Bump环强化柴油混合过程的数值模拟研究

对双模式HCCI燃烧，促进其主喷阶段燃油的混合速率至关重要。采用CFD数值模拟方法研究了一种新型燃烧室设计——BUMP燃烧室对直喷柴油机喷雾燃烧过程的影响。结果表明，bump环扰动缸内气流运动产生复杂的流谱，形成强烈的湍流。燃油喷雾撞壁后，bump环剥离壁面射流形成二次空间射流，减少了燃油在燃烧室壁面的沉积，湍流混合速率大大增加。自燃着火时刻，BUMP燃烧室内有38．2％的燃油处于碳烟生成门槛之外，而对比燃烧室仅为28．9％。数值模拟解释了BUMP燃烧室同时降低NOx和碳烟排放的实验现象。此外，模拟还发现燃油混合速率对喷油定时非常敏感，存在一个高湍流混合速率曲轴转角区间。

CFD Study of Mixing Enhancement by a Bump Ring in a Combustion Chamber

It is important to enhance the mixing rate of the fuel of main injection in dual-mode combustion of diesel HCCI.In this study,the influences on spray mixing by using the bump ring in the combustion chamber investigated by means of CFD modeling.The study showed that the bump ring would play an important role in changing the flow patterns.After spray impingement,the wall-jet reflects by the bump ring and the secondary spray is formed with induced eddies,resulting in the remarkable increase in evaporation and mixing rate.At ignition timing,the resultant fuel mass distribution shows that 38.2% of fuel in the bump bowl falls in the range of F/A ratio less than 1.67,which is beyond the F/A ratio for soot formation,while the value is 28.9% in the baseline bowl.Temperature distribution in the bump bowl reveals more even amd the maximum temperature during combustion is lower than that of baseline case.The results are helpful for explaining the reason for the simultaneous reduction both NO_x and soot emissions using the bump ring.The present work also suggests a high sensitivity of injection timing on air/fuel mixing rate for bump bowl chamber,and a range of crank angle for getting high turbulent mixing rate is existed.