Author(s): Sarai Diaz; Javier Gonzalez
Linked Author(s): Sarai Díaz García
Keywords: Irflow; Closed conduits; Free surface flow; CFD
Abstract: Shaft spillway or dam outlet works often produce high velocity water flow, which is conducted through a tunnel, where open-channel flow occurs. Into the tunnel, airflow is induced by water movement, which must be allowed by suitable air vents. Therefore, the induced airflow quantification is necessary for air vents’designing. Airflow in a partially filled conduit may occur in two forms: free air flowing above the water surface because of momentum exchange, and air flowing in small bubbles dragged into the water flow, with different air-water concentrations that depend on flow characteristics. Both kinds of airflow are estimated usually by empirical expressions or charts. Soft analytical procedures are thus available but depend on gross assumptions. Computational Fluid Dynamics models allow modeling both air and water flows occurring into closed conduits. This work presents the application of a CFD code to the problem of modeling water flow occurring downstream a shaft spillway gate through a tunnel, and the induced airflow movement above the water surface. The numerical model is implemented in Ansys CFX 12. 0and computed as a steady state simulation. The geometry is simplified as 2D using a hybrid grid composed mainly by hexahedral cells. The k-εmodel is used to predict turbulence caused by rough walls, and a High Resolution Scheme is used for discretization of the convective terms. In regard to the multiphase option, a homogeneous free surface model is adopted to reproduce the airflow within the tunnel in order to balance the computational expense of the simulation and the accuracy of the results. The application demonstrates the ability of this kind of tools to reproduce the composed two-phase flow patterns, and therefore its potential use in airflow quantification. Nevertheless, some limitations in the formulation of this CFD models are pointed out, which should be addressed in order to improve results accuracy.
Year: 2014