Heat transfer during drop impact onto wetted heated smooth and structured substrates: Experimental and theoretical study

Spray cooling is a very effective means of heat removal from hot surfaces. Its efficiency can be further improved using structured wall surfaces. One of the fundamental processes governing spray cooling is an impact of a single cold droplet onto a heated wetted wall. The hydrodynamics of drop impact governs the transient heat transport in the film and in the wall. We study hydrodynamics and heat transfer during impact of a single drop onto heated smooth and structured heated plates. The temperature distribution in the heated plates has been measured with seven thermocouples. The splash dynamics and the evolution of interface temperature distribution have been visualized using high-speed infrared thermography. The film thickness evolution in the inner region has been measured using chromatic confocal imaging technique. Initial film thickness and drop impact parameters have been varied in the experiments. The evolution of the temperature distribution at the liquidgas interface and the instationary temperature distribution in the heated plate depend on the initial film thickness, impact parameters and the plate topography. A self-similar analytical solution of the full Navier-Stokes equations and of the energy equation is obtained for the velocity and temperature fields in the spreading film. The theory allows prediction of the contact temperature and the residual film thickness.