不凝性气体及湍流扰动对喷嘴孔内空化现象的影响

Effects of Noncondensable Gases and Turbulent Fluctuations on Cavitation Phenomenon in Injector Nozzle

摘要:

利用ANSYS_Fluent 15.0软件，在两相流计算采用均相流模型(HEM)、湍流计算采用realizable k-ε模型基础上，分别基于SS及Singhal空化模型对喷嘴孔内空化现象进行数值模拟，并将模拟结果与试验结果进行对比分析，其中前者忽略了不凝性气体及湍流扰动对孔内空化现象的影响，而后者考虑了这2个因素。其后在两相流计算仍然采用均相流模型的基础上，将Singhal空化模型与不同的湍流模型组合应用，并与试验结果进行对比分析。结果显示：不凝性气体及湍流扰动对喷嘴孔内空化现象数值模拟存在显著影响，不考虑这2个因素时计算所得喷孔内气相区域分布规律与试验结果一致，然而计算所得气相体积分数仅为试验结果的42%，考虑这2个因素时计算所得气相区域分布规律同样与试验结果一致，且计算所得气相体积分数约为试验结果的96%，即考虑不凝性气体及湍流扰动的影响会使喷嘴孔内空化现象数值模拟更加准确；考虑不凝性气体及湍流扰动时计算所得气相区域在其末端出现小幅扩散现象，这种扩散现象源于不凝性气体在气相区域末端的膨胀作用；湍流粘度的计算结果通过影响场内流速、静压的方式最终对计算所得喷孔内气相区域分布产生一定影响。

Abstract:

Fuel spray atomization strongly affected engine economic performance and emissions, which in turn was significantly influenced by nozzle cavitation phenomenon with the high injection pressure in diesel and GDI engine. Aiming to study the influence of noncondensable gases and turbulent pressure fluctuations on modeling of nozzle cavitation, Schnerr and Sauer cavitation model and Singhal cavitation model were used to model nozzle cavitation, which were both coupled with homogeneous equilibrium multiphase model and realizable k-ε turbulence model. Then, the Singhal cavitation model was used to model nozzle cavitation with homogeneous equilibrium model and different turbulence models. The comparison of simulation results and experimental results showed that noncondensable gases and turbulent pressure fluctuations had significant impact on modeling nozzle cavitation. The simulation result ignoring noncondensable gases and turbulent pressure fluctuations seriously underestimated the volume fraction of gas phase in nozzle, which was just 42% of the experimental results, while the volume fraction of gas phase calculated by the one considering turbulent pressure fluctuations and noncondensable gases agreed well with experimental results, which was 96% of the experimental results. The results indicated that considering noncondensable gases and turbulent pressure fluctuations would improve accuracy of nozzle cavitation simulation. The calculated gas phase considering noncondensable gases and turbulent pressure fluctuations was diffused to some extent at the end of gas phase region, which was caused by the expansion of noncondensable gases in fluid. Turbulence viscosity slightly affected the distribution of gas phase by affecting the velocity and pressure of fluid.