Experimental and numerical studies of whirling fires
Motivation of this study stems from the need to understand the physical mechanisms of whirling fires that occur in an open space and within enclosures. Buoyant whirling flames may be potentially more destructive than ordinary fires due to greater burning rate, higher concentration of heat release in a small region of the plume core, increased radiative output and unexpected smoke movement. The effects of rotation upon the structure and behaviour of buoyant flames have not yet been thoroughly studied and understood. Investigation of this phenomenon is therefore required to allow techniques to be developed that will counter the threat of such outbreaks. Also, the mechanisms controlling the development and stability of whirling flames are of fundamental interest for refined modelling of coherent and self-organised flame behaviour. This work, is an experimental, theoretical and numerical study of whirling fires. Experimental results, a modified CFD model and simulations of whirling flames are presented within this Thesis. The work aims to overcome the limitations of the previous research of whirling fires which is insufficient from both an experimental and theoretical point of view. Firstly, experimental studies of intermediate (room-size) scale whirling fires have not yet been comprehensively reported, despite a great deal of attention devoted to both large scale mass fires and smaller laboratory flames. Experimental studies undertaken using a facility at the Greater Manchester Fire and Rescue Service Training Centre fill this gap, thus demonstrating that whirling flames may develop within a compartment. The periodic precession, formation and destruction of the whirling flame and the increase of the time-averaged burning rate (compared to non-whirling flames in the open space) have been observed. Three fuels with significantly different burning rates (diesel, heptane and ethanol) were investigated in this work. Secondly, previously published results of theoretical analysis of rotating flames were oversimplified and based on strict limitations of the integral model or the inviscid flow assumption. Also there have only been few attempts to undertake CFD modelling of whirling flames. In published studies, radiative heat transfer was not modelled and the burning rate was not coupled with the incident heat flux at the fuel surface. To overcome these limitations, the CFD fire model Fire3D, developed in the Centre for Research in Fire and Explosion Studies, has been adapted to allow numerical simulations of rotating buoyant turbulent diffusion flames. The turbulence model was modified to take into account stabilisation of turbulent fluctuations due to the centrifugal acceleration within the rotating flow. Theoretical analysis of the vorticity equation revealed the physical mechanisms responsible for vorticity concentration and amplification in the rising plume affected by externally imposed circulation. This explains the significant flame elongation (when compared to non-rotating cases) observed in the experiments. Computational results have also been compared to video-recordings of the experimental flames produced; flame elongation was replicated and similar stages of oscillating flame evolution, including formation and destruction of the vortex core, have been identified. Implications of the phenomena studied in relation to fire engineering are also provided. This study contributes to a performance based framework for an engineering approach, which is reliant upon detailed quantitative analysis and modelling. Such an approach is encouraged by modem fire safety legislation including the guides to fire safety engineering BS9999-21 and BS79742 'British Standard 9999-2 Draft Code of Practice for fire safety in the design, construction and use of buildings. BSI, 2004. UK. 2 British Standard BS7974 Application of fire safety engineering principles to the design of buildings. BSI, 2001-2003. UK.