Transition from Surface Fire to Crown Fire and Effects of Crown Height, Moisture Content and Tree Flower

At present, most of the previous work focused on the effects of crown bulk density, crown base height, and wind on the initiation and behaviors of crown fire using tree branches. However, very few investigations have been carried out related to the initiation and behaviors of crown fire using a real tree considering moisture content, crown height and tree flower. In this paper, a series of burning experiments are carried out using real trees to investigate the effects of moisture content, crown height and tree flower on initiation conditions from surface fire to crown fire and crown fire behaviors. The variables of two crown heights (1.0 m, 1.5 m), three crown moisture contents (45 ± 2%, 55 ± 2%, 65 ± 2%) and tree flowers are selected. The main conclusions are as follows: when the tree has the higher crown base height and the larger moisture content, the crown fire initiation success from surface fire decreases, while the existence of tree flowers tends to lead to initiation success of crown fire. The critical fireline intensity can be used to determine the initiation of crown fire from surface fire for conditions with the absence of tree flowers, whereas the critical value can not be applied to cases with the existence of tree flowers. Moreover, the lower moisture content and higher crown height are, the maximum values of the flame height, fire plume temperature, and radiation heat flux of crown fire are larger. Besides, when the tree is covered with tree flowers, the flame height, fire plume temperature, and radiation heat flux of crown fire increase significantly due to the increases of fuel load and flammability. This work can not only provide theoretical basis for crown fire, but also supply technical guidance for the prevention and extinguishment of crown fire.