Enhanced saturation and subcooled boiling of FC-72 dielectric liquid

Experiments are conducted which investigated enhancements in nucleate boiling of FC-72 dielectric liquid on porous graphite and compared results with those on a smooth copper surface of the same dimensions (10 × 10 mm). Also investigated is surface temperature excursion at boiling incipience and the obtained values of CHF are compared with those of other investigators on copper, silicon, and micro-finned silicon surfaces and micro-porous coatings. Results showed no temperature excursion at boiling incipience on porous graphite but as much as 14.0 K in saturation and 9.0-13.3 K in subcooled boiling on copper. The nucleate boiling heat transfer coefficients on porous graphite are significantly higher than on copper and the values of CHF (27.3, 39.6, 49.0, and 57.1 W/cm² in saturation and at ΔT_sub = 10, 20, and 30 K, respectively) are 63-94% higher than on copper (16.9, 21.9, 26.9, and 29.5 W/cm², respectively). The surface superheats at CHF on porous graphite (11.0 K in saturation and 17.2-19.5 K in subcooled boiling) are lower than on copper (21.3 K and 22.9-24.9 K, respectively). The nucleate boiling heat transfer coefficient increase with subcooling and CHF increased linearly with ΔT_sub. The subcooling coefficient of CHF on porous graphite (0.041) is slightly smaller than those on micro-porous coatings (0.044 and 0.049) but much higher than those on micro-finned silicon surfaces (0.022, 0.036), and on Cu, Si, and enhanced SiO₂ (0.018) surfaces.