Elimination of boiling incipience temperature drop in highly wetting ~uids using spherical contact with a ~at surface

This work examines the e}ectiveness of the zero!angle cavities created by curved contact with a ~at surface in promoting vapor embryo capture\ low temperature boiling incipience\ and reduction or elimination of the incipience temperature overshoot with highly wetting liquids[ Using FC!61 as a test liquid\ both single and multiple spherical point contacts on a ~at heater in both horizontal and vertical orientations are examined[ The e}ects of contact force\ material conductivity\ and ~uid ~ow obstruction as well as overall boiling behavior resulting from the surface contact are addressed[ Boiling incipience behavior\ for contact forces between 3[34 and 15[6 N\ showed independence of force for all contact materials[ Multiple contact points of insulative material with conductive tips showed the greatest improve! ment\ virtually eliminating the temperature excursion often associated with incipience while still allowing su.cient liquid access to the boiling surface such that the critical heat ~ux was within only 5) of that for a bare surface[ Because such features are large\ it is unlikely that prolonged boiling or submersion would signi_cantly degrade the boiling enhancement e}ects\ as is the case with the small features resulting from surface roughening techniques[ Þ 0888 Elsevier Science Ltd[ All rights reserved[ Nomenclature CHF critical heat ~ux k thermal conductivity P pressure qýs heat ~ux across heater surface T temperature[