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Transient CFD Analysis
Fire Simulation and Risk Modeling
 
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Background

Fire engineering and risk assessment are integral parts of the design process within the Built Environment industry. Besides the obvious structural design, today’s buildings pay special attention to designing a comfortable atmosphere for inhabitants, be it in terms of acoustic noise, sunlight or dispersal of dust particles exiting an air conditioning duct. Predicting temperatures and convection flow fields within an open plan office are typical engineering parameters that most designers are concerned with these days. In certain circumstances however, CADFEM UK CAE Ltd. have been involved with modelling more severe scenarios such as fire and smoke dispersal.

Analysis Set Up

Given that the fire spreads spatially with time, it only makes sense to conduct a transient analysis, rather than a steady state solution. In the example shown to the right, a fire is initiated through the ignition of a gas leak on the lower right hand corner.

The gas is assumed to be Methane that undergoes combustion within air to yield CO2, CO, water vapour and NOx gases. Using ANSYS CFX, the user can customise the reaction with forward reaction rates, pre-exponential factors or activation energies. Alternatively, a user can simply opt to use one of the predefined combustion reactions that are already within CFX.

As mentioned earlier, a transient solution with an appropriate time step needs to be used. This largely depends on the burning rates of the reaction, the mesh size and initial conditions. In this particular simulation, an SST turbulence model was used to predict the flow of the gases and the burning of the fuel. The advantage of using an SST model is that it has been shown to be more accurate than standard K-omega and K-epsilon models in terms of boundary layer prediction and heat transfer. If we wanted to know the temperature distribution near a surface, predicting the flow near the boundary layer is vital.

During a combustion reaction, given the number of gases that are produced, it is important to consider the effects of buoyancy under density gradients within the flow domain. It is common practice to set the heaviest gas as the baseline reference density. By using a multiphase buoyancy model, we are ensured that each species of gas has its own fluid velocity, while also interacting with the other gases present. As it would be in reality, each gas would therefore follow a slightly different trajectory based on its density and temperature.

In this particular analysis, structural surfaces such as the stairs, walls and vertical poles are given specific material properties such as thermal conductivity. The figures below show the gradual increase in temperature of the floor near the fire within the first 10 seconds.

Design Benefit

Fire modelling finds use in many applications, one of which could be forensic studies. Using a time dependent analysis, we can determine the concentration of toxic gases at a certain point or even the time taken for a surface to reach a critical temperature (see figure to right). Alternatively, it can also go a long way in determining evacuation routes in the event of an emergency.

 

 

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