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Povzetek: Concurrent flame spread data for thermally-thin charring solid fuels are presented from Saffire and BASS experiments performed in habitable spacecraft for three duct sizes, five sample sizes, two materials, and two atmospheres. The flame spread rates and flame lengths were strongly affected by duct size even for the relatively large ducts (> 30 cm tall). A transient excess pyrolysis length (i.e., flame length overshoot) was observed for the cotton fabric that burned away, which indicates that the transient excess pyrolysis length phenomenon is caused by more than just the flame moving into the developing boundary layer thickness as was the case with the SIBAL sample. A burnout time, defined as the pyrolysis length divided by the flame spread rate, normalized the pyrolysis length histories into a single curve with a steady burnout time of 22 s for the SIBAL fabric. The transient excess pyrolysis length is hypothesized to be a post-ignition flame growth transient for the essentially two-dimensional flames where the burnout time becomes very long until the preheat and pyrolysis lengths develop. The three-dimensional flames over narrow samples have lateral thermal expansion and lateral oxygen diffusion which allows them to transition to a steady state length without the transient excess pyrolysis length. Surface temperature profiles, nondimensionalized by the pyrolysis length, indicate that the temperature profiles exhibit the same shape across the pyrolysis zone. A surface energy balance calculation in the preheat region revealed that the heat flux increased rapidly at the pyrolysis front to near the critical heat flux for ignition. An estimate of the acceleration of the inviscid core flow in the duct due to thermal expansion and developing boundary layers on the duct walls and the SIBAL sample surface seems to explain the observed spread rate trends across three duct sizes and multiple sample sizes.
Ključne besede: concurrent flame spread, microgravity, duct size, sample size, cellulose fabrics
Objavljeno v DiRROS: 08.01.2024; Ogledov: 167; Prenosov: 35
Celotno besedilo (2,13 MB)
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Povzetek: Flame spread experiments upon a BROOF(t4) compliant flat roof mock-up located below a vertical barrier were carried out for variations in gap height, inclination, subjacent insulation material, and the barrier type (stainless-steel board or photovoltaic (PV) module). A binary flame spread scenario was identified, where re-radiation from the flame facilitated self-sustained flame spread if the gap height to the horizontal panel was below 10 cm for the stainless-steel board and 11 cm for PV modules. These were defined as the critical gap heights. Inclination of the PV modules increased the critical gap height and caused a 25% faster flame spread rate (FSR) than the FSR below horizontal modules with the same gap height at the location of ignition. The faster FSR for inclined modules caused a 40% reduction of the maximum temperature measured at a depth of 70 mm in the insulation materials (242°C). Based on temperatures measured in the insulation materials, the 60 mm polyisocyanurate (PIR) insulation performed slightly better than the 50 mm mineral wool insulation. However, it is expected that the mineral wool would outperform the PIR insulation if tested with the same thickness, as it insulates significantly better at high temperatures. Finally, no sustained flame spread was observed on the back side polymer sheet of the PV modules, but one of the three PV module brands produced burning droplets. Based on the experiments, it can be concluded that the current standards are inadequate as the introduction of a PV system on a compliant roof construction enables flame spread.
Ključne besede: photovoltaic (PV) installations, flame spread, fire dynamics, property protection, open access
Objavljeno v DiRROS: 31.05.2023; Ogledov: 313; Prenosov: 241
Celotno besedilo (2,92 MB)
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