蒸汽喷射真空泵性能的CFD模拟研究

Numerical Simulation of the Performance of Steam-Jet Vacuum Pump

采用有限体积法离散控制方程,标准 k-ε湍流模型,近壁面处使用壁面函数修正的方法对蒸汽喷射真空泵的超音速混合过程进行数值模拟。计算并分析了第二喉管与工作蒸汽喷嘴喉管面积比、喷嘴出口截面与混合段入口截面间的距离及混合段的锥度等结构参数及工作蒸汽的压力和温度、引射流体及混合流体压力等热力参数对真空泵操作性能的影响。数值计算结果表明,几何参数的改变极大地影响着波系结构,在一定的设计工况下,总存在一个最佳的面积比及一个最优的相对位置对应于最大的喷射系数,其在物理上的表现形式为通过工作蒸汽喷嘴所产生的激波系刚好能够通过第二喉管。混合段的锥度在一定范围内对真空泵的性能无显著影响,等压混合理论较等面积混合理论具更优的操作性能。根据喷射泵内的物理现象及喷射系数的变化规律将蒸汽喷射真空泵的操作状态分为临界状态、亚临界状态和回流状态三类,同时指出临界点为最佳工作点。

The supersonic steam-jet ejector as a function of vacuum pump was simulated by using CFD. An explicit finite volume scheme was applied to solve the axisymmetric Navier-Stokes equations with a standard k-epsilon turbulence model. The performance of steam-jet vacuum pump was investigated by changing the ratio of secondary to primary throat area, relative position of the steam nozzle and the taper of the mixing section. The numerical results clearly indicate that when geometric parameters vary, the structure of shock also changes. There are optimum throat area ratio and ideal steam nozzle position for a given operating condition, which corresponds to the state that shock waves generating at the steam nozzle moves downstream and just passes the second throat. The taper of the mixing section has little effect on the performance of the ejector over a range and the constant-pressure mixing theory is better than constant-area one. The performance of steam-jet vacuum pump was also investigated by changing the primary fluid pressure and temperature, the secondary and the compressed fluid pressures. The ejector performance is divided into three operational modes according to the shock wave patterns and the variations of entrainment ratio, that is, critical mode, sub-critical mode and reversed-flow mode. And the critical point is the optimum working point.