Measurements of heat transfer in a separated and reattaching flow with spatially varying thermal boundary conditions

Abstract

Abstract The effect of highly varying thermal boundary conditions on the convective heat transfer in a backward-facing step flow was investigated in a two-dimensional (2D) boundary-layer tunnel in air. A modified form of the transient heat transfer technique was used that allowed variable surface temperature distributions to be imposed by heating the test surface with a radiative lamp prior to cooling. The experiments were restricted to axial variations in surface temperature. Surface temperatures were measured with both cement-on foil thermocouples and thermochromic liquid crystal thermography. A semianalytic superposition technique was developed that was capable of predicting the heat transfer for any arbitrary axial surface temperature distribution, given the general solution or “Green's” function. The Green's function was measured to a limited spatial resolution by solving a simultaneous set of equations using data from 30 experiments. The techniques were developed and tested in a 2-D flat-plate boundary layer with Re δ2 of 3500 at the leading edge and qualified against a well-verified boundary-layer code. The Green's function and inverse Green's function were then measured in a turbulent backward-facing step flow with Re based on step height of 26,000. In the separated flow, it was found that the effect of localized heating extended only a short distance upstream and downstream, and that the heat transfer was less sensitive to temperature boundary conditions than would be expected from attached flow behavior. This corroborates the conclusion of previous workers that the primary resistance to the heat transfer is localized near the wall. This localized behavior, and the spatial insensitivity of the Green's function, suggest that with proper scaling, the results could be extended to other separated flows.