Computational analysis of flow field on cross-flow hydro turbines

A better understanding of the flow field of the cross-flow turbine (CFT) will be useful in its design and operation. As far as is known, no comprehensive study carried out relating to the effect of Reynolds number to turbulent shear stress, shear wall, energy kinetic turbulent, dissipation rate and Reynolds stress, and the occurrence of vortices around the runners of the CFTs. This study was designed to investigate the flow field in the nozzle and runner of the CFT using computational fluid dynamics (CFD) method. CFD methods were chosen because they can visualize detailed flow patterns that other methods cannot. The setups used in the CFD method such as two-dimensional unsteady simulations, six degrees of freedom features, shear stress transport k-ω turbulent model, and pressure-based solver. Based on results, for the nozzle, the shape of the velocity profile shows that the highest momentum flux occurs at the end of the nozzle, near the runner. Distribution of shear wall was highest at the base and tip of the nozzle; it was lowest at the centre. The turbulent kinetic energy profile at the nozzle was proportional to the turbulent boundary layer profile, Reynolds stress and eddy viscosity. This indicated that nozzle shape affects the momentum flux; therefore, good nozzle geometry can transfer the maximum water energy into the blade. The nozzle’s optimum geometry can be achieved by discharge and direction, optimizing velocity magnitude. This minimizes energy loss due to friction between the stream, vortex and mass of wasted fluid. For the runner, the highest turbulent kinetic energy, dissipations rate and Reynolds stress were located at the runner. Not all the water’s energy converted into mechanic energy because the part of that energy was used in mixing between water and air. The establishment of lift force on the active blades was not caused by the flow field that crosses the upper part of the blade, but by the momentum of water that hit the lower part of the blade. A vortex formed due to separation of the flow from the blade significantly affected the runner’s performance rather than rotational flow (air phase) in the CFT.