Effect of surface wettability on evaporation using spray cooling

Abstract

In the age of Moore’s Law, high-power electronics are packing a greater number of transistors per area onto printed circuit boards. As these transistors decrease in size, their switching frequency is increased, which also increases the power losses. To meet the growing needs of thermal management in such electronics, the use of air-cooled heat sinks may be insufficient, and, thus, spray-based multiphase cooling is explored. In this study, spray cooling is investigated, and the effects of surface wettability (hydrophobic or hydrophilic) and geometry (micro-, nano- and hierarchically-structured) are quantified to determine optimal surface conditions. Additionally, this research presents methods for altering surface wettability and geometry for commonly used aluminum alloys. Spray cooling was characterized at flow rates of 0.012 mL/s and 0.0309 mL/s, for heat fluxes ranging from 9000 to 61000 W/m² on 99% aluminum surfaces. The maximum heat transfer coefficient observed was 1260 +/- 230 W/m²K on a hierarchically-porous hydrophobic surface. Moreover, improvements of up to 30% in heat transfer coefficient were observed when comparing fabricated surfaces to cleaned, plain samples. It was found that, on average, hierarchically-porous samples outperform both micro- and nano-porous, with nano-porous samples exhibiting the worst performance. In this study, hydrophobic surfaces had, on average, better heat transfer performance compared to hydrophilic samples, which may be a result of the downward-facing surface orientation.