Flow Structure Of A Fixed-frame Type Firewhirl

Abstract

This paper reports the results of both experimental and numerical investigations on a laboratory-scale fixed-frame type fire whirl. We designed a fixed-frame type fire whirl generator consisting of two vertically oriented split PMMA cylinders which are placed in an off-center location to create a small compartment with a slit at each end. At the center of the compartment floor, a 5 cm-diameter 1-propanol pool fire generated upward buoyancy flow and fresh air entering the compartment through the slits, generating swirl motion in the compartment. This is a fixed-frame type fire whirl. We measured transient 2-D radial, tangential, and axial velocity profiles with 2-D particle image velocimetry (PIV). The radial and tangential velocity profiles were measured at four different heights (5, 10, 15, and 20 cm) along the axis of the fire whirl, while the axial velocity profiles were measured at 10 cm in the axial height. We also measured relative temperature distributions of the fire whirl with an infrared thermograph technique. These measurements show that both the tangential velocity and the absolute value of radial velocity increase with an increase in the radial distance, while they remain relatively constant along the axis. Our IR data also show that the relative temperature map stays unchanged along the axis. The radial locations where the maximum radial and tangential velocity were measured approximately coincide and are at about 3.5 cm on the radial coordinate, while the visible spinning flame location is at about 1.2 cm. This shows a difference in the velocity structure between open pool fires and fire whirls, because the location of the maximum flow velocity and visible flame approximately coincides for laminar open pool fires. A simple laminar 2-D axisymmetric CFD model was developed to simulate these measured flow and temperature structures created by the fire whirl, and reasonable agreement was obtained.