Experimental and Numerical Study of Flame Spread Over Bed of Pine Needles

For the first time, on the basis of a systematic experimental study of the propagation of a model ground fire over pine needles bed at a low wind speed (in the range of 0.1 m/s-0.4 m/s), in which there have been no measurements reported so far, a nonlinear dependence of flame spread rate on the wind speed has been established. Further, the total heat flux and its radiation counterpart have been measured using compact cooled sensors placed in the needles bed. Furthermore, the influences of the bed width and its moisture content on the flame propagation rate are studied. A three-dimensional numerical model based on Fire Dynamics Simulator (FDS) is used to investigate the processes governing the pyrolysis of pine needles, oxidation of char, gas phase combustion and radiation, using parameters from literature. The model, with simplified moisture release and pyrolysis sub-model, is able to predict the experimentally measured flame spread rates for most of the cases well within the measurement uncertainties. A sensitivity analysis is done to demonstrate the importance of pyrolysis chemistry over char oxidation rate. The predicted flow, temperature and species fields are presented to bring out the physics involved in the flame propagation. The validated model, coupled with detailed turbulence and radiation models, can be used as a first-hand predictive tool for scaled up ground fire scenarios.