基于准稳态假设的混流泵启动特性分析

Starting characteristic of mixed-flow pump based on quasi-steady state assumption

为了研究混流泵启动过程的准稳态性能,该文以瞬态外特性试验性能参数为依据,获得了混流泵准稳态计算的外特性曲线和无量纲扬程瞬态性能曲线,通过对3种转速下的压力场、速度矢量分析,总结出准稳态计算混流泵内部流动的一般性规律并与粒子图像测速技术(particle image velocimetry,PIV)测量的瞬态内部流场进行对比。研究发现,准稳态计算扬程呈现直线上升趋势,并随着体积流量的增大逐步偏离试验扬程;3种转速下泵内压力具有相同的分布趋势,叶轮进口截面相对速度矢量近似满足相似定律,并在低转速下出现大尺度的叶顶泄漏涡。与无量纲瞬态性能一致,瞬态PIV测量结果呈现出明显不同于准稳态工况的瞬态效应:在0.75 s时准稳态计算中叶顶泄漏明显,在外缘形成泄漏涡和进口边回流,而瞬态PIV测量中端壁区边界层正从层流向湍流发展;在1.07 s时准稳态计算中叶轮进口速度分布较为均匀,而瞬态PIV测量中流体在惯性力作用下呈现从轮毂向轮缘运动的趋势,卷吸效应较为明显;在1.35 s时准稳态计算结果与1.07 s时的速度场分布较为相似,而瞬态PIV测量中加速的卷吸效应更为显著,外缘高速流动区域随着转速的增加不断增大。该研究可为考证准稳态假设方法的准确度和揭示混流泵启动过程瞬态内部流动特性提供参考。

Quasi-steady state flow characteristics are the approximately discrete superposition of the transient flow characteristics in the mixed-flow pump during starting period, and it is meaningful to understand the transient characteristics by researching the quasi-steady state performance. In order to study the quasi-steady state flow characteristics of mixed-flow pump during starting period, the external characteristic curve of quasi-steady state and the transient dimensionless head performance were obtained by numerical simulation and experiment based on the transient performance parameters of the external characteristic experiment. The overall flow field of mixed-flow pump was numerically simulated by the commercial software ANSYS-CFX, which was based on the time-averaged N-S equation, the standard k-ε two-equation turbulence model, and the SIMPLEC algorithm. Based on the analysis of pressure field and velocity vector under 3 different rotating speeds, the general laws of the quasi-steady state were summarized through numerical simulation of mixed-flow pump. The results were compared with the transient PIV(particle image velocimetry) experimental results. The calculated head presented a perpendicularly rising trend, and deviated from the experimental head with the increasing of the volume flow rate, which was due to the ignoring of blade acceleration effect in the quasi-steady state. The process of the experimental dimensionless head deviating from the quasi-steady state condition proved that the transient effect was caused by the impeller acceleration. The starting period of the mixed-flow pump was described by using the dimensionless head changing over time. The transient experimental dimensionless head curve decreased rapidly from a great value with the increasing of rotational speed, and then increased and coincided with the calculated quasi-steady state curve. Similar pressure distribution was presented in the mixed-flow pump under 3 different rotating speeds. The pressure and the relative velocity vector at the impeller inlet cross section met the similarity law approximately. Besides, the large-scale tip leakage vortex turned up at the low rotating speed. By comparing the flow field of the quasi-steady state and transient measurement at the time of 0.75 s, the tip leakage appeared under quasi-steady calculation, and the backflow and tip leakage vortex came up at the rim of impeller. But the boundary layer of wall ending region developed from laminar flow to turbulence flow in the experimental transient measurement. When the time was 1.07 s, the relative velocity distribution at the impeller inlet cross section was uniform, but the fluid had a moving trend from hub to rim, which was related to the impeller blade under the inertia force effects in the transient PIV measurement. Meanwhile, the entrainment effect at the impeller inlet flow field by the transient PIV measurement was more conspicuous. Consistent with the dimensionless transient performance, PIV measurement results showed that the transient effect of internal flow characteristics in the mixed-flow pump during starting period was distinct from that of quasi-steady state conditions. The results in this paper provide the academic basis and reference for verifying the accuracy of quasi-steady state method, and revealing the transient internal flow characteristics of mixed-flow pump during starting period.