Propagation of Precursor Flame Tip in Surrounding Airflow along the Ground Soaked with High-volatile Liquid Fuel

Abstract

The propagation of a precursor flame tip through a layered flammable gas mixture along a fuel-soaked ground is studied experimentally under the effect of the surrounding airflow. The behavior of the precursor flame tip is investigated by the simultaneous measurement of the images of color Schlieren photograph and a visualized chemiluminescence zone due to the OH radical in the flame tip. The effects of the ground conditions; the surface inclination, the ground temperature, and the surrounding airflow velocities, on the propagation velocity of the flame tip and on the thickness of the flammable gas layer are investigated. The thickness and the length of the OH luminescence zone in the flame tip are of the order of 1 and 2-3 cm, respectively. The tip of the OH luminescence zone propagates in the velocity boundary layer of the airflow. When the ground temperature is not so high, even above the stoichiometric temperature of combustion of the liquid fuel, no propagation of flame tip occurs in the counter airflow, the velocity of which is more than the flame propagation velocity with no airflow. Also in a high speed assist airflow, no propagation of the flame tip occurs. These results are in contrast to the flame propagation along the liquid pool surface in the surrounding airflow. This is mainly due to the low rate of formation of a flammable gas layer on the unburned ground and the blow-off of it by the surrounding airflow.