Multidimensional Simulation of the Two-Phase Flow Downstream of Spillways for Total Dissolved Gas Prediction

Abstract

Elevated supersaturation of the total dissolved gas (TDG) concentration in water may cause gas bubble disease (GBD) in fish. Spillway discharges at hydropower dams are the main source for TDG supersaturation in the Columbia and Snake basins in the Northwest. Since the most important source for TDG is the mass transfer from the bubbles, a proper model for TDG prediction must account for the two-phase flow found in the stilling basin. Some numerical studies had been conducted in the past to predict TDG, most of them relying on experimental models for the gas phase distribution. A better approach is possible through the use of mechanistic models that rely less on empirical information. We use a mixture two-phase flow model to predict the gas distribution, TDG concentration, and bubble number density on the stilling basin. The method used in this work assumes one variable bubble size, which may change due to mass transfer and local pressure. The equations of the proposed model were implemented into the commercial code FLUENT through UDS’s and UDF’s. The multidimensional fields of TDG, gas volume fraction, bubble sizes and velocities of the bubbles are presented and discussed. The TDG concentration downstream of spillways was calculated and compared against available field data. Effects on the overall flow caused by the aeration introduced by the spillways are discussed.