Numerical Studies on Pump Running in Turbine Mode

Abstract

The objective of the present study is to carry out low cost modifications on pump as turbine (PAT) viz. blade leading edge rounding, blade trailing edge sharpening, impeller eye enlargement, etc. In the present study, numerical simulations were carried out with the commercial package ANSYS-CFX to predict the performance characteristics of PAT under various flow conditions. The three dimensional computational model was discretized with tetrahedral and hexahedral grid elements. Boundary conditions used in the simulation were mass flow rate at the casing inlet and total pressure at the outlet of draft tube. Single precision version of the ANSYS-CFX solver was used considering the available computational facilities. Residual target of RMS 0.001 was given as a constraint to get the solution. Modelling of the turbulence effects were carried out using three turbulence models viz. Baseline Reynolds Stress, k-" and Shear Stress Transport turbulence models. Among these models, k-" model gave least variation with the experimental results. Various features like velocity contours, pressure contours, velocity vectors and streamlines were used to trace the flow behaviour in the PAT. After carrying out blade leading edge rounding modification, the part load efficiency rise of 0.12% was obtained in comparison to the base model. This may be due to decrease in flow separation losses. Blade trailing edge sharpening led to efficiency increament of 5.6% in comparison with the base model. This may be due to decrease in flow sepration losses at the round trailing edge. The modification of impeller eye enlargement led to an efficiency rise of 4.1 % in comparison with the base model. The reason could be decrease in turbulence losses on account of decrease in kinetic energy of fluid due to increase in flow area. In the present study, force analysis was carried out at different mass flow rates. Pressure force and viscous force acting on the impeller were taken into account and force vector distribution on the impeller area was studied by polar plots. The maximum force was found to be acting in the third quadrant of the casing in most of the cases. Moreover, static structural analysis of PAT casing was carried out and found that there are scope for reduction of casing thickness.