STEADY AND OSCILLATORY OPPOSING MIXED CONVECTION IN A SYMMETRICALLY HEATED VERTICAL CHANNEL WITH A LOW-PRANDTL NUMBER FLUID

Steady laminar and transient oscillatory mixed convection of a low Prandtl number fluid in a symmetrically heated vertical plane channel subject to an opposing buoyancy is investigated by solving the transient two-dimensional flow and energy equations. Results are particularly presented to illustrate the effects of buoyancy strength Gr/Re2 and Reynolds number Re. At a fixed Reynolds number, the flow is steady and unidirectional for a long time for a very small Gr/Re2. At a higher Gr/Re2, steady flow reversal is noted. An oscillatory flow with a single fundamental frequency is found when Gr/Re2 exceeds a critical value (which is about - 1.2 with Re = 500). Flow asymmetry appears at a certain time instant. Further increasing the opposing buoyancy, a secondary fundamental frequency sets in and the flow is quasi-periodic. The effect of increasing the Reynolds number is to stabilize the flow. Besides, in the initial transient the larger Reynolds number results in a longer duration in which a number of symmetric cells oscillate longitudinally. Finally, the transition of various flow patterns can be clearly distinguished by noting a sudden change of the average Nusselt numbers.