# Capturing the Swirling Vortex and the Impact of Ventilation Conditions on Small-Scale Fire Whirls
## 1 Introduction  
Fire whirls (also referred to 'fire devils') are a distinctive and potentially catastrophic form of fire, which can occur in forest fires, bushfires, or intense fires in large building voids or atrium structures. Unlike normal pool fires, the flame structure of fire whirls is sustained by a swirling vortex core along the axis of the plume [1]. The vortex core could be initiated by certain air entrainment conditions [2] and the upward movement of the heated gases [3]. Due to the circular motion within the core, the turbulent intensity, air entrainment, and the mixing between air and fuel are significantly enhanced, forming an intensified and prolonged flame structure with a burning rate that is several times higher than that of normal pool fires [4]. More importantly, fire whirls emit significant radiant heat that could further ignite surrounding combustibles, causing rapid fire spread and posing great threats to the residences and structures nearby. Due to the complex nature of fire whirls, only a few experimental measurements have been carried out over the past decades [5; 6; 7; 8]. Early experimental studies were carried out to investigate the vorticity strength within fire whirls using a rotating cylindrical screen [9] or by channelling air entrainment [10]. More recently, several small-scale experimental studies have been carried out, attempting to correlate the burning rate andflame length with the size of the fuel bed and type of fuels of fire whirls. [PERSON] and [PERSON] [11] demonstrated that the flame height of a fire whirl is proportional to the fuel burning rate by coupling the Burke-Schumann flame with the Burgers vortex theory. Similarly, based on their measurements, [PERSON] et al. [2] derived an empirical equation correlating the flame height with the heat release rate, Richardson number, and axial velocity. Furthermore, by imposing circulation using a rotating screen, [PERSON] et al. [12] demonstrated that fire whirls present a difference in flame shape under a variety of imposed circulation. These studies focused on the correlation of flame height with flow characteristic parameters for fully developed fire whirls. Through a series of small- and medium-scale internal fire whirl experiments, [PERSON] et al. [13; 14; 15] identified the four generalized stages in fire whirl development: (i) initial stage, where it resembles a free-standing fire with a relatively short flame height; (ii) flame rising stage, where the circulation motion of entrained air starts to prolong the flame height and intensifies the burning rate; (iii) fully developed stage, where the flame structure, height, and burning reach and stabilize at the maximum level; and (iv) decay stage, where the swirling motion and burning rate of the fire whirl gradually decay due to the limited remaining fuel. Furthermore, [PERSON] and [PERSON] [14] also investigated the influence of ventilation width and height on the onset behaviour of fire whirls. The vertical gap width was identified to be the critical factor in flame elongation and the rising of heat release rate. Unfortunately, no measurement of velocity field was conducted in the experimental studies. Only qualitative analyses on the swirling motion of the entrained air were presented. Later, [PERSON] and [PERSON] [7] presented a comprehensive particle image velocimetry (PIV) measurement on a fire whirl, where air entrainment is formed by a staggered transparent cylinder. Their study provided quantitative velocity field information on the vortex core of the fire whirl and the surrounding circulation structure. Nonetheless, only a few temperature measurement points were conducted in conjunction with the PIV measurement. The interaction between vortex circulation and combustion within the vortex core was not thoroughly studied. On the other hand, computational fluid dynamics (CFD) has the potential to reveal more detailed flow characteristics, particularly near the flame surface and the thin boundary layer above the fuel surface.