Effect of confinement on heat transfer characteristics of a microscale impinging jet

This study experimentally investigates the local heat transfer characteristics of a microscale confined impinging air jet on a heated plate. The experimental parameters included the Reynolds number (ReD = 1600–5600), the nozzle-to-plate spacing (H/D = 1–10), and the degree of confinement of the nozzle (DC/D = 3, 6, 9, 12, 24, 48). The degree of confinement of the nozzle is a novel parameter. A reduction in the heat transfer rate was found for nozzles whose DC/D values were 6, 9, 12, 24, and 48 as a result of the confinement effect at small nozzle-to-plate spacings. The confinement effect disappeared beyond H/D values of 2, 3, 4, 8, and 17 for DC/D values of 6, 9, 12, 24, and 48, respectively. Flow characteristics were investigated by measuring pressure distributions along the wall. Subatmospheric pressure, which is evidence of the confinement effect, was observed for the confined nozzles. Correlations of the stagnation and average Nusselt numbers are proposed on the basis of the experimental results. Finally, a contour map that depicts the ratio of the Nusselt numbers of the unconfined and confined jets is presented. The contour map confirms that the confined jets have a smaller Nusselt number than the unconfined jets whenever the degree of confinement of the nozzle is large and the nozzle-to-plate spacing is small.