Enhancing pool boiling in microgravity using micro-fins and an electrostatic field under different liquid subcooling

Abstract

Despite boiling heat transfer is an effective way to remove heat and its performances comply with the high requirements of the electronics cooling and aerospace devices, in reduced gravity levels environments a decrease of the Critical Heat Flux (CHF) occurs. In the past decades, several techniques were developed to enhance the performances of a boiling system, such as external force fields and surface modification. Adding micro-structures on a boiling surface can delay the CHF occurrence, due to capillarity. As capillary forces are independent of gravity, this technique is potentially useful in microgravity. In the present study, micro-finned surfaces of new geometry were tested during parabolic flights in microgravity conditions under the action of an electric field and at different subcooling levels ranging from 5 to 35 K. The experimental results, reported here for the first time, showed that the micro-fins alone (whose characteristic length ranges from 30 to 120 µm) were not effective to increase CHF in microgravity, even if the heat transfer coefficients were higher with respect to the smooth surface tested for comparison. The addition of an external electric field enhanced the CHF due to its vapor removal capability and the bubble breaking effect. Moreover, the combined effect of micro-fins and electric field showed a synergic effect, increasing the CHF value up to 2.2 times. The CHF increased linearly with the subcooling level. The results indicate that micro-fins enhancing action can be severely reduced by the accumulation of vapor in the proximity of the surface, due to lack of buoyancy. However, if vapor is removed with an alternative action (e.g., electric forces) the fin enhancement is restored.