An experimental study of laminar heat transfer downstream of backsteps

Heat transfer in a laminar air flow past a heated backstep is investigated experimentally in the 20 x 15 x 61-cm working section of a low-speed wind-tunnel of contraction ratio 12.5:1 by Mach-Zehnder interferometry. Top-wall steps with channel-expansion ratios 1.06, 1.03, and 1.02 are examined, and additional pressure, temperature, velocity, and (smoke-injection) reattachment-point determinations are obtained. Typical interferograms are given, and the results are presented in graphs and tables. Heat transfer downstream of the step is found to increase monotonically in the streamwise direction but to remain less than the flat-plate value (at most 56 percent for the 1.06-expansion-ratio step), as predicted by the cavity theory of Chapman et al. (1958). Smaller steps are shown to give smaller values of heat transfer, and an expression for the average transfer is derived.