| An experimental and analytical investigation of saturated and subcooled flow boiling under microgravity conditions was conducted to study gravity effects on two-phase flow boiling heat transfer. Optical measurement of Freon 113 two-phase flow patterns in a 0.5m long, 5mm x 5mm vertical channel uniformly heated with transparent conductive films identified bubbly, slug, and annular flow under microgravity conditions. A significantly different void distribution was observed in bubbly flow. Velocity and temperature distributions in the radial direction in bubbly flow were calculated by Sato and Sadatomi's method. The distributions were changed by the increased void fraction and coalescence of bubbles under microgravity conditions. Analysis of experimental results showed that the bubble-induced turbulence, accounting for a large fraction of heat transfer in normal gravity, became smaller under microgravity conditions. Effects of gravity on bubble agitation were less pronounced in turbulent flow than in laminar flow because of the inherent turbulence. A substantial increase in void fraction resulted in an increase in the accelerational and frictional pressure drops in the low-quality range, suggesting that the pressure drop in the subcooled region is also significant in microgravity.; Motivated by the observed changes in flow and heat transfer rates in the subcooled region under microgravity conditions, the author designed, fabricated, and tested an additional forced convective boiling rig aboard NASA's low-gravity Learjet. A 0.5m long, 11.3mm diameter vertical copper tube was heated uniformly with a constant heat flux of 23.9kW/m{dollar}sp2{dollar}. Freon 113 in the once-through flow line was pumped by air-driven pistons to reduce the system transient time. An increase in wall superheats observed near the test section exit during the 18 seconds of microgravity implies a decrease in bulk convection in the absence of buoyancy force. The heat transfer analysis showed that the reduced gravitational acceleration affected the subcooled two-phase heat transfer through an increase in void fraction and stagnation of coalesced bubbles. The contribution of evaporation heat transfer was small in the low heat-flux range. The overall two-phase flow heat transfer coefficient in vertical upward flow was decreased under microgravity conditions. |
