不同导叶开度下立式蜗壳离心泵失速特性分析

Stall Characteristics of Vertical Volute Centrifugal Pump at Different Guide Vane Openings

大型立式蜗壳离心泵是长距离输水的核心动力装备,为了研究小流量工况下泵内不稳定失速机理,基于精细化网格和SSTSAS湍流模型,数值分析了活动导叶开度在小开度、最优开度和大开度3种条件下的非定常流态及其诱导压力脉动特性,讨论了导叶开度对立式蜗壳离心泵失速特性的影响。研究结果表明,在不同导叶开度下泵内失速的特征工况点相近,流量扬程曲线均呈现典型的驼峰区,但在小导叶开度条件下流量扬程曲线对应正斜率最大,大导叶开度时最小。在3种活动导叶开度下,活动和固定导叶之间的无叶区内均出现了大尺度旋涡,且固定导叶工作面的流动分离不断扩散至相邻固定导叶背面。在深度失速工况下,不同导叶开度条件时叶轮内的流体熵产率分布规律呈现明显不同的特征。随导叶开度增大叶轮叶片靠近前盖板截面上的流动分离区域扩大,且在最优开度和大开度条件下叶轮叶片中截面上靠近叶轮流道出口位置出现了较大速度梯度,导致了局部熵产率高的特征。在导叶小开度时深度失速工况下压力脉动的主频为叶片通过频率7f_(n),而最优开度时主频为0.9f_(n),大开度时在0.7f_(n)~1.2f_(n)范围内频率对应振幅均较大,大开度下的压力脉动呈现宽频特性。通过导叶内非定常数值模拟,发现低频压力脉动的出现与导叶内存在的周期性大尺度旋涡密切相关。

The vertical volute centrifugal pump is widely used in the long-distance water division project.Flow separation and vortex phenomenon can be observed in the vertical volute centrifugal pump at stall operating conditions.This kind of unsteady flow structures would result in the instability of the pump unit which affects the safety and reliability of the unit operation.In order to obtain the relationship between the stall characteristics of the pump and the different guide vane openings,a comparative study of the guide vane openings and the stall phenomenon of the vertical volute centrifugal pump was carried out.Based on the SSTSAS turbulence model with refined mesh,the unsteady flow patterns and induced pressure fluctuation in the pump were simulated under the conditions of small opening,optimal opening and large opening of the guide vane,and the influence of the guide vane opening on the stall characteristics of the vertical volute centrifugal pump was analyzed.The research results showed that the SST turbulence model combined with the selected grid can be validated in the experimental verification during the steady calculation.The performance results predicted by the SSTSAS turbulence model showed quite good agreement with the experimental data,and the prediction errors were less than 5%.The stall characteristic operating points were the same at different guide vane openings.The flow-head curve showed a typical hump area,but the positive slope corresponding to the flow-head curve was the largest when the guide vane opening was small,and the smallest when the guide vane opening was large.Under the three kinds of guide vane openings,the vortex in the vaneless area between the guide and stay vanes was serious,and the flow separation on the working surface of the stay vane continued to spread to the back of the adjacent stay vane.At the same time,the distribution law of fluid entropy production rate in the impeller was significantly different at different openings under the deep stall condition.As the opening of the guide vane increased,the flow separation area in the impeller blade near the shroud was enlarged.And under the conditions of the optimal opening and large opening,a large velocity gradient appeared in the middle section of the impeller blade near the impeller flow channel exit,the characteristics of high local entropy production rate were caused.The main frequency of the pressure fluctuation under the deep stall condition at small opening was the blade passing frequency 7f_(n),while the main frequency at the optimal opening was 0.9f_(n),and the corresponding amplitude of the frequency was large in the range of 0.7f_(n)~1.2f_(n) at the large opening.The pressure fluctuation at the large opening showed broadband characteristics.Through the unsteady numerical simulation results,it was found that the appearance of low-frequency pressure fluctuation was closely related to the periodic large-scale vortices in the vane diffuser.