyl pairs. A similar structural impact was also observed for C9H12 isomers and their blends with n-heptane, while the influence of CH bond energy was more noticeable than C8H10 molecules. Kinetic simulations of the alkylbenzene/n-heptane blends highlighted the effect caused by adjacent methyl pairs that is referred to as the “ortho effect”. Analysis of ethylbenzene and o-xylene showed that o-xylene's intermediate/low-temperature pathways initiated by benzylperoxy radical – benzylhydroperoxide isomerization (RO2 – QOOH) produce additional active radicals such as OH and CH2O, which accelerates the oxidation chemistry of more reactive n-heptane. This study provides knowledge on the blending effect of alkylbenzene compounds with n-heptane on their ignition characteristics that is useful to develop surrogates that can better mimic the reactivity of real fuels.

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.

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 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.

Increasing energy demands and more stringent legislation relating to pollutants such as nitrogen oxide (NOx) and carbon monoxide (CO) from fossil fuels have accelerated the use of biofuels such as biodiesel. However, current limitations of using biodiesel as an alternative fuel for CI engines include a higher viscosity and higher NOx emissions. This is a major issue that could be improved by blending biodiesel with alcohols. This paper investigates the effect of a butanol–acetone mixture (BA) as an additive blended with biodiesel to improve the latter's properties. Macroscopic spray characteristics (spray penetration, spray cone angle and spray volume) were measured in constant volume vessel (CVV) at two injection pressures. A high-speed camera was used to record spray images. The spray's edge was determined using an automatic threshold calculation algorithm to locate the spray outline (edge) from the binary images. In addition, an engine test was carried out experimentally on a single-cylinder diesel engine. The engine's performance was measured using in-cylinder pressure, brake power (BP) and specific fuel consumption (SFC). Emission characteristics NOx, CO and UHC were also measured. Neat biodiesel and three blends of biodiesel with up to 30% added BA were tested. The experimental data were analyzed via ANOVA to evaluate whether variations in parameters due to the different fuels were significant. The results showed that BA can enhance the spray characteristics of biodiesel by increasing both the spray penetration length and the contact surface area, thereby improving air–fuel mixing. The peak in-cylinder pressure for 30% BA was comparable to neat diesel and higher than that of neat biodiesel. Brake power (BP) was slightly improved for 10% BA at an engine speed of 2000 rpm while SFC was not significantly higher for any of the BA-biodiesel blends because of the smaller heating value of BA. Comparing the effect on emissions of adding BA to biodiesel, increasing the amount of BA reduced NOx and CO (7%) and (40%) respectively compared to neat biodiesel, but increased UHC.

The ignition behavior of n-dodecane micro-pilot spray in a lean-premixed methane/air charge was investigated in an optically accessible Rapid Compression-Expansion Machine at dual-fuel engine-like pressure/temperature conditions. The pilot fuel was admitted using a coaxial single-hole 100 µm injector mounted on the cylinder periphery. Optical diagnostics include combined high-speed CH2O-PLIF (10 kHz) and Schlieren (80 kHz) imaging for detection of the first-stage ignition, and simultaneous high-speed OH* chemiluminescence (40 kHz) imaging for high-temperature ignition. The aim of this study is to enhance the fundamental understanding of the interaction of methane with the auto-ignition process of short pilot-fuel injections. Addition of methane into the air charge considerably prolongs ignition delay of the pilot spray with an increasing effect at lower temperatures and with higher methane/air equivalence ratios. The temporal separation of the first CH2O detection and high-temperature ignition was found almost constant regardless of methane content. This was interpreted as methane mostly deferring the cool-flame reactivity. In order to understand the underlying mechanisms of this interaction, experimental investigations were complemented with 1D-flamelet simulations using detailed chemistry, confirming the chemical influence of methane deferring the reactivity in the pilot-fuel lean mixtures. This shifts the onset of first-stage reactivity towards the fuel-richer conditions. Consequently, the onset of the turbulent cool-flame is delayed, leading to an overall increased high-temperature ignition delay. Overall, the study reveals a complex interplay between entrainment, low T and high T chemistry and micro-mixing for dual-fuel auto-ignition processes for which the governing processes were identified.

