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Computational Fluid Dynamics


CFD first requires that a geometrical model be constructed, to represent the type of fluid flow problem that is to be modelled. This is often accomplished by extracting the coordinates of the boundaries from drawings derived from a CAD analysis. The space (or domain) through which the fluid moves must then be split into many inter-connected volumes by means of a mesh which divides the domain into small elements. The governing equations, representing the physics of the flow problem, must be satisfied for each of these elements.

In practice, the mesh defines the precise locations where the equations will be solved. Both the quality of the mesh and the mesh density (or number of small elements) are important because they directly affect the quality of the results produced. These choices are made on the basis of experience and practice, and should be investigated as part of the modelling exercise.

The mesh is imported into the CFD package and the computational model is completed by specifying the physical and material properties, the boundary conditions, the turbulence model, the details of the physics to be resolved, and the strategy for obtaining a solution. Running the proprietary software leads to predictions for the values of all the required variables throughout the meshed domain. Currently, Zeta-pdm Ltd employs the Fluent software supplied by ANSYS, in both the UK and Dutch Offices.

In many situations, it will be convenient to interpret the CFD predictions by applying visualisation software. This provides the engineer with options for displaying the key variables such as the fluid velocity vectors, pressure, temperature, mass fraction, turbulence levels, or the derivatives of these, so as to reveal the variation of these properties in space and time. Additionally, CFD visualisation tools make it possible to determine integral quantities, such as the mass flow rate through a specified area, or the gas/liquid ratio in a particular location, before showing how these quantities vary from one location to another. Finally, it is common practice to present complex fluid flow predictions in the form of multi-dimensional graphs, or as a video representation.

Case Studies

Upstream Piping

The upstream piping (or isometrics) will always dictate the condition of the flow entering the vessel. Special care should therefore be taken when designing the piping in a gas production system to minimize the risk of maldistribution and avoid the droplet shatter that could reduce the performance of the downstream separation internals. By modelling the isometrics with CFD, maldistribution can be reduced and the size of any droplets entering the separator can be predicted. This is useful for troubleshooting existing vessels and optimising the design of the process internals.

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