Heat flux reconstruction by inversion of experimental infrared temperature measurements – Application to the impact of a droplet in the film boiling regime

An Inverse Heat Conduction Problem (IHCP) is considered in order to estimate the transient heat flux extracted from a hot solid surface by an impinging droplet. The resolution of the IHCP is made with the so-called quadrupole method, which provides an analytical expression of the temperature and the heat flux at the front surface of the solid wall, where the drop impact takes place. In the experiments, the thermal response of the front surface is recorded using IR thermography. For that, sapphire is chosen as the material of the solid wall, and the front surface is coated with a thin TiAlN ceramic layer (thickness of 300 nm). The latter is highly emissive and opaque in the IR while sapphire is transparent at the same wavelengths. This feature allows the surface impacted by the droplet to be viewed from the bottom by the IR camera. This approach has been implemented to gain some insights into the heat transfer from the solid surface as well as the formation and growth of the vapor film, which appears under the droplet in the regime of film boiling, when the solid temperature is much higher than the boiling temperature of the liquid. Due to the small thickness of the vapor film, heat conduction is predominant in the vapor layer. Hence, the thickness of the vapor film can be deduced from the value of the reconstructed local heat flux, assuming a linear profile of temperature between the liquid interface of the droplet at the saturation temperature and the solid surface measured by IR thermometry.