Temperature Uniformity, Correlations and the Flow Field in Swirling Impinging Jets

Infrared thermography, velocity and impingement pressure measurements alongside numerical modelling are used in this study to resolve (heated) surface temperature distributions of turbulent swirling impinging jets for two Reynolds numbers ( Re =11 600 and 24 600). Whilst building upon earlier discoveries for this same geometry, this paper provides three new contributions: (1) identifying the role of impingement distance ( H / D ) as a deciding factor in the trade-off between more efficient heat transfer (at high swirl numbers) and achieving better substrate temperature uniformity (lower gradients), (2) developing correlations to predict Nusselt number for swirling and non-swirling cooling jets, and (3) predicting the underlying mixing field in these jets and its interplay with the thermal distributions resolved. Results indicate substrate temperature uniformity varies based on H / D and swirl intensity ( S ) with a significant level of thermal non-uniformity occurring in near-field impingement ( H / D =1) at stronger swirl ( S =0.59 and 0.74). Four correlations describing the effects of S , Re , and H on the average heat transfer and stagnation heat transfer are developed and yield accuracies of 8% and 12%, respectively. Flow recirculation near the impingement surface is predicted at H / D =1 for stronger swirl jets which disappears at other substrate distances. The peak wall shear stress reduces and the flow impingement becomes radially wider at higher H / D and S . Stronger turbulence or eddy viscosity regions for non-swirling jets ( S =0) are predicted in the shear layer and entrainment regions at H / D =1, but such turbulence is confined to the impingement and wall jet regions for strongly swirling flows.