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It is well known that premixed and non-premixed flamelets coexist in the combustion system, especially for multiphase combustion (Luo et al., 2011; Bai et al., 2016). In this work, a multi-regime combustion model for partially premixed multiphase combustion (2PMC) is developed in the framework of large eddy simulation (LES). In this model, the multi-regime combustion mode is decomposed into two pure combustion regimes (i.e. premixed and non-premixed) through combustion regime indicator. Different combustion models are chosen for different combustion regimes. For example, for premixed combustion regime, the flamelet generated manifold (FGM) tabulation method in combination with the artificially thickened flame (ATF) approach is used. For non-premixed combustion regime, on the other hand, the thermo-chemical quantities will be extracted from the non-premixed chemtable generated by the flamelet/progress variable (FPV) approach. The proposed multi-regime flamelet model is then extended to adapt to the pulverized coal combustion (PCC) and applied to a laboratory-scale pulverized coal jet flame. The simulation results show that the percentage of the volume-weighted premixed combustion regime of this studied flame is up to 18%. Quantitative comparisons between the experimental data and the numerical results with the present multi-regime flamelet model, the FPV model, and the Eddy Break Up (EBU) model shows that the proposed multi-regime flamelet model has several advantages. It performs better than the FPV model in the regions where premixed combustion mode prevails and also much better than the EBU model in species concentration prediction.

To study how intake swirl affects the combustion performance of the lateral swirl combustion system (LSCS), experiments under different swirl ratios were carried out in a single-cylinder diesel engine to optimize the circumferential injection angle (CIA) and test the eventual combustion performance. Then, simulation under different swirl ratios was conducted to reveal the influence mechanisms of intake swirl on the fuel/air mixing characteristics of the LSCS. Experimental results show that, as the swirl ratio (SR) was increased from 0 to 1.0, the optimal CIA increased 2.5° in the opposite direction to that of the intake swirl. Besides, the LSCS obtained better combustion performance with SR = 0 than with SR = 1.0, especially at high loads and low excess air coefficients. Simulation results indicate that, generating intake swirl weakens the wall-flow-guided and interferential interactions of the LSCS, thereby deteriorating the in-cylinder air utilization and fuel/air mixing uniformity, especially at high loads and low excess air coefficients, in which the interactions between the LSCS chamber and spray jets is strong due to the enhanced spray penetration ability. Therefore, the LSCS without intake swirl obtains the optimal fuel/air mixing and combustion performance in DI diesel engines.

To study nitrogen oxides release, two model coals, which contain pyrrolic or pyridinic nitrogen, are prepared and burned in an entrained flow combustion system during 800–1300 °C. The influences of temperature, mineral, interaction between the two nitrogen forms, and oxygen concentration are discussed. The results show that NO is the dominant species, and its release increases drastically as temperature rises. N 2 O is only detected at 800 and 900 °C. The interaction between the two nitrogen forms gives rise to an increase of NO release at low temperatures but a decrease at high temperatures. For pyrrolic nitrogen, Ca and Na promote NO release above 900 °C, and Fe increases NO release for all temperatures investigated. As for pyridinic nitrogen, all minerals inhibit NO release above 900 °C. The NO release decreases when catalyst load rate increases. NO release rises with increasing oxygen concentration first and then decline. More NO 2 is released when oxygen concentration is above 20%.

The separated swirl combustion system (SSCS) is a new combustion system developed to improve fuel efficiency and reduce soot emissions in diesel engines. While soot emissions from the SSCS can be tested in a real-world single-cylinder engine, soot formation and oxidation cannot be; therefore, the mechanisms behind those processes have not yet been researched. To understand the mechanisms of soot formation and oxidation in the SSCS, soot evolution must be investigated using a simulation model. To bridge this gap, this study analyzed the combustion and emission performance of the SSCS under different speeds in a real-world single-cylinder engine. Then, a new phenomenological model was developed and used to simulate soot formation and oxidation and reveal the mechanisms behind those processes. The SSCS with the new model was validated against a double swirl combustion system (DSCS). The experiment results show that at 2100 r/min, the SSCS significantly reduced fuel consumption (about 6.54 g/(kW h)) and soot emission (0.17 FSN) by 2.89% and 6.31%, respectively. The simulation results show that the DSCS generates more incipient soot particles and soot mass than the SSCS, and surface soot oxidation is faster in the SSCS. The mechanisms analysis shows that the equivalence ratio is smaller in the SSCS than in the DSCS, so fewer incipient soot particles are generated, and the temperature is higher, which speeds up soot oxidation. Furthermore, the combustion duration is much shorter in the SSCS than in the DSCS, which means that the time between the end of combustion and the exhaust valve opening is longer, granting more time for soot oxidation. Due to the decreased soot formation and faster soot oxidation, the soot mass is also lower in the SSCS than in the DSCS.

