Free surface-pressurized flow in ceiling-sloping tailrace tunnel of hydropower plant: simulation by VOF model

Abstract

The ceiling-sloping tailrace tunnel is a new type of tailrace structure for underground hydropower stations and has been selected to replace the tailrace surge tanks in three large-scale hydropower plants in China. During the starting and load rejection conditions of turbine-generator units, the complex free surface-pressurized transient flow occurs in the tailrace tunnel. In this paper, a three-dimensional computational fluid dynamics research code with the volume of fluid (VOF)model is applied to simulate this special flow. After reviewing the ceiling-sloping tailrace tunnel structure and existing simulation methods, the mathematical models and numerical strategies are outlined. Then, the combined transient flow under a load rejection condition is simulated. Comparing with model test data, the flow patterns and pressure surge characteristics are analyzed intensively. The generation, development and breakup of the trapped large air bubbles in the interface region are also studied based on two-phase theory and the flow field results. It is shown that the flow is a special free surface-pressurized flow with a clear and regularly moving interface region, the interfacial large bubbles have no potential to generate large pressures harmful to the tunnel structure and the VOF model is a feasible and more advanced method to simulate this problem