Capturing the Swirling Vortex and the Impact of Ventilation Conditions on Small-Scale Fire Whirls

Xiang Fang,Anthony Chun Yin Yuen,Sherman Chi P Cheung,Eric Wai Ming Lee,Guan Heng Yeoh

doi:10.3390/app10103428

Abstract

The fundamental flow structure and temperature distribution of small-scale fire whirls, including tangential and axial velocities, temperature variation, and air entrainment in the lower boundary layer, were successfully captured using a generic fire field model with large eddy simulation (LES) turbulence closure. Numerical predictions were validated thoroughly against two small-scale experimental measurements, where detailed temperature and velocity distributions were recorded. Good agreement between numerical and experimental results was achieved. Normalization was also performed to compare the numerical predictions with the empirical correlations by Lei et al. (2015) developed from medium-scale fire whirl measurements. The transient development stages of small-scale fire whirls and the impact of air entrainment on the stability of the fire whirls were also investigated based on the validated numerical results. The numerical validations showed the potential of the current LES fire field model in capturing the dynamic behaviour of the fire whirl plume and performing a quantitative analysis on its onset criteria and combustion dynamics in future.

Highlights

Fire whirls 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]
Bare-wire K-type thermocouple trees were adopted for temperature measurements at the centre of the fire whirl
According to Chow and Han [14], the stainless steel thermocouple probes with a diameter of 1.5 mm were used throughout the experiment

Summary

Introduction

Fire whirls ( 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. The flame structure of fire whirls is sustained by a swirling vortex core along the axis of the plume [1]. 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]. 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]

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Conclusion