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ARTICLE:
C. M. K . Se
Fire Safety and Disaster Prevention Group, Department of Building and Construction, City University of Hong Kong, Hong Kong S.A.R., PRC S. C. P. Cheung
School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Vic. 3083, Australia Guan Heng Yeoh
Nuclear Analysis Section, Reactor Operations, Australian Nuclear Science and Technology Organisation, Menai, Australia 2234 A. L. K.. Cheung
Fire Safety and Disaster Prevention Group, Department of Building and Construction, City University of Hong Kong, Hong Kong S.A.R., PRC Richard Yuen
City University of Hong Kong K. Yuen
Fire Safety and Disaster Prevention Group, Department of Building and Construction, City University of Hong Kong, Hong Kong S.A.R., PRC
ABSTRACT Free-standing buoyant fire burnings always characterized by a unique, oscillating and periodic visible flame structure. This phenomenon is commonly referred as the “puffing effect” and considered as a distinguishing feature of free-standing fires. The occurrence of this oscillation is generally resulted from the instabilities of buoyancy driven turbulent flow which in turn leads to vortex shedding especially through the formation of large flaming vortices that rise up until they burn out at the top of the flame. Although the puffing effect exhibits a transient flickering behaviour altering the flame burning, experimental observations suggested that flames were burnt in a periodic manner with particular pulsation frequency. From a physical view point, the pulsation frequency governs the fuel-air mixing rate which thereby imposes significant turbulent fluctuations on the diffusion flame structure and interacts with the air entrainment, combustion efficiency and thermal radiation of the flame. Since all these processes are strongly coupled in nature, capturing the pulsation frequency is remarkably essential in modelling free-standing fire.
In the past decade, the Reynolds Averaged Navier Stokes (RANS) based fire field models have been widely adopted by researchers. Although encouraging results were obtained, the success of the RANS models in simulating the time-dependent fire dynamics especially capturing the pulsating behaviour of a buoyant fire has remained elusive. In this paper, in attempting to circumvent this shortcoming, a numerical model using the Large Eddy Simulation (LES) approach with considerations of turbulence, combustion, soot chemistry and radiation effects is presented and employed to capture the puffing effect of a medium scale free-standing buoyant fire. The puffing effect of the buoyant plume was appropriately captured. Predicted pulsation frequency obtained through Fast Fourier Transform (FFT) agreed well with the experimentally measured frequency. The predicted centreline temperature and velocity were found to be in good agreement with the experimental data as well.
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