EFFECT OF CIRCULATION ON CRITICAL SUBMERGENCE FOR PUMP INTAKES

Abstract

Strong vortices which entrain air reduce the carrying capacity of intake. Air entrainment in pump and power plant intakes causes cavitation, leading to vibration in turbine or impeller reducing their efficiency and life span. The submergence depth at which incipient air-entrainment occurs is called critical submergence. The accuracy of existing relations for determining the critical submergence for intakes in a sump or forebay with circulation is low and they do not cater for Froude number less than 1.1. Since large size intakes at power plant operate at Froude number less than1/3, experimental investigation was taken up in the present study to obtain a relation for critical submergence for low range of Froude number. Theoretical approaches do not consider the effect of viscosity and circulation. The experimental setup used for the study consists of rectangular channel draining into cylindrical sump. The walls of the tank were made of Perspex sheet to observe the air entrainment. A vertical downward intake was fixed at centre of bottom surface in the tank. Intakes of different diameters were used. Asymmetry of the channel with the sump creates circulation in the sump. Experiments were carried out using water for all runs. The circulation in the tank was calculated by measuring the tangential velocity in the sump using Acoustic Doppler Velocimeter (ADV). The condition of incipient air-entrainment in steady vortex flow at vertical intake pipe was obtained by observing the vortex at various submergences. The partial correlation of dimensionless Critical submergence show that for low Froude number the critical submergence depend more on Froude number followed by Circulation parameter and Viscosity. The present study reveals that for low range Froude number critical submergence increases with increase in circulation. The proposed predictor for critical submergence for Froude number less than1.1. predicts with in error of + 20%. The analysis of the existing data reveals that for zero circulation flow field critical submergence is exclusively a function of Froude number. A new predictor for Froude number greater than 1.1 has been proposed which predicts the critical submergence with in error of + 25% better than the accuracy of existing relations. The proposed formulae can be used for determining the minimum submergence requirement for an intake.