Hydrodynamic Performance Enhancement of a Lift-type Vertical-axis In-pipe Water Turbine Using a Flow Deflector: A CFD-based Parametric Study

The use of existing water distribution infrastructure for micro-hydropower generation offers a practical pathway toward decentralized and low-carbon energy production. Lift-type vertical-axis water turbines (VAWTs) are particularly attractive for in-pipe applications because they operate independently of flow direction and can achieve relatively high efficiency at moderate rotational speeds. However, their performance is often constrained by unsteady flow behavior, torque fluctuations, and associated pressure losses in confined pipeline environments. This study numerically investigates the effectiveness of a stationary flow deflector in enhancing the hydrodynamic performance of a lift-type vertical-axis in-pipe water turbine. Three-dimensional unsteady computational fluid dynamics (CFD) simulations were conducted to evaluate turbine operation with and without a deflector under gravity-fed pipeline conditions. The effects of blade number and tip-speed ratio were systematically examined. Key performance indicators, including instantaneous and time-averaged torque, power output, pressure drop, and hydraulic efficiency, were quantified and compared. The results show that the introduction of the flow deflector significantly improves flow guidance toward the windward blades, leading to stronger lift generation and reduced flow separation. Across the investigated operating range, the deflector-assisted turbine achieved torque increases of approximately 20-30% and power output improvements of up to 30-40% relative to the baseline configuration without a deflector. Peak hydraulic efficiency was observed at moderate tip-speed ratios, with efficiency gains of approximately 15-25%. At the same time, the additional pressure loss introduced by the deflector remained limited, typically below 5% of the equivalent pressure head. Furthermore, torque fluctuations were noticeably reduced, indicating more stable turbine operation. These findings demonstrate that flow deflectors can effectively mitigate the unsteady hydrodynamic limitations of lift-type in-pipe turbines while preserving acceptable pressure losses, providing new design insights for micro-hydropower energy recovery in water distribution networks.