Evaporation of Polar Liquids Under Low Pressure and/or Microwave Irradiation

Ethanol is evaporated under controlled pressure and under microwave irradiation, in a cylindrical or in a square cell located in a waveguide or in a resonant cavity (TE01, 2450 MHz). Volume (Rayleigh-Benard) and surface instabilities (Marangoni and differential vapor recoil) occur during evaporation under microwave. The sequence of heed transfer regimes observed (purely conductive, stationary convective, convective driven by surface tension and finally strongly accelerated by vapor recoil instability) as well as their localization (first in the volume phase and then in the surface) are analysed in relation with the absorbed field.A comparison between the results under microwave ivith the evolution of the same system under low pressure only shows the specificity of the interaction between the field and the material evaporated. The presence of the field favors the unstationary regimes first by increasing the thermal gradient near the surface (for the transition from conductive to convective regime) then by an amplification of temperature inhomogeneities through the dependance of the dielectric loss with T. Moreover the field increases the coupling between Marangoni and vapor recoil instabilities because of the increase of evaporation rate by the microwave heating.