Heat transfer characteristics of swirling and non-swirling impinging turbulent jets

Abstract

The effect of swirl on the impingement surface heat transfer is experimentally investigated in incompressible turbulent (Re=11,600–35,000) impinging air jets. A swirl nozzle (D=40mm) is used for seamless transition from a non-swirling (S=0) jet to a highly swirling (S=1.05) jet. The effect of swirl number (S), nozzle-to-plate distance (H) and Reynolds number (Re) on the radial uniformity and intensity of convective heat transfer on a heated thin foil is examined over the range H=1D–6D.The use of low-to-medium swirl numbers (S=0.27–0.45) at Re=35,000 is found to improve the magnitude of heat transfer (Nu) in the impingement region compared to non-swirling (S=0) jets over H⩽4D. When S further increases, significant enhancement in Nu occurs only at near-field impingement (H⩽2D), regardless of the impingement area (footprint). Larger nozzle-to-plate distance (H⩾4D) is found to be detrimental on the convective heat transfer, with a significant reduction of Nu intensity. In relation to spatial uniformity of heat transfer, imparting low values of swirl does not appreciably improve Nu uniformity compared to non-swirling jets over all the impingement distances (H) investigated. In contrast, a marked improvement in Nu uniformity (i.e. flat Nu profiles) on the impingement surface occurs for strongly swirling jets (S=0.77 and 1.05), but only at H=4D and 6D. The highest Nu regions are found to correlate well with Nurms. The magnitude of Nu significantly reduces across the imaged radial domain as Re decreases, and the relationship appears to be Nu(r)αRe0.47. It appears that a transitional swirl number (dependent on Re) exists which controls the impingement heat transfer characteristics.