SUBCOOLED BOILING OF DIELECTRIC LIQUIDS ON POROUS GRAPHITE AT DIFFERENT ORIENTATIONS
Experiments are performed which investigate saturation and subcooled boiling of FC-72 and HFE-7100 dielectric liquids on porous graphite measuring 10 × 10 mm at inclination, θ, from 0° (upward-facing) to 180°. The Nucleate Boiling Heat Transfer Coefficient (NBHTC) and CHF increase with increased subcooling, but decrease with increased θ. NBHTCs increase with increased surface superheat, ΔTsat, to maximums at or near the end of the fully developed nucleate boiling region, then drop with further increase in ΔTsat until reaching CHF. At ΔTsat < 15 K, NBHTCs for FC-72 are much higher than for HFE-7100, but slightly lower at higher surface superheats. Thus, for immersion cooling of electronics, FC-72 is far more effective than HFE-7100 at ∼ 10−15 K lower junction temperatures. However, CHF values for HFE-7100 are much higher than for FC-72. For both liquids, CHF increases linearly with liquid subcooling, ΔTsub, but decrease slowly with increased θ, from 0° to 90°, then decreases rapidly thereafter. The developed correlation for these dielectric liquids shows that CHF depends on the surface characteristics (smooth, porous, or microporous), ΔTsub and θ, and fits present and reported data by others to within ±10%.