Boiling at subatmospheric pressures on hydrophobic surface: Bubble dynamics and heat transfer

It is well-known that the physical and chemical properties of a heat exchange surface, especially wettability, significantly affect the pool boiling performance. Nevertheless, the influence of wetting properties on the evolution of vapor bubbles, the heat transfer rate and the crisis phenomena development, especially at subatmospheric pressures, remains a poorly studied research area. The aim of current research is to investigate the evolution of two-phase flows and heat transfer rate during water boiling on a hydrophobic surface within the pressure range 11.3-102.8 kPa. The experiments were carried out with the use of a specially designed transparent heated substrate with sprayable hydrophobic coating, and integrated high-speed experimental techniques including visualization and IR thermography. It was revealed that the regularities of the bubble evolution with pressure reduction during boiling on hydrophobic surfaces significantly differ from hydrophilic surfaces. In particular, it was shown that during boiling on a hydrophobic surface the bubble departure diameter does not change and bubble emission frequency increases significantly with the reduction of pressure, which is inconsistent with the trends observed during boiling on a hydrophilic surface. Moreover, the bottom side visualization allowed us to analyze the evolution of the void fraction and bubble site density, the sizes of the dry spots under the sessile bubbles during boiling on the hydrophobic surface with pressure change. It was shown that superheats corresponding to the onset of nucleate boiling and heat transfer rate on the hydrophobic surface don't depend on pressure, while the heat transfer enhancement was observed at low heat fluxes in comparison with hydrophilic surface.