To improve air efficiency in the combustion chamber, a new separated swirl combustion system (SSCS) was developed and is proposed herein. The SSCS performance was validated against a double swirl combustion system (DSCS) in an experimental test with a single-cylinder diesel engine. The experiment results show that the SSCS effectively reduces fuel consumption and soot emission, with a maximum decrease in fuel consumption of 5.41% (when the effective power was 17 kW) and a maximum decrease in soot emission of 20.48% (when the effective power was 43 kW). Then, simulation was used to investigate and reveal the mechanisms behind the fuel/air mixture in the chamber. The simulation results show that the angular fuel sprays collide with the unique circular ridges in the SSCS, which improves air utilization in the chamber and accelerates the fuel/air mixture, reducing soot emission.

A new, reduced reaction mechanism for DME combustion in internal combustion engines is proposed. The new mechanism is based on a detailed DME mechanism suitable for low to high temperature ranges. The reduced model consists of 29 species and 66 reactions and contains a detailed H2/CO mechanism and reduced C1–C2 chemistry. The reduced mechanism inherits the major reaction pathways of the detailed mechanism, which ensures its predictive capability when coupled into CFD simulations. The performance of the reduced mechanism was compared to simulation results of the detailed mechanism, ignition delay times from a shock tube and a rapid compression machine, and DME engine combustion data. Overall, the reduced mechanism shows a good balance between accuracy and computational efficiency necessary for use in combustion system design applications.

The paper describes an experimental study aimed to characterize the impact of the dual-fuel ethanol–diesel combustion system on size, number and chemical characteristics of the emitted carbonaceous particles. The tests were conducted on a single cylinder research engine provided with a modern architecture and properly modified in a dual-fuel (DF) configuration. In particular, the experimental campaign was aimed to evaluate in detail the effect of the use of the ethanol as port injected fuel in diesel engine on the size distribution function, morphology, reactivity and chemical features of the exhaust emitted soot particles. The engine tests were chosen properly in order to represent actual working conditions of an automotive light-duty diesel engine. The morphological and chemical features of soot particles were studied in dependence of the ethanol–diesel substitution rate by analyzing the soot collected at the engine exhaust. Results indicated a significant effect of ethanol fumigation on the concentration of the emitted particles but not on the average size, as well as, a negligible impact of ethanol premixed charge on the soot nanostructural features.

Experiments were conducted in a swirl stabilized premixed combustor at atmospheric pressure in order to study the stability and structure of lean premixed methane air flames. Swirl number of the combustor was 0.74. The lab scale combustor was operated up to 30 kW of thermal power during measurements. Experiments focused on two main aspects of the combustion system. First the relation between the cold flowfield and the flame stabilization is examined. It is observed that, the structure of the flame near the dump plane corresponds closely to the features of the non-reacting air flowfield. The flame is seen to be stabilized on the boundary of the recirculation region. Lean blowout limits of the combustor that operates within the flamelet regime were also recovered. A correlation linking the blowout equivalence ratio to laminar flame speed is presented. (C) 2012 Elsevier Ltd. All rights reserved.

The idea of evaluating combustion system performance based on the concept of reversible work is analysed. An expression for system efficiency is developed and applied to data obtained from an experimental coal combustor. It is shown that there is a region of optimal operating conditions that depends upon its intended use. Although the example treated here is specific, the methodology is quite general and can be used for any combustion system.