# Influence of Eddy-Generation Mechanism on the Characteristic of On-Source Fire Whirl
## 3 Results
### Temperature  
The thermal component of the flame is undeniably the most critical aspect to be investigated in fire modelling. It is more critical in this study of fire whirl, as the temperature and velocity are interrelated, and such coupling is essential for the understanding of the ensuing formation and sustenance of the swirling combustion phenomenon. The qualitative analysis of the fire whirl therefore starts by assessing the temperature profile of the formulated swirling reacting flow field. Figure 3: Numerical prediction of incoming y-velocity through the air gap, compared with that generated from prior study. Figure 2: Numerical prediction of temperature at domain centreline, compared with experimental measurements. The temperature iso-surface of the flame generated by the two cases, presented in Figure 4, provides a glance of the geometric characteristics of the fire whirl formulated under different entrainment boundary conditions. The figure shows that, with the side flow channelling slit(s) acting as the eddy-generation mechanism, a highly twisted flow field with relatively high temperature can clearly be observed in both geometric configurations. This temperature profile exhibits a helical form, implying the persistence of strong swirl motion within the core structure of both flames. To be more specific, the fire whirl generated from the baseline model is exhibited in the format of a tortuous and highly twisted string. The hot plume accumulates and exits up near exit region in the vertical orientation. Whereas the fire whirl of that generated via the two-slit configuration resembles a convoluted cylinder centred on the lower-mediate region of the computational domain. The temperature contour of a cross-sectional plane of the computational domains of the two cases is illustrated in Figure 5a,b. The contour plot shows that the maximum flame temperature of the case with the two-slit configuration is higher, i.e., increased by about 9.42%, when compared with the baseline. Similar to the temperature iso-surface presented above, the cloud of the hot plume, accumulated at the downstream region observed in the baseline case, is cleared out when the additional slit is introduced. The visible flame height of the two cases determined based on applying the cut the cut-off visible flame tip temperature of 600 \({}^{\circ}\)K is presented in Figure 5c,d. From the comparison, it is evident that the visible flame height for two-slit configuration grows significantly compared with that of the baseline model, i.e., it is increased by 84.38 % from 0.32 m to 0.59 m.  
Figure 4: Temperature iso-surface of the fire whirl formulated in 01 slit case (**a**) and the 02 slit case (**b**). The extension of the flame height of the comparison model is further verified by comparing the spatial temperature distribution, along with the lateral direction, as presented in Figure 6. Theoretically, the peak flame temperature of a typical diffusion flame should occur at the reaction layers, i.e., the intersection where the fuel and the oxidiser encounter each other and react due to diffusion. The reaction layer in the continuous and intermittent region of a diffusion flame is typically away from the flame centreline, and as a result, the spatial distribution of the flame temperature should resemble a hump-like profile. The hump-like profile tends to shift to a Gaussian distribution, with its value maximised at the centreline, as the flame flows further to the downstream, i.e., enters the plume region. The transition from a hump-like profile to a Gaussian distribution can be observed in the baseline model, as the HAB increased from 0.3 m to 0.5 m. This implies that the domain from HAB 0.5 m and beyond of this very flame can be identified as the plume region. On the other hand, the temperature profile generated by the 02 slit case features a hump-like profile for all monitoring HABs, means that the flaming structure up to HAB 0.5 m of this fire whirl can still be considered to be in the continuous and intermittent region.