Characterisation and analysis on the instantaneous development and dynamic vortex cores of fire whirls in a fixed-frame facility

Abstract In an outdoor fire accident, the fire whirl could significantly accelerate the spread of fires with the elongated flame and the rotating flame movement. Large eddy simulations were conducted to investigate the fire whirl formations in a medium-scale fixed-frame facility and their development processes under three heat release rates. Four approaches were developed to identify the size and location of fire whirl's vortex core. A hybrid swirling vector and tangential velocity refinement approach was found as the most promising approach to identify reliable vortex cores at various fire whirl development stages. The numerical outcomes revealed that the instantaneous vortex core expanded with the increased heat release rate in the fixed-frame facility, and the vortex core constructed the fire whirls' elongated flame. Also, the correlations between heat release rate and the imposed circulation could impact the flame shape and the transition process under different stages.