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Moderate or intense low-oxygen dilution (MILD) combustion is a novel technology owing to the ability to reduce NOx emissions from combustion significantly. The development of this technology and its combination with gasification are of increasing interest for syngas impurities control. Therefore, the present study aims to numerically assess the feasibility of implementing MILD combustion to a two-stage entrained flow gasifier for reducing nitrogen-containing impurities (mainly HCN, NH3, and NO) in the produced syngas. Specifically, a conventional combustion gasifier (CCG) is modified to a MILD combustion gasifier (MCG) by replacing the original burners with a conceptually designed MILD burner. Detailed comparisons have been made between CCG and MCG in terms of combustion characteristics and evolutions of the nitrogen-containing impurities. The results show that the present conceptually designed MILD burner can achieve more uniform temperature and species distributions in comparison with the conventional burner. Different from the preferable neutralization reactions with NO in the CCG combustor, HCN and NH3 tend to be oxidized to form NO in the MCG combustor, resulting in higher NO content at the MCG combustor outlet. As a result, concentrations of HCN and NH3 in syngas at the MCG gasifier outlet can be reduced by 22% and 9% respectively compared to those at the CCG gasifier outlet. Furthermore, results from the equilibrium-reaction-network model show that the final NO emission from MCG is 14% less than that from CCG.

Methyl decanoate, methyl-5-decenoate and methyl-9-decenoate have been widely adopted as biodiesel surrogates for engine simulation. However, these surrogates have relatively short chain length and very different physical properties compared with biodiesel molecules. Additionally, it is difficult to distinguish the unsaturation degree of different biodiesel fuels by using these surrogates. The direct use of the heavy methyl esters in real biodiesel fuels as surrogates is essential to accurately simulate biodiesel fuels. In this work, a four-part scheme to formulate skeletal mechanism for heavy saturated methyl esters has been proposed. Within the scheme, the oxidation mechanism is divided into four parts: low temperature oxidation, high temperature decomposition, ester group reactions and detailed C4-C0 chemistry. A skeletal mechanism for two of the five main methyl esters in real biodiesel fuels, i.e. saturated methyl palmitate and methyl stearate, has been constructed. The obtained skeletal mechanism contains only 6 fuel-dependent species and 13 fuel-dependent reactions for each heavy saturated methyl ester. Extensive validations were performed against shock tube experimental data for ignition delay timing under different initial pressure, temperature and equivalence ratio. The ignition delay behavior at high temperature has been well captured by the developed skeletal mechanism. As for ignition at low to medium temperature (from 650 K to 900 K), there is no experimental data due to the low vapor pressure and high melting point of heavy methyl esters. The comparison of ignition behavior at low temperature has been made between several models. Furthermore, the oxidation of n-decane/methyl palmitate and benzene/methyl stearate in a jet-stirred reactor has been utilized to validate the important species concentration. Good agreements have been observed through the validations. The results indicate that the developed skeletal mechanism is capable of predicting the combustion characteristics of methyl palmitate and methyl stearate.

In the present study, the numerical simulation of the performance of fuel injection and spray atomization on diesel engine combustion was conducted, which was validated against fuel injection test, spray test and single cylinder diesel engine test. Thus, the effect of the intake swirl on the fuel spray and diesel engine combustion performance was explored. Quantitative analysis of the characteristics of fuel-air mixing and combustion characteristics under different swirl ratio was given. It was found that the spray atomization and fuel-air mixing begins from the front of fuel beam which has deflection due to the presence of the intake swirl. The function of swirl is to accelerate the mixing and diffusion speed of fuel, thus promoting the combustion process in cylinder. Meanwhile, the NO and Soot emissions were quite sensitive to the swirl ratio, which leads higher NO but lower Soot as the swirl ratio is increased. In addition, the performance of varied swirl ratio on spray penetration and spray angle was well captured by numerical studies in the paper. Consequently, this study reveals the influence of different swirl ratio on the fuel spray performance, output power and fuel consumption, thus exploring the influence of inlet swirl of the controllable swirl diesel engine on the fuel atomization behavior as well as dynamic performance, and verifying the accuracy of the simulation results.

