基于密度修正的滤波器模型在轴流泵叶顶区空化数值模拟中的应用与验证

Application and verification of density correction method based filter based method for numerical simulation of cavitation in tip region of axial-flow pump

为了准确揭示轴流泵叶轮叶顶区的流场结构和空化形态,选用某一模型轴流泵进行数值模拟和空化可视化试验研究。研究结果表明,基于密度修正的滤波器湍流模型(Density Correction Method Based Filter Based Method,DCMFBM)可准确预测汽蚀余量NPSH值,预测最大误差比SST k-ω模型小3%;在小流量工况下,叶顶泄漏流和泄漏涡在叶顶区充分发展,随着流量的增大,泄漏流向叶顶中后部发展,且泄漏涡与叶片吸力面的夹角减小,泄漏流对相邻叶片压力面的影响减小;叶顶区轴向速度先减小后增大,泄漏流进入间隙时在压力面拐角处发生流动分离,在叶顶端面附近卷吸形成角涡,在离开间隙区前泄漏流又会重新附着在叶顶端面上;湍动能呈现先增大后减小的趋势,且峰值随着弦长系数的减小而增大。通过空化性能曲线和叶顶泄漏涡的空穴形态对比分析,验证了DCMFBM湍流模型的适用性。从高速摄影结果可见,随着空化的发展,叶轮叶顶区泄漏流空化、射流剪切层空化以及泄漏涡空化共同构成三角形云状空化结构,且叶片尾缘存在空泡脱落,揭示了叶顶泄漏涡不稳定的空化特性。

A model of axial flow pump was selected to deeply understand the structure of flow field and the cavitation morphology in the tip region of impeller by numerical simulation and visualized experimental study. The resutls indicate that the density correction method based filter based method（ DCMFBM） can accurately predict the value of cavitation number NPSHR. The biggest prediction error of DCMFBM is smaller by 3% than the SST k-ω turbulence model. The tip leakage flow and leakage vortex develop fully in the tip region,and then the leakage flow develops towards the middle and back of rim with the increase of flow rate,moreover the angle between leakage vortex and suction side decreases and the effect of leakage flow on the pressure side of neighboring blade is also decreased. The axial velocity in tip region decreases at first and then increases,separation occurs on the corner of pressure side while the leakage flow enters into the gap,forming an angular vortex near the wall of tip,and the leakage flow will re-attach to the wall of tip when it leaves the gap. The turbulent kinetic energy increases at first and then decreases,and the peak decreases with the increase of chord length coefficient. The cavitation performance curve and the comparison of tip leakage vortex cavitation morphologies verify the applicability of DCMFBM turbulence model. By using the high speed photography,it is shown that along with the development of cavitation the tip leakage flow cavitation,jet shear layer cavitation and leakage vortex cavitation constitute a triangular structure,and the cavity will fall off from the trailing edge,showing an unsteady characteristic of tip leakage vortex cavitation.