Fire Simulation for Engineers/FDS/Materials

Materials

Third, some properties are set up, as the properties of each material (MATL). This chapter covers RAMP (temperature dependent) namelist group, too.

Defining a material, MATL

The properties of each material used in the model are designated via the MATL namelist group. These properties indicate how rapidly the materials heat up, and how they burn. Each MATL entry in the input file must have an ID that can be referred by other namelist groups.

The following table summarizes some MATL parameters:

Parameter	Type	Description	Unit	Default
ID	String	Identifier
DENSITY	Real	Solid mass per unit volume	Kg/m3	0
EMISSIVITY	Real	Emissivity		0.9
CONDUCTIVITY	Real	Thermal conductivity	W/m/K	0.
CONDUCTIVITY_RAMP	String	Ramp ID for conductivity
SPECIFIC_HEAT	Real	Specific heat	kJ/kg/K	0.
SPECIFIC_HEAT_RAMP	String	Ramp ID for specific heat
HEAT_OF_COMBUSTION	Real	Heat of combustion	kJ/kg	0
HEAT_OF_REACTION	Real	Heat of reaction	kJ/kg	0
ABSORPTION_COEFFICIENT	Real	Absorption coefficient	1/m	50 000
BOILING_TEMPERATURE	Real	Boiling temperature	◦C	5000

Thermal properties

The MATL namelist group can be used to specify thermal CONDUCTIVITY (W m⁻¹ K⁻¹), DENSITY (kg m⁻³), SPECIFIC_HEAT (kJ kg⁻¹ K⁻¹), and EMISSIVITY (0.9 by default) of materials, for example:

 &MATL ID='steel', EMISSIVITY=.95, DENSITY=7850.,
     CONDUCTIVITY=45.8, SPECIFIC_HEAT=0.46, / 
 &MATL ID='concrete', DENSITY=2200.,
     CONDUCTIVITY=1.2, SPECIFIC_HEAT=0.88, /
 &MATL ID='copper', SPECIFIC_HEAT=0.38,
     CONDUCTIVITY=387., DENSITY=8940. /
 &MATL ID='gypsum plaster', CONDUCTIVITY=0.48,
     SPECIFIC_HEAT=0.84, DENSITY=1440. /

Thermal properties like conductivity and specific heat can vary significantly with temperature. In such cases, use the RAMP function like this:

 &MATL ID='steel', SPECIFIC_HEAT_RAMP='c_steel',
     CONDUCTIVITY_RAMP='k_steel', DENSITY=7850. / 
 &RAMP ID='c_steel', T=20., F=0.45 /
 &RAMP ID='c_steel', T=377., F=0.60 /
 &RAMP ID='c_steel', T=677., F=0.85 /
 &RAMP ID='k_steel', T=20., F=48. /
 &RAMP ID='k_steel', T=677., F=30. /&MATL ID='calcium silicate',
     CONDUCTIVITY_RAMP='k_casi', DENSITY=770.,
     SPECIFIC_HEAT=0.96 /
 &RAMP ID='k_casi', T= 25., F=0.18 /
 &RAMP ID='k_casi', T=200., F=0.19 /
 &RAMP ID='k_casi', T=500., F=0.20 /

For this kind of ramps the parameter F is the value of the actual physical quantity.

If CONDUCTIVITY_RAMP is used, there should be no value of CONDUCTIVITY given. Note also that for values of temperature, T, below and above the given range, FDS will assume a constant value equal to the first or last F specified.

Each set of RAMP lines must be listed with monotonically increasing T. The following table summarizes some RAMP (temperature) parameters:

Parameter	Type	Description	Unit	Default
ID	String	Identifier
T	Real	Temperature	°C
F	Real	Function value

Burning properties

Solids

The MATL namelist group can be used to specify the parameters employed in the solid phase pyrolysis process. As already explained in Chapter [cha:Combustion-and-radiation], pyrolysis is the decomposition or transformation of a compound caused by heat that produce the gaseous fuel, that is burned during gas phase combustion.

FDS contains a fairly general description of multi-layered, multi-component, multi-reaction solid: while burning, each material can undergo several reactions that may occur at different temperatures and consume different amounts of heat. Each individual reaction can produce a single solid residue, water vapor, or fuel gas.

Here is an example of a material that burns in the neighborhood of 350 °C, converting all its mass to fuel gases with NU_FUEL(1)=1.:

 &MATL ID='my fuel', SPECIFIC_HEAT=1.0, CONDUCTIVITY=0.1,
     DENSITY=100.0, HEAT_OF_COMBUSTION=15000., N_REACTIONS=1,
     NU_FUEL(1)=1., REFERENCE_TEMPERATURE(1)=350.,
     HEAT_OF_REACTION(1)=3000. /

See next Sections and [FDS5 user's guide] for broader description of the problem and its complexity.