The requirements on high efficiency and low emissions of internal combustion engines (ICEs) raise the research focus on advanced combustion concepts, e.g., premixed-charge compression ignition (PCCI), partially premixed compression ignition (PPCI), reactivity controlled compression ignition (RCCI), partially premixed combustion (PPC), gasoline compression ignition (GCI) etc. In the present study, an optically accessible engine is operated in PPC mode, featuring compression ignition of a diluted, stratified charge of gasoline-like fuel injected directly into the cylinder. A high-speed, high-power burst-mode laser system in combination with a high-speed CMOS camera is employed for diagnostics of the autoignition process which is critical for the combustion phasing and efficiency of the engine. To the authors’ best knowledge, this work demonstrates for the first time the application of the burst-system for simultaneous fuel tracer planar laser induced fluorescence (PLIF) and chemiluminescence imaging in an optical engine, at 36 kHz repetition rate. In addition, high-speed formaldehyde PLIF and chemiluminescence imaging are employed for investigation of autoignition events with a high temporal resolution (5 frames/CAD). The development of autoignition together with fuel or CH2O distribution are simultaneously visualized using a large number of consecutive images. Prior to the onset of combustion the majority of both fuel and CH2O are located in the recirculation zone, where the first autoignition also occurs. The ability to record, in excess of 100 PLIF images, in a single cycle brings unique possibilities to follow the in-cylinder processes without the averaging effects caused by cycle-to-cycle variations.

Partially premixed combustion (PPC) and reactivity controlled compression ignition (RCCI) are two new combustion modes in compression-ignition (CI) engines. However, the detailed in-cylinder ignition and flame development process in these two CI modes were not clearly understood. In the present study, firstly, the fuel stratification, ignition and flame development in PPC and RCCI were comparatively studied on a light-duty optical engine using multiple optical diagnostic techniques. The overall fuel reactivity (PRF number) and concentration (fuel-air equivalence ratio) were kept at 70 and 0.77 for both modes, respectively. Iso-octane and n-heptane were separately used in the port-injection (PI) and direct-injection (DI) for RCCI, while PRF70 fuel was introduced through direct-injection (DI) for PPC. The DI timing for both modes was fixed at –25°CA ATDC. Secondly, the combustion characteristics of PPC and RCCI with more premixed charge were explored by increasing the PI mass fraction for RCCI and using the split DI strategy for PPC. In the first part, results show that RCCI has shorter ignition delay than PPC due to the fuel reactivity stratification. The natural flame luminosity, formaldehyde and OH PLIF images prove that the flame front propagation in the early stage of PPC can be seen, while there is no distinct flame front propagation in RCCI. In the second part, the higher premixed ratio results in more auto-ignition sites and faster combustion rate for PPC. However, the higher premixed ratio reduces the combustion rate in RCCI mode and the flame front propagation can be clearly seen, the flame speed of which is similar to that in spark ignition engines but lower than that in PPC. It can be concluded that the ratio of flame front propagation and auto-ignition in RCCI and PPC can be modulated by the control over the fuel stratification degree through different fuel-injection strategies.

