(397f) Recent Cryogenic Propellant Transfer Line Steady State Flow Boiling Experiments in 1-g

NASA and the aerospace community are developing cryogenic fluid management technology to enable future manned and unmanned missions beyond Low Earth Orbit. Cryogenic propellants are beneficial over traditional storable propellants due to a relative improvement on safety and environmental concerns (storable propellants are toxic) and higher performance. Cryogens, however, exist as gases at standard conditions and are thus difficult to store and difficult to transfer as single-phase liquids due to a variety of reasons. Of particular interest in this work is the nuclear thermal propulsion system which relies on transferring liquid hydrogen from a storage tank to a nuclear reactor via pumps. The connecting transfer line and associated hardware must be chilled down to cryogenic temperatures and then maintained at steady-state flow with little-to-no boiling to avoid system instabilities due to two-phase flow. Due to the low normal boiling point of cryogens, phase change, complex flow patterns, two-phase flow boiling, and high heat transfer are inevitable.

Researchers at NASA and Purdue University are currently conducting ground and reduced gravity experiments to investigate the effect of gravity on steady-state cryogenic flow boiling in the transfer line. Understanding the flow conditions over which the onset of nucleate boiling or critical heat flux occurs allows designers to set limits on the allowable heat flux into the transfer line to prevent or limit boiling. This presentation covers recent steady-state flow boiling transfer line testing conducted in 1-g across five flow orientations: vertical upflow, vertical downflow, horizontal flow, 45â—¦ inclined upflow, and 45â—¦ inclined downflow across a wide range of mass flux and inlet pressures. High-speed video recordings were utilized to capture two-phase flow patterns and interfacial behaviors. Results show that vertical upflow yields the highest heat transfer coefficients, while vertical downflow exhibited the lowest. As mass velocity increases, the differences in heat transfer among orientations is less distinct as inertial forces dominate over buoyancy forces. Meanwhile, symmetrical flow patterns were exhibited in vertical orientations, and non-vertical orientations exhibited asymmetric flow stratifications, primarily due to the buoyancy force in 1-g.

This work is funded through NASA Flight Opportunities under the Space Technology Mission Directorate at NASA.