Behavior of pool boiling heat transfer and critical heat flux on high aspect-ratio microchannels

We conducted pool-boiling experiments and investigated the physical mechanism of boiling heat transfer and critical heat flux (CHF) on heating surfaces with top-opened rectangular microchannels. Through capillary wicking experiments for the boiling samples, it was revealed that, as the microchannel heights increase, the liquid-wicking capability is enhanced significantly for the same capillary pressure gradient, i.e., for the same channel width. This result can be explained by considering the balance between the capillary-pressure potential and the viscous friction by the channel walls. The pool boiling experiments showed that the higher aspect-ratio channel sample has a higher CHF and boiling heat transfer coefficient (BHTC), and it provides evidence that on the microchannel surfaces, additional liquid supply to the dry spot that is formed on the boiling surface by capillary wicking can lead to an enhancement of CHF and BHTC under pool boiling conditions. Expressions for the liquid mass-flow rate and the liquid-occupied region by capillary wicking were derived by the mathematical procedure with a one-dimensional, steady-state fluid momentum equation for liquid flow inside a single microchannel, and the parameters increased monotonically with an increase in channel height. Through numerical analysis, a simple relationship between the average boiling surface temperature (Ts,avg), the surface heat flux (qs), and the dry-spot diameter (Ddry) was derived as Ts,avg∝Ddry2qs. Hence, an exact solution to predict CHF on the boiling surface with microchannels was obtained, and it supports the strong relationship between the CHF and the capillary wicking capability on a boiling surface.