Evaluation of CFD Codes for Gas Dispersion Studies

Computational Fluid Dynamics (CFD) codes are widely used for gas dispersion studies on offshore installations. Majority of these codes use the porosity/distributed-resistance (PDR) approach to model small geometric details. In this approach, small scale obstacles are not resolved on the computational mesh, which leads to reduced computational costs. However, there are some uncertainties regarding this approach, especially in terms of turbulence generation, since a large part of the geometry is unresolved.

An alternative approach, which can be implemented in general purpose CFD codes, is to combine multiple small scale obstacles in close proximity and use porous media models to represent blockage effects. This approach is validated in this study, by comparing with large-scale gas dispersion experiments. Gas concentrations and gas cloud volumes obtained from simulations are compared with measurements. These simulations are performed using the commercially available ANSYS CFX, which is a general purpose CFD code. For further comparison, additional simulations are performed using CFX where small scale objects are explicitly resolved and using FLACS, which is a specialized CFD code used for process safety applications.

The aim of this work is to evaluate the modelling approaches available and quantify their accuracy.