Optimization Of Inlet Pier Type And Study On The Measures Of Air Entrainment And Cavitation Alleviation Of High Head Of Free-flow Tunnel

With the continuous emergence of high dam projects,the waterhead of the discharge structure is also increasing.For high head of free-flow tunnel,Adding the middle-pier in the entrance section,can overcome the limitation of working gate in designing,manufacturing and operation,but also can bring harmful water-wings in the middle-pier tail,impacting the roof and side wall;With the continuous improvement of the running head of the discharge tunnel,the maximum flow velocity is more than 25.00m/s,which is vulnerable to cavitation damage.The practice has proved that the technology of air entrainment and cavitation alleviation is the most effective method to solve the cavitation damage.The main contents of this paper include two aspects: structural optimization of middl-pier and study on the measures of reducing cavitation by aeration.The research results are as follows:(1)on the basis of the model experiments and observations,the traditional vertical piers in the upstream reservoir water level at all levels,have caused the strong harmful water-wings at the end of the pier combined,and constantly impact the roof and side wall.the flow pattern of tunnel body is poor,bringing a threat to the safe operation of the discharge tunnel.(2)Based onthe design idea of the middle-pier in the discharge tunnel of Sanbanxi,a new kind of middle-piers,called as energy distribution uniformity type,was designed,which can reduce the impact force caused by the dispersion of water flow gradually from top to bottom along the middle-pier wall.Based on the model experiments,the middle-pier of energy distribution uniformity type,could weaken water-wings efficiently,and improve the flow pattern in the discharge tunnel.(3)Combined with the actual working head and the application requirements in partial opening of 1# discharge tunnel in Hekou Village Reservoir,according to the experience of the construction projects,for Eccentric-hinge Radial Gate,the Sudden Lateral Enlargement and Bottom Drop Section is set up.According to the empirical formula to estimate the parameters of the hydraulic parameters,combined with the model experiments observation and analysis,the design of the Sudden Lateral Enlargement and Bottom Drop Section is reasonable,fully aerated,and can achieve the purpose of air entrainment and cavitation alleviation.(4)Observation and analysis by the model experiments,adding water guiding plate in working gate after both side walls,can solve the problem of water-wings impacting the gate hinges,in the upstream reservoir water level at all levels,and improve the flow pattern in the discharge tunnel.