Effects Of Transition Stair With Different Size Steps On Hydraulic Characteristics Of Stepped Spillway

The application of integrated energy dissipater (wide tail pier+stair overflow dam+stilling pool) makes new breakthrough in high-velocity flow problems caused by large discharge per unit width and higher water head. As the increase of unit discharge and water head, such problems as aeration, cavitation and damage and energy dissipation ratio become increasingly prominent. In order to solve the above problems, it is necessary to create aeration conditions in nappe bottom and overflow dam, in order to increase aeration cavity and density. In actual engineering application, aerator and transition stair will be set between the connection of WES curve and stair overflow dam, in order to add the aeration effect and change the pressure features and energy dissipation characteristics. Based on overflow surface outlet in Areva power station, the paper uses water vapor VOF to calculate RNG k-ε Turbulent flow numerical model. Conduct 3D numerical simulation on transition stair with different sizes. The paper discusses the influence of different combination schemes on dam aeration characteristics, pressure distribution, cavitation and damage, energy dissipation mechanism, etc, in order to further provide reference for perfecting integrated energy dissipater (wide tail pier+stair overflow dam+stilling pool). The main research achievements are shown as follows:(1) The combination scheme of transition stair with different steps has little influence on dam aeration cavity and high influence on the area of dam aeration cavity. With the increase of step size of transition stair, the stair overflow dam aeration cavity will increase gradually. The dam aeration will be more sufficient and prevent the stir dam from cavitation and damage.(2) The length of stair overflow dam surface increases with the increase of step size of transition stair, and the aeration density of stair overflow dam tends to decrease as a whole, while the fluctuation is huge. The aeration density in each scheme will reach a minimum in the 10-14 step. The aeration density distribution on the level wall of each stair has different distribution rules, while the aeration density of level wall tends to increase as a whole.(3) Each scheme will generate local negative pressure in aeration cavity. Except the negative pressure of transition stair consisting of 12 steps (height×width =0.5m×0.375m) is distributed in the former several ladders, the negative pressure of remaining schemes occurs in the first step. The pressure on vertical wall and level wall has the same distribution rules. The maximum negative pressure basically occurs in the same position (22.56m). and then the pressure will increase gradually. The maximum negative pressure of vertical and level wall of transition stair consisting of 6 steps (height×width=1m×0.75m) is respectively -4.469kPa and -3.898kPa, which is higher than other schemes, indicating the increase or decrease in step size of transition stair. It is conductive to decreasing the negative pressure of wall.(4) The energy dissipation rate will increase rapidly with the increase of step size of transition stair. The energy dissipation rate in each scheme has the same change rules and increases gradually along with the stilling pool, while the growth rate will decrease gradually. The energy dissipation rate of tradition stair scheme consisting of 2 steps (height×width=3m×2.25m) will reach the maximum. The energy dissipation rate increases 55.86% in the last section from 29.79% in the first section, which is higher than other schemes. The first step of transition stair has maximum influence on the energy dissipation rate of overflow dam.In summary,The tradition stair scheme consisting of 2 steps (height×width =3m×2.25m) Is the best option.