Author(s): Yameng Wang; Jun Deng; Wangru Wei
Keywords: Tmospheric pressure; Aerator; Air entrainment; Ventilation coefficient; Air-water flows
Abstract: Chute aerators are a measure to add air into the water flow to prevent downstream cavitation damage. With the development of hydraulic engineering in China, high-head dams are going to be built at high altitude areas of 3,000 meters and above. For hydraulic engineering constructions in high altitude areas, the effects of decreasing atmospheric pressure and rarefied air on the aeration efficiency are unknown. This may increase the hydraulic damage risk of high-speed flow in high altitude areas. In the present study, the effects of the atmospheric pressure on aeration performances in chute aerated flows are studied experimentally. Using a pressure-reducing tank, detailed subpressures in the air cavity and air entrainment discharges are measured for different atmospheric pressure conditions. The test results show that when the approaching flow Froude Number ranging from 4.47 to 6.93, as the atmospheric pressure decreases, the pressure difference between the inside of the cavity and atmosphere decreases gradually. This results in a reduction in the airflow velocity in ventilation shafts, and the air discharge decreases due to the atmospheric pressure decrease. The air discharge reduction can reach approximately 64.5% to 73.56% with an atmospheric pressure decreases from 96 kPa to 6 kPa. The effects of atmospheric pressure on the air ventilation coefficient and the air density variations are discussed. For the air density change due to the atmospheric pressure decrease, the ventilation coefficient gets reduced gradually from 0.76 to 0.31 with an atmospheric pressure decrease on the scope of experimental conditions, and an empirical relationship between the atmospheric pressure and ventilation coefficient is proposed. Compared to the normal atmosphere condition, the chute aerator of the same size has less air entrainment in high altitude areas, and the air entrainment capacity decays because of the lower atmospheric pressure. Therefore, the findings of experimental tests emphasized the improvement of the design of chute aerators and air supply systems in high altitude area.