AIR-JET IMPINGEMENT HEAT-TRANSFER AT LOW NOZZLE PLATE SPACINGS

The local heat transfer characteristics of air jet impingement at nozzle-plate spacings of less than one nozzle diameter have been examined experimentally using an infrared thermal imaging technique. Fully-developed nozzles were used in the study. The flow structure was investigated using laser-Doppler velocimetry and wall pressure measurements. The stagnation Nusselt number was correlated for nozzle-plate spacings of less than one diameter. The customary Nusselt number dependence on Re1/2 for impinging jet transport was observed. A power-law relationship between Nusselt number and nozzle-plate spacing of the form Nu0 is similar to (z/d)-0.288 observed experimentally is explored from theoretical considerations. The effects of accelerating fluid between the nozzle-plate gap as well as a significant increase in local turbulence leads to substantially increased local heat transfer with decreased nozzle-plate spacing. A stagnation point minimum surrounded by an inner and outer peak in the local heat transfer was observed for nozzle-plate spacings less than z/d = 0.25. These primary and secondary maxima are explained by accelerated radial flow at the exit of the jet tube and an observed local maximum in the turbulence, respectively. These conclusions are drawn from observations made relative to the turbulent flow structure and wall pressure measurements. The outer peak in local Nusselt number was found to move radially outward for larger nozzle-plate spacings and higher jet Reynolds numbers.