(27a) Nucleate Boiling Enhancements On Porous Graphite and Micro-Porous and Macro-Finned Copper Surfaces

Nucleate boiling of dielectric liquids is important in engineering applications such as refrigeration and immersion cooling of high power computer chips and central processor units (CPUs). These liquids are chemically compatible and environmentally friendly and have low saturation temperatures (54 - 64 ^oC) at atmospheric pressure.  Challenges in cooling high power computer chips using nucleate boiling of dielectric liquids are removing the dissipated thermal power of > 100 W, while keeping the junctions temperature < 85 ^oC, and mitigating the effect of the hot-spots developing on the chip surface. Hot spots’ local heat flux could be more than 3 times the surface average and induced thermal stresses could reduce service life and increase failure frequency of the chips.  Because they are high wetting (static contact angle on most metal and silica surfaces < 5^o), undesirable excursion in surface temperature prior to initiating nucleate boiling on smooth copper and silicon surfaces could reach or exceed 25 K.

Numerous investigations have been reported on enhancements of nucleate boiling of dielectric liquids on surfaces with reentrant cavities, micro- and macro-porous surfaces, macro- and micro-structured surfaces, surfaces with micro-porous coating and metal foams. In addition to reducing or eliminating the temperature excursion prior to boiling incipience, the high active sites density for bubbles nucleation increases heat transfer coefficient and Critical Heat Flux (CHF) flux and reduces surface superheat. While CHF is a practical limit in engineering applications, desirable operation is at or below the maximum nucleate boiling heat transfer coefficient; it is much higher than that at CHF and occurs at a lower surface temperature. 

Results of experimental investigations on enhancements of saturation and subcooled nucleate boiling of dielectric liquids FC-72, PF-5060 and HFE-7100 on porous graphite and micro-porous and macro-finned copper surfaces are presented, including the effect of surface inclination from 0^o (upward-facing) to 180^o (downward-facing). Results are compared with those reported by other investigators for macro-structured and micro-porous surfaces and surfaces with micro-porous coatings. In addition, results of CFD calculations are presented of sizing copper spreaders with thin surface layers of porous graphite and micro-porous copper for cooling 10 x 10 mm underlying chips using nucleate boiling of FC-72 and PF-5060 liquids.