## 3 Results
### The Formation and Evolution of Fire Whirl
#### 3.1.1 Stage A: Flame Development  
Stage A: flame development represents the period from starting the ignition until the full development of the buoyancy-driven diffusion flame. The flame temperature in the domain centreline of all HABs of both Slit 01 case and Slit 02 case during Stage A of the combustion process are presented in Figure 5 and Figure 8. The temperature iso-surface at representative instants of time are illustrated in Figures 6 and 7, and Figures 9 and 10. Figure 4: Centerline temperature of Slit 02 case at all monitoring height above burners (HABs), from 00.00 s to 50.00 s, consist of three main stages of the development and evolution of fire whirl, namely Stage A: flame development, Stage B: fire whirl development and Stage C: fire whirl formation and evolution. Figure 3: Centerline temperature of Slit 01 case at all monitoring HABs, from 00.00 s to 50.00 s, consist of three main stages of the development and evolution of fire whirl, namely Stage A: flame development, Stage B: fire whirl development and Stage C: fire whirl formation and evolution. The flame temperature at domain centerline during Stage A of Slit 01 case is shown in Figure 5. Combined with the temperature iso-surface presented in Figures 6 and 7, the time duration for the stage of flame development can be approximately defined as from \(t=0.00\) s to \(t=2.55\) s. The stage A could be further splitted into two periods, the period flame developing upwards from the time of ignition, demonstrated in Figure 6, and the period flame fluctuation propagates from flame tip downwards to near fuel pan surface, presented in Figure 7.  
Figure 5: Centerline temperature of Slit 01 case at all monitoring HABs, from 00.00 s to 05.00 s (example of Stage A: flame development). Figure 6: Temperature iso-surface at representative instant of time during Stage A of the combustion process: from the ignition to the fully developed buoyant diffusion flame, of Slit 01 case. Figure 6 demonstrates a typical development of a buoyant diffusion flame during the period of \(t=0.00\) s to \(t=0.60\) s. The flame proporgates in the vertical direction, due to the dominant of momentum at the initialisation stage of the combustion. The flame front reaches the three monitoring HABs, \(0.1\ m,\ 0.3\ m\) and \(0.5\ m\), at approximately \(t=0.25\ s,\ t=0.50\ s\) and \(t=0.60\ s\). Upon the extension of the flame height to its maximum, the flame with the straight profile starts to flicker, from flame tip and gradually propagated to the further upstream region, as demonstrated in Figure 7. It can be seen from the figure that, the region where flame starts to fluctuate starting from the flame tip at approximately \(t=1.00\ s\), reaches the low-intermediate region at approximately \(t=1.70\ s\) and finally proporgates to near fuel pan surface at approximately \(t=2.55\ s\). The fluctuation of the flame is developing towards upstream along the domain centreline axis, as the figure demonstrated that, the flame structure that beneath the flickering region remains straight and in a relatively regulated shape. The transition of the flaming behaviour could be contributed to momentum-driven upward motion is overwhelmed by the thermo-dynamic of the buoyant diffusion upon the completion of its development. It should be highlighted that the fluctuation of the flame pattern at this particular stage is highly randomised but relatively centred with respect to the location of fuel pan, i.e., differing from the structure of a formulated fire whirl, no tendency of tilting, rotation can be observed explicitly at this instant of time. The flame temperature at domain centerline during Stage A of Slit 02 case is shown in Figure 8. Alongside with the temperature iso-surface presented in Figures 9 and 10, the time duration for this stage of flame development can be approximately defined as from \(t=0.00\) s to \(t=2.55\) s. Similar to what observed in Slit 01 case, the Stage A of Slit 02 could be further split into the flame vertical development period and flame flickering period.