BOTTOM AIR CONCENTRATION OF AERATED CHUTES

摘要

High-speed flows on dams have a potential for cavitation damage of the concrete surface. Air introduced by so-called aerators is known to reduce those damages. These aerators are placed in regions where flow velocities are high and local pressures are low. Free surface aeration is normally not taken into account when positioning aerators. Moreover the air detrainment process downstream of the aerator is not yet fully understood. However, if the spacing between two aerators is too large, the air detrainment process reduces the bottom air concentration below the critical value required for chute protection. If the distance between two aerators is too short, super-saturation may occur, requiring uneconomically high sidewalls and increasing flow velocity due to reduced friction. An improved design guideline for aerator spacing is thus required. To understand the influence of various hydraulic parameters, an experimental approach is presented in this paper. A 14 m long hydraulic chute model with variable bottom slope built at the Laboratory of Hydraulics, Hydrology and Glaciology VAW, ETH Zurich, using remote controlled fiber-optical instrumentation was employed. Pre-aerated discharge was supplied to focus the air detrainment gradients det(C) and the bottom air concentration C_b, which is most important for cavitation protection. Results from selected measurements with a variable bottom slope between 10 % and 50 % and inflow Froude numbers up to 11 are discussed.