FDS6 - Soot Deposition for Polyurethane

[Jaimee Choi]

Hi there,

I’ve started to explore the soot deposition function in FDS6. I was able to follow the example codes from the FDS User guide 'propane_flame_deposition' and successfully run a model using propane as a fuel.

 

However, I am having trouble running a model with polyurethane (chemical formula C6.3H7.1NO2.1) as the reaction fuel.

 

I have used the chemical equation balancer from Kris's website (http://www.koverholt.com/chemical-equation-balancer/) to determine the stoichiometric coefficients for polyurethane.

C6.3H7.1NO2.1 + 5.9391(O2 + 3.7619N2) --> 5.2141CO2 + 3.55H2O + 22.8421N2 + 1.0859C

 

Here are the REAC and SPEC lines using the equation above:

&SPEC ID = 'NITROGEN',        LUMPED_COMPONENT_ONLY = .TRUE. /
&SPEC ID = 'OXYGEN',          LUMPED_COMPONENT_ONLY = .TRUE. /
&SPEC ID = 'CARBON DIOXIDE',  LUMPED_COMPONENT_ONLY = .TRUE. /
&SPEC ID = 'WATER VAPOR',     LUMPED_COMPONENT_ONLY = .TRUE. /
&SPEC ID = 'POLYURETHANE' /

&SPEC ID = 'SOOT', AEROSOL=.TRUE. /

&SPEC ID = 'AIR', SPEC_ID ='NITROGEN','OXYGEN',
      VOLUME_FRACTION = 3.7619,1.000, BACKGROUND = .TRUE. /
&SPEC ID = 'PRODUCTS', SPEC_ID ='NITROGEN','CARBON DIOXIDE','WATER VAPOR',
      VOLUME_FRACTION = 22.8421,5.2141,3.55/

&REAC FUEL = 'POLYURETHANE', HEAT_OF_COMBUSTION=2270.,
      SPEC_ID_NU = 'POLYURETHANE','AIR','PRODUCTS','SOOT',
      NU=-1.,-5.9391,1.,1.0859/

When I try to running this input file, I get an error saying:

ERROR: Problem with REAC           1. Mass of products, 946.351 does not equal mass of reactants, 844.776

This tells me that the stoichiometric coefficients are incorrect, is there any chemistry guru out there that might be able to correctly balance this equation?

Any help would be greatly appreciated, thanking you in advance.

Kind regards,

Jaimee Choi

Exova Warringtonfire Aus

[Randy McDermott]

Try looking at the example given by Eq. (12.5) in the user guide, in Sec. 12.2 on Complex Chemistry.

In the way I think about lumped species, your NUs for Air and Products are wrong.  You need to group the lumped species (like you did with the parens around the Air components) and normalize their coefficients to sum to unity.  Then the coefficient outside the parens will be the correct lumped species NU.

I am confused by several of the examples given in the user guide, including the propane soot deposition case.

Craig or Jason: I know you internally normalize the mass fractions for the lumped species definitions.  But do you then also correct the NUs?  This needs to be clarified in the guides.  I see no mention of it between giving the correct specification of a lumped species reaction balance (12.5) and subsequent simplistic representations.

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[Craig Weinschenk]

FDS does correct the NUs as well. One thing I do not see if the chemical formula definition of polyurethane in your input file text posted. This would need to be done either on the SPEC line or the REAC line as polyurethane is not one of the defined species in Table 11.1 in the User Guide. There should be error trapping when the fuel MW is not defined but for some reason this is passing through unless there is other text on your input file that is not included.

Randy - I will try to get clarifying text into the Guide as soon as possible. If the MW/chemical formula addition solves this problem I will take a look at the error trapping as well.

[dr_jfloyd]

You haven't defined the FORMULA or the MW for POLYURETHANE.  So as far as FDS is concerned it has the properties of air or an MW of 28. and the chemical formula of approximately 0.79N0.21O

[Jaimee Choi]

Thank you Randy, Craig and dr_jfloyd for your helpful comments.

I managed to figure it out and run the soot deposition model successfully using polyurethane as the fuel.

I have realised that the stoichiometric coefficients from Kris's website (http://www.koverholt.com/chemical-equation-balancer/) needs to defined down to the 6th decimal place for the chemical equation to be balanced.

C6.3H7.1NO2.1 + 5.939058(O2 + 3.7619N2) --> 5.214058CO2 + 3.55H2O + 22.842143N2 + 1.085942C

I have attached my FDS file for those who may be interested, thank you.

Kind regards,

Jaimee Choi

Exova Warringtonfire Aus