Lifted flame in fire whirl: An experimental investigation

Abstract

This paper presents an experimental study on the flame and flow characteristics of the lifted flame in a fire whirl. A small-scale rotating screen facility was used for experiments with finely controlled imposed circulation (Γ) and heat release rate (Q˙=2.0−8.0kW), by using methane as the fuel. The instantaneous three-dimensional velocity fields in the vertical plane passing through the flame centerline were measured by stereo particle image velocimetry (SPIV) technique. It is found that the flame of fire whirl can be lifted off the burner under relatively weak Γ, for which, the lifted flame is mainly blue partially premixed flame under a certain lower heat release rate (Q˙=2.0kW in this work), while when the heat release rate is beyond a certain value (Q˙≥4.0kW in this work), the lifted flame is structured by blue partially premixed flame at the bottom and yellow diffusion flame downstream. For these two different lifted flames, the flame length and the flame base diameter have significantly different dependence on Γ. Experimental data indicate that under a specified Q˙, the flame liftoff height firstly increases slowly and then decreases quickly with increasing Γ after reaching the maximum value. The maximum liftoff height decreases with Q˙ slightly and Γ dominates the variation of liftoff height. It is also found that the instantaneous axial velocity in the lifted fire whirl increases quickly in the bottom inflow layer, then decreases with height upstream of the flame base and increases again in the hot flame region. The flow field diverges as the instantaneous flame base is approached. The mean axial velocity conditioned on the instantaneous lifted flame base generally increases with Q˙ and is less than two times the maximum laminar flame speed. The lifted fire whirl and the triple flame share similar characteristics of the flame front and velocity fields.