Air Transport Caused by Chute Aerators

Abstract

Chutes with flow velocities in excess of some 20 m/s are typically prone to cavitation. In order to avoid damages, these flows are aerated using chute aerators. The current literature describes the efficiency of these aerators mainly in terms of the air entrainment coefficient as the ratio of the entrained air and water discharges. However, this is a global coefficient neither specifying the precise air distribution in the flow nor its detrainment rate. As cavitation damages occur along the chute boundaries, the associated air concentration is of prime interest. The present investigation focuses on the flow structure and the air transport downstream of chute aerators. Systematic hydraulic model tests were conducted including a data analysis of the spatial air concentration distribution in the near and the far fields downstream of chute aerators. Based on these and other measurements, general air transport zones were described. Three flow zones were introduced, namely the: (1) Jet zone; (2) Re-attachment and spray zone; and (3) Far-field zone. It was further found that aerators have primarily an effect on the average air concentration, whereas they increase the bottom air concentration only slightly. A large de-aeration gradient was found near the bottom downstream of the re-attachment point. It was concluded that the bottom air concentration downstream of chute aerators is smaller than generally assumed. Nevertheless, these air concentrations obviously suffice to inhibit cavitation damages on spillways equipped with aerators.