Thermal Flux Required for Piloted Ignition Wooden Products

Context

The assignment of establishing the region of origin and the cause of a fire requires methodology application. At Technorm, we apply scientific method and basic methodology as proposed in NFPA 921, ‟Guide for Fire and Explosion Investigations”.

With some exceptions, the appropriate method for a fire investigation is to first determine its origin and then to establish its cause.

The cause of a fire is defined by the circumstances and the conditions which all led to the ignition source, combustible and oxidant agents to cause the fire.

Therefore, a significant part of the process to establish the cause of the fire is to identify the combustible materials present in the region of origin and to understand their properties. The nature and the properties of these combustibles will have an impact on the fire scenarios that will be evaluated.

Ignition Properties of Combustible Materials

The properties of materials, and more particularly their ignition properties, are a well-studied subject in the field of fire safety and investigation. Indeed, most of them are obtained through standard tests, for instance, the use of equipment as cone calorimeter.

Most of the tests to achieve ignition and to verify combustible ignitions are based upon exposition to a heat source for a period of up to 30 minutes. However, what is the situation with combustible materials exposed to heat sources for a period longer than 30 minutes?

Not much research exists concerning the above tests in fire science community. Thereby, Technorm, in collaboration with the University of Waterloo, made an experimental research on the ignition of some wood products to help define their ignition properties on prolonged duration. We have to make a parenthetical comment on the thermal flux, so to better understand the following.

Thermal Flux 

The thermal flux may be defined as an amount of energy per unit time (J/sec) transmitted for a given area (m²).

It is a very important parameter to consider in the thermal flux models. Actually, a simple temperature measurement is not enough to quantify the power of a heat source.

For example, the temperature could be compared to the speed of a car, while the power of the car could be assimilated to thermal flux. Thus, a Porsche 911 and a Toyota Corolla can both reach 100 km/hour, the difference is that the Porsche 911 can reach it in much lesser time than the Toyota Corolla by considering the difference in power.

Hence, for a given heat source, the incident thermal flux will depend on the distance for which the source is exposed. For example, at 20 mm from a halogen bulb, a thermal flux around 16 kW/m2 will be perceived, while at 80 mm, the thermal flux will be around 7.5 kW/m2.

Experimental Research on the Ignition of Some Wood Products

Concerning the ignition of combustible materials, two (2) main factors affect the behaviour; that is the thermal flux and the exposition duration.

Therefore, so to better understand the behaviour of some wood products, we have performed a series of tests on wood (spruce), plywood (spruce), Oriented Strand Board (OSB) and Tentest wall. More than 115 samples were submitted to thermal flux varying between 4 and 14 kW/m2 for periods of up to 4 hours.

We have therefore compared our results with those we have found in scientific literature. The obtained results have permitted us, at a time, to validate our experimental method, but also to obtain strong remarkable results.

In the context of wood and plywood, the results we obtained were very similar, if not almost identical, to those of the scientific literature. This is not surprising considering that wood products were subject of several studies and were documented many times by other organizations.

In the OSB and Tentest case, the results obtained for the thermal flux to ignite these panels accounted for more than half of the results found in the records. Thus, in the case of these two materials, less powerful heat sources are required to achieve their ignition compared to what we could assume from the values obtained from the scientific literature.

During our tests, we also observed some interesting phenomena. For example, knots, cracks, imperfections and, more surprisingly, traces of ink on the different materials we tested accelerated the thermal decomposition and shortened the ignition time to a given heat flux. Thus, for the same thermal flux on a piece of wood lacking imperfection versus a piece of wood provided with a knot, we thoroughly observe an accelerated thermal decomposition at the piece displaying the knot.

Conclusion

As a result, as part of the laboratory tests we performed, we obtained the minimum flux required to ignite wood and plywood, and we were able to establish minimum flux required for ignition for OSB and Tentest which were, to date, not documented in the scientific literature.

The application of this test principle can be applied for a much larger range of existing products and materials. In addition, we could perform these same tests on even longer than 4 hours in order to obtain ignition values that could be even lower than the ones we obtained during our tests.

Do you have questions about this article? Contact the author!

By Christian Bernier, P.Eng. M. Eng., Origin, Cause and Fire Propagation

Christian Bernier will present the full results during the IFireSS 2019 (International Fire Safety Symposium), on June, 7 2019, in Ottawa.

By |2019-06-14T16:38:35+00:00May 21st, 2019|News|0 Comments