Heat transfer enhancement by low amplitude forcing in unsteady confined impinging jets

A finite-difference model, derived using a control-volume approach, was used to compute the flow and heat transfer characteristics in a two-dimensional confined laminar air jet impinging on an isothermal surface. Several cases were studied using Reynolds numbers of 650 and 750 with a nozzle-to-plate spacing, H/W, of 5. The behavior of the jet and the corresponding heat transfer from the target wall were investigated when the jet was forced by fluidic excitation at the nozzle exit. At a Reynolds number between 585 and 610, the unforced jet exhibits a transition to an unsteady regime, leading to asymmetric vortex shedding and jet flapping. An investigation of the velocity spectra found distinct dominant modes; the lowest frequency is associated with the jet flapping while the highest frequency is associated with the asymmetric vortex formation that causes buckling of the jet column. As a result of the two combined modes, the peak heat transfer is enhanced and the extent of the lateral cooling is broadened. The jet was subjected to forcing by the introduction of numerical excitation on each side of the jet. This was used to simulate fluidic excitation with the jet being forced on both sides at the exit. Both in-phase and out-of-phase modes were considered. At a Reynolds number of 750, forcing with an out-of-phase mode near the highest frequency leads to a complete stabilization of the jet. The forcing suppresses the low-amplitude, low-frequency flapping mode leaving only a high-frequency vortex formation mode. The suppression of the jet flapping leads to a decrease in the peak heat transfer, but because separation is suppressed, the average wall heat transfer is enhanced.