Turbulent heat transfer from a flat surface to a swirling round impinging jet

Impinging jets have been widely used for the heat transfer augmentation in a variety of engineering applications such as cooling of hot steel plates, tempering of glass, drying of papers and films, cooling of turbine blades and electronic components. The common feature of impinging jet is the heat transfer enhancement in the stagnation region and a rapid decay of heat transfer in the wall jet region due to the boundary layer buildup on the target surface. One of the important parameters affecting impinging jet heat transfer is the flow condition at the nozzle exit. The swirl flow at the nozzle exit would alter the jet-spreading rate, flow entrainment and turbulence characteristics before its impingement on the surface. Swirl flows result from the effect of a spiraling motion (i.e., tangential component) imparted in a flow which could be generated by swirl vanes or axial-plus-tangential flow entry to the main axial flow. It is important to understand the swirling effect on flows and heat transfer so that distinctions between favorable and undesirable effects of swirl to many flow and heat transfer processes could be made. In the swirling jet, the degree of jet growth, entrainment of ambient air, and jet decay are affected by the degree of swirl. A few papers have been published to investigate heat transfer characteristics with a swirling impinging jet. Ward and Mahmood [1] used naphthalene sublimation technique to investigate the effect of mass and heat transfer on a flat surface from a swirling impinging jet. They concluded that the swirl has an unfavorable effect on heat transfer in terms of both local and average value for the parameters tested. Huang and El-Genk [2] used a swirl generator made of a cylindrical plug with four narrow channels to provide swirl to a single and multiple air impinging jets. They reported that for the single jet with swirl, the radial heat transfer distributions become more uniform with higher degree of swirls and larger nozzle-to-surface distance. Owsenek et al. [3] carried out numerical investigations of impinging jet with superimposed swirls. However, their study was limited to the laminar flow regime. In the present study, the local Nusselt numbers are measured for a swirling round turbulent jet impinging on the flat plate. The experiments are made for the jet Reynolds number (Re = 4m/pimud, where m is the mass flow rate measured by the orifice flow meter and d is the pipe nozzle diameter) of Re = 23,000, nozzle-to-plate d