CFD耦合化学动力学模拟EGR对柴油机燃烧的影响

Investigation of the Effects of EGR on Diesel Engine Combustion Based on CFD Coupling with Chemical Kinetics

研究了柴油机低速部分负荷工况引入不同EGR对缸内燃烧排放特性的影响.将CHEMKIN-Ⅱ化学反应求解器集成到KIVA 3V Release 2程序中,用正庚烷化学反应机理替代柴油燃烧,建立柴油机缸内燃烧数值模拟模型;结合试验数据,模拟分析喷油时刻保持不变,EGR率(废气再循环)从0%增加到60%的燃烧过程、NOx和碳烟排放.结果表明:引入大比例EGR后点火延迟明显增长,燃烧相位推迟,燃烧温度降低;较低燃烧温度避开了NOx的高浓度生成区,EGR率60%时NOx排放比无EGR时降低93.5%;但高EGR率未使燃烧路径避开碳烟生成区,加之较低的氧浓度不利于碳烟的氧化,碳烟排放增高.

The combustion processes of a diesel engine operating at low speeds and low loads with dif- ferent EGR rates were investigated. The CHEMKIN- Ⅱ chemistry solver was integrated into the KIVA 3V Release 2 code and a reduced n-heptane kinetics mechanism was employed as the surrogate of the diesel oxi- dization mechanism to model diesel combustion. Combined with the experimental data, simulations and a- nalysis were performed for EGR rates sweeping from 0% to 60% while the injection timing was un- changed. The results have shown that the combustion phase is retarded obviously with high EGR rates. The combustion temperature is decreased and the combustion path avoids the high NOx formation regions. The NOx emission of the case with EGR 60% is reduced by 93.5%. But combustion path with high EGR rate does not avoid the high level soot formation regions and the diluted air is unfavorable to the oxidization of the soot. So the soot emission is much higher.