Formation Mechanisms And Scale Effects Of Hydraulic Intake Vortices

The vortex have been observed frequently at hydraulic intakes such as hydroelectric power station, pumps, navigation locks, flow and level regulation and withdrawal from sumps in Emergency Core Cooling Systems (ECCS) of nuclear power stations. Movement forms of vortex are complicated including large-scale quasi-stability vortex and small-scale stochastic vortex. These two kinds of vortex could be generally classified as surface dimple, vortex pulling air bubbles to intake and full air-core to intake.The presence of air-core vortex at an intake can cause discharge reduction, increase the fluctuation of hydraulic structure, reduce pumping efficiency, and in extreme cases, completely disable the intake. Despite the universal familiarity of the problem, existing literature provides little on the detailed mechanics of vortex formation and mathematical simulation. No agreement has been reached referring to the scale effects on modeling vortex. An analytical, computational, and experimental study was conducted to describe formation mechanisms and scale effects of vortex at hydraulic intakes. The principal contents and achievements of this dissertation are as follows:(1) The theoretical analysis of quasi-stability vortex at intakes was completed. The reason for vortex stretching at the bottom and the side intakes was explained. According to Rankine's combined vortex model and Cascade structure of energy, free surface vortex energy-balance process and the quasi-stability vortex forming, developing and varying were probed.(2) An experiment was conducted with turbulent strength distribution, wave height and fluctuating pressure at inflow section to study mechanics of stochastic vortex formation. Experimental study of flume shows that the variation of the number of stochastic vortex with time in the intake has the characteristics of turbulent coherent structures.(3) Based on the experimental study, formation mechanizes of stochastic vortex was completed. It is indicated that stochastic vortex near the intake are caused by short-period wave induced by turbulent strength on the water surface.(4) Experiments were conducted using serial models of scale1:1, 1:2, 1:4 to determine whether scale effects distort the predictive ability of hydraulic models. The relational expression between the formed type of vertical vortex and the intake Froude number and Reynolds number is founded. On the certain geometric boundary conditions, the quantitative relationship between flow increasing to overcome scale effects and submergence and model scales was provided.(5) The calculated results of standard k -ε, RNG k -εand Relizable k -εturbulent models with VOF model are compared. The results show that RNG k -εmodel is more suitable than standard k -εand Relizable k -εmodel for the vortex as the rapidly strained and the great curving streamline flows. The RNG k -εare applied in an actual project to simulate the change of free surface with the reservoir level under the pumping condition and flow characteristics. The simulated results are consistent with the experimental results.