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

摘要

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 Nu_(rms). The magnitude of Nu significantly reduces across the imaged radial domain as Re decreases, and the relationship appears to be Nu(r)αRe~(0.47). It appears that a transitional swirl number (dependent on Re) exists which controls the impingement heat transfer characteristics.