Numerical study of wind effects on the characteristics of flames from non-propagating vegetation fires

摘要

A variant of a three-dimensional two-phase model was proposed to address the central question of how wind affects the geometric properties of flames from steady vegetation fires. The pyrolysis rate and the spatial distribution of burning vegetation were representative of the heat release rates of fires ranging from single tree or tree group "torching" to continuous surface fires. Wind speed, U_∞, was varied from 0.5 to 8 m s~(-1). Results revealed that about one third of the heat produced by combustion is released within the fuel bed, and this occurs almost independently of wind speed and vegetation distribution. Predicted values of flame height, flame length and tilt angle were found to be consistent with available literature data. Model results showed that the flame length and height normalized by the flame depth, D, scale with the modified Froude number Fr_D = U_∞~2/gD and the dimensionless heat release rate. Predicted mean velocity flow streamlines appeared to be nearly straight in both the flaming region and the inert plume, supporting the relevance of a far field analysis to determine the flame tilt angle. This analysis showed that the tangent of the flame tilt angle may be expressed in terms of the Froude number based on the characteristic length scale of the plume. The numerical model was then used to quantify these relationships. The relevance of scaling parameters was supported by both field and laboratory-scale experiments.