A numerical-experimental study of heat transfer enhancement using unconfined steady and pulsating turbulent air jet impingement

Pulsating flows can yield an additional enhancement in heat transfer rate compared to steady flows; however numerical modeling of pulsating jet impingement remains challenging. There is no clear consensus in the literature on (i) the most reliable computational methodology for this case, and (ii) the magnitude and dependencies of the heat transfer enhancement due to flow pulsation. Using detailed experimental measurements for validation, this study has established an accurate computational fluid dynamics (CFD) methodology to predict the fluid dynamics and heat transfer of an axisymmetric pulsating jet impinging on a heated plate. A comprehensive sensitivity study is presented on the effect of grid density, spatial discretization scheme, turbulence model and time-step. The final model is in excellent agreement with the experimental local Nusselt number distribution for the steady jet, to within a maximum normalized deviation of 5%. Although other numerical studies typically use a fully turbulent model in the entire domain, this paper employs the transitional turbulence model Gamma-Theta. This approach captures the laminar-turbulent transition in the wall jet for small nozzle-to-surface distances (H ≤ 2D), and thus the intensity and extent of the secondary peak in the radial Nusselt number distribution. The effects of pulsation frequency (9Hz ≤ f ≤ 55Hz; 0.017 ≤ fD/U_m ≤ 0.102) and nozzle-to-surface distance (1D ≤ H ≤ 6D) on heat transfer enhancement are discussed for time-averaged Reynolds number of 6,000. The effect of flow pulsation on Stagnation and area-averaged Nusselt numbers are respectively quantified as -10% ≤ δNu₀ ≤ 19% and 10% ≤ δNu_ave ≤ 63%.