A multiple flow regime CFD methodology for modelling liquid nitrogen chilldown in vertical pipes

Cryogenic fuels are central to the decarbonisation of heavy-duty transport and the deployment of next-generation propulsion systems. However, their widespread adoption is constrained by the lack of reliable predictive tools for transient thermofluid processes. A critical bottleneck is pipe chilldown, in which system hardware undergoes transient cooldown before system operation. Chilldown is a highly coupled, non-linear process governed by rapid transitions between film, transition, and nucleate boiling regimes, which remain challenging to model accurately and efficiently. This study presents a novel, computationally efficient 2D axisymmetric CFD framework for liquid nitrogen (LN 2 ) cryogenic chilldown, validated against LN 2 experiments. The key distinguishing feature of the approach is the use of distinct, regime-specific phase change models within each flow sub-regime, rather than a single correlation applied across the full chilldown transient. The model integrates this multiple flow regime boiling approach within STAR-CCM+ using heat transfer-based phase change, with two heat transfer correlations governing the liquid-vapour phase change investigated: the Hughes–Duffey and Coste models. Both models produce wall temperature predictions within 10% across all validated cases. The Hughes–Duffey model demonstrated better overall agreement with the experimental data, producing more physically consistent boiling curve predictions and more accurately capturing the characteristic features of the chilldown transient. The results demonstrate that a multiple flow regime approach can deliver accurate and generalisable chilldown predictions, providing practical guidance for CFD practitioners and supporting the design of next-generation cryogenic transfer systems.