Micro direct-injection (DI) strategy is often used to extend the operation range of the reactivity controlled compression ignition (RCCI) to high engine load, but its combustion process has not been well understood. In this study, the ignition and flame development of the micro-DI RCCI strategy were investigated on a light-duty optical engine using formaldehyde planar laser-induced fluorescence (PLIF) and high-speed natural flame luminosity imaging techniques. The premixed fuel was iso-octane and an oxygenated fuel of polyoxymethylene dimethyl ethers (PODE) was employed for DI. The fuel-air equivalence ratio of DI was kept at 0.09 and the premixed equivalence ratio was varied from 0 to 1. RCCI strategies with early and late DI timing at –25° and –5° crank angle after top dead center were studied, respectively. Results indicate that the early micro-DI RCCI features a single-stage high-temperature heat release (HTHR). The combustion in the low-reactivity region shows a combination of flame front propagation and auto-ignition. The late micro-DI RCCI presents a two-stage HTHR. The second-stage HTHR is owing to the combustion in the low-reactivity region that is dominated by flame front propagation when the premixed equivalence ratio approaches 1. For both early and late micro-DI RCCI, the intermediate-temperature heat release (ITHR) of iso-octane, indicated by formaldehyde, takes place in the low-reactivity region before the arrival of the flame front. This is quite different from the flame front propagation in spark-ignition (SI) engine that shows no ITHR in the unburned region. The DI fuel mass is a key factor that affects the combustion in the low-reactivity region. If the DI fuel mass is quite low, there is more possibility of flame front propagation; otherwise, sequential auto-ignition dominates. The emergence of the flame front propagation in micro-DI RCCI strategy reduces its combustion rate and peak pressure rise rate.

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.

The study of auto-ignition under temperature stratification is of great interest. Indeed, further understanding of the thermo-kinetic interactions and its influence on the combustion propagation regime is needed. In a previous work [1], experiments in a flat piston Rapid Compression Machine (RCM) demonstrated that the apparent propagation of reaction fronts is highly influenced by the typical temperature stratification observed at inert conditions. Nevertheless, the influence of low temperature heat release (LTHR) on the internal aerodynamics and temperature of the RCM is not well understood. In the present study, we first address the LTHR-flow interaction then address the LTHR-temperature interaction. We performed 2D-PIV experiments at 10 kHz for inert and reactive lean isooctane mixtures. We averaged spatially the acceleration to present the time evolution during the cool flame period. We found that the normalized acceleration has a decreasing trend in both inert and reactive tests. No significant effect of the cool flame was observed on the trend. We performed temperature measurements using thin wire (7.6 µm) type K thermocouples at inert and reactive n-hexane mixtures (same test conditions of Fig. 7 in [1]). The temperature evolution of the hot (adiabatically compressed) and the colder gases were recorded when cool flame occurs. The corrected gas temperature showed good agreement with the theoretical adiabatic core temperature as well as previous measurements with toluene LIF. In the tested case, we found that the cool flame induces an equal temperature rise of approximately 110 K in both the adiabatically compressed and the colder vortex gases. These results confirm quantitatively that LTHR does not significantly affect the mixing of the temperature stratification of our flat piston RCM. In the studied test conditions, the temperature stratification is conserved globally despite the LTHR.

Low temperature combustion (LTC) is a potential thermodynamic pathway to maximize the thermal efficiency of internal combustion (IC) engines. However, high exergy loss is also observed within this combustion concept. The present study focuses on the homogeneous combustion process and examines the detailed exergy destruction mechanisms under representative LTC engine conditions. By varying both equivalence ratios (φ) and temperatures (T) at initial pressure of 50 bar, it is found that the decreased total exergy destruction fraction (fED) with increasing initial temperature mainly results from the decreased exergy destruction in the high temperature heat release stage, while using rich mixture can significantly reduce the fED in the ignition delay stage, which is dominated by the reactions involving large molecules (C7 species). Reaction pathway analysis reveals that the detailed exergy destruction sources are significantly affected by the reaction pathways. Furthermore, a qualitative exergy loss φ-T map was created to illustrate the exergy loss reduction potential. It is concluded that the combustion pathway that reforming the rich fuel/air mixtures before ignition followed by the low temperature combustion of lean reforming products offers the potential to simultaneously reduce exergy destruction and avoid soot and NOx formation. However, the potential advantages of this exergy reduction combustion concept still require further work.

