Estimation of the radiation extinction coefficient of natural fuel beds

. The standard formula used to estimate the radiation extinction coefficient, kb (m–1), of solid porous natural fuel beds is examined and tested against laboratory experiments with isotropic beds of pine needles in the range of packing ratio, β, of 0.01–0.02. To measure kb in the tests, a setup using a parallel beam of white light radiation was constructed. The error of the standard formula is observed to be smaller than 10%. Similar tests were performed for a non-isotropic bed of pine needles with β ≈ 0.02, in which the maximum angle of inclination of the needles was limited to 30°, for two directions of incidence of radiation: horizontal and vertical. For each one of these two cases, original estimation formulae for kb are proposed alternative to the standard one. In these cases it is concluded that, while the standard formula may be in error by up to 20%, the new formulae have errors around 5% or smaller.