Fire Simulation for Engineers/FDS/Extra Gas Species

Extra gas species

Third, some properties are set up, as the properties of extra gas species (SPEC).

Defining extra gas species, SPEC

Gases that are introduced into the domain that are neither reactants nor products of combustion, like carbon dioxide from an extinguisher, are tracked separately from the mixture fraction model for gas phase combustion via an additional scalar transport equation. In fact, there does not need to be any fire at all, as FDS can be used to transport a mixture of non-reacting ideal gases.

The namelist group SPEC is used to specify each additional species. Each SPEC line should include at the very least the name of the species via a character string called ID.

The following gases are predefined in FDS and do not need any property to be set up:

Predefined extra species
AIR, ARGON, CARBON DIOXIDE, CARBON MONOXIDE, HELIUM, HYDROGEN, METHANE, NITROGEN, OXYGEN, PROPANE, WATER VAPOR.

For example:

 &SPEC ID='HELIUM' /

adds the predefined HELIUM gas as an additional specie that can be injected in the domain.

To specify a gas not included in the list, the user should input several chemical properties. See [FDS5 user's guide] for broader description.

If the ambient initial mass fraction of an extra gas specie is something other than 0, then the parameter MASS_FRACTION_0 is used to specify it. For example, the line:

 &SPEC ID='ARGON', MASS_FRACTION_0=0.1 /

specifies that 10% in mass of ARGON is to be included in the calculation, in addition to the 90% unlisted default ambient specie named AIR.

The following table summarizes some SPEC parameters:

Parameter	Type	Description	Unit	Default
ID	String	Identifier
MASS_FRACTION_0	Real	Initial mass fraction		0

CARBON DIOXIDE and carbon dioxide

These extra gas species are identified by an uppercase name, and are not to be confused with the lowercase species implicitly defined by FDS when doing a mixture fraction calculation for gas phase combustion, as fuel, oxygen, nitrogen, water vapor, carbon dioxide... See Section [sec:Modeling-gas-phase-comb-REAC] for reference.

If the user introduces an extra gas in the calculation that is the same as a product of combustion, as in:

&SPEC ID='CARBON DIOXIDE' /

FDS will take into account two different gases: the implicitly defined carbon dioxide and the extra gas species CARBON DIOXIDE, injected for example to simulate a CO₂ extinguisher.

The first is a product of combustion, while the second is just another gas: it does not participate to combustion, but it can dilute oxygen and contribute to fire suppression.

The two gases are tracked separately: carbon dioxide is tracked via the mixture fraction variable and CARBON DIOXIDE is tracked via its own transport equation.