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On the importance of spectrally resolved absorptivity data in fire technology
RISE, SP – Sveriges Tekniska Forskningsinstitut, SP Sveriges tekniska forskningsinstitut, SP – Sveriges Tekniska Forskningsinstitut / Brandteknik, material (BRm).ORCID iD: 0000-0002-7140-4737
2009 (English)Report (Refereed)
Abstract [en]

Radiation is one of the two main heat transfer processes in transporting energy from flames to a surface. The other process is convection and was not considered in this report. The spectral absorptivity of 72 products was measured in the wavelength region 0.3-20 mm. This wavelength region contains virtually all radiation energy from a fire. Based on the measurements, and on the theory for radiative heat transfer, the effective absorptivity for fire induced heat radiation for the different products were calculated. These typically lies in the range 0.75-0.95. It was also found that the effective absorptivity varies significantly with the temperature of the heat source. The reason for this is that the spectrum of the emitted radiation from a heat source, such as a fire, changes with temperature. This has limited effect on the heating of a surface. The dependence of the effective absorptivity on heat source temperature is important when the absorptivity is used as input for calculations of ignition temperature and thermal inertia. Using existing models for predicting ignition temperature and thermal inertia, and correcting these models with the new information on effective absorptivity, it could be shown that the effects were significant but not very large. It was considered that the uncertainties in the model are so high that corrections for the effective absorptivity might be of minor importance compared to the other uncertainties and assumptions. An interesting observation was that the absorptivity of radiation from fires for products exposed to irradiation in the cone calorimeter decreased with increased exposure time, that is, the absorptivity decreased when the products darkened due to heat. This is surprising since, for example, wood that is darkened when exposed to heat obviously has a higher absorptivity in the visual part of the spectrum than fresh non-darkened wood. This is an important observation since it opposes the general view that heat transfer increases with increased thermal exposure due to darkening Finally, it was concluded that none of the studied materials showed a particularly low absorptivity in the infrared region and therefore none of the products stands out as particularly good reflector of radiation from fires. Several ideas were presented for how such spectrally tailored surfaces, with low absorptivity for radiation from fires, can be produced.

Place, publisher, year, edition, pages
2009.
Series
SP Rapport, ISSN 0284-5172 ; 2009:48
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:ri:diva-4881Local ID: 10508ISBN: 978-91-86319-35-9 (print)OAI: oai:DiVA.org:ri-4881DiVA: diva2:962520
Available from: 2016-09-07 Created: 2016-09-07 Last updated: 2017-07-06Bibliographically approved

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Försth, Michael
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