Application of Flow Boiling for Thermal Management of Electronics in Microgravity and Reduced-Gravity Space Systems

Large density differences between liquid and vapor create buoyancy effects in the presence of a gravitational field. Such effects can play an important role in two-phase fluid flow and heat transfer, especially critical heat flux (CHF). CHF poses significant risk to electronic devices, and the ability to predict its magnitude is crucial to both the safety and reliability of these devices. Variations in the gravitational field perpendicular to a flow boiling surface can take several forms, from flows at different orientations at 1 g _e to the microgravity environment of planetary orbit, to the reduced gravity on the Moon and Mars, and the high g ´s encountered in fighter aircraft during fast aerial maneuvers. While high coolant velocities can combat the detrimental effects of reduced gravity, limited power budget in space systems imposes stringent constraints on coolant flow rate. Thus, the task of dissipating the heat must be accomplished with the lowest possible flow velocity while safely avoiding CHF. In this paper, flow-boiling CHF is investigated on Earth as well as in reduced gravity parabolic flight experiments using FC-72 as working fluid. CHF showed sensitivity to gravity at low velocities, with microgravity yielding significantly lower CHF values compared to those at 1 g _e. Differences in CHF value decreased with increasing flow velocity until a velocity limit was reached above which the effects of gravity became inconsequential. This proves existing data, correlations, and models developed from 1 g _e studies can be employed with confidence to design reduced gravity thermal management systems, provided the flow velocity is maintained above this limit. This paper discusses two powerful predictive tools. The first, which consist of three dimensionless criteria, centers on determination of the velocity limit. The second is a theoretically based model for flow boiling CHF in reduced gravity below this- - velocity limit.