Critical Heat Flux of Cryogenic Flow Boiling under Terrestrial, Partial, and Reduced Gravity Conditions

Abstract

Cryogenic fluid management (CFM) technology has been recognized as a critical area requiring substantial research to ensure the safe and reliable development of in-space cryogenic architectures, such as Nuclear Thermal Propulsion (NTP) systems. The urgency of this research is underscored by the lack of Critical Heat Flux (CHF) data for cryogenic fluids under reduced gravity conditions, which significantly hampers the development of accurate design tools. This study aims to elucidate the gravitational effects on cryogenic two-phase fluid physics and CHF by conducting the first-ever experimental measurements of cryogenic flow boiling using a steady-state heating method in a reduced gravity environment. Using liquid nitrogen (LN₂) as the working fluid, parabolic flight experiments were conducted to obtain both CHF measurements and high-speed video recordings of interfacial behavior under varying gravity levels, including microgravity and both Lunar and Martian gravities. Additionally, terrestrial ground experiments were performed across five distinct flow orientations: vertical upflow, vertical downflow, horizontal flow, 45° inclined upwards, and 45° inclined downwards. The study employed a circular heated tube featuring an inner diameter of 8.5 mm and a heated length of 680 mm. Vertical upflow exhibited the most enhanced CHF performance, while vertical downflow showed the poorest performance. Increasing gravitational acceleration—from microgravity to Lunar to Martian—reduced CHF due to intensified flow stratification caused by the strengthening buoyancy force. The gravitational effect was mitigated by increasing the mass velocity above a threshold of 700 kg/m²s. Finally, existing CHF correlations were evaluated, revealing a need for a new correlation specific to cryogenic fluids under reduced gravity conditions. Consequently, a new CHF correlation was developed and tested using datasets from microgravity as well as both Lunar and Martian gravities. The new correlation shows excellent predictive accuracy, evidenced by a mean absolute error (MAE) of 7.54%, against eleven newly acquired microgravity, Lunar and Martian CHF datapoints.