Contribution of wall heat transfer in the transition region between microlayer and macroscopic regions to the growth of a single boiling bubble

It is known that microlayer forms beneath a growing boiling bubble and evaporation of the microlayer considerably contributes to the bubble growth. In addition, a transition region attributable to the surface tension of the liquid-vapor interface lies between the microlayer and the macroscopic regions, where the wall heat flux rapidly decreases along the radial direction. To the best of the authors knowledge, no experimental study has quantitatively evaluated the contribution of heat transfer in the transition region to single-bubble growth. Here, the liquid thickness profiles and the local heat flux distributions in the microlayer and transition region were simultaneously measured, respectively, to estimate their heat transfer contributions to single-bubble growth. The entire geometry of the liquid-vapor interface of a boiling bubble (from microlayer though the transition region to the macroscopic region) and the corresponding wall temperature and heat flux distributions were measured using the integrated shadowgraph, total internal reflection, laser interferometry, and infrared thermographic imaging technique. The thickness profile of the liquid layer in the transition region was calculated by substituting the local wall temperature and heat flux distribution data into the one-dimensional heat conduction equation. The thickness profile obtained for the transition region reasonably connected the liquid-vapor interface geometries in the microlayer and macroscopic regions. The obtained results showed that the transition region occupied a considerable areal fraction in the bubble base, reaching up to 57 % on average and 69 % at maximum. The resulting contributions of heat transfer in the microlayer and transition region to the growth of a boiling bubble were estimated as approximately 30 % and 20 %, respectively.