喷孔几何特征对孔内流动及近孔区域燃油雾化的影响

Influence of Nozzle Geometry on Internal Flow Characteristics of Injection Nozzle and Near Nozzle Region Injection and Atomization

采用混合多相流模型与空穴模型相结合的方法,对喷孔内部流动特性及近孔区域燃油喷射及雾化进行数值模拟。计算结果表明,喷孔几何特征对其内部流动特性及近孔区域燃油喷射及雾化具有重要影响,对于截面渐扩的喷孔,孔内空化效应、湍流度均增强,喷孔出口流速提高,喷油压力增大时效果更显著,空穴强度增大能够促进近孔区域燃油快速分裂,使燃油液滴雾化更细小,有利于柴油机油气混合以及性能的提高。对于截面渐缩的喷孔,孔内空化效应受到抑制、湍流度和喷孔出口流速均降低,喷孔出口液柱较长、液块较大,近孔区域燃油雾化程度降低。

The presence of cavitation and turbulence in a diesel injector nozzle has significant effects on the subsequent spray characteristics. The influence of nozzle hole shape on internal flow and near-nozzle region fuel injection and atomization under stationary conditions was studied. Five standard-sac nozzles with the same hole inlet diameter but different hole shapes （cylindrical, divergent, convergent, divergent - convergent and convergent - divergent） were used for the investigation of influence of nozzle hole shape on internal flow and spray. Large eddy simulation （LES） along with a two phase homogenous mixture model were employed. From research results, three important conclusions can be drawn. Firstly, the geometry characteristics Of the orifice had a great impact on internal flow of injection nozzle. The cavitation can raise the effective velocity at the nozzle exit and strengthen flow disturbance of the nozzles, and such effect became even more obvious with higher injection pressure, e. g. , IOOMPa. Secondly, the diverging-shaped nozzle was more prone to cavitate, that＇ s the very opposite of the converging nozzles. Stronger outlet cavitation intensity was found in hyperbolic-shaped nozzles, and stronger inlet cavitation intensity was found in elliptic-shaped nozzles. Thirdly, the cavitation intensity had a great impact on the near-nozzle region fuel break-up and atomization, especially the outlet cavitation such as the diverging- shaped nozzle and hyperbolic-shaped nozzle under the condition of high injection pressure, which were beneficial to the performance of diesel engine, the fuel injection and atomization.