02 Jul
02/07/2021 09:15

Sciences & Société

Soutenance de thèse : Karolina WOJTASIK

Soutenance d'une thèse de doctorat de l’Université de Lyon en cotutelle internationale entre Wroclaw University of Science and Technology (Wroclaw, Pologne) et l’INSA Lyon

Boiling of water at low pressure: the role of subcooling and thermophysical properties on the bubble dynamics and heat transfer 

Doctorante : Karolina WOJTASIK

Laboratoire INSA : CETHIL

Ecole doctorale : ED162 : Mécanique, Energétique, Génie Civil, Acoustique de Lyon

Boiling process is a common heat transfer mode used in various applications, among which air-conditioning and refrigeration. Recently, there has been a sparkling interest in the development of efficient cooling technologies which utilize environmentally-friendly refrigerants. Water could be one of them, but it requires to operate under low pressure. Close to the triple point, the physics of boiling significantly differs from atmospheric pressure. Although the knowledge on subatmospheric boiling is slowly growing, many of its aspects remain unknown. In order to extend the fundamental knowledge concerning the boiling process, several experimental activities were carried out. Initially, the experiments were conducted on a polished surface with single cavity, and then on a rough surface with multiple nucleation sites. Boiling took place on a heated wall equipped with a novel heat flux sensor, under various vapor pressures. The height of liquid in the experimental reservoir was also varied: the induced static pressure creates a significant liquid subcooling that was shown to strongly affect the boiling behavior. The analysis of the dynamics of isolated bubbles allowed to discover three types of bubbles and explain their associated thermal phenomena. Statistical methods were applied for boiling on a rough surface because of the random character of this process. Along with camera visualization, these methods allowed to distinguish four distinct boiling regimes, whose conditions of occurrence were summarized owing to an original boiling regime map. To enhance the phase change heat transfer at low pressure, an alternative option to boiling could be to spray multiple droplets on the heated wall. There is lack of knowledge concerning evaporation process at subatmospheric conditions. For that purpose, the last experiments were focused on the evaporation of a drop or a layer of liquid from a horizontal surface. It allowed to describe the various thermal processes occurring during evaporation phenomenon.