Prediction of the Total Dissolved Gas Downstream of Spillways Using a Two-Phase Flow Model

Abstract

One of the possible negative environmental effects of hydropower installations is the supersaturation of gas in the stilling basin, responsible for the bubble diseas e in fish. Large amount of bubbles are entrained when the plunging jet impacts the pool of water. These bubbles might travel deep into the water pool transferring mass to the liquid and causing an increment of the concentration of total dissolved gas (TDG) . Some numerical studies have been conducted in the past to predict TDG downstream of spillways, most of them based on experimental correlations for the gas volume fraction. A better approach to predict the TDG is possible using a two -phase flow model. This method is based on the two-fluid model to calculate the gas volume fraction and velocity of the bubbles. A transport equation for the TDG is solved considering the mass exchange with the bubbles. We assume one variable bubble size, which may change due to local mass transfer and pressure. The simultaneous solution of a bubble number density equation allows the prediction of the bubble size. The two-phase equations were implemented CFDShip-Iowa, a parallel, multiblock RANS solver developed at IIHR. Results of TDG, gas volume fraction, bubble number density are discussed. The numerical results of TDG are compared with available field data in the stilling basin of Wanapum Dam on Columbia River.