Depressurization of CO2-rich mixtures in pipes: Two-phase flow modelling and comparison with experiments

We present and discuss two-phase flow models to simulate transient flow of CO2-rich mixtures in pipes, which is of relevance for design, operation and safety. The model predictions are compared to data from five depressurization experiments from three facilities. Two flow-model formulations are considered. One is a homogeneous equilibrium model in which the phases travel at the same velocity. The other is a more complex two-fluid model in which the slip between the phases is modelled. The thermodynamic equilibrium of the multi-component mixture, constrained by energy and volume, is computed simultaneously with the flow equations. In general, good agreement with the experiments is obtained, including the dry-out point where the liquid in the pipe has evaporated. For the friction and heat-transfer models tested, the two-fluid model did not provide substantially better predictions than the homogeneous equilibrium model. The effect of different heat-transfer models is also discussed. In our case, it is necessary to take the pipe heat capacity into account.