The Design Fire: Selecting Fire Characteristics for a CFD Model
As performance-based design becomes more popular and valuable the importance of properly characterizing fire scenarios (design fires) in these analyses has never been more critical. A six-story atrium was built using Fire Dynamics Simulator (FDS) to evaluate the effect of varying fire characteristics on model results. In order to evaluate the impacts of specific fire characteristics it is important to keep other parameters constant. Visibility conditions were used to evaluate the influence of varying fuel load compositions and determine failure.
After testing wood and polyurethane foam blends at varying percentages, the analysis found that the more concentrated the polyurethane foam, the faster untenable conditions developed. The point of this analysis was to provide an understanding of the order of magnitude that varying fuel composition could have on assessing tenability in a space. It is very important to properly characterize design fires in terms of material properties.
The majority of design fire characteristic issues are based on just three things. The first being an internal preset value utilized by a modeler for all fire models. For example, a specific soot yield. The second being the application less demanding material properties based on limiting fuel loads in a space. For example, the rationalization that “they won’t have anything else in this space except wood chairs.” And finally having no specification of material yields. This causes potential for overestimating the performance of a system.
As fire protection engineers it is our responsibility to design the smoke control system based on the representative fire scenarios, using our engineering judgement to properly identify these scenarios.
INFLUENCE OF THE BUILDING ON A DESIGN FIRE
Design fires are a valuable tool for evaluating performance-based designs and they are used all over the world. Engineers must be careful when using them however, because they are simplified and if used outside of the boundaries of their original intent, can cause erroneous conclusions in regards to fire safety.
To investigate the influence of different building characteristics on design fires, a simple 10x10x3 meter room was created in the Fire Dynamics Simulator (FDS). This was used as the default and the simulations were performed with varying building materials, floor area, room heights, and openings. Wahlqvist and van Hees used a simple numerical model to predict the changes in growth rate and maximum heat release rate based on radiative feedback to the fuel and oxygen depletion. Where the lowered mass loss rate close to the fire source is due to lowered oxygen levels and the increased mass loss rate is due to radiation from external sources.
The results showed that there were minor changes in growth rate due to building materials except for insulated walls which caused a large increase in growth rate. It was also noted that the growth rate was hampered by the lack of oxygen being supplied to the compartment. The growth rate was larger in all cases where four doors were present compared to only one door. The floor area was found to have a significant impact on the maximum heat release rate. The largest floor area with one door reaches the highest heat release rate for only a brief time, while the smallest floor area with four doors reaches the highest heat release rate over a much longer period of time. Finally, the ceiling height had a significant effect on the growth rate as well. With a large floor area, the growth rate drastically decreases as height increases, however it does not have the same effect if the floor area is small.
This experiment shows that building materials, floor area, room heights, and openings can have a substantial impact on design fires and should be considered when using design fires for performance based designs.