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- Fjellgaard Mikalsen, Ragni
(författare)
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Fighting flameless fires : Initiating and extinguishing self-sustainedsmoldering fires in wood pellets
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Smoldering fires represent domestic, environmental and industrial hazards. This flameless form of combustion is more easily initiated than flaming, and is also more persistent and difficult to extinguish. The growing demand for non-fossil fuels has increased the use of solid biofuels such as biomass. This represents a safety challenge, as biomass self-ignition can cause smoldering fires, flaming fires or explosions.Smoldering and extinguishment in granular biomass was studied experimentally. The set-up consisted of a cylindrical fuel container of steel with thermally insulated side walls. The container was closed at the bottom, open at the top and heated from below by a hot surface. Two types of wood pellets were used as fuel, with 0.75-1.5 kg samples.Logistic regression was used to determine the transition region between non-smoldering and self-sustained smoldering experiments, and to determine the influence of parameters. Duration of external heating was most important for initiation of smoldering. Sample height was also significant, while the type of wood pellet was near-significant and fuel container height was not.The susceptibility of smoldering to changes in air supply was studied. With a small gap at the bottom of the fuel bed, the increased air flow in the same direction as the initial smoldering front (forward air flow) caused a significantly more intense combustion compared to the normal set-up with opposed air flow.Heat extraction from the combustion was studied using a water-cooled copper pipe. Challenges with direct fuel-water contact (fuel swelling, water channeling and runoff) were thus avoided. Smoldering was extinguished in 7 of 15 cases where heat extraction was in the same range as the heat production from combustion. This is the first experimental proof-of-concept of cooling as an extinguishment method for smoldering fires.Marginal differences in heating and cooling separated smoldering from extinguished cases; the fuel bed was at a heating-cooling balance point. Lower cooling levels did not lead to extinguishment, but cooling caused more predictable smoldering, possibly delaying the most intense combustion. Also observed at the balance point were pulsating temperatures; a form of long-lived (hours), macroscopic synchronization not previously observed in smoldering fires.For practical applications, cooling could be feasible for prevention of temperature escalation from self-heating in industrial storage units. This study provides a first step towards improved fuel storage safety for biomass.
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- Fjellgaard Mikalsen, Ragni, et al.
(författare)
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Synchronized smoldering combustion
- 2018
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Ingår i: Europhysics letters. - : IOP Publishing. - 0295-5075 .- 1286-4854. ; 121:5, s. 50002-p1-50002-p2
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Tidskriftsartikel (refereegranskat)abstract
- Synchronized, pulsating temperatures are observed experimentally in smoldering fires.The entire sample volume (1.8 l) participates in the pulsations (pulse period 2–4 h). The synchronylasts up to 25 h and is followed by a spontaneous transition to either disordered combustion orself-extinguishment. The synchronization is obtained when the fuel bed is cooled to the brink ofextinguishment. Calculations for adiabatic conditions, including heat generation from combustion(nonlinear in temperature) and heat storage in sample (linear in temperature), predict divergingsample temperature. Experimentally, heat losses to surroundings (linear in temperature) preventtemperatures to increase without bounds and lead to pulsations.
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