Pressure and heat transfer characteristics in a backward-facing step flow with a large expansion ratio

Abstract

The effect of the expansion ratio on a backward-facing step flow is investigated experimentally. The expansion ratio ER ranges from 2.0 to 5.0. The Reynolds number based on the mean velocity and the hydraulic diameter of the inlet channel ranges from 2.0 × 10³ to 7.0 × 10³. This Reynolds number range includes laminar, transition and turbulent flows. The forward flow fraction, the wall static pressure coefficient and the local Nusselt number on the lower wall behind the step are measured. When the position of the reattachment point is nondimensionalized by the step height, it moves to the step side as the ER increases in the laminar region. However, the reattachment length is approximately the same in the turbulent region. For all Reynolds numbers, the pressure loss increases and the Nusselt number decreases as the expansion ratio increases. When the upstream flow transitions from a laminar to a turbulent flow, the pressure loss coefficient increases and the maximum Nusselt number decreases for all expansion ratios. For ER = 2.0, the position where the maximum Nusselt number appears is downstream of the reattachment point in the laminar flow and is approximately the same in the turbulent flow. In the case of ER = 4.0 and 5.0, the position is uptream from the reattachment point and is close to a constant value, regardless of the Reynolds number.