Effects of Upstream Wall Heating on Mixed Convection in Separated Flows

Measurements and predictions are reported for buoyancy-assisting, laminar mixed convection boundary-layer flow of air along a two-dimensional, vertical backward-facing step, in which the upstream wall and the step are heated to a uniform temperature while the downstream wall is heated to the same or different uniform temperature. The experiments were performed on a backward-facing step geometry, with a step height of 0.8 cm, over a range of freestream velocities 0.26 ≤ u∞ ≤ 0.41 m/s and a range of temperature differences 0 ≤ ΔT ≤ 34°C between the heated walls and the freestream. Laser Doppler velocimeter and cold-wire anemometer were utilized to measure, respectively, the air velocity and the temperature simultaneously. Flow visualizations were also performed to determine the flow reattachment length behind the backward-facing step. Reported measurements are limited to the case where the upstream wall and the step are heated to the same uniform temperature as the downstream wall, but numerical results are presented for the cases where these two temperatures are different and also for the situation when the upstream wall and the step are maintained as adiabatic surfaces. These results reveal that heating of the upstream wall and the step significantly affects the reattachment length, the velocity and temperature distributions, and the rate of heat transfer downstream of the backward-facing step