IMPINGEMENT HEAT-TRANSFER WITH A SINGLE ROSETTE NOZZLE

This investigation examined the magnitude and uniformity of the convective heat transfer coefficients for a single rosette nozzle impinging air normally to a heated surface. The local heat transfer coefficient was measured using a heated 0.025-mm-thick stainless steel foil impingement surface coated with liquid crystals. The temperature distribution along the surface was determined by measuring the reflected light from the liquid crystal with the use of bandpass filters and an electronic digitizer. Measurements were obtained for low nozzle-to-plate spacings (H = 3.81, 0.635, and 0.159 cm) with and without a confinement surface. Local and average convective coefficients distributions illustrate the uniformity or nonuniformity of the heat transfer and assist in understanding the variations in the magnitude of the average convective coefficient. The convective coefficients for the rosette nozzle were also compared to values obtained with axisymmetric jets. Results show that the local convective coefficients are highly nonuniform for the rosette nozzle. From the comparison of the rosette nozzle with the axisymmetric jet, it would appear that the enhancement of the average heat transfer by this type of specialty nozzle design compared to an axisymmetric jet occurs only for small flow rates, small operating separation distances, and specific averaging areas. Small separation distances (H less-than-or-equal-to 0.635 cm) are required to minimize the convective coefficient degradation due to entrainment and velocity decay for the rosette nozzle.