CROWN FIRE DEVELOPMENT IN A FOREST STAND

This work examines the phenomenon of the transition from a surface fire to a crown fire, using a numerical code (WFDS) specifically conceived to simulate fire propagation in forest environments. In detail, it explores the role of fuel bulk density (ρ) and of the tree spacing (d) in controlling the rate of spread of surface fire and the onset of a crown fire. Results show that ρ affects both the transport of oxygen through the surface fuel layer towards the flame front and the amount of thermal power, generated by the combustion of the surface fuel, transferred to the tree crowns. As a result, it plays a fundamental role in determining the potential of the system to generate a crown fire. On the contrary, results show that the effect of the tree spacing (d) on the transition from a surface fire to a crown fire is negligible. However, such a parameter affects the characteristics of the fire propagation across the domain. In particularly, if the tree spacing is relatively large, it is possible to observe peaks of acceleration of the flame front, corresponding to the presence of the tree, with successive slowdown. Contrariwise, if the tree spacing is low or very low, there is an initial acceleration, when the flame front reaches the first tree, without a subsequent slowdown. The comparison of the results with experimental and/or phenomenological issues reported in the literature shows that WFDS code output agrees with the predictions of some previous semi empirical models and does not has an inherent underprediction bias, as found by Cruz and Alexander [1] in the case of many fire models.