Thermomechanical behaviour of a solid plate undergoing a rapid, local cooling process

In the experiments to analyze the dynamical and the thermal behavior of the drop impact onto heated solid surfaces, the various impact regimes are characterized through the surface temperature before the impact. Although such “nominal temperature” may represent a valid and consistent parameter to describe such phenomena, other thermal aspects must be considered to understand deeply the physics of the impact. It is clear for example that the surface effusivity has an important role and determines the instantaneous temperature during the drop spreading. Furthermore if the drops are falling onto the heated surface with a given frequency, the time lag between following impacting droplets must be determined in order to obtain the same given “nominal temperature” at each impact event. A drop interaction onto a heated wall generates a transitory, local cooling of the solid surface, i.e. a transient heat conduction in a finite medium subject to a pulse cooling. The software FlexPDE® is a commercial code based on a finite element method, which include a simple solver with an automatic re-gridding routine. The Fourier equation is solved assigning a time dependent boundary condition on the plate, corresponding to the convection coefficient under the wetted area, both for the monophase and for the phase transition regime. The convection coefficient is evaluated from literature data using a conservative value. The wetted area is estimated directly from own experiments using CCD images of drop impact onto heated surfaces. The temperature evolution inside the plate is determined.