Analysis of the Gas–Liquid Two-Phase Flow Characteristics of Multistage Centrifugal Pumps Under Different Rotational Speeds

Performance deterioration and unstable operation are common when multistage centrifugal pumps handle gas–liquid mixtures. Here, we investigate a two-stage centrifugal pump over a wide speed range and inlet gas volume fractions (IGVFs) using experiments and CFD. The two-phase flow is simulated with a Eulerian–Eulerian two-fluid approach (liquid as the continuous phase; gas as a dispersed bubbly phase with a representative bubble diameter of 0.3 mm). Turbulence is closed using the SST k–ω model for the liquid phase and the built-in dispersed-phase turbulence treatment in ANSYS CFX. Transient pressure signals are analyzed in the time and frequency domains (FFT) to assess how rotational speed affects void-fraction distribution, overall performance, and the dominant unsteady components within the adopted modeling framework. The results show that IGVF primarily controls gas accumulation in the impeller passages: as IGVF increases, the gas phase evolves from dispersed bubbles to a central core, whereas speed mainly alters the detailed morphology via centrifugal effects. Similarity-law scaling is strongly speed-dependent in this pump: agreement is better at higher speeds and deteriorates at lower speeds where viscous effects become more influential. The dominant unsteady content also changes with speed, shifting from low-speed broadband features associated with gas redistribution to high-speed periodic components linked to blade–vane rotor–stator interaction (RSI). In addition, the downstream stage exhibits more uniform void fraction and more regular periodic signatures, consistent with an inter-stage flow-rectification effect. These observations provide practical guidance for hydraulic design and variable-speed operation of multistage pumps under gas entrainment.