Characterizing cryogenic chilldown and boil-off: A liquid nitrogen experimental study

Cryogenic chilldown and boil-off directly impact the efficiency, safety and operational margins of any system that handles cryogenic fluids, including propulsion, storage and transfer applications. These processes arise from the interaction between wall heat transfer, boiling regimes, and vapor-space dynamics, and are strongly influenced by geometry, initial thermal conditions, and operating procedures. As a result, well-resolved experimental data remain essential for the development and validation of transient two-phase models. This work presents an experimental investigation of liquid nitrogen chilldown and boil-off in a small-scale test facility composed of two cryogenic vessels, a feed tank and a return tank, connected by a horizontal transfer line. The setup is instrumented to measure temperature evolution, pressure and liquid levels, under controlled valve operations. Wall heat fluxes and mass flow rates are derived from these measurements through energy and mass balances. The influence of initial thermal conditions is examined by comparing chilldown into a pre-cooled transfer line with that of an ambient-temperature configuration under unpressurized flow. To assess the role of the driving mechanism, both unpressurized and pressurized flow transfer conditions are then considered for an ambient temperature transfer line and return tank. Boil-off behavior in the feed tank is characterized at different liquid fill levels and further investigated through controlled heating using an immersed electrical heater. A quantitative mass and energy balance framework is used to reduce the measurements to transfer efficiency, thermodynamic minimum chilldown demand, residual losses, and closed tank energy contributions. The results demonstrate that while pressure-driven transfer shortens the time by more than an order of magnitude at the expense of liquid retention, pre-cooling increases the transfer efficiency and reduces wall enthalpy demand by approximately a factor of three. Over the examined fill level range, sensible heating of the liquid dominates the closed tank energy budget for boil-off, providing a quantitative basis for evaluating the effect of liquid inventory on subsequent thermal evolution and hold-time. These findings quantify the trade-off between transfer speed and cryogen consumption and provide an experimentally supported basis for selecting chilldown strategies under different time and inventory constraints.