Heat transfer of single drop impact on a film flow cooling a hot surface

A water drop impacts a thin heated wafer which is being cooled by a film flow generated from water jet impingement. The drop impact breaks the steady-state cooling and causes the local temperature around the drop landing location to change. This transient heat transfer process is experimentally investigated using an IR camera to record the surface temperature (Ts) of the wafer's underneath. The measured temperature shows two stages of the process: a response stage when the temperature quickly decreases as a result of the drop impact, followed by a recovery stage during which the temperature returns to the steady state. It is found the recovery time decreases with increasing the film flow rate. Although during the entire process Ts is lower than the steady state, the heat transfer coefficient (ht) is revealed to change by three steps. The first step is the increase of ht across the impact area, which indicates enhanced convection. In the second step, ht decreases toward and eventually below the steady state. In the third step the heat transfer coefficient increases toward and returns to the steady state. An enhancement factor based on the change of heat transfer coefficient rather than the temperature change is introduced to evaluate the enhancement of convection. The distribution of local maximum enhancement (ηmax) along the center line of the impact area and the peak value of ηmax are used to investigate the enhancement effects of film flow rate, drop diameter, and drop impact velocity. It is found ηmax does not follow a monotonic trend with increasing the impact velocity. The peak enhancement is found to be proportional to the square root of the ratio of the drop flow rate to the film flow rate.