# Influence of Eddy-Generation Mechanism on the Characteristic of On-Source Fire Whirl
## 1 Introduction
Such interrelation between the involved physical and chemical aspects makes the understanding of the not-well-understood fundamentals of the swirling reacting flow more unfathomable. For instance, the existence of the whirling flame within the enclosure alters the heat feedback from the flame sheet to the fuel surface, and results in an unevenly distributed heating profile over the fuel surface. In a pool fire configuration, such ununiformed heat feedback could affect the burning rate as it influences the evaporation of combustible gas mixture converted from liquid-based parent fuel, and hence varies the ensuing combustion process and heat release rate [32; 33; 34; 35]. Similarly, the residence of the whirling flame escalates the circulation within the enclosure, enhances the mixing of the oxidiser and reactant, and promotes a more completed combustion process. Theoretically speaking, the formation of soot species, the fine particles resulting from incomplete combustion, and the coupled radiative heat transfer to and from the surrounding, should be suppressed, when compared with non-swirling freestanding flame [36; 37; 38; 39]. Nevertheless, the aggravation of the combustion process increases the flame temperature, which often excesses the threshold of soot nucleation, therefore aid the inception of in-flame generated soot species [40; 41]. The excess generated soot species could arguably intensify the radiative heat transfer to the fuel surface and further boost the heat release rate, which contradicts with the previous speculation. In addition to the intricate physical and chemical coupling process, the uncertainty caused by the numerical modelling configuration could make the characterisation of fire whirl more cumbersome. For example, the combustion region of the fire whirl is likely to swirl around the domain which may involve tilting, stretching, converging etc. in a highly unpredicted manner. Ifthe numerical domain is discretised non-uniformly, the variation in spatial resolution of the discretised region may significantly affect the evaluated result, particularly for the prediction of the sensitive flame temperature [42]. The ambiguous correlation, involving various aspects, has nevertheless not been decoupled or restrained in many of the previous studies. Strategies, e.g., heavy sooty flame, pool-based flame generation mechanism where the burning rate depends on the evaporation rate, locally refined meshing algorithm with a denser grid at the domain centre, where a freestanding fire is expected to occur and coarse mesh at the fringes, etc., have commonly been adopted in previous studies, both experimental and numerical. Those implementations make quantitative analysis of the fire whirl behaviour unattainable. It is, therefore, critical to construct a numerical framework with the isolation and consideration of the preceding coupling parties, and focus on establishing the correlation between the eddy-generation mechanism and the characteristic of the formulated fire whirl. In light of the abovementioned gap, the current study aims to characterise fire whirl formulated under various entrainment conditions via a numerical approach. The proposed numerical model, elaborated in more detail in the next section, is deliberately designed to isolate and restrain the coupling effect between some primary physical, chemical and numerical factors. The proposed model adopts soot-free and clean-burning alcohol-based fuel with a set injection speed, and aims to eliminate the soot radiative heat transfer to and from the surrounding, as well as the potential variation in the burning rate. In addition, the numerical domain of the entire enclosure, where both combustion and dynamic flow motion are expected to occur, is discretised with uniformed grid size to minimise the impact of the spatial resolution of computational control volume on the numerical result. The baseline case with one flow channelling slit is built and validated against experimental data.