Effect of wind on smoldering combustion limits of moist pine needle beds

Abstract

We studied moist pine needle beds burning under the effect of wind, inorder to determine the upper moisture limit for which there is fire propagation fordifferent wind velocities. For this purpose we built a wind tunnel that allowed us toburn a 600 mm by 150 mm by 40 mm bed under wind velocities between 0.5 m/s and5.0 m/s and controlled air temperature. Results show an increase in moisture limitfrom 54% to approximately 140%, for the velocity range indicated. Combustion atlimiting conditions proceeds mainly by smoldering with some periods of flaming combustion.It was observed that, for conditions close to extinction, the smoldering frontis not quenched at the surface. Additionally, it was also observed that a strong flowof hot gases exit from the fuel bed at the free surface. These two observations lead tothe conclusion that the main heat sink is moisture evaporation and that heat losses tothe surroundings is reduced by the blowing effect of the hot gases coming off the bed.A dimensional analysis suggests a correlation between moisture limit and wind velocityof the form M = A – B/vw 2 , where M is moisture limit for fire propagation, Aand B are constants, and vw is wind speed. Two dimensionless numbers helped toplot the smoldering temperature and fire propagation velocity in a more meaningfulway. They are P1 ¼ Tsmlcp;gv2w and P2 ¼ Mhfgv2w, where Tsml is smoldering temperature,cp,g is the gas specific heat, M is fuel moisture content and hfg is the latent heatof water evaporation. A relatively high moisture limit at 5 m/s wind velocity is possibledue to the relatively high air flow into the smoldering front and the efficient heatfeedback produced in forward smoldering.