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Reactions
Chapter 12. Reactions
This chapter provides an overview of how to specify combustion (the reaction of fuel vapor and oxygen)
using PyroSim. A more detailed discussion of this topic is provided in the Fire Dynamics Simulator User’s
Guide (McGrattan, et al., 2013) and the Fire Dynamics Simulator Technical Reference Guide (McGrattan,
et al., 2013).
As described in the Fire Dynamics Simulator User’s Guide (McGrattan, et al., 2005), a common source of
confusion in FDS is the distinction between gas phase combustion and solid phase pyrolysis. The former
refers to the reaction of fuel vapor and oxygen; the latter the generation of fuel vapor at a solid or liquid
surface. In an FDS fire simulation, there is only one gaseous fuel that acts as a surrogate for all the
potential fuel sources. The reaction is defined using the Edit Reactions dialog in PyroSim.
The PyroSim interface supports only the single-step, mixing controlled combustion model to account for
the evolution of the fuel gas from its surface of origin through the combustion process. The alternative
provided in FDS6 is the finite-rate approach, where all of the individual gas species involved in the
combustion process are defined and tracked individually. This finite-rate approach is recommended only
for Direct Numerical Simulation. The mixture fraction model is the most frequently used approach.
Mixture Fraction Combustion
In FDS, there are two ways of designating a fire: the first is to specify a Heat Release Rate Per Unit Area
(HRRPUA) as part of a surface, the other is to specify a HEAT_OF_REACTION, along with other thermal
parameters, as part of a material. In both cases, the mixture fraction combustion model is used.
The heat release rate approach is the simplest way to specify a fire. All that needs to be done is create a
burner surface with the desired heat release rate (see Chapter 6). If no other reaction is specified,
propane will be used as the surrogate fuel. If a reaction is specified, that reaction will be used to
calculate the combustion products.
In the mixture fraction model, the reaction is assumed to be of the form:
Figure 12.1. Reaction equation
By including a reaction in the model, the species AIR, PRODUCTS, and the FUEL used by the reaction
become tracked species. As a result, their components, OXYGEN, CARBON DIOXIDE, WATER VAPOR,
CARBON MONOXIDE, SOOT, and NITROGEN can be referenced by output data. It is important to
understand that these instances of species are not explicitly tracked, and are somewhat distinct from
other species which may share a name. For instance, adding an additional tracked OXYGEN species to a
PyroSim model will not result in more oxygen being available for the combustion model. In this case,
only the oxygen included as part of the AIR lumped species is reactive. For more information about
species, see Chapter 11.
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