Heat Transfer Characteristics of Swirling Impinging Jets

Impinging jets are used in a wide number of industrial cooling applications due to their high heat and mass transfer abilities. The current research is concerned with the effect of swirl on the heat transfer characteristics of jet impingement cooling. Two inserts were designed order to generate swirling flow. These two designs, “Swirl Insert A” and “Swirl Insert B”, were tested at various Reynolds numbers, between 8000 and 16000 inclusive, and at H/D=0.5 and 1. The jet was directed downwards onto a 25μm thick stainless steel foil which was ohmically heated. Images were recorded using a thermal imaging camera focused on the underside of the foil. These images were then analysed using Matlab and the Nusselt number profile was obtained. It was found that, while both swirl inserts establish an improvement in the heat transfer by comparison to that of a jet with no swirl, the “Swirl Insert B” design performed better that the “Swirl Insert A” design for high Reynolds number at H/D=0.5 and consistently for H/D=1. It was also discovered that, while both the “No Insert” and “Swirl Insert B” results did not change dramatically with an alteration in H/D, “Swirl Insert A” decreased by ~10%. INTRODUCTION Impinging jets are used in a wide number of industrial cooling applications due to their high heat and mass transfer abilities. Such applications include the tempering of glass, cooling of turbine blades and thermal management of electronics in high power systems amongst others. Impinging jets are also quite versatile since they can be arranged in arrays to cover specific areas and shapes, their heights and impingement angles can be altered and the flow rate of the fluid can be changed. Furthermore, a degree of swirl can be incorporated in the flow Many different approaches have been taken into the understanding of the effect on heat transfer caused by different types of jet conditions; jet arrays, synthetic jets and swirling jets. The discussion and experimentation of the advantages of swirling jets over normal impinging jets has been around for many years. Over this period of time many different designs have been tested in order to initiate a swirling flow. Bakirci and Bilen [1] developed helical channels which were placed inside a hollow aluminium tube. These inserts caused the fluid flow to be divided up into four separate streams and then rotated along a centre column a specific number of times, depending on the swirl number. These designs did have a certain draw back; when the flow was divided up into four separate flows, they were not given the time to recombine into one single swirling flow. Thus at certain swirl numbers the flows act as a multi-jet array and as they impinged the surface at an angle, the heat transfer at r/D=0 was a lot lower than a nonswirling impinging jet. Hee Lee et al. [2] attempted something quite similar to this also. They designed swirl generator inserts for a long straight pipe which involved a centre core with guide vanes along the outside to rotate the flow. This design gave results comparable to that of Bakirci and Bilen. At a distance of r/D=0, L/D=2 and large swirl angles the heat transfer was diminished by comparison to that of a jet with no swirl. However this would then surpass the non-swirl jet at r/D≈0.5. At higher values of L/D the higher swirl angled jets would perform worse than the non-swirl jet at