Modelling chemical reaction

[Pec Nova]

Hi all.

I am trying to learn about combustion modelling in FDS. In the FDS user guide it is stated:

-For simple chemistry model each reaction is assumed to be of the form of Eq. (12.1) where the following needs to be specified:
 
 -the chemical formula of the fuel (number of C, H, O and N atoms)
 -yields of CO and soot
 -the volume fraction of hydrogen in the soot

-For any other situation, where the fuel molecules contain not only C,O,H and N, complex stoichiometry model and a detailed chemical formula (an example of such is Eq. (12.6) in the FDS user guide) is needed.

If one is not a chemist and the material treated is an entire unknown (but let's say the reaction is complex and a detailed model is required), what are the possibilities of defining a corresponding chemical reaction for an FDS input?
Can the entire formula be somehow defined from, e.g., cone calorimeter experiments joined with suitable FTIR analysers or something similar?

In such a case, what heat flux should be established in the experiment?
Can somebody recommend some literature where such tests were performed and a suitable reaction was determined accordingly for an FDS fire simulation?

All help will be really appreciated.

Best regards.

[Randy McDermott]

I believe you want Sec 8.5 of the FDS User Guide: Complex Pyrolysis Models.

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[Pec Nova]

Hi.

Thank you for the quick response.

But I am affraid I don't quite understand the answer.

Section 8.5 in the User Guide discusses pyrolysis models.

But my question was about combustion: how to define a suitable gas-phase chemical reaction (chemical formula of the fuel and the recation products).

Regarding this I was wondering what kind of experiments are usually performed to define these formulas if the material that is burning is a complete unknown (so, no corresponding reaction can be adopted directly from, e.g., literature). Also, I was wondeirng if their are some reference papers, books etc. discussing such tests that you could recommend.

Section 8.5 that you are referring to (first paragraph) suggests SFPE handbook for further explanation on pyrolysis modelling. Is this reference also suitable for collecting some answers to my questions? I will try to collect this book in the library when possible...

Kind regards.

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[Randy McDermott]

Well, you've hit on a key research question.  The gas phase pyrolysate is elusive.  Maybe someone else on the forum has a better answer.  But to my knowledge this composition is not measured in experiments.  Hence the use of surrogate fuels like propane (with the correct heat release rate) for gas phase combustion.  Should you have a specific surrogate gas phase fuel in mind, the user guide discusses how to set this up for gas phase combustion in FDS.  But I guess that is not your question.

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[Simo]

I think the composition of the gases resulting from the pyrolysis of real materials is rarely measured in fire tests. Some measurements are availale in the field of bio-fuel research, and they indicate that the pyrolysis of, e.g. cellulose produces a range of different hydrocarbon molecules, ranging between one and six carbon atoms (Lin et al., J.Phys.Chem C 2009, 113, 20097-20107). So what is the appropriate mixture of cases you would specify as an inflow boundary condition?  In most fire safety engineering studies, the heat release is the primary driver. Therefore, carrying out Micro-Scale Combustion calorimetry for the sample would provide you the apparent heat of combustion of the pyrolysis products, serving as a basis for choosing the surrogate fuel.

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