# Influence of Eddy-Generation Mechanism on the Characteristic of On-Source Fire Whirl
## 3 Results
### Velocity
#### 3.3.2 Radial Velocity and Tangential Velocity  
The profile of U, V and radial velocity, plotted as a function of the radial position at three monitoring HABs, is presented in Figure 15. The transformation of the profile of U, V and radial velocity from hump-like to a plateau with the increase of flame height was observed in the baseline model, whereas that hump-like velocity profile is inherently persistent in all HABs in the 02 slit case. Furthermore, the radial location at which the velocity of interest peaks shifts away from the centreline axis with an increase in the flame height in the baseline case, whereas this is nearly identical at all HABs in the comparison case. It has been described in prior studies that the maximum radial velocity occurs adjacent to the surface and reverses direction close to the upper parts of the boundary layer, indicating the presence of the circulation zone [26]. The departure of the boundary layer of the circulation zone away from the centreline, observed in the baseline case, implies the dissipation of the fire whirl core region. In other words, the flame departs from the continuous and intermittent region and enters the plume zone at the elevated height of 0.5 m. The near-constant radial position of the peak radial velocity observed in the 02 slit case depicts a well-maintained fire whirl core structure, i.e., the flame at high  
Figure 14: Plot of axial velocity, along with the radial direction at three monitoring HABs: 01 slit case (**a**), and 02 slit case (**b**). elevations can still be considered to be within the continuous region. The combustion region of the comparison case, therefore, is demonstrated to be well-extended compared with that of the baseline. Circulation is indeed the unique feature distinguishing fire whirls from non-swirling fires. The presence of the circulation disturbs the flow field and alters the ensuing combustion process, as a result significantly affecting assorted properties, such as flame temperature, combustion dominant region, as well as the velocity profile. The circulation is characterized by assessing the azimuthal velocity, or tangential velocity, in this study. Herein, the tangential velocity against the radial position at the selected HABs of the cases of interest is plotted in Figure 16.  
Similar to the profile of radial velocity, The humped-like profile turns to flat and the radial location where the tangential velocity peaks departs away from domain centreline with the increase of flame height in the baseline model, while those of the comparison case remains relatively unchanged at all monitored heights. Figure 15: Plot of U, V and radial velocity along with the radial direction at three monitoring HABs: U-velocity of the 01 slit case (**a**), U-velocity of the 02 slit case (**b**), V-velocity of the 01 slit case (**c**), V-velocity of the 02 slit case (**d**), radial velocity of the 01 slit case (**e**), radial velocity of the 02 slit case (**f**). To further elaborate the transition, the mean whirl vortex core radius at the elevated heights of the two cases are evaluated and listed in Table 2. The mean whirl core radius is determined based on the azimuthal velocity profile, i.e., the radial location at which the tangential velocity is at its maximum, and beyond which the circulation becomes nearly constant. The results reveal that the mean whirl core radius of the baseline case departs significantly from the centreline axis, i.e., it increases by 136.91% from 2.33 cm at HAB 0.3 m to 5.52 cm at HAB 0.5 m, while that reported for the 02 slit case is nearly identical at all monitoring HABs. Previous experimental study examining the correlation between the fire whirl radius and the stage of combustion suggests a reasonably constant whirl core radius in the continuous flame region, which increases with flame height in the intermittent and plume regions [33].