Influence Mechanism of Particle Diameter and Volume Fraction on the Solid–Liquid Two-Phase Flow Performance of Semi-Open Impeller Sewage Pumps

Semi-open impeller sewage pumps are widely used in fields such as municipal wastewater treatment. However, they often face performance degradation and operational instability when conveying solid–liquid two-phase flows containing solid particles. This study aims to systematically elucidate the influence mechanisms of particle diameter (0.5–3.0 mm) and volume fraction (1–20%) on the external characteristics and internal flow field of semi-open impeller sewage pumps, providing a theoretical basis for optimizing their design and operational stability. Using an 80WQ4QG-type sewage pump as the research subject, this study employed a combination of numerical simulation and experimental research. The standard k-ε turbulence model coupled with the Discrete Phase (Particle) approach was adopted for multi-condition solid–liquid two-phase flow simulations. Furthermore, two-way analysis of variance (two-way ANOVA) was utilized to quantify the main effects and interaction effects of the parameters. The results indicate that the pump head and efficiency generally exhibit a decreasing trend with increasing particle diameter or volume fraction, with particle diameter exerting a more pronounced effect (p < 0.01). When the particle diameter increased to 3.0 mm, the head decreased by 5.66%; when the volume fraction rose to 20%, the head decreased by 4.17%. It is noteworthy that the combination of a 0.5 mm particle diameter and a 20% volume fraction resulted in an abnormal increase in head, suggesting a possible flow pattern optimization under specific conditions. Analysis of the internal flow field reveals that coarse particles (≥1.5 mm) intensify the pressure gradient disparity between the front and rear shroud cavities of the impeller, thereby increasing the axial thrust. A high volume fraction (≥10%) promotes pronounced flow separation in the volute tongue region and exacerbates the risk of localized erosion at the outlet.