CRITICAL SUBMERGENCE FOR HORIZONTAL INTAKES

Abstract

Air entrainment by means of a free air-core vortex occurring at a water-intake pipe is an important problem encountered in hydraulic engineering. When the submergence of the intake pipe is not sufficient, air enters the pipe and decreases the intake's efficiency. The submergence depth at which incipient air entrainment occurs at a pipe intake is called the critical submergence. As vortex formation is a surface phenomenon, similarity of Froude number (F) in model and prototype is essential to ensure dynamic similarity. Dagget and Keulgan (1974) reported that in the range of Weber number (W) between 615 to 9000 it _has no effect on critical submergence. Based on experimental study, Jain et al (1978) concluded that there is no influence of surface tension on critical submergence when W>120. Hughes (1975) contented that air entrainment should depend on surface tension. Odgaard (1986) has shown that in case of air entraining vortices in a still water body, for W>720 and Reynolds number (R) greater than 1.1 x105, the effects of surface tension and viscosity can be neglected. Padmanabhan and Hecker (1984) proposed for W > 600 and R> 7.7 x 104 for neglecting the surface tension and viscosity effects. Many empirical relations have been given by different investigators for estimating the critical submergence for vertical intakes in presence of uniform flow but only little effort has been made to study the critical submergence for horizontal intakes in the presence of uniform flow. Yildirim and Kocabas (1995) derived an equation for the critical submergence of an air entraining vortex occurring at a vertical intake in a uniform flow by using potential flow theory and the theory of Critical spherical sink surface (CSSS). They showed that the results of the derived equation for critical submergence compared well with the experimental results. Thus the potential of potential theory and the theory of CSSS can be used to study the critical submergence for horizontal intakes in the presence of uniform flow. The objective of the present study is to predict the critical submergence at horizontal intakes for the different flow conditions by considering the potential flow theory and the theory of CSSS. Experiments were performed to validate the analytical equation for 111 critical submergence for bottom clearance equal to zero and half of the diameter of intake. Analysis of experimental data reveals that Froude number and ratio of intake velocity and uniform flow velocity are the dominant parameters, which affect the critical submergence. Based on the statistical analysis, relationship for critical submergence for bottom clearance equal to zero and half of the diameter of intake have been proposed and the same is validated with the experimental data. Existing relationships for critical submergence have also been checked for their accuracy using the experimental data. The proposed relationship gives more accurate prediction of critical submergence compared to the exiting relationships.