Comparison of thermal characteristics of confined and unconfined impinging jets

In the present study, we compared the thermal performance of the two types of impinging jets: confined and unconfined impinging jets. In order to obtain the thermal characteristics of impinging jets, experimental investigations for air and water are conducted. The effects of dimensionless pumping powers image on the Nusselt number are considered. The focus is on the cases where the nozzle-to-plate spacing is equal to or less than one nozzle diameter (H/d ⩽ 1). The results show that the thermal performance of the confined jet is similar to that of the unconfined jet under a fixed pumping power condition, while the thermal performance of the confined jet is 20–30% lower than that of the unconfined jet under a fixed flow rate condition. Based on the experimental results, generalized correlations for the stagnation and average Nusselt numbers for both confined and unconfined impinging jets are presented as a function of the dimensionless pumping power and the Prandtl number.