Aligned with the European Union’s ambition to achieve net zero greenhouse gas emissions by 2050, a plethora of carbon dioxide abatement technologies have been developed and are being deployed in industries. Among these, Carbon Capture, Utilisation and Storage (CCUS) technologies play a pivotal role in decarbonising hard-to-abate sectors where carbon dioxide emissions are inevitable. Geological carbon storage is an effective option for long-term carbon dioxide sequestration, especially in regions with substantial sub-surface capacity for large scale permanent storage.

In this technology, transporting carbon dioxide in supercritical conditions enhances the transport efficiency due to its high density and low viscosity. However, the rapid depressurization of supercritical carbon dioxide across control valves prior to injection into geological storage reservoirs can trigger complex phase transition phenomena, including flashing, cavitation and shock wave formation. This study investigates the expansion dynamics of supercritical carbon dioxide using converging-diverging nozzles, as a simplified yet representative geometry for control valves, to develop modelling strategies for the depressurization process.

Building on earlier work, the simulation framework was upgraded from modelling dense liquid phase to supercritical phase which is characterised by the sharp thermodynamic gradients in the vicinity of the critical point. This study employs the Span-Wagner Equation of State to capture the real gas behaviour of carbon dioxide. The implementation in Ansys Fluent was carried out via a non-uniform lookup table, refined near the critical region and smoothed using the Savitzky-Golay filter to ensure numerical robustness of the solver while preserving the thermodynamic fidelity. Simulations were performed using the Mixture multiphase model in Ansys Fluent, incorporating interphase slip velocity to represent mechanical non-equilibrium between the liquid and vapour phases. The phase transition was modelled using the
Zwart-Gerber-Belamri (ZGB) cavitation framework tuned to simulate the non-equilibrium phase transition behaviour. Both quasi-one-dimensional and two-dimensional simulations were performed and the results were validated against experimental data from converging-diverging nozzle studies.

The results highlight the significance of metastable effects, sensitivity to cavitation model coefficients and the necessity of incorporating slip velocity between phases to accurately capture the phase transition behaviour. While the quasi-1D simulations provide qualitative trends, 2D simulations resulted in better agreement with the experiments. Overall, this study establishes a scalable and robust simulation framework for modelling the depressurization behaviour of supercritical carbon dioxide, which can be extended to complex geometries such as control valves used in CCS infrastructure.