喷嘴空化模型建立与有效性试验验证

Establishment and Experimental Validation of Nozzle Cavitation Model

提出了一种针对喷嘴孔内空化现象的数值计算模型——喷嘴空化模型,并进行了可视化试验以验证其模型有效性,其中喷嘴空化模型考虑了液相与气相之间相变、气泡动力学、湍流扰动及不凝性气体影响等诸多因素,并采用喷孔内空化数对相变速率方程进行了修正。将基于喷嘴空化模型的数值计算结果与试验结果及基于Schnerr-Sauer模型的计算结果进行对比分析,结果显示:在2种空化模均采用默认参数的前提下,喷嘴空化模型与Schnerr-Sauer模型计算所得喷孔内空化现象变化趋势均与试验结果吻合良好,即喷嘴孔内空化现象随喷射压力提高而加强,且在发展空化至超空化流态过渡过程中空化特征长度涨幅远高于其他流态过渡情况;喷嘴空化模型计算所得空化特征长度在各流态下均与试验结果吻合良好,Schnerr-Sauer模型计算空化特征长度则低于试验结果,两者与试验值的最大误差均发生在超空化阶段,其中喷嘴空化模型计算值为试验值的92%,Schnerr-Sauer模型计算值为试验值的65%。该结果表明,所建立喷嘴空化模型可用于较为准确地模拟喷嘴孔内空化现象的变化趋势及空化特征长度。

Fuel spray atomization strongly affects engine economic performance and emissions, which in turn is significantly influenced by nozzle cavitation phenomenon with high injection pressure in diesel and GDI engine. A new cavitation model named ＂ nozzle cavitation model＂ was presented to specifically simulate nozzle cavitation while the corresponding visual experiment was made to validate this model. The presented model considered phase change, bubble dynamics, turbulent pressure fluctuations and noncondensable gases while the equation of phase-change rate was amended by cavitation number. The comparison of simulation results with ＂nozzle cavitation model＂, simulation results with Schnerr- Sauer cavitation model and visual experimental results showed that the development trend of nozzle cavitation from ＂nozzle cavitation model＂ and Schnerr-Sauer cavitation model both agreed well with experimental results, that was, the normalized cavitation length was increased with the enhancement of injection pressure and the maximal increase appeared on the transition from development cavitation to super cavitation. The normalized cavitation length from ＂nozzle cavitation model＂ agreed well with experimental results while that from Schnerr - Sauer cavitation model was obviously less than that of experimental results. The maximum errors of normalized cavitation length simulated with the two cavitation models both appeared at super cavitation stage, which were 8% with ＂nozzle cavitation model＂ and 35% with Schnerr- Sauer cavitation model. The conclusion that the predictive capability of ＂nozzle cavitation model＂ was superior to that of Schnerr- Sauer cavitation model for simulation of nozzle cavitation was mainly because of the turbulent viscosity in near-wall region calculated from the former was lower than that from the latter, the threshold pressure value to produce phase change from the former was higher than that from the latter, the bubble number density from the former was amended by volume fraction of noncondensable gases and the equation of phase-change rate from the former was amended by cavitation number.