Experimental study of impingement heat transfer at small jet to target surface distances

• Online Access: http://hdl.handle.net/2047/d20000907

Jet impingement study is of significance because of its various applications in different industries, ranging from electronic chip cooling, cryogenic tissue freezing to food packaging industry, de-icing of aircraft wings to cooling of gas turbine shrouds, combustor liners, blades and hot moving parts with close spaces between them. In the current study, thermo-chromatic liquid-crystal technique was used to measure impingement heat transfer coefficient for a single jet issued from a hole with round inlet corners. Experiments were conducted for target-plate distance to jet diameter (Z/D) ratios of 0.3 to 3 and for jet Reynolds numbers ranging from 15,000 to 30,000. Nusselt numbers were measured and plotted against the non-dimensional radial distance varying from the jet stagnation point to about 12 times the jet diameter. Comparisons were made between the results of the present study and those reported in open literature for holes of the same geometry with sharp inlet corners under identical flow and heat transfer conditions. Major conclusions of this study were: There is a substantial increase in discharge coefficient when the inlet corners are rounded. When the target plate was very close to the hole exit (Z~0.3 to 0.5D), it was observed that the jet after impingement filled the gap between the jet and target plate and formed a channel flow, thus giving rise to the heat transfer coefficient at higher radial distances. This behavior was not observed when the target plate was at a bigger distance (Z=3D) from the hole exit, since the impinging air did not form a channel flow. Also one of the observations made was that, in the presence of the copper foil, the liquid crystal thermographs were of smoother texture, and the cross-conduction in the copper foil caused a significant variation in local heat transfer coefficients.