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Ignition of Cellulosic Fuels by Heated and Radiative Surfaces.


pdf icon Ignition of Cellulosic Fuels by Heated and Radiative Surfaces. (8944 K)
Pitts, W. M.

NIST Technical Note 1481; NIST TN 1481; 105 p. March 2007.

Keywords:

cellulosic fuels; ignition; mufflers; catalytic converters; scenarios; surface temperature; wind effects; air flow; fuel beds; combustion; flaming combustion; ignition time; thermal conduction; thermal radiation; cone calorimeters; polyurethane foams; surface ignition; newsprint; grasslands

Abstract:

Experiments designed to characterize the ignition behavior of typical outdoor fuels by heated mufflers and catalytic converters found on outdoor power equipment are described. Ignition by direct contact with a heated surface and by radiation from a heated surface were considered. For the first scenario the fuel was placed in contact with an electrically heated surface, and the times required for ignition measured as a function of surface temperature. The effects of a wind on heated plate ignition were studied by passing air flows of 1.1 m/s and 2.5 m/s over the fuel bed. Fuels tested included shredded newsprint, four types of grass, pine needles, a grass/pine needle mixture and three types of dry leaves. Both glowing combustion and flaming were observed. The transition to flaming required glowing combustion to be present. Ignition times generally increased with decreasing surface temperature until a temperature was reached where ignition was no longer observed. Ignition times for given applied wind and surface temperature conditions and transition to flaming behavior were fuel dependent. An applied wind generally reduced the time required for ignition, with the reduction being greater for the higher velocity. A commercially available cone calorimeter operated in the non piloted ignition mode was used to impose known radiative heat fluxes on the fuel surface, and ignition times were measured as a function of applied heat flux. Shredded newsprint, two types of grass, pine needles, and non fire-retarded flexible polyurethane foam were studied. Ignition times for radiative heating increased with decreasing applied heat flux until levels were reached where ignition did not occur. The polyurethane foam had a distinctly different behavior than the cellulosic fuels. Comparison of the heated plate and radiative heating experiments suggests that they can be related using the black body temperature corresponding to the applied radiative heat flux.