(54af) Liquefied Natural Gas Vapour Dispersion Modelling Using Computational Fluid Dynamics Approach

Releasing at cryogenic temperature, Liquefied natural gas (LNG) dispersion is one of the most complicated problem in dense gas dispersion. Atmospheric boundary condition is an important factor affecting the dispersion process of LNG vapour by the effect of wind speed, surface roughness and atmospheric stability. Heat transfer from the surrounding and ground surface to the cold LNG vapour cloud will increase cloud temperature and reduce cloud density. The major effect of heat transfer to the LNG dispersion is the increasing of turbulent mixing process. Another relating heat transfer phenomenon is heat addition or heat removal due to the condensation or evaporation of water vapour. When the dispersion process occurs at sloping terrain with presence of obstructions, these factors will enhance gravity-driven flow and turbulent mixing. Computational Fluid Dynamics (CFD) is a powerful approach to take into account all of these factors for a sound modelling of LNG dispersion process.

Monin-Obukhov similarity theory assumes horizontally homogeneous atmospheric boundary layer and is widely used to model the profiles of velocity, turbulent kinetic energy and eddy dissipation rate in surface layer. These profiles are used as boundary conditions for the CFD simulation. Proposed model includes heat transfer from the ambient air to the vapour cloud by the diffusion of air to the boundary of the cloud. Change of material properties on temperature is also included. Parametric study of the proposed model is presented including (i) Effect of ground heat transfer models (ii) Effect of turbulent models. A study of the effect of impoundments in mitigating the distance to lower flammability limit (LFL) of LNG vapour cloud shows that impoundments can reduce the distance to LFL of the cloud to 40%.