Spraying a dielectric liquid such as PF-5060 (95% pure FC-72) has been shown to be an effective method of cooling high power electronics. Recent studies have illustrated the potential enhancement of spray cooling by the addition of extended structures, particularly straight fins, to the heated surface. In the current work, these studies are extended to finer fin widths and pitches and longer fin lengths. Four such heat sinks were EDM wire machined. These 1.41 × 1.41 cm2 heat sinks featured a fin pitch of 0.86 mm; a fin width of 0.5 mm; and fin lengths of 0.5 mm, 1 mm, 3 mm, and 5 mm, which substantially increase the total area, allowing more residence time for the incoming liquid to be heated by the wall. The four enhanced surfaces and a flat surface with the same projected area were sprayed with a full cone nozzle using PF-5060 at 96 mL/min, 24°C, and 3.65 atm (38.5 psig). In all cases, the enhanced surfaces improved thermal performance. Longer fins were found to outperform shorter ones in the single-phase regime. Adding fins also resulted in two-phase effects and higher heat transfer at lower wall temperatures than the flat surface. Finally, the two-phase regime appeared to be marked by a balance between added area, changing flow flux, channeling, and added conduction resistance. Although critical heat flux (CHF) was not reached for the finned surfaces, fin lengths between 1–3 mm appeared to be optimum for heat fluxes as high as 131 W/cm2 and the range of conditions studied.
