Previous researches has shown that inlet backflow may occur in a centrifugal pump when running at low-flow-rate conditions and have nonnegligible effects on cavitation behaviors (e.g. mass flow gain factor) and cavitation stability (e.g. cavitation surge). To analyze the influences of backflow in impeller inlet, comparative studies of cavitating flows are carried out for two typical centrifugal pumps. A series of computational fluid dynamics (CFD) simulations were carried out for the cavitating flows in two pumps, based on the RANS (Reynolds-Averaged Naiver-Stokes) solver with the turbulence model of k-ω shear stress transport and homogeneous multiphase model. The cavity volume in Pump A (with less reversed flow in impeller inlet) decreases with the decreasing of flow rate, while the cavity volume in Pump B (with obvious inlet backflow) reach the minimum values at δ = 0.1285 and then increase as the flow rate decreases. For Pump A, the mass flow gain factors are negative and the absolute values increase with the decrease of cavitation number for all calculation conditions. For Pump B, the mass flow gain factors are negative for most conditions but positive for some conditions with low flow rate coefficients and low cavitation numbers, reaching the minimum value at condition of σ = 0.151 for most cases. The development of backflow in impeller inlet is found to be the essential reason for the great differences. For Pump B, the strong shearing between backflow and main flow lead to the cavitation in inlet tube. The cavity volume in the impeller decreases while that in the inlet tube increases with the decreasing of flow rate, which make the total cavity volume reaches the minimum value at δ = 0.1285 and then the mass flow gain factor become positive. Through the transient calculations for cavitating flows in two pumps, low-frequency fluctuations of pressure and flow rate are found in Pump B at some off-designed conditions (e.g. δ = 0.107, σ = 0.195). The relations among inlet pressure, inlet flow rate, cavity volume, and backflow are analyzed in detail to understand the periodic evolution of low-frequency fluctuations. Backflow is found to be the main reason which cause the positive value of mass flow gain factor at low-flow-rate conditions. Through the transient simulations of cavitating flow, backflow is considered as an important aspect closely related to the hydraulic stability of cavitating pumping system.

先前的研究表明，离心泵在低流量条件下运行时可能会出现入口回流，并对空化行为（例如质量流量增益系数）和空化稳定性（例如空化喘振）产生不可忽视的影响。为了分析叶轮进口回流的影响，对两种典型离心泵进行了空化流对比研究。基于RANS（Reynolds-Averged Naiver-Stokes）求解器，采用k - ω剪切应力传递的湍流模型和均匀多相模型，对两台泵中的空化流进行了一系列计算流体动力学（CFD）模拟。泵A（叶轮进口逆流较少）的腔体容积随着流量的减小而减小，而泵B（叶轮入口回流明显的）腔体容积在δ =0.1285处达到最小值，然后随着流量的增加而 增大率下降。对于泵 A，在所有计算条件下，质量流量增益因子均为负值，并且绝对值随着空化数的减少而增加。对于泵 B，质量流量增益因子在大多数情况下为负值，但在某些低流量系数和低空化数的情况下为正值，大多数情况下在σ  = 0.151时达到最小值。发现叶轮进口回流的发展是造成这种巨大差异的根本原因。对于B泵，回流与主流之间的强烈剪切导致进口管中产生空化。随着流量的减小，叶轮内腔容积减小，进口管内腔容积增大，使得总腔容积在δ = 0.1285处达到最小值，此时 质量流量增益因子变为正值。通过对两台泵空化流的瞬态计算，发现B泵在某些非设计工况（如δ  =0.107、σ  =0.195）下存在压力和流量的低频波动。详细分析入口压力、入口流量、腔体体积和回流之间的关系，以了解低频波动的周期性演化。研究发现回流是造成低流量工况下质量流量增益系数为正值的主要原因。通过对空化流的瞬态模拟，认为回流是与空化泵送系统水力稳定性密切相关的一个重要方面。