Pool boiling enhancement through hierarchical texturing of surfaces

Pool boiling is an important technology that can be used for the thermal management of microelectronics. Previous studies have shown that pool boiling enhancement can be obtained by controlling the surface wettability, by patterning surface features, and by controlling micro/nano porosity characteristics of the heater surface. While many of these studies have focused on the increase in critical heat flux (CHF), both an increase in CHF and heat transfer coefficient are desired for application to thermal management. In this study pool boiling experiments are conducted on hierarchical copper microporous structures in order to maximize the heat transfer coefficient and critical heat flux (CHF). This was accomplished by investigating the boiling curve for DI water on flat and structured microporous copper surfaces fabricated with spherical copper powder. The surfaces were tested as-fabricated and by machining channels into the coated surfaces to control vapor and liquid flow paths. Unpatterned (as-fabricated) microporous structures showed that the CHF can be increased over 220% compared to that of flat surfaces, both with a significant rise (>100°C) in the surface superheat. The large increase in superheat was found to arise from the existence of partial dry-out in the microporous copper. By patterning the microporous copper, vapor was allowed to escape and resulted in a 412% increase in CHF with significant decrease in surface superheat (<49°C) over those of flat surfaces. This large enhancement is believed to be attributed to the effective removal of the partial dry-out within the patterned porous copper as well as the separation of liquid and vapor flow over the surface. Moreover, it was found that both the thickness of the microporous layer and depth of the channel played an important role in both CHF and heat transfer coefficient enhancement. Both can be easily controlled during packaging to improve thermal management of devices cooled through pool boiling.