正十二烷喷雾火焰中碳烟的瞬态生成特性

A Study on Transient In-Flame Soot Evolution of n-Dodecane Spray

针对压燃式发动机中碳烟演化过程难以被实验准确捕捉的问题,研究了高温高压环境中正十二烷喷雾燃烧焰中碳烟瞬态生成特性及动力学行为,在实验方面,应用消光辐射联合技术以及燃烧成像测速技术,同步获得高温高压环境中单孔喷油器正十二烷喷雾燃烧火焰中的碳烟体积分数、碳烟温度和碳烟速度场的瞬态分布;数值计算方面,基于OpenFOAM环境,发展欧拉喷雾模型和非稳态火焰面进度变量湍流燃烧模型的耦合模型,喷雾燃烧过程的数值计算结果与实验数据有高度的一致性。研究结果表明:相比于参考工况,低喷油压力工况火焰浮起长度更短,碳烟在当量比Φ>2区间驻留时间更长,推动了碳烟的生成,碳烟峰值质量增加了80%;高温环境工况碳烟在当量比Φ>2区间驻留时间没有明显差异,但更短的火焰浮起长度使得燃料在更高的当量比开始燃烧,导致其碳烟峰值质量增加了88%;富氧工况的燃烧温度最高,碳烟粒子运动速度增大,使得碳烟在当量比Φ>2区间驻留时间明显减少,同时火焰内部当量比较小,因而碳烟峰值质量减少了42%。

Capturing the transient evolution of soot under diesel-like conditions through experimental methods is challenging.The transient formation characteristics and kinetic of soot in-flame of n-dodecane spray under high-temperature and high-pressure conditions is investigated in present study.Experimentally,the combined extinction and radiation(CER)methodology and combustion image velocimetry(CIV)are applied to capture the transient distributions of soot volume fraction,soot temperature,and soot velocity field of n-dodecane spray flames from a single-hole injector under high-temperature and high-pressure conditions.Numerically,a coupled model combining the Eulerian spray model and unsteady flame progress variable combustion model is developed in the OpenFOAM environment.The numerical results of the spray combustion process exhibit a high level of agreement with experimental data.The results show that,compared to the reference condition,in the lower injection pressure condition,the shorter lift-off length(LOL)and the longer soot residence time within the interval equivalence ratioΦ>2 significantly propel the formation of soot,leading to an 80%increase in peak soot mass.In the higher-temperature condition,there is no significant difference in the soot residence time within the intervalΦ>2,while the shorter LOL leads the fuel to ignite at higher equivalence ratios,resulting in a significant increase in soot formation,with an 88%increase in peak soot mass.In the oxygen-rich condition,the flame temperature is the highest,which increase the particle velocity of soot.Consequently,the residence time of soot within the intervalΦ>2 is significantly[JP3]reduced.Additionally,owing to the lower equivalence ratio in the flame,peak soot mass is reduced by 42%.