Phase-change heat transfer

Micro/nanostructured surfaces for enhanced pool boiling 

Micro-nano hybrid structure (MNHS) that comprises of microcavities and nanowires is a specific class of MNHS that is considered to be ideal for two-phase boiling heat transfer applications. However, realizing such structures has always been difficult and time-consuming. Here, we demonstrate a simple, robust, rapid, and photolithography-free route to fabricate MNHS that consists of individual microcavities and copper nanowires on a large area. We show that this MNHS can be extremely beneficial in boiling heat transfer compared to the nanowire surface. 

Publications:
Shin et al., Appl. Phys. Lett. 101 251909 (2012); Kim et al. Int. J. Heat Mass Transf. 70 23 (2014)

Flow boiling heat transfer using highly wetting liquid

It is commonly believed that surface roughening leads to higher boiling heat transfer performance due to the increased nucleation sites and enhanced wettability. Here, we observe an interesting behavior where the presence of nanowires reduces the wetting and wicking that in turn leads to significant decrease of heat transfer performance compared to the plain surface, which opposes the current consensus in phase-change heat transfer. Also, the effects of nanowire length and Reynolds number on the boiling heat transfer are shown to be highly non-monotonic. We explain such an unusual behavior on the basis of wetting, nucleation and forced convection, and we show that such factors are highly coupled in a way that lead to unusual behavior. 

Publications:
​Shin et al., Energy 76 428 (2014); 

Enhanced Boiling Heat Transfer using Self-Actuated Nanobimorphs

We present a new concept of a structured surface for enhanced boiling heat transfer that is capable of self-adapting to the local thermal conditions. An array of freestanding nanoscale bimorphs, a structure that consists of two adjoining materials with a large thermal expansion mismatch, is able to deform under local temperature change. Such a surface gradually deforms as the nucleate boiling progresses due to the increase in the wall superheat. The deformation caused by the heated surface is shown to be favorable for boiling heat transfer, leading to about 10% of increase in the critical heat flux compared to a regular nanowire surface. A recently developed theoretical model that accounts for the critical instability wavelength of the vapor film and the capillary wicking force successfully describes the critical heat flux enhancement for the nanobimorph surface with a good quantitative agreement.

Publications:
​Shin et al., Nano Lett. 18 6392 (2018);