VORTEX FORMATION AT VERTICAL PIPE INTAKES

Abstract

The formation of vortices is a phenomenon of frequent occurrence at intakes operating under certain combinations of submergence and discharge. Strong vortices which entrain air reduce the carrying capacity of the intake and cause many other undesirable effects like loss of efficiency of hydraulic machines} vibrations, increased head loss and increased susceptibility to cavitation. It is therefore apparent that intake installations should be designed and operated in such a way that the formation of strong vortices is eliminated. In order to predict the safe submergence of intake for given discharge, recourse has usually to be taken to hydraulic model studies. Vortex formation at intakes being a free-surface phenomenon* many hydraulic engineers suggest that similarity of Froude number is essential for dynamic similarity, although different opinions have been expressed by many investigators(1 ,10 ,11 ,18,20 J22i3if) t As such this investigation was taken up with the object of providing a clear picture of the parameters influencing vortex formation at intakes > thereby enabling in setting out the criterion for hydraulic modelling of the phenomenon. Experiments were carried out in two geometrically similar cylindrical vortex tanks each installed with vertical pipe intakes centrally located in the tank bottom. Pumped outflow, which was recirculated back into the tank, was used. The circulation was generated at a large radial distance from -iiithe intake by means of a number of guide vanes. The experi ments were carried out using liquids of different viscosities and surface tensions. The viscosity of the liquid was varied by mixing varying amounts of CEPOL (chemical name being Carboxy-Methyl Cellulose) to water, whereas addition of iso-amyl alcohol to water enabled variation of surface tension. The experiments were also carried out for radial flow by placing the vanes in the radial direction. The condition of incipient air-entrainment in vortex flow at the vertical pipe intake (for a given liquid and given geometry) was obtained by observing the vortex at various submergences for a constant discharge. Such observations were made for different discharges, liquids, intake sizes and circulations. The analysis of data so collected showed that within the range of experiments , the__surface tension did not affect the critical submergence. The change in the direction of circulation also did not affect the critical submergence. The analysis enabled determination of condition for the onset of air-entrainment in vortex flow and the dimensionless critical submergence was found to depend on the circulation parameter, viscosity parameter and the Froude number. The influence of viscosity on vortex formation has been quantitatively determined and correction for distortion due to change in Reynolds number evaluated. Condition for incipient air-entrainment in radial flow has also been determined. The analysis of available experimental data on horizontal intake(1) showed that the condition for the onset - 1Vof a4r air-entraining vortices depended on the circulation parameter and the Froude number but the relationship was slightly different from that determined for the vertical intakes. The information made available as a result of this investigation is useful in correctly interpetting the results of hydraulic model studies on vortex formation at intakes.