A primary challenge in cryogenic fuel management is the chilldown (or cool-down) process. Before a vessel or pipeline can be fully loaded, the hardware must undergo a transient cooling phase to reach stable operating conditions. This process is inherently difficult to predict, involving rapid transitions through multiple boiling flow regimes with strongly non-linear heat transfer behaviour. As cryogenic liquid contacts the warm pipe wall, it flashes to vapour, forming an insulating film. As the wall cools, the flow transitions from film boiling through transition boiling and nucleate boiling before reaching a steady liquid phase, each regime exhibiting differing heat transfer characteristics.
Physical testing of chilldown can be expensive particularly for fluids such as liquid hydrogen. However, numerical methods offer a cost-effective solution for assessing transient behaviour of these physical systems, underpinned by validation using well established experimental datasets.
This study presents a CFD modelling framework for the chill-down validated against LN2 experimental datasets established by Darr et al. [1]. The model demonstrates strong agreement with experimental wall temperature profiles, capturing the correct thermal behaviour to within ±5% across all validated cases.
The modelling approach employs a multiple flow regime sub-model, implemented within STAR CCM+, using heat-transfer-based phase change rather than the Kinetics based formulations such as the Lee model commonly used in existing literature [2]. A key novelty of this work is that the heat transfer correlations are applied directly without case-specific tuning, yet the model achieves consistent agreement across all tested conditions. In the modelling approach, three distinct regimes are defined based on the local vapour volume fraction. The transition thresholds between these regimes are kept fixed across all cases, although the sensitivity of the predictions to these values is acknowledged and explored within this work. In the dispersed liquid regime, the liquid is the continuous phase, and vapour is dispersed where slip between phases is modelled but no phase change is applied. In the intermediate regime, heat-transfer based phase change is active, where two surface renewal theory models are compared. This includes the Hughes-Duffey and the Coste models, both originating from the nuclear industry. In the vapour-dominant regime, film boiling heat transfer is captured using the Ranz-Marshall correlation alongside an analytically derived Nusselt number formulation. This structured, physics-based approach addresses a shortcoming of existing models in the literature, which typically rely on the Lee model with tuned empirical coefficients that limit their generalisability [2].
Building on the validated LN2 framework, the methodology is subsequently applied to a liquid hydrogen case which covers forced convection heat transfer through a vertically mounted heated tube from Tatsumoto et al. [3]. The translation to liquid hydrogen modelling introduces several additional thermodynamic complexities, including a lower boiling point, distinct liquid to gas property ratios, and the ortho-to-para hydrogen conversion, which acts as an additional heat sink during the chilldown process.
The results of this study provide a validated, computationally efficient approach to transient cryogenic transfer line chilldown. By demonstrating that a multiple flow regime-based approach can produce accurate predictions, this work offers practical guidance for CFD practitioners and supports the reliable design of next generation cryogenic transport systems.
[1] Darr, S.R. et al., 2016, "An experimental study on terrestrial cryogenic transfer line chilldown I. Effect of mass flux, equilibrium quality, and inlet subcooling," Int. J. Heat Mass Transf., 103, pp. 1225–1242.
[2] Kassemi, M. et al., 2023/2024, "Predictions of Line Chilldown Boiling Regime Transitions by a Coupled CFD-Subgrid Boiling Model Validated against 1G LN2 Experiments," Int. J. Heat Mass Transf. (published 2024, presented at 30th Space Cryogenics Workshop, 2023).
[3] Tatsumoto, H. et al., 2015, "Forced convection heat transfer of subcooled liquid nitrogen in a vertical tube," J. Phys.: Conf. Ser., 234.
Acknowledgement: This work was funded by the Engineering and Physical Sciences Research Council (EPSRC) and Airbus UK (EP/Y52778/1). Additionally, we are grateful for access to the University of Nottingham’s Ada HPC service.

Keywords
Chilldown|Liquid Nitrogen|Computational Fluid Dynamics|Multiple Flow Regime|Two-phase Flow