Jet fuel has been recognized as a potential alternative for traditional diesel engines because of its ability to reduce particulate matter (PM) emissions while retaining engine power output. In this study, the particulate formation process has been studied in detail using diesel/Jet A-1 blends with evenly staggered ratios. The number concentration of the accumulation mode particle decreases exponentially when additional Jet A-1 is introduced to the blends under 30% engine load, as more fuel and particle precursors are oxidized. Additionally, the comparison of PM emissions with pilot-main and single main two injection strategies is conducted to better understand the particle formation process. The phenomenon of ‘particle saturation’ of nucleation mode particles is observed using the pilot-main injection strategy. With these supporting findings, we strengthen the point that the pilot-injection strategy has the potential weaken the oxidation process during the combustion process. Furthermore, this research quantifies the impact of Jet A-1 on combustion and gas emission characteristics by extracting the change rate from the data. In general, Jet A-1 tends to delay the ignition and shorten the combustion duration. The results also reveal that the rise in NOx emissions is due to a higher proportion of premixed combustion, while the increase in HC emissions is attributed to a longer ignition delay and shorter combustion time. Ministry of Education (MOE) National Research Foundation (NRF) We gladly appreciate the support of the Singapore Ministry of Education via research grant R-265-000-681-114. The National Research Foundation (NRF), Prime Ministers' Office, Singapore, is also funding this initiative via its Campus for Research Excellence and Technological Enterprise (CREATE) programme.

In this study, an optimization methodology was developed for formulating both jet fuel and diesel fuel surrogates along with their associated skeletal oxidation mechanisms. Based on this methodology, new jet fuel surrogate (JFS) and diesel fuel surrogate (DFS) are formulated with same five components (n-dodecane/iso-octane/iso-cetane/decalin/toluene) by emulating practical fuel properties including liquid density, viscosity, surface tension, cetane number (CN), hydrogen-carbon (H/C) ratio, molecular weight (MW), lower heating value (LHV) and threshold sooting index (TSI). Based on the newly developed JFS and DFS, their associated chemical skeletal mechanisms were developed, which are described with same chemical reaction structure: five skeletal sub-mechanisms are employed into the skeletal fuel mechanisms, including decalin (C10H18), n-dodecane (C12H26), iso-cetane (C16H34), iso-octane (C8H18) and toluene (C7H8) sub-mechanisms. By describing the chemistries for the oxidation of large molecules C4-Cn and small H2/CO/C1 molecules respectively, the skeletal mechanisms are significantly compacted into 74 species and 189 reactions, which makes them practical to be used in 3-D engine combustion simulations. In addition to sufficient 0-D validations of ignition delay times, species concentrations and laminar flame speed in various environments, 3-D validations of constant volume spray and engine combustion were also performed for the new JFS and DFS skeletal mechanisms. It can be observed that the agreements between the computational results predicted by the present mechanisms and all the experimental data are reasonably good, which proves that the newly proposed JFS and DFS skeletal mechanisms are robust and accurate to be used in engine combustion computational fluid dynamics (CFD) studies.

Being one typical cyclo-alkane class component of commercial jet and diesel fuels, decalin has two fused six-membered rings and so far few studies of decalin were accessible to its oxidation mechanism. So that this study aims to propose a skeletal mechanism of decalin oxidation that facilitates the formulating of surrogate fuels applicable in power device like engines. In this study, a skeletal mechanism of decalin oxidation was developed based on the decoupling methodology. It contains 42 species and 163 reactions, in which a simplified mechanism for the oxidation of C2–C10 is used to model the oxidation of heavy hydrocarbons for the prediction of the ignition characteristics, while the mechanism of H2/CO/C1 was considered in details for the prediction of intermediate species concentrations. Among all intermediate combustion products, C2H2 is designated as the most important soot precursor and will be used to predict soot tendency in the future study. The present skeletal mechanism was extensively validated against available fundamental experiments in regard of decalin oxidation under engine relevant conditions, wherein the temperature and pressure were relatively high. The computational results indicated that the predicted ignition delay in shock tubes and species concentrations in jet stirred reactors as well as in laminar premixed flames agree reasonably well with the measurements.

In this work, the effects of boot injection rate-shapes on the combustion process and emissions formation of a direct injection compression ignition engine fueled with kerosene and diesel are investigated through numerical simulations. Boot injection rate-shapes with varying boot injection velocity and boot injection duration are used. The KIVA4-CHEMKIN code is used in conjunction with a phenomenological soot model and an improved kerosene-diesel reaction mechanism to study the combustion process and emissions formation. The phenomenological soot model consists of a number of sub-models from literature that accounts for soot particle inception, soot coagulation, soot surface growth via the hydrogen-abstraction-carbon-addition (HACA) mechanism and soot surface oxidation by oxygen (O 2 ) and hydroxyl radical (OH). It should be noted that the improved kerosene-diesel reaction mechanism is robust enough to predict the combustion and emissions trends of kerosene with respect to diesel. From this study, boot injection rate-shapes are seen to cause combustion phasing and cause lower nitrogen oxide (NO) emissions in general. Furthermore, it is observed that when kerosene replaces diesel, engine efficiency and NO emissions increase while carbon monoxide (CO) and soot emissions decrease. Soot mass quantity, soot particle number and soot particle size are the lowest for pure kerosene combustion. Finally, detailed analyses of the effects of boot injection rate-shapes on soot particle dynamics are also presented.

