柴油发动机燃油跨/超临界喷射特性数值仿真

Numerical Simulation of Transcritical/supercritical Injection Characteristics of Diesel Engine Fuel

针对柴油发动机工作条件下喷雾特性变化复杂的问题,利用计算流体动力学软件CONVERGE,采用大涡模拟(Large Eddy Simulation,LES)方法进行跨/超临界柴油机燃料正庚烷的射流喷雾特性仿真分析,对LES中湍流亚格子模型进行深入研究,并从质量分数、密度和温度三方面对比跨/超临界射流的喷雾特性。研究结果表明:随机湍流分散模型开启会使喷雾贯穿距的预测值更接近试验值,当湍流分散系数c_(ps)=0.16时模型预测的贯穿距比c_(ps)=0.05更加贴合试验的结果;跨临界射流存在更加类似于液态的区域,升温时从环境吸收更多热量;与跨临界射流相比,超临界射流喷雾的温度扩散更加迅速,并且喷雾前端温度接近环境温度;跨临界射流中类似液态区域形成了较大的密度分层,抑制喷雾扩散,超临界射流喷雾外层的絮状结构比跨临界射流出现得更早,絮状结构范围更大,径向扩散作用更强;两种状态下温度、密度和质量分数分布呈现一定的对应关系,即温度、质量分数和密度都在中心轴线靠近喷油器处最大,随着轴向距离和径向扩散的增大而减小。

For the complex changes in spray characteristics of diesel engine under working conditions,this paper uses CFD software CONVERGE to simulate and analyze the jet spray characteristics of n-heptane fuel in transcritical/supercritical diesel engine by using large eddy simulation method,and conducts an in-depth study on turbulent subgrid model of LES.The spray characteristics of the transcritical/supercritical jet are compared from mass fraction,density and temperature.The results show that the stochastic turbulence dispersion model can make the predicted values of gas phase and liquid phase penetration distances closer to the test values,and the predicted gas and liquid phase penetration distances for turbulent dispersion coefficient c_(ps)=0.16 are more consistent with the test results than those for c_(ps)=0.05.Compared with the transcritical jet,the temperature diffusion of supercritical jet spray is faster,and the temperature of spray front is close to the ambient temperature.The similar liquid region in the transcritical jet forms a larger density stratification,which inhibits spray diffusion.Therefore,the flocculent structure in the spray outer layer of supercritical jet appears earlier than that of the transcritical jet.The flocculent structure has a larger range,and the density is closer to the surrounding environment.It is also found that the distributions of temperature,density and mass fraction in the two states show a certain corresponding relationship,that is,the temperature,mass fraction and density are all maximum at the central axis close to the injector,and decrease with the increase of axial and radial distances.