Bark char patterns in forest fires: A heat transfer analysis of wind effects

Background/Question/Methods Char is a black, carbonaceous solid produced by pyrolysis of organic materials.  Following forest fires, bark char patterns can be used to estimate fire behavior variables because they vary with fire spread direction, wind direction, and wind velocity.  In low velocity winds, charring occurs on the side of the tree from which the fire approaches; as wind velocity increases, charring shifts to the leeward side, regardless of fire spread direction.  Here we model bark surface temperature rise to pyrolysis on a local basis around a tree stem circumference as one-dimensional conduction with forced convection boundary conditions.  Convection coefficients are estimated from empirical local convection (Nu) data across a range of stem diameters and wind velocities (0 ≤ Re ≤ 400000) typical of forest fires. 

Results/Conclusions The model shows that increasing wind velocity induces a leeward shift in bark charring, because increased convection rates result from the formation of a turbulent boundary layer and leeward vortices.  These processes predict bark char patterns that are qualitatively similar to those commonly observed in forest fires.  Consequently, the model provides a theoretical basis for reconstructing fire behavior from post-fire bark char patterns.