EFFECT OF RAMP ANGLE ON AERATOR PERFORMANCE M.B. JAVANBARG Power and Water Institute of Technology, Tehran, Iran. Email: javan@pwit.ac.ir A.R. ZARRATI Assistant Professor, Department of Civil and Environmental Engineering, Amir Kabir University of Technology, Tehran, Iran. Email: zarrati@aut.ac.ir M.R. JALILI Ph.D student, Department of Civil and Environmental Engineering, Amir Kabir University of Technology, Tehran, Iran. Email: m.r.jalili@aut.ac.ir ABSTRACT Ramp angle is one of the most important factors, which affects the performance of aerators. Though the importance of ramp angle is mentioned in the literature, its effect on aerator performance has not been independently studied. In this experimental work, 3 ramps with different angles were installed on an aerator and in each ramp angle, air discharge into the aerator was measured in a range of flow discharges. Results of these tests showed that when the ramp angle was first increased, the air discharge also increased. However, further increase in the ramp angle decreased the air discharge. KEYWORDS: air entrainment-aerator-ramp angle-spillway-physical model INTRODUCTION Aeration is known as the most efficient and economical method for prevention of cavitation in high-speed flows over chute spillways (Pinto 1984, wood 1991). With 8% air near the concrete surface damage of cavitation attack is completely prevented (Peterka 1953, Zhang 1991). Surface aeration takes place in spillways but in this way usually not enough air is introduced near the concrete surface (May 1987). Therefore, with regard to the danger of cavitation attack, forced aeration of flow is recommended (Falvey 1990, Pinto 1991). Aerators, which locally create an air cavity at the lower boundary of the flow, have been found to be an effective and cheap way of promoting air entrainment into the flow. Aerators cause the flow to separate from the surface of the spillway and form a nappe (Fig.1). Air will then be entrained along the lower surface as well as along the upper surface of the nappe (Chanson 1991).The entrainment on the lower surface introduces air bubbles near the bed of the spillway for some distance downstream (Chanson 1989, Zarrati and Hardwick 1995). AERATORS CHARACTERISTICS AND MECHANISM OF AERATION Fig.1 shows the main flow regions above an aerator device. These flow regions can be named as: (1) the approach flow region, which characterizes the initial flow conditions over the aerator, (2) the transition zone which coincides with the length of the aerator ramp, (3) the aeration region along the nappe (4) the impact region that the flow is subjected to a change of momentum direction and to a rapid change of pressure distribution from a low negative pressure below the nappe to a high pressure at the impact point, and (5) downstream flow region.