压力室结构对断油阶段空化及气体倒流的影响

Effect of Pressure Chamber Structure on Cavitation and Gas Ingestion in Diesel Nozzle After End of Injection

断油阶段的空化现象对喷油间隔期的流场有重要影响,空化引起的高温燃气倒流会造成孔内结焦,进而影响缸内射流雾化.基于可视化试验和流体体积(VOF)方法两相三组分多相流求解器,对柴油喷嘴断油阶段的流动过程进行试验和模拟,分析3种不同压力室壁面曲率结构的喷嘴对断油阶段空化及气体倒流的影响.结果表明:在断油阶段喷孔和压力室依次发生空化现象,其中压力室内空化量较大,并呈现为涡空化和带状空化两种形态,壁面曲率小的狭直结构以带状空化为主;压力室结构较为狭直的喷嘴空化体积变大,但其低压持续时间延长使柴油流出量变小,最终导致倒流气体量下降;在发生涡空化时,速度梯度张量第二不变量Q与涡空化体积呈正相关,涡核区域越小则流场中磨擦耗散越少,越易于压力室内空化的形成.

The cavitation phenomenon after the end of injection(EOI)has important influence on flow field in the injection interval.The high temperature ingestion caused by cavitation will cause coking in the hole,which will affect the atomization of the jet in the cylinder.Based on the visual test and volume of fluid(VOF)two-phase three-component multiphase flow solver,the flow process of the diesel nozzle after the EOI was tested and simulated.The wall curvature structure of three different pressure chambers was analyzed to analyze the cavitation and gas ingestion after the EOI.The results show that cavitation occurs in the nozzle hole and the pressure chamber after the EOI.The cavitation in the pressure chamber is large,and it appears as vortex cavitation and strip cavitation.The strip cavitation is dominant when the narrow structure is small wall curvature.The cavitation volume of the nozzle with a narrower pressure chamber structure becomes larger,but the extension of the low pressure makes the diesel outflow smaller,eventually causing the ingestion to decrease.In the case of vortex cavitation,Q and the vortex cavitation volume are positively correlated.The smaller the vortex core region is,the less frictional dissipation is in the flow field and the easier the formation of pressure chamber cavitation is.