离心泵不稳定工况流场及性能分析

R. Prunières, C. Kato
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引用次数: 0

摘要

以局部凹痕为特征的离心泵性能曲线不稳定可能是旋转或静止部件流动不稳定的结果。这种流动不稳定往往会导致异常的运行条件,造成压力脉动增大、噪音和振动等严重问题,从而损坏泵和系统。为了使泵可靠运行,有必要了解导致性能曲线不稳定现象的发生和机理。本文研究了低比转速(ωs = 0.65, Ns = 1776)离心泵的性能曲线不稳定性,旨在更好地理解部分负荷试验中观察到的水头下降的机理。为此,计算流体动力学(CFD)使用大涡模拟(LES)方法进行。本研究使用的几何结构实际上是多级离心泵的第一级,由吸入室、封闭式叶轮、叶片扩散器和下一级的返回导叶(不包括在内)组成。为了考虑其对泵稳定性的潜在影响,磨损环和级衬套的泄漏也被包括在计算几何中。计算流体力学中的不稳定性发生在比实验中更高的流量下。结果表明,预旋角被低估了几度,从而导致叶轮工况的改变。尽管如此,对CFD结果的分析仍然有助于更好地理解水头下降的开始。当水头下降时,观察到叶轮出口低径向和轴向速度从轮毂一侧切换到叶冠一侧。这种流型的变化伴随着扩散器入口喉道再循环的强烈增加和失速的发展,这损害了叶轮出口和扩散器入口之间的压力恢复。随着泵流量进一步降低到扬程降流量以下,扩压器喉部的再循环向叶轮出口延伸,冲击欧拉扬程。相反,当流速再次减慢时,从叶轮出口到扩压器入口喉部的压力恢复再次增加。因此,泵扬程再次增大。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Flow Field and Performance Analysis of a Centrifugal Pump During Unstable Operating Conditions
Centrifugal pump performance curves instability, characterized by a local dent, can be the consequence of flow instabilities in rotating or stationary parts. Such flow instabilities often result in abnormal operating conditions, causing severe problems such as increased pressure pulsation, noise and vibration which can damage both pump and system. For the pump to have reliable operation, it is necessary to understand the onset and the mechanism of the phenomenon resulting in performance curves instability. Present paper focuses on performance curves instability of a centrifugal pump of low specific speed (ωs = 0.65, Ns = 1776) and aims at a better understanding of the mechanism leading to the head drop observed during tests at part load. For that purpose, Computation Fluid Dynamic (CFD) was performed using a Large-Eddy Simulation (LES) approach. The geometry used for present research is in fact the first stage of a multi-stage centrifugal pump and is composed of a suction chamber, a closed-type impeller, a vaned diffuser and return guide vanes to next stage (not included). Leakages at wear ring and stage bush were also included in the computed geometry in order to consider their potential influence on pump stability. The occurrence of the instability in CFD is found at a higher flow rate than in the experiments. It is observed that the pre-swirl angle is under-predicted by several degrees which leads to change the impeller operating conditions. Nevertheless, the analysis of the CFD results is still useful to have a better understanding of the onset of the head drop. When the head drops, a switching of low radial and axial velocities at the impeller outlet from the hub side to the shroud side is observed. This change of flow pattern goes along with a strong increase of the diffuser inlet throat recirculation and the development of stall, that impairs pressure recovery between the impeller outlet and the diffuser inlet. As the pump flow rate is further decreased below the head drop flow rate, recirculation at the diffuser throat extend toward the impeller outlet and impact Euler head. Conversely, the pressure recovery from the impeller outlet to the diffuser inlet throat increases again as the flow velocity slowdown can be effective again. Consequently, the pump head increases again.
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