Investigation of transient flow boiling heat transfer physics and system-level thermal-hydraulic responses during line chilldown

Efficient, safe, and reliable cryogenic liquid fuel transport within in-space cryogenic propellant storage and transfer systems is critical to enable long-duration deep-space missions to the Moon, Mars, and beyond. Based on the orbital locations of these systems, the propellant transfer lines are at an elevated temperature due to radiative heating from the surroundings. Hence, for transferring liquid cryogenic propellant successfully, these transfer lines must first undergo a complete line quenching or chilldown process to prevent any undesired boil-off of the liquid fuel. This transient flow boiling process associated with line chilldown involves complex two-phase spatial and temporal thermal and hydrodynamic interactions between the propellant liquid, vapor, and transfer line wall. In the past, extensive research efforts have been devoted to elucidating the mechanisms of flow regime transition and the corresponding heat transfer behaviors during line chilldown. However, they have been limited by low spatio-temporal resolution in heat transfer measurements and limited transient responses of system hydrodynamic parameters. In this work, in-tube line chilldown experiments using n-Perfluorohexane (n-PFH) were conducted at different mass flow rates and inlet liquid subcoolings in horizontal stainless-steel tubes under terrestrial conditions. Thorough analyses on the high-fidelity experimental data, encompassing wall and fluid temperature, mass flow rate, and pressure, provide fundamental insights into the independent and combined effects of liquid subcooling and mass flow rate on the thermal and hydrodynamic responses and their associated transient flow boiling heat transfer physics. The analysis provides insights into interfacial instability induced re-wetting phenomena, quench front propagation velocities, and local heat transfer coefficients in each flow boiling regime from inverted annular film boiling to termination of nucleate boiling via the transition points of minimum heat flux and critical heat flux. Finally, two key design parameters are developed and analyzed to quantify the efficiency of the entire chilldown process through analysis of the chilldown rate and liquid consumption. This work provides a deeper understanding of the complex transient two-phase flow physics associated with the cryogenic propellant transfer process and provides valuable design and operational guidelines for safe and efficient liquid propellant transfer.