Buoyant pool fires under imposed circulations before the formation of fire whirls

This paper presents the first comprehensive experimental study on the dynamical behaviors of small scale buoyant pool fires under imposed circulation before the formation of fire whirls. A rotating screen facility was used for experiments. It was found that as the screen rotated slowly, the original vertical buoyant pool fire inclined with a certain angle (α) and revolved around the facility central axis steadily. The deviation of the flow field from axial symmetry is proved to be induced by the additional Coriolis force in the rotating coordinates. A correlation is proposed to couple the tangent of inclination angle to the screen angular speed and fire characteristics. The correlation agrees well with the experimental data in this work. The flame revolution was entirely dominated by aerodynamics and its angular speed was strictly equal to that of the rotating screen. It was found that the inclined flame surface was steady at the base and the oscillation was originated at a higher position due to suppression of vorticity generation. The nominal averaged pulsation frequency of the inclined flame was always larger than that of the free buoyant fire. The flame inclination and the change of pulsation mode can affect the flame heights and lengths significantly. At a critical screen angular speed, the inclined flame became vertical again with self-swirling and the fire whirl formed finally.