Gravity Independence of Microchannel Two-Phase Flow

Highly integrated electronic systems and onboard processing capabilities are critical to future NASA missions, but their development and operation is constrained by poor thermal management. Compact, two-phase coolers offer the heat transport capacity, small volume, low mass, and low power consumption necessary to efficiently remove heat from these systems, but the present lack of understanding of boiling heat transfer in microchannels in microgravity severely limits NASA's implementation of this technology. The hypothesis is that in microchannel flow boiling, shear and surface tension forces dominate and gravitational forces play a minor role. The proposed effort will systematically determine the microchannel two-phase flow conditions for which the flow behavior is gravity independent. Most of the amassed two-phase flow and heat transfer knowledge comes from experiments conducted in Earth's gravity. Space missions span varying gravity levels, thus demanding thorough understanding of gravitational effects on two-phase flow. Alternatively, it is believed that in microchannel flow boiling, shear and surface tension forces dominate thermal and momentum transport and gravitational forces play a minor role. The multi-year effort will systematically explore the dependence of microchannel two-phase flow behavior on gravitational acceleration and determine the flow parameters for which the effect of gravitational force is insignificant. This objective requires the development and assembly of a state-of-the-art flow boiling test facility that can be rotated relative to earth's gravitational acceleration vector and can be installed in a plane capable of achieving variable gravity via parabolic maneuvers. Measurements from the test facility will enable calculation of critical heat flux, local heat transfer coefficients, and pressure drop, all of which can be compared to existing correlations for flow boiling in microchannels in terrestrial gravity to assess gravity effects. Visualization of the evaporator using a high-speed camera will provide flow pattern assessments that will be correlated with measurements and calculations. Several recent studies provide evidence of the link between flow regimes and key two-phase parameters and inspire confidence that regime-informed approaches can improve the predictive accuracy of two-phase formulations. The goals of the ground studies are to identify boundaries separating gravity-dependent and gravity-insensitive two-phase flows and to identify boundaries separating flow regimes and determine the extent to which these boundaries are applicable to different boiling phenomena like pressure drop, heat transfer coefficients, and critical heat flux. The results from the ground studies will be validated via testing in reduced gravity. More »