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 (P-pump* = 1.35 x 10(10) - 4 x 10(13)) 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. Crown Copyright (C) 2010 Published by Elsevier Ltd. All rights reserved.