Flow Boiling in Microgravity: Part 1 – Interfacial Behavior and Experimental Heat Transfer Results

Space agencies worldwide are being confronted with the challenges of more distant manned space missions, which will demand greater energy efficiency and reduced weight and volume. One method being considered to reduce the weight and volume of a long duration mission spacecraft is to replace present single-phase Thermal Control Systems (TCSs) with ones that rely on flow boiling and condensation. This transition will require a thorough understanding of the influence of reduced gravity on flow boiling and condensation, and the development of predictive tools for both. This study is the first part of a two-part study investigating flow boiling of FC-72 in microgravity, which is simulated in a series of parabolic flight maneuvers. Flow boiling experiments are conducted in a rectangular channel fitted with two opposite heating walls. The operating conditions include liquid inlet velocities of 0.1–1.9m/s, liquid mass velocities of 224.2–3347.5kg/m2s, and inlet subcoolings ranging from 2.8 to 8.1°C. The study includes both high-speed video analysis of interfacial features and heat transfer measurements. A dominant wavy vapor layer behavior is encountered for most operating conditions. Boiling is sustained mostly in ‘wetting fronts’ corresponding to contact regions between the wave troughs and the wall, and abated near the wave peaks. During a flight parabola, the heated wall temperatures decrease slightly as the aircraft enters the hypergravity ascent phase, then increase slightly during the microgravity phase, and decrease once again during the hypergravity descent. These temperature variations point to enhancement in flow boiling heat transfer with increasing gravity, and conversely a reduction with microgravity.