Investigation of the effect of gaps between the blades of open flume Pico hydro turbine runners

This study analyzes the impact of gap size for two reaction turbine runners, to provide recommendations for the maximum of design and manufacture of open flume turbines. Two number of blades were tested: five blades (Runner A) and six blades (Runner B). Three methods were used for this investigation: the theoretical analysis to design the turbine, experiments to determine the actual turbine performance and to validate the design and the manufacturing processes, and computational fluid dynamics to study physical phenomena and re-check runner velocity triangles. From the results, it can be seen that gaps between blades can alter the velocity vector at the outlet, thus they can unbalance the runner rotation; this could cause cavitation as a result of the decrease in the velocity of the water on the outlet with the increase in pressure. This pressure increase causes the draft tube not to function optimally, and, consequently, the torque decreases because the water pressure in the draft tube approaches the atmospheric pressure. Two conditions must be satisfied to maximize the turbine performance: minimizing the radial velocity at the outlet by flow after passing through the turbine is made swirl; and avoid forming gaps between the blades.