The oscillatory behaviour of medium-scale pool fires

This paper describes an investigation of the formation of the large-scale structures which govern air entrainment and mixing in the continuous flame zone of a 30 cm diameter acetone pool fire. Flow visualization was combined with Laser Doppler Anemometer velocity and thermocouple temperature measurements to determine the pulsation frequency of the fire and to identify the regions of the fire involved in the evolution of these structures. The visible fire structure and the pulsation frequency were also examined when the boundary conditions on the burner were varied. The flow visualization studies and the velocity and temperature profiles suggested a detailed description of the stages occurring during the development of large-scale structures in the fire. The frequency of oscillation of the fire varied only slightly with the reduction of fire radiation back to the surface of the fuel and with minor blockages in the entrainment of air at the edges of the fire. When the flow of air into the base of the fire was altered more significantly, the visible structure of the fire and its frequency of oscillation were affected to a larger extent. It appears that the large-scale structures are more a result of instabilities in the fluid dynamics external to the fire, than the variations in flame chemistry due to the local accessibility of fuel and air. An initial phenomenological model of the entrainment processes and their effect on fire behaviour formulated as a result of these experiments has provided further insight into the complex processes which interact in the continuous flame zone of a medium-scale pool fire.