Numerical And Experimental Investigation Of Soot Mechanism Of Diesel And Acetone-Butanol-Ethanol Blends

Compression ignition engine, which is usually referred to diesel engine, is one of the main power sources of vehicles and power machineries because of its high thermal efficiency, reliability and durability. However, the excessive reliance on diesel and other fossil fuels can lead to a series of critical issues, such as energy supply crisis, global environmental deterioration, and human diseases. Soot, which is one of the major pollutants emitted out from diesel engine, is also the main source of inhalable particle. Although a number of studies have been carried out in this field, uncertainties are still existed in soot mechanism, and there are many difficulties in achieving in-cylinder purification technology in diesel engines. Meanwhile, as the upstream product of bio-butanol, acetone-butanol-ethanol blends(ABE) have drawn great attention in nowadays. However, investigations of the combustion and soot formation mechanism are still in its infant stage. Based on the above considerations, a method which combines experiment and numerical simulation is adopted to investigate the soot formation mechanism of diesel and ABE in this thesis.Based on a constant volume chamber and an automotive diesel engine, two sets of tests are conducted in present study. In constant volume chamber, the Forward-Illumination Light-Extinction method is used for exploring the transient mass and distribution of soot particles, with conditions of various initial temperatures(800K, 900 K, 1000K) and oxygen concentrations(21%, 16%, 11%)。 Parallelly, for the engine test, the steady soot emission are measured with different EGR rates(0%, 15%, 30%, 45%, 55%, 60%)。For the aspect of numerical modeling, a phenomenological soot model of ABE is proposed in present study, which is validated based on the experiment results in constant volume chamber with various operation conditions. Moreover, based on the principle of mass conservation and the competitive relationship between soot surface growth and oxidation, a correction model is developed for considering the effect of surface oxidation on soot number density, based on which the original nine-step phenomenological soot model of diesel is improved. Additionally, the correction method is also integrated into ABE soot model. Further comparisions of the original and revised model are made on the predictions of particle size, and also on the differences between the revised nine-step model, HNS model and Fusco’s model. Based on KIVA coupling with above soot models, multi-dimensional numerical simulations of spray combustion process are conducted under the same condition as experiments to investigate the soot mechanism and the influences of initial temperature, oxygen concentration and EGR rate, based on which the differences of the soot evolution process between diesel and ABE.have been discussed.In present study, the results reveal that:(1) After calibration by experiment, the ABE soot model and revised diesel soot model can capture the trend of soot formation in a wide range of operation conditions.(2) When encountering strong oxidation effect in flame regions, the soot number density correction model can predict more realistic particle number and size, and avoid the numerical error. Additionally, compared to HNS model and Fusco’s model, the recised nine-step model can more accurately reflect the influence of EGR on soot emission.(3) In constant volume chamber, lower initial temperature prolongs the ignition delay, and the combustion mode starts to transform form diffusion combustion to premix combustion. In this case, the local high temperature and fuel rich region starts to shrink, which lead to strong suppression of soot formation mechanism, and thus the yield of soot will decrease monotonously for both diesel and ABE blends. As oxygen concentration decrease from 21% to 11%, the sooting tendency of diesel and ABE increase with different influencing mechanisms. For diesel, the rising sooting tendency results from the enhancement of soot formation mechanism and suppressed oxidation mechanism. As for ABE blends, both soot formation and oxidation mechanism are suppressed, however, the oxidation rate decreases faster. At 11% oxygen concentration, the yield of soot turns to decrease due to the dominant effect of suppressed soot formation rate.(4) In present diesel engine, EGR shows a segmented influence on soot emission. Small amount of EGR can reduce the combustion temperature, which contributes to emission control. As EGR rate increasing, soot oxidation reaction is suppressed at the same time, thus soot emission decrease slowly or even increase slightly. When EGR rate increases to a certain range, combustion temperature drop to a lower level in which combustion may avoid the limitation of soot formation, which leads to significant reduction of soot emission. On this basis, however, combustion begins to deteriorate when EGR continues to increase, and sharp increasing of soot emission can be observed.(5)Affected by the molecular structure and functional group, ABE leads to less amount of acetylene generated during the fuel pyrolysis process. Furthermore, the oxygen contained in the fuel molecular will accelerate the formation of OH radicals, which enhances the soot oxidation rate. In conclusion, ABE has lower sooting tendency than diesel at the same operation condition.In this thesis, a generalized ABE phenomenological soot model is proposed, and further improvements are made to original diesel soot model. Based on the experiments in different combustion devises with various operation conditions, these models are well calibrated, which provides theoretical basis and practical guidance for in-depth understanding of soot mechanism and improvement of soot emission. Moreover, the experiment and simulation results contribute to revealing the ABE combustion and soot formation characteristics, which lays a foundation for future researches on the usage of ABE as diesel surrogate.