Two-mode bubble detachment mechanisms: Enhancing critical heat flux and the heat transfer coefficient in shallow liquid boiling

The two-mode bubble detachment mechanism describes a phenomenon on a boiling surface where bubble detachment from the heated surface is influenced both by the surface tension of the liquid interface and by other factors. This study utilizes microchannels integrated with a cylindrical confined space to facilitate shallow liquid boiling, creating distinct macroscopic upper and lower surfaces with a height difference. By adjusting the liquid height, two boiling modes are established: one involves bubble detachment suppressed confined boiling on the upper surface, while the other features bubble detachment active confined boiling on the lower surface. Systematic experimental investigations were conducted on the boiling heat transfer curves of the two-mode boiling surface (TMBS) with a height difference of 3 mm. The maximum heat transfer capacities of TMBS were found to be 1114 W and 1425 W for liquid heights of 1 mm and 3 mm, respectively. Observations of bubble dynamics revealed that the bubble departure frequency in the cylindrical confined space of the lower surface was higher than that of the upper surface, while the bubble departure diameter was smaller compared to the upper surface. The results indicate that the bubble detachment suppressed confined boiling on the upper surface enhances the heat transfer coefficient (HTC) through increased microconvection from bubble disturbances, improved evaporation of the bubble microlayer, and reduced thermal resistance. Conversely, bubble detachment active confined boiling on the lower surface generates hydrodynamic liquid inflow and vapor jets due to the confined space, thereby improving the stability of boiling heat transfer and increasing the critical heat flux (CHF). Predictive CHF models at various liquid levels for surfaces with smooth plain, microchannels, cylindrical confined spaces, and a combination of the latter two were developed for confined boiling, successfully predicting experimental data within an acceptable error range of ±35 %. The mean relative error (MRE) between the predicted and experimental CHF was calculated to be 16.3 %, 19.3 %, 5.8 %, and 18.5 %. It is anticipated that the conclusions of this paper will provide valuable insights for engineering applications in boiling heat transfer technology.