An experimental study of the flame intermittent frequencies of wind-driven line fires

摘要

While forward heating from flames in wind-driven conditions has previously been studied to help understand flame spread, past measurements only report time-averaged values that do not give a picture of the truly dynamic processes occurring. In this study, we investigate the intermittent movement of flames, namely the flame forward pulsation frequency, which may play an important role in forward convective heating during flame spread. A stationary burner (25 by 5 cm~2) using propane gas as the fuel was used to model the fire source. Three fire sizes (6.3, 7.9, 9.5 kW) were employed in the experiments, with a wind flow velocity, measured by a hot-wire anemometer, ranging from about 0.5 to 2.5 m/s. High-speed videos were taken of the side-view of the flame, followed by thresholding image analysis to track the forward movement of the flame on each frame. The flame forward pulsation frequency was then determined from the flame position using a variable interval time average (VITA) method. Derived scaling laws indicates that the flame forward pulsation frequency, f, is a function of the Froude number and a non-dimensional fire heat release rate, which is confirmed with experimental measurements to form a semi-empirical power-law trend of the form f = 12.9 (Fr/Q~(*1/2))~(0.35).