Laminar coupled flow downstream of an asymmetric sudden expansion

The effect of the expansion ratio and temperature varying viscosity on the hydrodynamic and heat transfer Characteristics of the laminar flowfield over a backward facing step is studied numerically. Both walls are maintained at uniform but different temperatures, while the flow at the step is assumed to be hydrodynamically and thermally fully developed. Calculations are carried out for different Reynolds numbers in the laminar flow regime and for a Prandtl number of 5.3. It is shown that an increase in the expansion ratio results in the augmentation of the heat transfer coefficient along both walls, the augmentation becoming substantial at high expansion ratios. Empirical correlations are obtained for the average Stanton number in the recirculating zone as a function of the Reynolds number and the expansion ratio. The variation of viscosity with temperature is shown to have a substantial effect on both the reattachment length and the local Nusselt number distribution along both the step and flat walls, the effect becoming stronger as the Reynolds number increases.