柴油机燃用水乳化柴油着火时刻的化学动力学特性

以4100QBZL-2型增压直喷柴油机为研究对象,利用美国劳伦斯利弗莫尔国家实验室(LLNL)的正庚烷第三版详细模型进行化学动力学计算,对被测发动机上燃用纯柴油,体积分数为:10%。、20%、30%的水乳化柴油时缸内的着火时刻进行模拟研究,并将化学动力学计算结果与试验结果及计算流体力学与正庚烷简化模型(CFD-SKLE)的耦合计算结果进行对比分析。研究结果表明:水乳化柴油所引起的着火时刻延迟是由于乳化燃料中的水分在缸内的物理现象所引起。从化学动力学的角度来分析,乳化燃料内的水分可以促进H、O、OH、HO2等自由基在低温反应(冷焰)阶段的大量生成,为随后的高温(热焰)反应提供了条件,最终使着火时刻提前。通过对掺水后正庚烷氧化反应的敏感性分析可知,水分会在正庚烷氧化的低温反应时期对过氧烷基的异构化过程与过氧化氢酮的分解过程产生了促进作用,从而加速正庚烷氧化的链式反应的进行。

Chemical kinetics of ignition timing of diesel engine fueled with water emulsion diesel

Based on the third edition of Lawrence Livermore National Laboratory (LLNL) n-heptane oxidation detail model, the chemical kinetics simulation was conducted using the parameters of a 4100QBZL-2 DI engine. The ignition delay time was investigated and simulated, with the engine fueled with 0%(pure diesel), 10%, 20% and 30%(by volume fraction) water-diesel emulsion. The chemical kinetics calculation results were compared with the experimental results and the computational fluid dynamics coupled with n-heptane reduced model (CFD-SKLE) calculation results. The results indicated that, the ignition time was delayed due to the physical phenomena of water in emulsified fuel. From a chemical view, the water in emulsion fuel can promote the formation of free radicals H, O, OH, HO2 in n-heptane's low temperature reaction regime (cool flame). Those free radicals can create advantageous conditions for n-heptane high temperature reaction (hot flame) and advance the ignition timing. The sensitivity analysis results of n-heptane oxidation with water showed, the water will promote isomerization of alkyl peroxide and decomposition of hydrogen peroxide ketones in n-heptane low temperature reaction regime and consequently accelerate the n-heptane oxidation chain reactions.