## 3 Results
### The Formation and Evolution of Fire Whirl
#### 3.1.3 Stage C: Fire Whirl Evolution
Figure 20: Centerline temperature of the Slit 01 case at all monitoring HABs, from 10.00 s to 20.00 s (example of Stage C: fire whirl evolution). therefore, reasonable to subdivide the Stage C of Slit 02 case into separate periods and investigate the characterisation of each period individually. The first period of the fire whirl evolution, Stage C\({}_{1}\), of the Slit 02 case is characterised as the period when the raise of domain centerline occurs frequently. From the observation of Figure 4, such period can be identified as from \(t=6.00\) s to \(t=12.00\) s, and the centerline temperature profile at all monitoring HABs is plotted in Figure 22. It is apparent that, by compared with that of Stage C of the Slit 01 case report previously, the occurrence of peak temperature at the domain centerline is observed to be more recurrent. For example, compared with peaks three or four times in every five seconds in the baseline case, the pattern of temperature rise at centerline demonstrated a two to three peak occurs in every second. This trend is more predominant during the period between \(t=7.50\) s to \(t=9.00\) s, where the centerline flame temperature at HAB 0.1 \(m\) remains roughly constant at a maximum value. This observation agrees well with the previous analysis in comparing the nascent fire whirl formulated in both cases. Due to the intensified eddy-generation mechanism from the second entrainment slit, the nascent fire whirl generated in the Slit 02 case is more spatially centralised in a confined space in the vicinity of domain centerline as well as with a more extended flame height, and thus persists with a relatively smaller radius and leads to higher centerline flame temperature, when compared with that of the baseline case. At this stage, the fire whirl in a fairly stable status, as the temperature iso-surface at representative instant of time during Stage C\({}_{1}\) is presented in Figure 23 is nearly identical in terms of all primary parameters of interest during this period. Figure 23: Temperature iso-surface at representative instant of time during Stage C\({}_{1}\) of the combustion process: the first phase of the evolution of fire whirl, of the Slit 02 case. The red solid line approximately illustrates the shape of the flame core region structure. Figure 22: Centerline temperature of the Slit 02 case at all monitoring HABs, from 06.00 s to 12.00 s (example of Stage C\({}_{1}\): fire whirl evolution). As the fire whirls revolving under the symmetrical and intensified entrainment configuration, it gradually moves to the second stage of fire whirl evolution, Stage \(C_{2}\): the fire whirl gradually deviates from the previously observed confined region and departs from domain centerline in an outbound direction. With a close inspection of the centerline flame temperature profile as well as the temperature iso-surface at representative instant of time, presented in Figures 24 and 25 respectively, the time duration of the Stage \(C_{2}\) can be defined as from \(t=12.00\ s\) to \(t=20.00\ s\). From the observation of flame centerline temperature, it can be concluded that the occurrence of a temperature peak at domain centerline are tending to become scatted during this period, compared with that reported in the Stage \(C_{1}\), i.e., the frequency of the flame temperature peak at the domain centerline reduced from two to three appearances every second into approximately three appearances in every five seconds, which resembles the nascent fire whirl formulated in the baseline case. The reduction in the frequency that high flame temperature arises at the domain centerline could be explained by assessing the iso-surface of representative instant of time during this very period. It can be seen from Figure 25 that, over time, the lower-intermediate region, as well as the downstream plume region of the fire whirl, starts to deviate from the confined space that observed in previous period and departs from domain centerline towards the enclosure.