基于空穴初生的柴油机喷孔内部流动

Flow in diesel nozzle holes based on initial cavitation

为揭示喷油嘴内空穴流动机理,设计可视化试验装置,观察了喷孔内空穴流动.采用混合多相流模型、空穴模型、标准k-ε方程模拟了气液两相的空穴流动.分析了不同喷油压力、气核体积分数、液体黏性、壁面物性、表面张力对空穴流动的影响.结果表明:随着入口压力的增加,空穴初生逐渐发展为超空穴;当空穴参数小于临界空穴参数时,流量系数随空穴参数的增大而增大;当空穴参数大于临界空穴参数时,流量系数变化很小,不再出现空穴现象;流动介质的气核体积分数越大,空穴现象越明显,并随着进口压力的增加,这种影响越强烈,而对应的临界空穴压力越小;流动介质的黏性大小对空穴的发展有一定的限制作用,黏度越大空穴初生所需临界空穴压力越大;壁面物性和表面张力对空穴现象影响不大.

In order to investigate the flowing mechanism of cavitation in nozzle,a visualization test was designed to directly observe cavitation in nozzle holes.Mixed multi-phase model,cavitation model and k-ε equation were used to simulate the two-phase cavitation flow.The effects of injection pressure,bubble volume fraction in liquid,liquid viscosity,wall physical properties and surface tension on the cavitation flow in nozzle were investigated.The results show that the initial cavitation flow developed gradually to be super cavitation with the increasing of entrance pressure.When the cavitation parameter is lower than the critical cavitation parameter,the discharge coefficient increases with the increasing of cavitation parameter.However,on contrary condition,the change of discharge coefficient can be negative with no cavitation.The larger the bubble volume fraction in liquid is,the more obvious the cavitation is.With the increase of inlet pressure,the bubble volume fraction has more influence on cavitation with smaller critical cavitation pressure.The liquid viscosity has a certain restriction on cavitation development.Enhanced liquid viscosity needs high critical cavitation pressure.The wall physical properties and surface tension has slight impact on cavitation.