HEAT TRANSFER AT TURBULENT SEPARATED FLOW IN MODELS OF COMPACT HEAT EXCHANGER

In the present work, results of an experimental study of a turbulent flow over a crossflow cavity and behind a backward-facing step with inclined wall at low (Tu_o= 1,5 %) and high level (Tu_o =6,6 and 14,2 %) of the free stream turbulence are reported. The inclination angles of cavity sidewalls φ varied in the range from 30° to 90°. The evolution of the flow has been traced in visualized flow patterns obtained by the oil-film and termography techniques. It is shown that, as the inclination angle φ increases, the vortex formation structure changes drastically. At an inclination angle of 70°, the main-vortex structure transforms from a single-cell two-dimensional to a double-cell three-dimensional one. The flow at φ = 70° is highly unstable, and this instability may be classed to the so-called elliptic type. In this case, the heat-transfer coefficients averaged over the entire cavity surface are increased appreciably. The step wall inclination angle β varied from 15° to 90°. The initial tunnel turbulence has been found to exert an intensifying on the distribution of heat-transfer coefficients behind the inclined steps. The effect is approximately similar at β = 45°-90° and considerably diminishes when approaching the flow without separation over the step. The turbulence decreases the length of the separated region behind the step, suppresses the flow separation, and increases the stalling angle.