Nozzle Opening Angle Optimization on Submerged Pico Hydropower Banki Turbine

Abstract This study investigates the influence of nozzle opening arc angle on internal flow recirculation and performance of a Banki turbine using two-dimensional CFD. Using ANSYS Fluent with a VOF multiphase formulation and a four-equation SST transition turbulence model, transient simulations were performed on a previously optimized turbine geometry for nozzle arc angles of 50°, 80°, 90°, 100°, and 110°. Mesh sizes ranged ∼60,000–80,000 elements; boundary conditions included a total-pressure inlet (18,688.05 Pa) and pressure outlets representing vertical free-surface and horizontal discharge. Convergence used residuals of 1e-3 with up to 200 iterations per timestep. Results show a clear optimum at a 90° nozzle arc: peak efficiency ≈70.5% at 300 rpm and a predicted maximum of 71.07% near 325 rpm, compared with ∼59.98% for the existing 50° nozzle. Larger arcs increase the number of effective energy-converting blades (from six to eight), but excessive discharge at 110° reduces efficiency. Flow-field analysis attributes the 90° advantage to reduced recirculation eddy viscosity, a more isotropic vortex core, and lower turbulence kinetic energy dissipation near the second-stage blades. The optimized design aligns with several prior studies and narrows the gap with experimental findings (≈2% difference for the 90° case). These CFD results support adopting a 90° nozzle arc to minimize internal recirculation losses and improve Banki turbine performance.