Numerical Simulation of Buoyant Pulsating Exchange Flow through Circular Opening in Horizontal Partition

Abstract

An interesting transport phenomenon is observed through openings between two compartments separated by a thin, vented, horizontal partition such as those between containment internals in nuclear power systems, in industrial installations in event of fire, passive cooling of heated structures and in natural building ventilation. A heavier fluid located on the top of a lighter fluid and separated by a horizontal vent constitutes a gravitationally unstable system. Horizontal vents produce flow, which are unstable with irregular oscillatory behavior. The objective of the present work was to simulate such type of flow across a circular opening in horizontal partition in presence of buoyancy force. Unsteady, axisymmetric Navier-Stokes equations have been solved with Finite Volume Method. The equations were solved using the in-house CFD code based upon the well-established pressure-based finite volume methodology. In terms of temporal differencing second order accurate Crank-Nicolson scheme was used. Interpolation to cell faces for the convective terms was performed using a third order QUICK scheme, second order central differencing was used for the viscous terms. Pressure-velocity coupling was based on the SIMPLE procedure. The upper chamber was filled with salt water and the lower chamber with fresh water, creating a density differential between the two chambers. Opposing forces at the interface created a gravitationally unstable system, and an oscillating exchange of fluid developed. Three different cases for vent length to diameter ratio (L/D) 0.008, 0.0376 and 0.106 from a reported experiment were examined. The pulsation frequencies and their decay with time have been determined. The flow coefficients were computed and compared with experimental results.