Multi-Phase CFD Modelling of CO2 Releases From High-Pressure Pipelines

Abstract

An accurate prediction of the ‘source strength’ of CO2 released from high-pressure pipelines is of great importance for risk assessment, because this parameter determines the subsequent dispersion in the atmosphere. The widely used method is to employ a one-dimensional discharge model describing the conservation of mass, momentum and energy. The fluid is usually considered to remain at thermal and mechanical equilibrium during the depressurisation process, while the non-equilibrium liquid/vapour transition phenomena are ignored. Although efforts have been made to model non-equilibrium, two-phase CO2 depressurisation in recent years, possible improvement can be made by using a more precise Equation of State (EOS) and more detailed models. In this paper, a multi-phase Computational Fluid Dynamics (CFD) model is presented to simulate CO2 releases from high-pressure pipelines. A real gas EOS (GERG-2008) was incorporated into the CFD code. This enabled accurate modelling of the thermodynamic properties to achieve a more precise source strength estimate. The non-equilibrium liquid/vapour transition was modelled by introducing source terms for mass and latent heat. A ‘time relaxation factor’ was used to control the inter-phase mass transfer rate. The CFD model was validated against experimental results from the British Petroleum (BP) CO2 release trials. The optimum relaxation time (for the CO2 depressurisation cases tested) was obtained through simulations by varying the time relaxation factors and comparison with experimental data. In addition, the effect of the relaxation time on the source strength prediction is discussed.