Experimental Investigation Of Flow Boiling And Spray Cooling On Enhanced Surfaces In FC-72

In this study the effect of a recently developed, thermally conductive microporous coating on flow boiling and spray cooling performance was studied using FC-72 as the working fluid. The overall goal is to further increase heat transfer in forced convection systems and in doing so, provide a viable option for future cooling applications. In flow boiling, the coating increased heat transfer coefficients by as much as 400% and produced values exceeding 40,000 W-m-2 K-1. This enhancement is believed to be a result of both an increase in active nucleation sites and a decrease of the superheated liquid layer. It is believed that the thinner superheated layer combined with the highly conductive coating produces high liquid temperature gradients and is the reason for the slightly negative wall superheats recorded in subcooled tests. Additionally, the coatings ability to wick fluid delays the onset of CHF and results in dual temperature excursions at high heat fluxes. The application of the coating to spray cooling was found to increase heat transfer coefficients by ~45% throughout the entire boiling curve. Even though this enhancement is less than that achieved with flow boiling; it is still significant considering the already high heat transfer coefficients of spray cooling. In addition to enhancing nucleate boiling, the coating is also believed to facilitate evaporation in spray cooling. Evaporation efficiency is increased as a result of the wicking effect of microporous coating which pulls the liquid toward the surface and thus decreases film thickness and increases the three-phase contact line length. The unique flow characteristic of spray cooling allows it to outperform flow boiling by producing higher CHF and h-values (especially at low heat fluxes). However, this is at the expense of pumping power which can be three orders of magnitude higher than those of flow boiling. Finally, in both flow boiling and spray cooling, the new coating was found to outperform a previously developed aluminum based (ABM) coating.