Fire Simulation for Engineers/FDS/Limitations

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Although FDS can address most fire scenarios, there are limitations in all of its various algorithms. Some of the more prominent limitations of the model are listed here.

Low speed flow assumption

The use of FDS is limited to low-speedMach numbers less than about 0.3 flow with an emphasis on smoke and heat transport from fires. This assumption rules out using the model for any scenario involving flow speeds approaching the speed of sound, such as explosions, choke flow at nozzles, and detonations.

Rectilinear geometry

The efficiency of FDS is due to the simplicity of its rectilinear numerical grid and the use of a fast, direct solver for the pressure field. This can be a limitation in some situations where certain geometric features do not conform to the rectangular grid, although most building components do. There are techniques in FDS to lessen the effect of “sawtooth” obstructions used to represent non rectangular objects, but these cannot be expected to produce good results if, for example, the intent of the calculation is to study boundary layer effects. For most practical large-scale simulations, the increased grid resolution afforded by the fast pressure solver offsets the approximation of a curved boundary by small rectangular grid cells.

Fire growth and spread

Because the model was originally designed to analyze industrial-scale fires, it can be used reliably when the Heat Release Rate (HRR) of the fire is specified and the transport of heat and exhaust products is the principal aim of the simulation. In these cases, the model predicts flow velocities and temperatures to an accuracy within 10% to 20% of experimental measurements, depending on the resolution of the numerical grid. It is extremely rare to find measurements of local velocities and temperatures from fire experiments that have reported error estimates that are less than 10%. Thus, the most accurate calculations using FDS do not introduce significantly greater errors in these quantities than the vast majority of fire experiments.

However, for fire scenarios where the heat release rate is predicted rather than specified, the uncertainty of the model is higher. There are several reasons for this:

1. Properties of real materials and real fuels are often unknown or difficult to obtain;
2. The physical processes of combustion, radiation and solid phase heat transfer are more complicated than their mathematical representations in FDS;
3. The results of calculations are sensitive to both the numerical and physical parameters. Current research is aimed at improving this situation, but it is safe to say that modeling fire growth and spread will always require a higher level of user skill and judgment than that required for modeling the transport of smoke and heat from specified fires.

Combustion

For most applications, FDS uses a mixture fraction-based combustion model.

The mixture fraction is a conserved scalar quantity that is defined as the fraction of gas at a given point in the flow field that originated as fuel. In its simplest form, the model assumes that combustion is mixing-controlled, and that the reaction of fuel and oxygen is infinitely fast, regardless of the temperature.

For large-scale, well-ventilated fires, this is a good assumption. However, if a fire is in an under-ventilated compartment, or if a suppression agent like water mist or CO_{2} is introduced, fuel and oxygen are allowed to mix and not burn, according to a few empirically-based criteria.

The physical mechanisms underlying these phenomena are complex, and are tied closely to the flame temperature and local strain rate, neither of which are readily-available in a large scale fire simulation.

Subgrid-scale modeling of gas phase suppression and extinction is still an area of active research in the combustion community.

Until reliable models can be developed for building-scale fire simulations, simple empirical rules are used by FDS that prevent burning from taking place when the atmosphere immediately surrounding the fire cannot sustain the combustion.

Radiation

Radiative heat transfer is included in the model via the solution of the radiation transport equation (RTE) for a gray gas, and in some limited cases using a wide band model. The RTE is solved using a technique similar to finite volume methods for convective transport, thus the name given to it is the Finite Volume Method (FVM).

There are several limitations of the model:

• First, the absorption coefficient for the smoke-laden gas is a complex function of its composition and temperature. Because of the simplified combustion model, the chemical composition of the smokey gases, especially the soot content, can effect both the absorption and emission of thermal radiation.
• Second, the radiation transport is discretized via approximately 100 solid angles, although the user may choose to use more angles. For targets far away from a localized source of radiation, like a growing fire, the discretization can lead to a non-uniform distribution of the radiant energy. This error is called “ray effect” and can be seen in the visualization of surface temperatures, where “hot spots” show the effect of the finite number of solid angles. The problem can be lessened by the inclusion of more solid angles, but at a price of longer computing times. In most cases, the radiative flux to far-field targets is not as important as those in the near-field, where coverage by the default number of angles is much better.