Prediction Of Total Dissolved Gas Downstream Of Spillways Using A Multidimensional Two-Phase Flow Model

Abstract

Elevated supersaturation of total dissolved gas concentration in water (TDG), which may cause gas bubble disease in fish, constitutes an important negative environmental effect of dams. Spillway discharges at hydropower dams are the main source for TDG supersaturation in the Columbia and Snake basins in the Northwest USA. The most important source for the TDG is the gas transferred from the bubbles, therefore a proper model for TDG prediction must account for the two-phase flow generated in the stilling basin. Most of the numerical studies on TDG downstream of spillways found in the literature are based on experimental correlations for the gas volume fraction. A better approach involves the use of a multiphase flow model that rely less on empirical information. In this work, an algebraic slip mixture model that accounts for the drag and turbulent dispersion forces and employs the modified k epsilon- µ model for the turbulence is used to calculate the gas volume fraction and velocity of the bubbles. A bubble number density transport equation is implemented to predict the bubble size, which can change due to bubble/liquid mass transfer and pressure. The TDG is calculated with a two-phase transport equation whose source is the bubble/liquid mass transfer which is a function of the gas volume fraction and bubble size. The equations of the proposed model were implemented into the commercial code FLUENT using the available multiphase flow algorithm based on the finite-volume method. The multidimensional fields of TDG, gas volume fraction, bubble sizes and velocities of the bubbles are presented and discussed. Quantitative agreements between the numerical results and field data for the TDG in the stilling basin of Wanapum Dam on the Columbia River are obtained.