FLOW PAST SPILLWAY AERATORS

Abstract

Cavitation erosion of spillway surfaces may be prevented either by improvement of the surface quality or by aeration of the flow. Experience has shown that protection of the spillway surface from cavitation erosion with good quality surface finish is no longer economical at velocities greater than 20 m/s. Further, such attempts have not been completely.successful either. This makes it necessary to aerate su.Ch flows. A characteristic of high velocity flows over a spillway is the phenomenon of self-aeration in which turbulence induced at the - intersection of the boundary layer with the water surface will entrain air from atmosphere into the flow and mix with it. The entrained air from the free surface may protect the spillway surface if it provides sufficient air concentration near the bottom. A concentration of about 5% near the boundary is considered adequate to prevent cavitation. If there is not enough surface aeration to achieve such a value, air can be artificially introduced into the flow by devices called aerators. This process is known as forced aeration. The present study is aimed at understanding clearly the distribution of air concentration of self-aerated and forced aerated flows. A series of experiments was carried out by the writer on both such flows in the Hydraulics Laboratory of the University of Roorkee. A deflector with a groove was used as the aerator assembly in these studies. The data collected by the writer have been used along with those of the earlier investigations in this thesis. Firstly the available data on air concentration in selfaerated flows have been analysed to evolve a quantitative criterion to distinguish between the developing and developed regions of the flow. Subsequently, the analysis of Rao et al (1970) has been extended to obtain empirical relations for the various scaling parameters used therein, thereby making calculation of the air concentration distribution possible for any given flow condition. The data on flow past aerators were analysed with the objectives of (i) determining the air demand of different types of aerators and (ii) predicting the distribution of air concentration downstream of these aerators. Air demand of different aerator geometries using extensive laboratory data has been analysed and this has resulted in a relationship among aerator geometry, flow conditions and pressure in the aerator. This can be used for the design of the air supply system, as indicated in the thesis. The flow downstream of an aerator has been divided into different regions. In each of these regions a portion of the flow near the surface has the characteristics of self-aerated flow. The depth below the foregoing zone is one of forced aeration. A (ii) similarity analysis including predictions of the scaling parameters has been carried out for this zone in all the regions using the available data. This would enable determination of the distance from the aerator where the near boundary concentration is less than 5% and , as such, the appropriate place for locating the next aerator in the series. (