Knowledge of the autoignition characteristics of diesel fuels is of great importance for understanding the combustion performance in engines and developing surrogate fuels. Here ignition delays of China's stage 6 diesel, a commercial fuel, were measured in a heated rapid compression machine (RCM) under engine-relevant conditions. Gas-phase autoignition experiments were carried out at equivalence ratios ranging from 0.37 to 1.0, under compressed pressures of 10, 15, and 20 bar, and within a temperature range of 685–865 K. In all investigated conditions, negative temperature coefficient (NTC) behavior of the total ignition delays is observed. The autoignition of the diesel fuel exhibits pronounced two-stage characteristics with strong low-temperature reactivity. Experimental results indicate that the total ignition delays shorten with increasing compressed pressure, oxygen mole fraction and fuel mole fraction. The first-stage ignition delays are mainly controlled by compressed temperature and also affected by oxygen mole fraction and compressed pressure but show a very weak dependence on fuel mole fraction. Correlations describing the first-stage ignition delay and the total ignition delay were proposed to further clarify the ignition delay dependence on the multiple factors. Additionally, it is found that the newly measured ignition delays well coincide with and complement the diesel ignition data in the literature. A recently developed diesel mechanism was used to simulate the diesel autoignition on the RCM. The simulation results are found to agree well the experimental measurements over the whole temperature ranges. Species concentration analysis and brute force sensitivity analysis were also conducted to identify the crucial species and reactions controlling the autoignition of the diesel fuel.

Exhaust gas recirculation (EGR) technology can be used in internal combustion engines to reduce NOx emission and improve fuel economy. However, it also affects the end-gas autoignition and engine knock since NOx in EGR can promote ignition. In this study, effects of NOx addition on autoignition and detonation development in dimethyl ether (DME)/air mixture under engine-relevant conditions are investigated. Numerical simulation considering both low-temperature and high-temperature chemistry is conducted. First the kinetic effects of NOx addition on the negative temperature coefficient (NTC) regime are assessed and interpreted. It is found that NOx addition greatly promotes both low-temperature and high-temperature ignition stages mainly through increasing OH production. Then the autoignitive reaction front propagation induced by either local NO accumulation or a cold spot within NTC regime with different amounts of NO addition is investigated. For the first time, supersonic autoignition modes including detonation induced by local NO accumulations are identified. This indicates that local accumulation of NOx in end gas might induce super-knock in engines with EGR. A new parameter quantifying the ratio of sound speed to average reaction front propagation speed is introduced to identify the regimes for different autoignition modes. Compared to the traditional counterpart parameter used in previous studies, this new parameter is more suitable since it yields a detonation development regime in a C-shaped curve which is almost unaffected by the initial conditions. The results in this study may provide fundamental insights into knocking mechanism in engines using EGR technology.

Post injection has significant benefit in the reduction of diesel soot emissions. Therefore, there is a need to understand the effect of post-injection strategy on soot physicochemical properties and reactivity because they play an important role in soot oxidation process that governs the final soot emissions. This work focuses on the impact of post injection on the physicochemical properties and reactivity of diesel in-cylinder soot using a main plus post injection (M*P) and a single injection (M) strategy. The soot was sampled by a developed total cylinder sampling system, and the dividing points of soot formation-dominant and oxidation-dominant phases were used for studying the impacts of post injection on the characteristics of in-cylinder soot. The physicochemical properties of the soot samples, including primary particle size, nanostructure, carbon chemical state and surface functional groups, were characterized. The soot reactivity was evaluated in terms of peak temperature, burnout temperature and apparent activation energy. In the oxidation-dominant phase, the M*P soot initially possesses smaller primary particle size, shorter fringe length, larger tortuosity, lower sp2/sp3 hybridization ratio of carbon atoms and higher content of aliphatic CH groups than the M soot. The beneficial influence of physicochemical properties on soot reactivity when using post injection is validated by the thermogravimetric data, which shows that the M*P soot is more reactive than the M soot at the onset of the oxidation-dominant phase. In the M*P case, the soot generated from the main-injection combustion has lower reactivity than the soot from the post-injection combustion after they experience the soot formation-dominant phase. The results indicate that the use of post injection leads to in-cylinder soot with physicochemical properties that favor reactivity. The enhancement of reactivity means that the soot will be more readily oxidized in the subsequent combustion process, and consequently contributes to a reduction in final soot emissions.

