轴流泵叶轮叶顶区空化流的数值模拟与实验研究

Numerical simulation and experimental study on impeller tip region cavitation in axial flow pump

为分析大流量工况下轴流泵叶顶区空化流场特性,采用修正的空化模型和SST k-ω湍流模型以及结构化网格技术,对南水北调工程某一轴流泵模型叶顶区流场进行数值模拟,并分析其大流量工况下叶顶间隙内的轴向速度和湍动能分布特性,揭示了叶片不同弦长截面的空穴分布和压力场的关联性。采用高速摄影技术,对叶顶区不同空化数下的空化流场进行实验测量,并与数值计算结果进行对比分析。研究结果表明,叶轮叶顶区空化主要分布在叶顶间隙区、泄漏涡卷吸区和涡带区域。在大流量工况下叶顶区空泡分布起始于叶顶叶片弦长中间位置,随着叶顶翼型弦长系数的增加,由叶顶角涡引起的间隙空化从叶顶压力面拐角处发起,并覆盖叶顶间隙区;泄漏涡空化区随着泄漏涡的产生、增强和耗散,与之对应也具有空化初生、发展和溃灭的过程。在大流量空化严重工况下,流动分离诱导叶片压力面前缘约50%区域出现片状空化,叶片吸力面中后部也形成了片状附着空化,同时叶顶区刮起涡空泡、泄漏流空泡和叶顶泄漏涡空化涡带并存,严重堵塞了叶轮流道。

Based on modified SST k-to turbulence model and homogeneous cavitation model, a scaling mod- el pump was simulated at large flow rate conditions. The tip clearance axial velocity and turbulent kinetic energy distribution were analyzed. The correlations between cavitation bubbles and pressure distribution of the different chord length cross-section in fixed cavitation number were studied as well. Combined with cavi- tation and high-speed photography experiments, the experimental results show good agreement with the cal- culated values. Numerical results show that tip cavitation bubbles are mostly located at the tip clearance re- gion, vortex rolling region and tip leakage vortex region. With the increasing of the chord length coeffi- cient, tip clearance cavitation caused by the corner vortex migrates downward along blade tip clearance from pressure side, and then covers the blade tip region. With the development of tip leakage flow, there is also a process of inception, development and burst in the leakage vortex cavitation. Experimental results show that cavitation bubbles of tip leakage vortex occur in the middle location of the chord at large flow rate conditions. Under serious cavitation conditions, cavitation bubbles occur which are induced by flow separation at pressure side at large flow rate conditions, and the cavitation bubbles also gradually cover the blade suction side, and then form attached sheet cavitation, together with tip leakage vortex, corner vortex and blowing vortex cavitation, which clog the flow channel seriously.