Étude expérimentale des phénomènes fondamentaux régissant la vaporisation d'une goutte sur une paroi chauffée sous une faible pression de saturation

Due to its high heat transfer capacity, liquid vaporization is utilized in numerous industrial processes, such as refrigeration. This sector is currently undergoing a renewal in practices, driven by the need to adopt environmentally friendly fluids. Among potential fluids, water emerges as a promising alternative, offering an effective solution for the decarbonization of cooling and refrigeration systems. However, water requires low saturation pressures (typically around 1 kPa) to achieve evaporation temperatures suitable for air conditioning or domestic refrigeration. At low pressures, the physical phenomena associated with boiling-induced vaporization differ significantly from those observed at atmospheric or higher pressures. Specifically, nucleation leads to the formation of large bubbles, driven by physical parameters that amplify bubble growth prior to detachment. This process degrades heat transfer by drying the heat exchange surface. The present research aimed to exploit the significant bubble size to atomize liquid drops. The spreading of daughter drops from a mother drop increases the contact area between the liquid and solid surface, thereby enhancing the heat transfer.First, the study focused on the vaporization of water drops deposited on a heated wall under low pressure. A parametric analysis revealed different vaporization regimes, including an evaporation regime and several boiling regimes characterized by bubble nucleation within the drop. Experimental results demonstrated that, under certain conditions (referred to as the “bursting bubble boiling” regime), a real atomization (i.e. thermally generated spray) occurs, where multiple daughter drops are created from a mother drop. The occurrence of the various regimes strongly depends on surface temperature, height of injection, drop volume, and saturation pressure. The transition between evaporation and boiling regimes is primarily governed by the surface temperature, which must be high enough to trigger nucleation, and height of injection. The latter influences the impact velocity of the drop, its post-impact hydrodynamics, thereby altering heat diffusion and nucleation. The conditions for these regimes were mapped using dimensionless numbers. To generalize the findings obtained with water, similar experiments were conducted with isopropanol, selected for its favorable properties for forming bubbles of large diameter. Some regimes were identified to be common to both fluids, but a specific regime for isopropanol (referred to as the “expulsion boiling” regime) was also discovered and shown to be related to this fluid's high viscosity. The bursting bubble boiling regime was found to be less effective in enhancing heat transfer for isopropanol compared to water. Finally, a preliminary experimental study was conducted involving multiple drop depositions (successive or simultaneous), providing insights for future work closer to industrial applications. This study helped clarify the conditions under which drops can be considered independent and highlighted phenomena specific to interactions between neighboring or successive drops.