Modelling soot formation and oxidation in diesel engines has been a long-standing challenge. In this study, soot particle characteristics in terms of particle dynamics, particle size and number density were modelled by integrating a multi-step phenomenological soot model into the KIVA-CHEMKIN CFD code for compression ignition engine combustion simulations. This semi-detailed soot model is dedicated to solving rate equations using sub-models to account for precursor formation, soot particle inception and coagulation as well as soot surface growth and oxidation. Soot growth and oxidation is inherently derived from the concentration of certain species found in the chemical reaction mechanism used, namely H, O2, C2H2 and the nucleating polycyclic aromatic hydrocarbon (PAH). Acetylene is taken as the core precursor species in nucleating PAH, soot inception as well as acetylene-assisted soot surface growth. The integrated multi-step phenomenological soot model has been validated under gasoline and diesel dual-fuel engine conditions. The predicted histograms of soot particle number along with size distribution contribute towards the understanding of dominant factors that affect soot formation. The factors affecting soot particle under varying engine loads and fuel conditions were extensively investigated. Generally, the predicted soot particle size was larger for heavy-sooting conditions as compared to low-sooting conditions. For port-injected gasoline-dominated combustion, there is less soot being discharged. This might be attributed to two reasons. First, a more homogenous mixture is realized with less diesel fuel, thus effectively reducing the formation of soot precursors. The other reason is the intensive heat release which enhances the depletion of soot precursors, thereby impeding the growth of soot particles in terms of size and mass.

Considering climate change and energy crisis, the high-carbon alcohol has become a popular substitute fuel for diesel engines. In this study, diesel (D100), n-octanol/diesel (D50O50), and n-butanol/diesel (D50B50) blends were tested in a single-cylinder CI engine to evaluate the effects of n-octanol and n-butanol on the engine combustion characteristics, performance, and emissions formation. During the testing, the engine was operated at two engine loads (30% and 50%) with two injection pressures (40 and 60 MPa) at 1200 rpm. Both single injection and split injection strategies were used. Experimental results suggest that D50O50 and D50B50could result in serious knocking combustion under certain conditions with the single injection strategy. In contrast, the split injection strategy can lower the peak pressure and peak heat release rate, and consequently mitigating the knocking tendency for D50O50 and D50B50. Besides, noticeable two-stage combustion was observed for D100 and D50O50 due to the NTC behavior of diesel and n-octanol. Generally, because of the lower fuel reactivity of n-octanol and n-butanol, the ignition delay periods of D50O50 and D50B50 were significantly extended with the single injection; however, the difference in ignition delay was not evident with split injection strategy. Moreover, D50O50 can exhibit the highest thermal efficiency with proper injection pressure, and combustion phasing, etc. NOx and black carbon can be simultaneously reduced by D50O50 with split injection at an injection pressure of 60 MPa. Nevertheless, adding n-octanol and n-butanol increased the HC emissions due to their lower fuel reactivity at all tested conditions.

Polyoxymethylene dimethyl ether 3 (PODE3) is an up-and-coming renewable fuel. In this study, the effects of PODE3 addition and injection pressure on combustion performance in a diesel engine have been investigated for the first time, with an emphasis on PM emissions in terms of both volume and number concentrations to account for ultrafine particles. Nitrogen oxides have been found to decrease with PODE3 addition at low injection pressure, although an increment has been observed at high injection pressure. The particle size distributions in terms of particle number concentration (PNC) are characterized by a bi-modal distribution, with the higher peak representing accumulation mode particles. At medium and high injection pressures, declines in PNC have been noticed. Furthermore, geometric mean diameter of particles has been found to decrease with the increase in PODE3 due to its additional oxygen content, stronger spray penetration and lack of C–C bonds. Overall, PODE3 has been concluded to be of high potential in terms of soot reduction. Among the diesel/PODE3 blends, although both P30 and P10 exhibit significant soot reductions at 600 bar and 800 bar, P10 is the most promising candidate given its ability to curtail ultrafine particles.

