Flow boiling enhancement of FC-72 from microporous surfaces in minichannels

The microporous coatings can remarkably enhance the liquid boiling heat transfer. Therefore, they are promising to be introduced into minichannels in the design of the cooling system of high-power microchips. However, the flow boiling heat transfer characteristics from microporous surfaces in the minichannels have not been extensively studied, and the pertinent knowledge is rather fragmentary. The present research is an experimental investigation on flow boiling of a dielectric fluid FC-72 from microporous coating surfaces in horizontal, rectangular minichannels of 0.49, 0.93 and 1.26 mm hydraulic diameter. Effects of coating structural parameters, such as the particle diameter and coating thickness, were investigated to identify the optimum microporous coating for heat transfer enhancement. All microporous surfaces in this paper were found to significantly enhance FC-72 flow boiling heat transfer in minichannels. With the optimum coating, the heat transfer coefficients could be 7-10 times those of the uncoated surface, and the boiling wall temperature was reduced by about 10 K. The flow boiling phenomena in the present minichannels were distinctly different from those in conventional-sized channels, due to the wall confinement effect on vapor bubbles. The confinement effect was evaluated by taking the contributions of the liquid mass flux and channel size into consideration. It was found that the very strong confinement effect was unfavorable with respect to flow boiling enhancement of the microporous coatings in the minichannels.