Transient heat transfer characteristics of swirling and non-swirling turbulent impinging jets
Experimental Thermal and Fluid Science
School of Engineering
A multitude of research has already been undertaken to examine the effects of imparting swirl (S), Reynolds number (Re), and nozzle-to-plate distance (H/D) on the steady-state heat transfer characteristics of turbulent impinging jets. However, no studies to date have compared the transient development of such jets in both swirling and non-swirling conditions. The current paper addresses this gap by using highly resolved (time series) imaged (infrared) data in conditions spanning Re = 11,600, 24,600, and 35,000. The experiments are based on an electrically heated foil (0.025 mm) with jets over S = 0–1.05 and nozzle-to-plate-distances of H/D = 2, 4, and 6.
For non-swirling impinging jets, at greater Reynolds numbers, the location of the highest Nusselt number remains fixed for all the time steps in contrast to jets with low Reynolds number. The transient characteristics for H/D = 4 are distinct compared to H/D = 2 and 6, with impingement heat transfer taking more time to reach steady-state for H/D = 4. It is also seen that heat transfer distribution over the impingement plate changes significantly over the small interval of time. For swirling jets, the impingement plate heat transfer characteristics develop simultaneously for the stagnation and wall jet region. The peak Nusselt number shifts to the wall jet region as the intensity of swirl increases. Two correlations (first for non-swirl and low-swirl, later for moderate to high swirl) are proposed to predict the time needed to reach steady-state for Re = 35,000.