Effects of the Swirl Number, Reynolds Number and Nozzle-to-plate Distance on Impingement Heat Transfer from Swirling Jets

This paper reports on a parametric study of the impingement heat transfer from swirling jets issuing from a circular nozzle. The swirl velocity component is imparted to the flow via a tangential entry into a re-circulation chamber located upstream of the nozzle; the degree of swirl is varied by changing the angle of the channels leading to the chamber with respect to the radial direction. The effects of the dimensionless impingement distance (H/D = 1 , 2, 3, 4, 6, 8, 10, 12, 14 with D being the nozzle diameter), the swirl number ( S = 0 , 0 . 078 , 0 . 22 , 0 . 37 , 0 . 54 ) and the Reynolds number ( Re = 20,800 , 31,100 , 41,500 ) on the spatial distribution of the heat transfer are investigated by means of infrared thermography and the heated thin foil sensor. The present results show that the swirl number and the impingement distance significantly influence the structure of the heat transfer distribution, while the Reynolds number affects essentially the magnitude. Four distinct ranges with different behaviours can be identified by distinguishing between weakly or non swirling jets ( 0 <= S <= 0 . 22 ) and moderately swirling jets ( 0 . 37 <= S <= 0 . 54 ) and between short and long impingement distances ( 1 <= H/D <= 4 and 6 <= H/D <= 14 respectively). A comparative assessment of the performance of the swirling jets is carried out by focusing on the area-averaged Nusselt number distributions. It is found that in the range of short impingement distances and over relatively small target areas an enhancement of the heat transfer rates can be obtained at a small extent by adding a weak swirl with no detrimental effect on the uniformity of the distribution and at a larger extent by a moderate swirl with a deterioration of the uniformity. For long impingement distances no enhancement is observed, although swirl yields a significant reduction of the non-uniformity. Finally, correlation laws of the area-averaged Nusselt number as a function of the control parameters are derived in the four regimes identified. (C) 2022 Elsevier Ltd. All rights reserved.