Modeling Fire in Pyrosim

Created with software version 2020.1

To follow along with this tutorial, download the relevant files here.

1. Simple Chemistry Fires

All PyroSim (FDS) models that include combustion must define a gas phase reaction that converts fuel to combustion products. We review the “simple chemistry” approach in FDS.

In the video below, we discuss the “simple chemistry” approach in FDS. The fuel is assumed to consist of Carbon, Hydrogen, Oxygen, and Nitrogen that reacts with Oxygen to form Water, Carbon Dioxide, Soot, Carbon Monoxide, and Nitrogen. The reaction of fuel and oxygen is assumed to be infinitely fast and controlled only by mixing.

2. Heat Release Rate Defined Fires

In the video below, we discuss what it means to define a fire using the Heat Release Rate. When we specify a Heat Release Rate, we are really specifying the rate at which fuel is released into the fire simulation. The fuel mass flow rate is calculated so that combustion will release heat at the desired rate.

We show how to use the stoichiometric reaction calculations in a spreadsheet, which is available here, and how to verify that FDS is using the same calculation.

3. Heat Release Rate Defined Fires - VTT Experiment

In the video below we show how to model a fire using a specified heat release rate (HRR).

One of the strengths of FDS, the Fire Dynamics Simulator, is that it can be a useful tool for both a fire protection engineer and a fire researcher. The challenge is that this can result in a bewildering array of options with respect to modeling the combustion process. In many cases, describing a fire using the heat release rate is both the simplest and most reliable approach.

To guide this discussion, we use an experiment and analysis performed at the VTT Research Center of Finland. I chose this example because the paper clearly describes the details of both the experiment and the supporting analysis and because one of the experiments burned a wood crib. The SFPE Handbook of Fire Protection Engineering provides equations that predict the heat release rate for such a fire. This gives us the opportunity to compare experimental values with the SFPE calculation.

4. Heat Release Rate Per Unit Area Defined Fire

In the video below we show how to model a fire using a specified heat release rate per unit area (HRRPUA), an ignition temperature, and burn away. This is about as complicated as models can be made without also simulating pyrolysis. To guide this discussion, we use an experiment and analysis performed at the VTT Research Center of Finland. I chose this example because the paper clearly describes the details of both the experiment and the supporting analysis and because one of the experiments burned a wood crib. The simulation shows both the benefits and drawbacks of this approach.

5. Variable Heat Release Rate Per Unit Area Defined Fire

This is an addendum to the Part 4 video on fire modeling. We re-analyzed the problem, changing the Heat Release Rate Per Unit Area (HRRPUA) from a constant value to a function (or curve). With this change, the new calculated total Heat Release Rate (HRR) is a reasonably close match to the measured experimental curve. The level of detail in the model is about as complicated as can be made without also simulating the pyrolysis process.

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