# Capturing the Swirling Vortex and the Impact of Ventilation Conditions on Small-Scale Fire Whirls
## 1 Introduction
[PERSON] and [PERSON] [9] conducted an early study of fire whirls and attempted to predict the flame swirling motion within a vertical square enclosure using their in-house field model. Numerical simulations were carried out to investigate the effect of vertical gap size, fuel area size, and enclosure height on swirling flame height. Without considering radiation in their calculation, the intense rising heat and radiation feedback of fire whirls were neglected. Moreover, in simulating fire whirls, it is crucial to capture the transient effect of the buoyancy and the swirling motion within the fire core. Due to the limitation of time-averaging techniques, traditional RANS (Reynolds-averaged Navier-Stokes) models tend to underpredict periodic motion, while large eddy simulation (LES) models, in general, yield more accurate results [16]. [PERSON] et al. [17] presented the first numerical study on fire whirl simulation using the LES framework. The LES turbulent model and a subgrid thermal element model were formulated to represent the swirling flame and these models were validated against the experiment of [PERSON] and [PERSON] [9]. The predicted mean temperatures for non-swirling and whirling fires were in satisfactory agreement with the experimental data. [PERSON] et al. [3] employed the widely adopted fire dynamics simulator (FDS) for fire whirl simulation. However, the predicted flame height, temperature, and velocity profile had considerably large discrepancies. As flame elongation is the most apparent characteristic of the fire whirl, the corrections of imposed circulation, velocity, and heat release rate with flame height have recently been of great concern [2; 8; 18; 19]. Over the past decades, more sub-modelling components were incorporated into the FDS, such as the detailed chemistry mechanism and the radiative feedback model. [PERSON] et al. [20] adopted the new version of FDS to simulate a small-scale fire whirl measured by [PERSON] and [PERSON] [15]. They implemented a liquid fuel model considering radiation and convection heat feedback from the flame and qualitatively captured the four development stages of the fire whirl. Their study has marked a crucial step in capturing the combustion behaviour and onset of fire whirls. Unfortunately,the predicted velocity field has not been thoroughly validated due to the lack of experimental data, providing only qualitative analyses of the swirling motion. More recently, [PERSON] et al. [21] presented a validation study of fire whirl using Reynolds-averaged Navier-Stokes (RANS) turbulence closure. Nevertheless, it is well known that the RANS model filters the instantaneous velocity and temperature fluctuation, which in essence is the nature of the fire dynamics. A thorough validation study of fire whirls using LES turbulence closure, where instantaneous fluctuations are resolved, is currently missing in the literature. Therefore, the purpose of this paper is to thoroughly validate our numerical prediction on the transient temperature and velocity field against three small-scale fire whirl experiments measured by [PERSON] and [PERSON] [14; 15] and [PERSON] [7]. With the validated result, capturing the swirling vortex core and a detailed analysis of the onset mechanism and stability of fire whirls under different circumstances are also presented.