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Förbättrat brandmotstånd hos fönster med lågemissivitetsbeläggningar
2016 (English)Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

In a room fire wall openings are playing an important role for how the fire is developed since the size of the fire, the heat release rate, is often limited by the flow of oxygen from these openings. When a window breaks and falls out, this sudden geometric change and increased access of oxygen can lead to backdraft and/or flashover which is not desirable.Glass breakage in windows mainly occurs due to differential heating of the glass pane. The literature review suggest that it is the temperature difference between the exposed central pane and the shaded edges that is causing critical stresses and breakage. Glass breakage tend to initiate at edge flaws and then propagating to the central of the glass causing fallout of the glass pane. Temperature rise in materials caused by radiation is depending on the size of absorbed heat flux which is depending on the absorptivity of the material. According to Kirchhoff’s law the spectrally resolved absorptivity is equal to the emissivity for opaque surfaces. But since glass is a semi-transparent material that absorbs and emits energy in depth Kirchhoff’s law is not valid and the in depth absorption and emission has to be taking into account.Using low emissivity coatings, Low-E coatings, has been discussed for several decades, mainly for energy saving purposes. But what about the positive effects in the fire protection area? Does reducing the absorptivity prevent rapid and early temperature rise and therefore stop or delay windows from breaking? And if this is the case, how effective are low emissivity coatings for this purpose?In this study the efficiency of Low-E coatings on glass exposed to radiation caused by an enclosure fire has been investigated. Theory and physics about heat transfer, windows in buildings, Low-E coatings, glass breakage and spectral properties of in depth absorptivity and emissivity has been studied. Simplified and more advanced models for calculations of the windows breaking time, with and without low emissivity coatings, has been investigated and calculations has been performed.Abaqus, an advanced FEM (Finite Element Method) program, was chosen for the thermal analyses and a simplified calculation method presented by Joshi and Pagni was used to evaluate the critical temperature difference. To get accurate input data spectral properties of the glasses was measured.Two glass types has been studied and compared, one with a Low-E coating (K-glass) and one without (F-glass). The glasses were exposed to two cases, one with an incident radiation from a flame of 23.4 kW/m2 and one with an incident radiation from a hot gas layer of 14.3 kW/m2. The result showed that reducing the absorptivity is preventing the rapid and early temperature rise and therefore stop or delay windows from breaking. In the case where the glasses was exposed to 23.4 kW/m2 the F-glass breaks after 42 seconds and the K-glass after 137 seconds, an increased integrity of 226 %. In the other case the F-glass breaks after 59 seconds while the K-glass is keep intact. The importance of in depth absorption was also examined and the results show that in depth absorption reduces the early and rapid temperature rise in the glass, the temperature rise is delayed at an early state, compared to surface absorption. This is probably since the glass is more uniformly heated which is believed making the heat flux done by conduction less rapid. The retardation of increased temperature makes the critical temperature difference exceeding later. It was also shown that taking in depth absorption into account gives computational results closer to the experimental result, but whether in depth absorption has to be taking into account or not is hard to tell since only one comparison was made. Further examination of the phenomena has to be performed. Surface absorption is considered to be a good first approximation since it will give conservative results but in depth absorption may be a more accurate boundary regarding absorption for semi-transparent materials.

Place, publisher, year, edition, pages
Keyword [en]
Keyword [sv]
Teknik, Glasbrott i fönster, värmeöverföring genom strålning, spektral absorptivitet, absorption på djupet, emissivitet för semitransparenta material
URN: urn:nbn:se:ltu:diva-51220Local ID: 86f9f263-6b30-4a0d-bc1a-65a307f635fdOAI: diva2:1024582
Subject / course
Student thesis, at least 30 credits
Educational program
Fire Engineering, master's level
Validerat; 20160823 (global_studentproject_submitter)Available from: 2016-10-04 Created: 2016-10-04Bibliographically approved

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