基于沸腾空化模型的跨临界CO2喷射器性能优化

PERFORMANCE OPTIMIZATION OF CO EJECTOR BASED ON CAVITATION AND BOILING MODEL

喷射器内存在的激波效应对其性能有重要影响,现有的一维模型无法实现对激波的捕捉。在处理非平衡相变、超音速流和激波等复杂现象时,建立CO2两相喷射器三维模型来捕捉喷射器超音速喷嘴处的激波特性,采用空化和沸腾相变模型来模拟流体的相变过程。经与文献的实验验证,沸腾空化模型能够准确分析喷射器的性能,喷射系数的最大误差为4.3%;进行5种湍流模型的对比,其中SST k-ω模型能很好地预测内部流动,喷射系数的最大误差为5.8%。进一步对设计工况下喷射器的几何尺寸混合段与扩散段进行优化,分析其与激波间的关系。结果表明:1)分析扩散角变化的影响,存在一个使摩擦损失与湍动能损耗都较小的最佳扩散角;2)在一定的工况下,存在最佳的混合室长度和直径使喷射系数较高,当混合室直径一定时,增加混合室长度有助于增加激波链的长度,但超过最佳混合室长度时,激波的强度逐渐减弱;在混合室长度不变时,减小混合室直径会使工作喷嘴出口处的激波幅度减弱,扩散室入口处的激波增强,但过多的增加混合室直径会使混合室的作用降低,失去升压效果。

The shock effect that exists inside the ejector has an important effect on the performance of the ejector,and the existing one-dimension theoretical model cannot achieve the capture of the shock.For this deficiency,in the process of nonequilibrium phase change,supersonic flow and shock waves phenomena etc.,a 3-D CFD model of CO2 two-phase ejector was built to capture the shock characteristics of the supersonic nozzle.The cavitation and boiling phase transition models were used to simulate the phase change of fluids.Compared with the experimental results in the literature,the established model could accurately predict the performance of the ejector,the maximum error of the entrainment ratio was 4.3%.Five RANS turbulence models have been compared,as a result,the SST k-ωmodel shows better performance,the maximum error of the entrainment ratio was 5.8%.Moreover,the model is used to study the geometry on the ejector performance and analyze its relationship with shock waves.The results show that 1)with the change of the diffusion angle,there is a balance between friction loss and turbulent kinetic energy loss,corresponding to the optimal value of the diffusion angle.2)Under certain conditions,there is an optimum mixing chamber length and diameter to maximize the entrainment ratio,increasing the length of mixing section with a fixed mixing section diameter would increase the length of the shock chain.but when the optimum mixing chamber length is exceeded,the intensity of the shock wave began to decrease;when the length of the mixing chamber is constant,decreasing the diameter of the mixing chamber will weaken the shock intensity at the outlet of the working nozzle and increase the shock at the inlet of the diffusion chamber,while excessively increasing the diameter of the mixing chamber will reduce the function of the mixing chamber,losing pressure lift effect.