Kerosene combustion in diesel engines is getting more prevalent due to the push for kerosene use in diesel engines by the North Atlantic Treaty Organization military. The study of kerosene combustion in diesel engines has primarily been carried out through engine experiments. Negligible study has been carried out through engine simulations possibly due to the lack of a compact and reliable kerosene reaction mechanism. A compact and well validated kerosene reaction mechanism with embedded soot chemistry for diesel engine simulations is still lacking. In this work, a kerosene–diesel reaction mechanism, containing only 123 species and 586 reactions, is developed. Diesel is represented by C14H28 while kerosene is represented by C12H24. Both C14H28 and C12H24 are assumed to be oxidized via global reaction steps. The kerosene sub-mechanism is well validated for its ignition delay times under different initial shock tube conditions of 20 atm at equivalence ratios of 0.25, 0.5, 1.0, 1.5 and 2.0 for temperatures between 700 and 1400 K. Moreover, constant volume combustion validations were carried out under ambient conditions of 900 K/6.0 MPa and 1000 K/6.7 MPa. It is seen that the newly developed mechanism is able to closely replicate the heat-release rates and flame lift-off lengths under these ambient conditions. In addition, constant volume ignition delay validations were done under ambient densities of 14.8 kg/m3 and 30.0 kg/m3 for temperatures between 800 and 1250 K. The simulated and experimental constant volume ignition delays are very similar. Furthermore, the reaction mechanism is able to predict the combustion characteristics and soot trends of kerosene and diesel well under real engine conditions. In all, this newly developed kerosene–diesel reaction mechanism is suitable to be used for diesel engine simulations.

Development of detailed soot model independent of empirical expression is supposed to reasonably replicate the soot formation and oxidation dynamic kinetics over a wide range of diesel engine operations, thus facilitates exploring crucial factor of soot forming. Based on comprehending of varying mixture inhomogeneity on soot forming under engine operations, a new multi-step soot model oriented in surface growth dynamic had so far been updated in this study. By correlation analysis, the new soot model is able to display the influence of both temperature and mixture inhomogeneity on soot surface activity by revising the parameter of the fraction of active sites. Hence the dominant factor of soot forming is conducted by sensitivity study in aspect of three distinctive phases: fast soot inception, soot growth equilibrium and soot oxidation prevailing, under varying mixture inhomogeneity. The soot inception is more influenced by temperature under uniform mixture combustion, but the dominant factor is switched to mixture inhomogeneity in stratified mixture wherein the acetylene formation rate gets the maximum. Similarly, less oxygen helps soot abatement due to weaker chemistry reactive in soot surface growth caused by lower temperature in homogenous mixture. Nevertheless, stimulated soot surface growth by activated surface activity and vast precursor deposit under heterogeneous mixture conduces to soot forming. Simultaneously, impaired soot surface oxidation under nonuniform mixture accelerates the deteriorated engine-out soot in comparison with discharged soot in less homogenous mixture.

This work presents a compact and robust chemical reaction mechanism for modeling the combustion of saturated furans including tetrahydrofuran, 2-methyltetrahydrofuran and 2-buthyltetrahydrofuran under engine relevant conditions. A decoupling method is adopted to construct the mechanism. The oxidation reaction for the small species is described by a mature and detailed H2/CO/C1 sub-mechanism, based on which the skeletal sub-mechanisms for tetrahydrofuran, 2-methyltetrahydrofuran and 2-buthyltetrahydrofuran are incorporated via a compact yet robust C2-C3 sub-mechanism. The sub-mechanisms for the three tetrahydrofuranic fuel components are selected from the detailed chemical mechanisms in the literature via a series of species rate of production analysis, sensitivity analysis, isomer lumping and reaction lumping. The Arrhenius pre-exponential A factors for these selected reaction pathways are then optimized via a single objective genetic algorithm. The resulting mechanism is rather compact consisting of only 56 species among 183 reactions. The performance of the developed mechanism for predicting the combustion chemistry for the three fuel components has been evaluated against the experimental measurements in the literature. Reasonable agreement between the predicted ignition delay times, speciation profiles and laminar flame speeds with the experimental data is achieved for all the cases considered, indicating the high accuracy and robustness of the developed mechanism.

