Ski-jump-step energy dissipater hydraulics

Shangtuo Qian

Monday 29 june 2015

16:15 - 16:30h at Africa (level 0)

Themes: (T) Water engineering, (ST) Experimental facilities and instrumentation

Parallel session: 3F. Engineering instrumentation

With the explosive development of the hydropower projects, problems caused by high-speed flow become the focus of concern day by day. The ski-jump-step energy dissipater, combining the advantages of both the ski jump and stepped energy dissipaters, pre-aerates the flow through the bucket and aeration basin before entering the step section. Therefore adequate aeration and energy dissipation ratio are obtained and problems like cavitation, energy dissipation and atomization are well solved. According to the variation of flow states in ski jump and step section under different unit discharges, there are two types of flow regime in the ski-jump-step energy dissipater: nappe flow and aerated skimming flow. In the nappe flow regime, nappe flow occurs both in the ski jump and step section. In the aerated skimming flow regime, free jet flow occurs in the ski jump section, meanwhile, flow in the step section performs as aerated skimming flow. In this paper, theoretical analysis and physical experiments were conducted with 3 cases for the ski jump section, 3 cases for the aeration basin and 6 cases for the step section. It presented a review of the hydraulic characteristics in each section of the energy dissipater with different unit discharges, discussed the effect of energy dissipation, described the methods to prevent adverse flow states like water fin, outlet flow separation, subpressure occurrence and flow instability and provided a series of design specification for structure safety. The following conclusions were demonstrated: (1) comparing with the traditional stepped chute, the ski-jump-step energy dissipater has much higher energy dissipation ratio and its superiority increases with unit discharge; (2) for various step types, energy dissipation ration has close relationship with Sv/Ss, where Sv and Ss are the areas of flow recirculation and skimming flow respectively; (3) by arranging a ventilation shaft downstream of the bucket and a WES outlet for the aeration basin, adverse flow states are controlled; (4) by data analysis, the maximum unit discharge qwm = 183.99m2/s and the length of aeration basin lb should meet the equation as lb _ 5.517ls for safety consideration, where ls is the step length.

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