Influence of Nonflow Zone (Back Cavity) Geometry on the Performance of Pumps as Turbines

The larger objective of this research comes from the fact that optimization studies in “pumps operated as turbines” have concentrated only within flow zones without any physical perception regarding the influence of nonflow zones such as back-cavities in standard end-suction pumps. Four pumps of different designs are selected and their back cavities are reduced by inserting solid material, leaving a very small axial clearance. The effects are investigated on an experimental platform, which reveal unique phenomena taking place. The first is associated with the reduction of expected disk friction (hence improvement in shaft torque), while the second is more intricate considering the effect on fluid momentum through reorganization of tangential velocities, based on the mixing zone theory proposed in the paper. The net effect of reducing the volume of nonflow zones (i.e., filling of cavity) is the enhancement of efficiency in the range of 1.3% to 3.6% (±0.4%) in turbine mode. The experimental disk friction coefficient as a function of blade Reynolds number is corroborated with the established theory proposed by different researchers. A significant phenomenon observed was the elimination of vibration and noise at overload operating conditions with the minimal axial clearance.