Internal Flow Analysis of Centrifugal Pump in Pump and Turbine Modes

Abstract

Energy is the basic need of any country for their economic growth. The electricity requirements of the world are increasing at alarming rate and the power demand has been running ahead of supply. It is also now widely recognized that the fossil fuels (i.e., coal, petroleum and natural gas) and other conventional resources, presently being used for generation of electrical energy, may not be either sufficient or suitable to keep pace with ever increasing demand of the electrical energy of the world. Still the electricity could not reach to many of rural areas. Thermal and large hydro plants cannot be installed in such rural areas due to their high initial cost and limited space. In small hydro power plants, centrifugal pump can be used as turbine because the cost of turbines are much higher than pump cost and operation of PAT is exactly reverse as Francis turbine.

In the present study, CFD analysis of single stage end suction centrifugal pump (Head: 21 m, Discharge: 29 kg/s) was done using commercial CFD package Fluent in pump and turbine mode. The 3-D computational model was discretized using unstructured tetrahedral grid and steady state simulations were carried out using Reynolds Averaged Navier Stokes equations. To consider the rotational effects of impeller, moving reference frame model was used and SIMPLE algorithm was applied as pressure-velocity coupling. The performance was analyzed at 7 different operating conditions using different turbulence models. The operating characteristics in both modes were predicted and validated with model testing results provided by the manufacturer. The maximum efficiency in pump and turbine modes were found to be 84.35% and 79.71% respectively. The cavitation analysis was done in pump and turbine modes in ANSYS CFX software. The study revealed that the suction side of the blades were more prone to cavitation and it resulted in somewhat decrease in efficiency. To improve the part load performance of PAT numerical experimentations were carried out to optimize the profile, angle and number of guide vanes. Among various options, 7 number of guide vanes (NACA 4414 profile) with 241 mm diameter impeller at an angle of 20° was found to be most appropriate. By provision of guide vanes the efficiency was improved in the range of 7.66% to 10.5% at different operating conditions.