Influence of Draft Tube Diffusion Angle on the Hydraulic Performance of a Double-suction Centrifugal Pump as a Turbine

Pumps as turbines (PATs) are widely applied in micro-hydropower stations and chemical industries as economical and efficient energy recovery devices. This study used numerical simulations and entropy dissipation theory to investigate the influence of four draft tube diffusion angles (β0, β1, β2, and β3) on the hydraulic performance and energy losses of a double-suction centrifugal PAT. The results show that the diffusion angle exhibits a complex nonlinear effect on head, efficiency, and shaft power across different flow rates. Under low-flow conditions (0.6Qd), β2 reduces efficiency by 9.27%, whereas under high-flow conditions (1.4Qd), β3 improves efficiency by 1.01%. Entropy analysis reveals that total entropy generation follows an approximately parabolic trend with flow rate, reaching its minimum near the design condition. Among the schemes, β1 and β3 effectively reduce total entropy generation, with β3 achieving a 5.51% reduction under high-flow conditions. The impeller is identified as the dominant source of energy loss (approximately 53% of entropy production), followed by the draft chamber (approximately 30%). Further analysis indicates that entropy generation in the draft chamber primarily arises from turbulent dissipation and wall friction and is highly sensitive to the diffusion angle. Under high-flow conditions, β3 reduces draft chamber entropy production by 7.44%, whereas under low-flow conditions, larger diffusion angles increase entropy production. Thus, optimizing the diffusion angle not only improves impeller flow conditions but also effectively reduces system energy losses, particularly under high-flow and off-design operating conditions. This work provides theoretical insights and engineering guidance for the optimized design of PAT draft tubes.