Features of Boiling Heat Transfer at VariousPressures on Hydrophilic/Hydrophobic Surfaces

The effect of surface wettability and pressure on multiscale heat transfer characteristics at liquid boiling was studied. The experiments were carried out at saturated water boiling on surfaces with different wettability in the pressure range of 8.8–103 kPa. The usage of transparent ITO film heater deposited on a sapphire substrate and high-speed visualization showed the nucleation site density to reduce with decreasing pressure and to significantly increase on heaters with hydrophobic fluoropolymer coatings. The results on the vapor bubble growth rate, bubble emission frequency, and evolution of the triple contact line at spreading of dry spots are also analyzed in detail. In particular, the rate of dry spot growth on a hydrophilic surface was shown to have a non-monotonic dependence with the lower extremum at pressures in the range of 22–42 kPa depending on the heat flux. The usage of high-speed infrared thermography enabled measurement of the temperature field of the heating surface and determination of the heat transfer rate at boiling depending on the pressure and surface wettability. The heat transfer was shown to decrease with the pressure at boiling on hydrophilic heaters, whereas it can be significantly enhanced due to a hydrophobic coating at atmospheric pressure in the range of low heat fluxes.