Vortex evolution and energy dissipation mechanisms under inlet distortion in a vertical inline pump based on Liutex and multiresolution dynamic mode decomposition

Abstract

Inline pumps are widely used in space-constrained fluid transport systems, where energy efficiency and operational stability directly affect system safety and economy. The elbow-shaped inlet of a vertical inline pump can induce pronounced non-uniform inflow and alter the impeller inlet conditions. This study investigates a vertical inline pump with a specific speed of ns = 132 under three representative conditions, 0.8Qd, 1.0Qd, and 1.2Qd, using unsteady simulations combined with Liutex vortex identification, multiresolution dynamic mode decomposition (mrDMD), and entropy generation analysis. At 0.8Qd, backflow separation interacts with curvature-induced secondary flow, producing pronounced asymmetric inlet vortical structures and a strong broadband unsteady response. The integrated oscillatory modal intensity reaches 3.847 × 107, 83.49 times that at 1.0Qd, with the 97–388 Hz band contributing 96.33%. The total entropy generation is 1.391 W/K, the highest among the three conditions. At 1.0Qd, inlet backflow, flow distortion, and unsteady response are substantially weakened. At 1.2Qd, no pronounced backflow is observed, while the high-speed mainstream strengthens tip leakage, near-wall shear-related vortical structures, and wake effects. The mrDMD response is mainly concentrated near the blade-passing frequency. Total entropy generation decreases to 1.069 W/K, while the relative contribution of the impeller increases. Overall, part-load operation is characterized by broadband unsteadiness and high dissipation associated with backflow separation and secondary flow, whereas high-flow operation exhibits localized dissipation mainly associated with high-speed shear, tip leakage, and wake effects.