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
ðP
pump ¼ 1:35 1010 4 1013Þ 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 6 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.