This study shows how soot particles inside the cylinder of the engine are reduced due to high pressure fuel injection used in a light-duty single-cylinder optical diesel engine fuelled with methyl decanoate, a selected surrogate fuel for the diagnostics. For various injection pressures, planar laser induced incandescence (PLII) imaging and planar laser-induced fluorescence of hydroxyl (OH-PLIF) imaging were performed to understand the temporal and spatial development of soot and high-temperature flames. In addition, a thermophoresis-based particle sampling technique was used to obtain transmission electron microscope (TEM) images of soot aggregates and primary particles for detailed morphology analysis. The OH-PLIF images suggest that an increase in the injection pressure leads to wider distribution of high-temperature flames likely due to better mixing. The enhanced high-temperature reaction can promote soot formation evidenced by both a faster increase of LII signals and larger soot aggregates on the TEM images. However, the increased OH radicals at higher injection pressure accelerates the soot oxidation as shown in a higher decreasing rate of LII signals as well as dramatic reduction of the sampled soot aggregates at later crank angles. The analysis of nanoscale carbon layer fringe structures also shows a consistent trend that, at higher injection pressure, the soot particles are more oxidized to form more graphitic carbon layer structures. Therefore, it is concluded that the in-cylinder soot reduction at higher injection pressure conditions is due to enhanced soot oxidation despite increased soot formation.

A new model has been developed to describe the size-dependent effects that are responsible for transient particle mass (PM) and particle number (PN) emissions observed during experiments of the active regeneration of Diesel Particulate Filters (DPFs). The model uses a population balance approach to describe the size of the particles entering and leaving the DPF, and accumulated within it. The population balance is coupled to a unit collector model that describes the filtration of the particles in the porous walls of the DPF and a reactor network model that is used to describe the geometry of the DPF. Two versions of the unit collector model were investigated. The original version, based on current literature, and an extended version, developed in this work that includes terms to describe both the non-uniform regeneration of the cake and thermal expansion of the pores in the DPF. Simulations using the original unit collector model were able to provide a good description of the pressure drop and PM filtration efficiency during the loading of the DPF, but were unable to adequately describe the change in filtration efficiency during regeneration of the DPF. The introduction of the extended unit collector description enabled the model to describe both the timing of particle breakthrough and the final steady filtration efficiency of the hot regenerated DPF. Further work is required to understand better the transient behaviour of the system. In particular, we stress the importance that future experiments fully characterise the particle size distribution at both the inlet and outlet of the DPF.

This study shows how the structure of soot particles within the flame changes due to the relative direction of the swirl flow in a small-bore diesel engine in which significant flame–wall interactions cause about half of the flame travelling against the swirl flow while the other half penetrating in the same direction. The thermophoresis-based particle sampling method was used to collect soot from three different in-flame locations including the flame–wall impingement point near the jet axis and the two 60° off-axis locations on the up-swirl and down-swirl side of the wall-interacting jet. The sampled soot particle images were obtained using transmission electron microscopes and the image post-processing was conducted for statistical analysis of size distribution of soot primary particles and aggregates, fractal dimension, and sub-nanoscale parameters such as the carbon layer fringe length, tortuosity, and spacing. The results show that the jet-wall impingement region is dominated by many small immature particles with amorphous internal structure, which is very different to large, fractal-like soot aggregates sampled from 60° downstream location on the down-swirl side. This structure variation suggests that the small immature particles underwent surface growth, coagulation and aggregation as they travelled along the piston-bowl wall. During this soot growth, the particle internal structure exhibits the transformation from amorphous carbon segments to a typical core–shell structure. Compared to those on the down-swirl side, the soot particles sampled on the up-swirl side show much lower number counts and more compact aggregates composed of highly concentrated primary particles. This soot aggregate structure, together with much narrower carbon layer gap, indicates higher level of soot oxidation on the up-swirl side of the jet.