Effects of wind velocity and slope on flame properties

The combined effects of wind velocity and percent slope on flame length and angle were measured in an open-topped, tilting wind tunnel by burning fuel beds composed of vertical birch sticks and aspen excelsior. Mean flame length ranged from 0.08 to 1.69 m; 0.25 m was the maximum observed flame length for most backing fires. Flame angle ranged from -46 degrees to 50 degrees. Observed flame angle and length data were compared with predictions from several models applicable to fires on a horizontal surface. Two equations based on the Froude number underestimated flame angle for most wind and slope combinations; however, the data support theory that flame angle is a function of the square root of the Froude number. Discrepancies between data and predictions were attributed to measurement difficulties and slope effects. An equation based on Byram's convection number accounted for nearly half of the observed variation in flame angle (R(2) = 0.46). Byram's original equation relating fireline intensity to flame length overestimated flame length. New parameter estimates were derived from the data. Testing of observed fire behavior under a wider range of conditions and at field scale is recommended.