背压对喷油嘴断油过程中气体倒流现象的影响

Influence of Ambient Pressure on Gas Ingestion in Diesel Nozzle after End of Injection

基于流体体积(VOF)模型和动态重叠网格技术对针阀关闭过程的喷油嘴内流场进行了瞬态模拟,分析发现在断油过程中,喷油嘴压力室和喷孔入口两个位置都会发生空化现象,与试验现象一致。计算得到的最大空化体积与倒流气体体积基本相等,说明空化溃灭是造成外部气体倒流的主要原因。压力室内的空化溃灭是引起倒流气体进一步流入压力室的必要条件。进一步从理论推导出新的变量——空化数,用以评估针阀关闭过程喷嘴内空化的程度,得出背压增大,断油过程的空化程度减弱,使倒流的气体体积减少,较好地解释了背压对气体倒流的影响规律。

For researching phenomena of the cavitation and the air ingestion in the nozzle under different ambient pressures, a visualization experiment with a transparent injector nozzle was carried out, which used the stroboscope, long-distance microscope, CCD camera, high-pressure spray chamber etc. The VOF method and the overset grid technology were used to simulate transient state flow in the spray orifice and the sac. The simulation results showed that the strong cavitation phenomenon would happen in the orifice and the sac of nozzle after the end of injection. The void volume of the cavitation bubble collapsing was filled with ingested air. And the cavitation volume was roughly equal to the ingested air volume based on the calculated data which can prove the conclusion. This showed that the cavitation collapse was the main cause of air ingestion. The cavitation collapse in the sac was a necessary condition for the ingestion air further flowing into the sac. And the smaller the cavitation area in the sac was, the less hot combustion gas was ingested into the sac. The cavitation number was proposed which represented the degree of cavitation in the nozzle after the end of injection. Further studies found that an increase in ambient pressure can cause the cavitation number to increase which led to the cavitation volume and the ingested air volume tend to decrease. And the cavitation number explained the influence of ambient pressure on air ingestion, and this was consistent with the experiment and simulation results.