进水均匀性对大型轴流泵装置空化特性的影响

Influence on cavitation characteristics of large axial-flow pumping unit by uniformity of conduit inflow

针对肘形进水流道对大型轴流泵装置空化特性的影响,从进口速度三角形理论出发,推导泵汽蚀余量与进口速度均匀性之间的关系.采用CFD技术对大型轴流泵外特性曲线进行模拟,与试验结果进行比较,结果表明:在设计工况下,扬程误差为3.58%,效率误差为3.31%.并对泵装置性能和空化特性进行预测,受肘形进水流态的影响泵装置进口速度均匀度变差,致使其扬程下降了7.83%,汽蚀余量增大了12.04%;通过比较进口速度均匀性,发现在叶轮进口截面处进口来流并没有垂直均匀地进入叶轮内部,而是与轴向流线成一定角度,且泵装置空化时来流速度加权平均角θ最大;将θ带入推导公式,发现泵装置的汽蚀余量误差能得到减小,且小于5%,达到工程应用要求.

Regarding to the effect on cavitation characteristics of large axial-flow pumping unit caused by the elbow-inlet conduit ,establishment of accurate prediction methods is necessary .In this paper , based on the velocity triangle theory ,the relationship of net positive suction head (NPSH) and uni-formity of inlet velocity was firstly deduced .And then computational fluid dynamics (CFD) technolo-gy was used to simulate external characteristic curve of large axial-flow pumps and the results of ex-periment and simulation were compared .In conclusion ,the research shows that under the optimal conditions ,the head error is 3 .58% w hile the efficiency error is 3 .31% ,w hich indicates the correct-ness of numerical simulation .What’s more ,properties and cavitation characteristics of pumping unit were predicted ,and uniformity of inlet velocity got bad due to the influence of elbow-inlet flow ,which lead to 7 .83% reduction of head and 12 .04% increment of NPSH .By comparing uniformity of inlet velocity ,it can be find that at impeller inlet section ,the inlet flow dose not enter into inside impeller vertically and uniformly ,but at an angle with axial streamline ,and weight average angle of inflow ve-locity (θ) become maximum when cavitation occurred .Actually ,putting θinto formula ,the NPSH error of both axial flow pump model and pumping unit are reduced to less than 5% ,w hich meets the requirements of engineering application .