Pyrolysis Model in FDS

In the next few weeks we plan to write a few articles to alert you to some changes made in FDS 5.4. This article focuses on the solid phase pyrolysis model; that is, where you specify a reaction or reactions that the solid undergoes in releasing fuel gases. There is still confusion among users as to the various ways that you can describe a fire. Most simply specify a heat release rate per unit area (HRRPUA). In that case, there is no need to provide more details about the solid phase reaction. HRRPUA is equivalent to a gas burner that you control via RAMPs or similar parameters.

However, if you specify material (MATL) properties, including kinetic parameters, to describe one or more reactions that occur as your solid heats up, be aware that the definition of the parameter REFERENCE_TEMPERATURE has changed in 5.4. In previous versions of FDS, this parameter, along with REFERENCE_RATE, was used to calculate the Arrhenius parameters A and E in the reaction rate expression. These parameters are typically found via thermo-gravimetric analysis (TGA) or similar small-scale measurement techniques. Since these measurements are most often unavailable for a particular material of interest, FDS provides a way, via these "reference" values, of estimating the kinetic parameters. You have also probably seen discussion of genetic algorithms that also are intended to estimate various kinetic parameters when only a partial number are measured directly.

The change made in FDS 5.4 to the definition of REFERENCE_TEMPERATURE was intended to make the FDS pyrolysis model more consistent with current trends in materials testing and analysis. For a good introduction, read Jose Torero's chapter, "Flaming Ignition of Solid Fuels," in the Fourth Edition of the SFPE Handbook. Then read the FDS User's Guide section entitled, "Solid Fuels that do NOT Burn at a Specified Rate." All of this development is focused on the long-term goal of standardizing the process of obtaining material property data. A necessary first step is to understand the meaning of typical TGA results (a good example can be found in Torero's chapter). Then we need to translate this information into FDS inputs.

We'd like to continue the dialog on pyrolysis modeling that started last year, but has recently stalled a bit. We are starting to notice yet again that many users are simply cutting and pasting lines of input from the User's Guide and sample cases without really understanding their meaning. We are developing a suite of verification cases that can be used to check the basic kinetic parameters (a simulated TGA experiment), as well as cases to assess the overall solid phase model (a simulated cone calorimeter measurement). Nick Dembsey, Marc Janssens and Morgan Hurley are continuing their multi-year effort to develop a standard guide for obtaining material properties. Our work in FDS will hopefully move us closer to the goal.

So for those of you with an interest in this area, it would be very useful to get your feedback. I will post this blog to the Discussion Group so that it will appear near the top of the list of issues.