In this study, an intelligent model to formulate fuel surrogates was proposed which includes intelligent surrogate blend optimizer and optimization module of chemistry kinetic mechanism. The components of surrogates were selected from the library of candidates with given compositions by the intelligent surrogate blend optimizer based on the targeted fuel properties. Hence, new jet fuel surrogate (JFS) and diesel fuel surrogate (DFS) models were formulated consisting of five components (n-dodecane/iso-octane/isocetane/decalin/toluene), with specified compositions of each component, respectively. The new surrogates contain major components of the target practical fuels and inherently emulate the key properties including liquid density, viscosity, surface tension, cetane number, hydrogen-carbon ratio, molecular weight, lower heating value and threshold sooting index. Moreover, a robust skeletal oxidation mechanism of fuel surrogate composed of five components was updated and multi-objective optimization algorithm of non-dominated sorting genetic algorithm II (NSGA-II) was introduced to achieve self-adaptive tuning of Arrhenius pre-exponential factor in fuel-related sub-mechanism. Thus, the optimal rate constants could satisfy the predictions of ignition delay times and species concentrations simultaneously. Then Generalized Polynomial Chaos (gPC) was employed to minimize the uncertainty of stochastic input coefficients and its propagation towards ignition behaviours. Consequently, the new surrogate models were well validated against available tested data from practical fuels, and better performance of current JFS model was highlighted when compared with the prediction results by previous model. It is feasible to employ this intelligent model to formulating surrogates for more practical fuels.

In this study, the synergy effects of fuel injection system, intake system and combustion system on the combustion characteristics of the controllable intake swirl diesel engine were investigated. A research platform consisting of CFD numerical simulation and experiment was established in this study. In ways of combination of numerical simulation and experiment based on fuel injection test bed, intake steady flow test bed and single cylinder engine test, the prediction accuracy was improved. As such, the complex fuel spray and combustion phenomenon can be theoretically explained from the views of fluid dynamics and heat transfer, thus leading to further improvement of the engine combustion system. The optimal designs of fuel injectors were proposed and the corresponding fuel injection rate was derived which was used as initial input parameters for CFD numerical studies. Meanwhile by optimization of structure of intake port volute radius and the inlet and outlet section dimensions of spiral intake port, the swirl ratio is increased by 16.5% while the flow coefficient is increased by 4%. Finally, with the optimized fuel injection and air intake system, the effective power was found increased by 3.5% compared to the original diesel engine while the BSFC is reduced by 1.8 g/kW·h. NO emission is slightly increased while soot emission is reduced compared to original diesel engine combustion.

Investigation of spherical expansion flame is an important method to study laminar flame. Processing schlieren pictures to obtain high-precision spherical flame radius is of great significance for understanding the combustion characteristics and modeling laminar flame. The objective of this study is to establish a systematical research method to evaluate the radius measurement center with scientific index factors. Four typical calculation methods of radius measurement center were compared and the adaptability of the four method was further investigated based on the typical schlieren pictures of flame floating and deformation. It was noted that as the increasing of proportion of the original edge contour points in the samples, the calculation accuracy of the flame radius is increased when considering the floating of the radius measurement center and non-fitting of the original edge contour points. For schlieren pictures of flame floating and deformation, flame floating is found to be the main factor leading to the errors of radius measurement center and radius measurement. However, the existence of flame deformation may further enlarge the measurement error of radius.

In this work, a reduced toluene-decalin reaction mechanism containing 108 species and 566 reactions was proposed for computational fluid dynamics (CFD) simulations and polycyclic aromatic hydrocarbon (PAH) formation predictions in combustion engines. The present mechanism was validated with the available literature data including ignition delay time (IDT) determined in shock tubes and rapid compression machines (RCM), species mole fractions measured in premixed flames and jet stirred reactors (JSR), and laminar flame speeds. Moreover, a sensitivity analysis was performed on toluene and decalin flames to further investigate the main formation pathways of four representative PAHs: benzene (A1), naphthalene (A2), phenanthrene (A3) and pyrene (A4). In general, the simulation results exhibited a reasonable agreement with the experimental data. The negative temperature coefficient (NTC) behaviors of decalin IDTs were accurately reproduced by the proposed mechanism. The PAH species concentrations were also well captured for ethylene and benzene flames. The sensitivity analysis results indicated that the main formation reactions for PAHs have a strong link with ring structures. The decompositions of toluene and decalin primarily contributed to the formation of A1. For toluene, A2 was formed by the reactions A1- + C4H3 = A2 and IC4H5 + A1 = A2 + H2 + H, while for decalin, the self-combination reaction of C5H5 became the main pathway. In addition, the reactions of the aromatic molecules and radicals significantly promoted the formation of A3 and A4 for both toluene and decalin.