伞喷喷嘴内空化现象的数值模拟

摘要

The cavitation flow inside the conical-spray injector is numerically investigated using the mixture multiphase model and full cavitation model. Results indicate that the cavitation evolution significantly affects the liquid sheet thickness and velocity at the nozzle exit, which will further change the spray angle and droplet Sauter mean diameter. Based on the cavitation distribution inside the nozzle, the cavitation flow inside the conical-spray nozzle can be classified into four regimes, i.e.no cavitation, cavitation inception at inlet, developing cavitation at nozzle exit and super cavitation. The extension of cavitation to nozzle exit in the super cavitation regime significantly improves the fuel atomization by increasing injection velocity and decreasing thickness of liquid sheet. A cavitation map for the conical-spray injector was developed by simultaneously varying the ambient pressure and injection pressure. It is found that the phenomenon of super cavitation only occurs in a narrow region where ambient pressure is very low.Therefore, the start of injection timing should maintain before the top dead center to ensure the occurrence of super cavitation inside the nozzle in order to create better homogeneous fuel/air mixture for diesel PCCI engines.%使用混合多相模型和全空化模型通过数值模拟研究了伞喷喷嘴内的空化流动过程.结果发现,伞喷喷嘴内的空化发展显著影响了喷嘴出口处的燃油液膜厚度和喷射速度,进而直接改变喷射锥角和液滴的平均索特直径.基于喷嘴内燃油蒸气的分布,空化可以划分为无空化、入口空化、出口发展空化和完全空化.完全空化中燃油蒸气延伸至喷嘴出口,可以显著增加喷射速度和减小液膜厚度,进而有效提高喷嘴的雾化性能.通过同时变化背压和喷射压力,构造了伞喷喷嘴内的空化分布图.结果显示,完全空化仅发生在背压较低的一个狭小范嗣内.因此,为实现伞喷喷嘴的完全空化,柴油PCCI发动机的喷射时刻需要远在上止点之前,以便产生更均匀燃油混合气.