# Influence of Eddy-Generation Mechanism on the Characteristic of On-Source Fire Whirl
## 1 Introduction  
Fire whirl is a unique swirling diffusion flame that often occurs in urban and wildland fires with disastrous effects [1]. The whirling combustion flow limits the radial flame dispersion and stretches the hot plume to rise upon an elevated vertical angular path, and as a result significantly increases the burning rate, flame height, and flame temperature and radiation flux to the surroundings [2]. Owing to the unique features associated with this particular combustion dynamic behaviour, the formation of the fire whirl in a fire scenario can be devastated. For instance, the intense heat energy contained within the elongated fire whirl core combined with the radiative heat transfer to the surrounding, promote the fire propagation to the neighbouring points. The swirling motion associated with the fire whirl could also intensify the combustion process by enhancing the mixing of the combustible gas mixture with the oxidiser, while at the same time making the unsteady movement of the hot plume regioneven more spatially unpredictable [3]. The presence of such phenomena associated with fire whirl has been observed and reported in many notorious fire cases [4; 5], and it has been identified as the core factor in intensifying the combustion process and making the scenario unmanageable and untenable. For this very reason, fire whirls, in terms of their formation mechanisms, characteristic features, and physical behaviours, are of great interest to both the research and industrial sectors, and hence have been identified as the topic of interest for this study. Previous experimental and theoretical studies have characterised and identified the three most critical criteria for the formation of a fire whirl, namely, a thermally driven fluid sink, a surface drag force to create a radial boundary layer, which facilitates air entrainment to the generated vortex column, and an eddy-generation mechanism [6; 7; 8]. In a typical buoyancy-driven diffusion flame, the flame can be considered as the fluid sink and the generated hot plume naturally drives horizontal flow radically towards the vortex column [9; 10], which fulfils the first two requirements. Therefore, the most critical element in fire whirl formation is the presence of a vortex generation mechanism. The generation of vortex could occur in a wildland fire situation, induced, for example, by topological obstacles, leeward slope, unpredicted weather conditions, etc. [11; 12; 13]. The formation of such circulation can also be observed in a compartment fire scenario [14; 15; 16], particular with the current trend of needing to construct high-density, high-rise buildings with complex geometric configurations. A typical example of a high-risk region for fire whirl formation is an enclosed space with entrainment from the flow channelling slit to introduce the circulation, for example, atrium, lift pit, spiral staircase etc., which are commonly featured in modern constructions. Despite the recently developed bio-based flame-retardant materials that effectively restrain the fire from spreading [17; 18; 19; 20], the occurrence of the spinning flaming flow in skyscraper fires, such as the recent incidents in Beijing Television Cultural Center, Plasco Building, Grenfell Tower, Marina Torch, FR Tower, etc., could aggressively promote an alarming fire propagation within the confined space and cause catastrophic damages [21]. This poses a severe risk to the occupants and makes the fire suppression process extremely challenging [22; 23; 24]. Therefore, the characterisation of fire whirl formulated for enclosed configurations was targeted as the investigation case. Fire whirl formulated in enclosed configurations has been intensively studied, both experimentally and numerically [7; 25; 26; 27; 28; 29; 30; 31]. The literature has revealed an entangled coupling between the combustion process and the dynamic flow, i.e., the temperature, buoyancy, vortex, the combustion reaction, etc., are interrelated and interact during the formation of the fire whirl.