Heat transfer in the stagnation region of an excited plane impinging jet

Heat transfer and turbulence characteristics in the stagnation of an excited plane impinging jet have been investigated experimentally. Jet excitation was utilized to lock the phase of vortex motion and temperature variation of both fluid and wall. The velocity and temperature field were measured by digital particle image velocimetry (DPIV) and laser-induced fluorescence (LIF). In the mean flow field, the excitation makes closer the location of the impinging plate which has a maximum heat transfer coefficient. The local Nusselt number with excitation along the stagnation line is larger than that without excitation for an impinging plate set at a distance of 5-6 times the nozzle width, where the intensity of the fluctuating velocity normal to the wall and turbulent heat flux component normal to the wall are also increased. The turbulent heat flux and Reynold's stress that are mainly generated by counter-rotating vortex pairs contribute to reduce velocity and thermal boundary layer thickness. When the spanwise vortices arrive in the stagnation region, vorticity of the counter-rotating vortex pairs is intensified, and this enhances the intensity of the turbulent heat flux and Reynold's stress which causes high heat transfer instantaneously.