Large-eddy simulation of jet impingement heat transfer using a lobed nozzle

The jet impingement heat transfer issuing from a lobed nozzle constructed using three circular orifices at a Reynolds number (Re) of 10,000 is investigated intensively with large-eddy simulation (LES). A comparative view was obtained for three nozzle configurations with different ratios of the orifice center offset (a) to the orifice radius (b) (i.e., a/b = 0, 0.8, and 1.15) at two nozzle-to-wall distances (H/De = 2 and 4). A constant equivalent diameter De is fixed for all of the configurations to ensure the nozzles' constant cross-section area. Good agreement of the LES data with the results obtained with temperature-sensitive paint (TSP) and particle image velocimetry (PIV) is established for the azimuthal-averaged Nusselt number on the impingement wall and the velocity distributions in the wall-jet and impingement zones, respectively. For all three nozzle configurations at H/De = 2, the LES results delineate two heat removal mechanisms of the impinging jet. Near the second-peak circle, the heat transfer is enhanced by the secondary vortices near the wall, whereas beyond the second-peak circle the instantaneous flow impingement onto the wall plays a significant role in heat transfer enhancement. The secondary vortices and instantaneous flow impingement are strengthened significantly in the configuration with a/b = 0.8 at H/De = 2, giving rise to a substantial increase in the Nusselt number in the region 1 < r/De < 4. For the three configurations at H/De = 4, the instantaneous flow impingement is the main mechanism for heat transfer enhancement in the region r/De < 0.5, whereas the increase in a/b results in frequent activation of the intense flow impingement along with high turbulent kinetic energy, yielding better heat transfer on the heated wall.