Liquids

The MATL namelist group is also used to specify the parameters for burning liquids.

For a liquid fuel, the thermal properties are similar to those of a solid material, with a few exceptions. The evaporation rate of the fuel is governed by the Clausius-Clapeyron equation. The only drawback of this approach is that the fuel gases burn regardless of any ignition source. Thus, if a liquid fuel is specified, the fuel begins burning at once.

As an example:

 &MATL ID='ethanol', EMISSIVITY=1.0, NU_FUEL=0.97,
     HEAT_OF_REACTION=880.,
     CONDUCTIVITY=0.17, SPECIFIC_HEAT=2.45, DENSITY=787.,
     ABSORPTION_COEFFICIENT=40., BOILING_TEMPERATURE=76. /

The inclusion of BOILING_TEMPERATURE on the MATL line tells FDS to use its liquid pyrolysis model.

It also automatically sets N_REACTIONS=1: the only reaction is the phase change from liquid to gaseous fuel. Thus, HEAT_OF_REACTION in this case is the latent heat of vaporization. The gaseous fuel yield, NU_FUEL, is 0.97 instead of 1 to account for impurities in the liquid that do not take part in the combustion process.

The thermal conductivity, density and specific heat are used to compute the loss of heat into the liquid via conduction using the same one-dimensional heat transfer equation that is used for solids. Obviously, the convection of the liquid is important, but is not considered in the model.

Note also the ABSORPTION_COEFFICIENT for the liquid. This denotes the absorption in depth of thermal radiation. Liquids do not just absorb radiation at the surface, but rather over a thin layer near the surface. Its effect on the burning rate is significant.

HEAT_OF_COMBUSTION in a MATL line?

The HEAT_OF_COMBUSTION is the energy released per unit mass of fuel gas that mixes with oxygen and burns. This has nothing to do with the pyrolysis process. This parameter would better be used in gas phase combustion!

If you remember what was said in Chapter [cha:Combustion-and-radiation], whereas there can be many types of combustibles in an FDS fire simulation, only one gaseous fuel can be simulated by FDS. The stoichiometry of the predominant reaction is specified via the REAC namelist group.

In fact, the HEAT_OF_COMBUSTION specified on the REAC line pertains to the only gaseous fuel modeled in gas phase combustion.

The HEAT_OF_COMBUSTION specified on the MATL line is that specific to gaseous fuel produced by pyrolysis.

If the HEAT_OF_COMBUSTION is specified on the MATL line, FDS automatically adjust the mass loss rate of the gaseous fuel injected by the pyrolyzing material, so that the corrected mass loss rate multiplied by the single, global, gas phase heat of combustion produces the expected heat release rate.

If, for example, the HEAT_OF_COMBUSTION specified on the REAC line is twice that specified on the MATL line, the mass of pyrolyzing material contained within wall cell will be decremented by that determined by the pyrolysis model, but the mass of fuel gas added to gas phase would be reduced by 50%.

Properties hell

The scientific community agrees that there is no standardized way of obtaining all of the parameters needed to run FDS. This is especially true of materials that burn. There are various devices used to measure various properties, but there is no consensus on the exact physical and mathematical description of these, and thus, no standard way of taking bench-scale data and converting it into an FDS input file.

Recently Nick Dempsey of WPI, Marc Janssens of Southwest Research, and Morgan Hurley of the SFPE were awarded a three year grant to develop an engineering guide that will document the standard test methods used to obtain material properties, and more importantly the physical and mathematical interpretation of these methods that will enable us all to understand what to do with measurements made in the various bench scale devices.

A prediction is called blind, if the results are not compared to experimental measures. Grid sensitivity and uncertain material properties make blind predictions of fire growth on real materials beyond the reach of the current version of the model.

However, the model can still be used for a qualitative assessment of fire behavior as long as the uncertainty in the flame spread rate is recognized.

For engineering level applications, it's strongly advised to recur to simplified fire modeling, directly prescribing the HRRHeat Release Rate (HRR) of the fire scenario taken from literature or direct experimentation, as shown in Section [sub:Prescribing-an-HRR].

Resources for material property data

Here are some web resources for material property data; a broader list of links is maintained on FDS web site:

• NIST Chemistry Webbook:[1]

• ChemFinder:[2]

• Parital INSC Material Properties Database:[3]

• Cone calorimeter data from Worcester Polytechnic Institute:[4]

• MatWeb:[5]

• Engineering Toolbox:[6]