…by Dr. Vytenis Babrauskas, Fire Science and Technology Inc.
Nowadays, many fire cases will need a specific fire science expertise: fire modeling. For attorneys, obtaining adequate expertise in fire modeling is not a trivial issue. The reason is that this is a highly specialized technical area, yet one without clear means of qualifying the individuals. Especially, it is important to note that professional qualifications such as: fire investigator, fire protection engineer, fire marshal, etc. etc., do not imply expertise in fire modeling.
Part of the problem arises from the fact that it is quite easy to acquire the trappings of a fire modeler, without actually having the background necessary for a full understanding of the subject. For a very nominal fee one can obtain NIST’s HAZARD, CFAST, and FPETOOL. The programs are sufficiently user-friendly that answers may be cranked out by an individual who is not at all aware of the physics that he is actually representing. When this author was with NIST, we agonized quite a bit over the release of the first version of HAZARD: how to avoid putting tools into unqualified hands. The issue was, unfortunately, never resolved. The attorney in a fire case, however, has to be able to determine whether the individual is qualified or not.
To this end, we offer a list of questions which can be helpful in determining the individual’s qualifications:
- What courses have you taken in heat transfer and combustion?
Computer fire models are exceedingly difficult to adequately comprehend for a person who has not studied either of these topics on a graduate level. At least several courses in heat transfer (convection, conduction, radiation) and at least an introductory course on combustion should be looked for.
- What fire models do you have available on your computer?
There are several dozen useful models to choose from. It would be hard to claim expertise in fire modeling if one had only the models from NIST in one’s toolkit. The person should have at least some models from abroad (Sweden, Finland, UK, Australia, etc.)
- Have you ever commissioned heat release rate fire tests for use as input data to fire models?
Today’s fire models require that heat release rate data (from small-scale or large-scale tests) be supplied as part of the input to the problem. A certain amount of public-domain HRR data exists. This, however, is not likely to solve very many needs in actual fire cases, since the data do not refer to the specific products which burned in this particular fire. Thus, someone who uses fire models in a meaningful way is going to need to obtain lab test results for HRR on exemplar materials.
- What HRR tests have you commissioned?
Appropriate tests would include the Cone Calorimeter (small-scale tests), a large-scale open-burning calorimeter, and a large-scale room calorimeter. They would not include tests such as UL 94, Steiner Tunnel, or various Bunsen burner type ignitability tests.
- How do you represent flame spread in a fire model?
Appropriate answers would include: (a) by conducting a full-scale sectional mockup on the geometry in question; or (b) by using a fire model which can represent flame spread.
- If you use a fire model with flame spread features, what computation is the model making?
There are only a very limited choice of conditions where today’s fire models can represent flame spread; the details should be understood by the expert.
- Do you ever use fire models to predict the smoke in fires? Can you explain the units that are used to represent smoke input data for smoke in fire models?
Most models use either m2/kg or m2 as the basic smoke characteristic of the material. The former is known as the specific extinction area while the latter is known as the smoke production.
- Why is the fire plume an important feature of room fire models?
Because this is the mechanism for moving gas from the lower (cold) to the upper (hot) layer.
- What determines the amount of carbon monoxide being produced in a fire?
Of most importance: ventilation (i.e., air supply). Of secondary importance: chemical nature of the combustible being burned.
- If you double the height of window opening from a room on fire, will you double the outflow from it?
No, you will more than double it. The flow rate through an opening is not linearly proportional to the height.
This article Copyright © 1997 by Vytenis Babrauskas.