Pool boiling in saturated and subcooled HFE-7100 dielectric fluid from a porous graphite surface

Experiments investigated the effects of subcooling on pool boiling of HFE-7100 dielectric liquid from porous graphite measuring 10×10 mm and 3.0 mm thick. Results indicate no temperature excursion at incipient boiling and that subcooled nucleate boiling occurs at surface temperatures well below saturation. Conversely, for a smooth copper surface the temperature overshoot at incipient boiling is as much as 37.4 K in saturation and 7.5-16 K in subcooled nucleate boiling. Nucleate boiling ensues on the porous graphite at 0.8 K in saturation, and -2.0 to -11.7 K in subcooled boiling, compared to 5.0 to 10.9 K on the smooth copper surface. The nucleate boiling heat fluxes are more than 500% higher and the surface superheats are significantly lower than those for the copper surface in same experimental setup. The critical heat flux (CHF) for the porous graphite surface is 31.8, 45.1, 55.9, and 66.4 W/cm² for saturation, 10 K, 20 K, and 30 K subcooled boiling, and the values for the smooth copper are 21.5, 28.1, 33.7, and 37.3 W/cm², respectively. In addition, the rate of increase in CHF for the porous graphite with increased subcooling ∼50% higher than for the smooth copper surface.