## 3 Results
### The Formation and Evolution of Fire Whirl
#### 3.1.1 Stage A: Flame Development
The emerging of the incipient rotating reacting flow indicates the starting of the second period of Stage B, the formation of nascent fire whirl, defined from \(t=6.00\) s to \(t=8.50\) s. During this period, the flame structure rapidly transformed from whirling flow rotating in a relatively large radius, i.e., at \(t=6.06\) s, into the nascent fire whirl that is shifting towards domain centreline with reduces rotating radius and increased flame height, i.e., at \(t=7.45\) s, and eventually developed into a fire whirl that is centred with respect to fuel source and confined with a relatively small rotating radius, i.e., at \(t=8.15\) s. Unlike of that observed in the first period of Stage B, during this period, the evolution of the fire whirl will not be revolved back to the previous stage, and the formulated fire whirl remains in a quasi-steady state. It should be highlighted that the domain centreline temperature of all monitoring HABs during the first period, is relatively low, i.e., near-ambient condition, compared with that of Stage A. This agrees well with the abovementioned description made based on temperature iso-surface, as the flame at this stage are likely to be tilting away from the domain centreline, thus resulting the observed low-temperature profile at all HABs. The raise in centreline temperature starts as the transition moves to the second period, which is again consistent with the tendency that the core structure of the flame is shifting towards domain centreline with reduced rotating radius and extended flame height. The flame temperature at domain centerline during Stage B of the Slit 02 case is shown in Figure 4:7 Alongside with the temperature iso-surface presented in from Figures 16-18, the time duration for this stage of flame development can be approximately defined as from \(t=2.50\) s to \(t=5.50\) s. Similar to what observed in Slit 01 case, the Stage B of the Slit 02 could also split into two period, the period of transition from randomly flickering flame to the emerging of rotating reacting flow, presented in Figures 16 and 17, and the period of formation of nascent fire whirl, shown in Figure 4:8  
Figure 4:9 Centerline temperature of Slit 02 case at all monitoring HABs, from 02.50 s to 06.00 s (example of Stage B: fire whirl development and formation). Figure 3:0 Temperature iso-surface at representative instant of time during Stage B of the combustion process: from randomly flickering flame to the emerging of rotating reacting flow, of the Slit 02 case. The red solid line approximately illustrates the shape of the flame core region structure. Figure 3:1 Temperature iso-surface at representative instant of time during Stage B of the combustion process: flame restoring from emerging of the swirling reacting flow back to that flickering randomly, of Slit 02 case. As illustrated in Figure 16, the three-phase transition pattern previously demonstrated in the Slit 01 case, is observed in Slit 02 case. The three-phase transition pattern can be identified as: flame tilting towards one side with relatively straight line format, i.e., at \(t=2.72\) s; flame tilting towards one direction in the low-intermediate height and bending towards the opposite direction in the downstream, i.e., at \(t=3.94\) s; and the emerging of the rotating reacting flow with zigzag flame structure in the low-intermediate region and straight in vertical direction in the plume region, i.e., at \(t=4.31\) s.  
Despite the general agreement with that observed in the Slit 01 case during this transition period, there are some noticeable difference need to be highlighted. Firstly is the difference regarding the flame tilting in a straight line yield at \(t=2.72\)s compared with that of Slit 01 case. It can be seen that the flame is split into two streaming and tilting towards the respective air entrainment slit, compared with that tilting towards one side demonstrated in Slit 01 case. This symmetrical tilting pattern again could be potentially attributed to the symmetric geometrical configuration of the Slit 02 case.