Flow and heat transfer characteristics of a pulsed jet impinging on a flat plate

The flow and heat transfer characteristics of a pulsed jet impinging on a flat plate under various excitation conditions were experimentally studied. The flow behaviors and time-averaged velocity fields of the characteristic flow structures were examined using a laser-assisted smoke flow visualization method and particle image velocimetry (PIV). The heat transfer of a pulsed impinging jet was measured by a thermal imaging camera. Two primary flow modes were observed in the domain of the jet pulsation intensity and excitation Strouhal number. The coherent vortices and vortex breakup modes appeared in the regimes of low and high jet pulsation intensities, respectively. In the coherent vortices mode, two submodes (contiguous vortices and discrete vortices) were additionally classified. The vortex ring maintained its coherency before impinging on the flat plate in the coherent vortices mode, while the vortex ring lost its coherency and broke up quickly before impinging on the flat plate in the vortex breakup mode. The increase rates of the axial traveling distance and axial convective velocity of the vortex rings in the vortex breakup mode were greater than those in the coherent vortices mode. Pulsating the impinging jet in vortex breakup mode, the effect of a large convective velocity of the vortex rings caused large mean axial velocities and axial velocity fluctuation intensities. The convective heat transfer was enhanced by increasing the convective velocity of the vortex ring and fluctuation intensity.