A comprehensive study of fire whirls and optimization of smoke exhaust in the large scale atrium using CFD
Creators: name: FANG, Xiang
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401204
subject: Computational methods in fluid flow, heat and mass transfer (incl. computational fluid dynamics)

description: An asymmetric ventilation condition in tall buildings could induce fire whirls with intensified heat release rate and flame height, causing massive destruction to buildings and immense risks to residents and firefighters near the front line. Over the past decades, a substantial amount of experimental and numerical works had carried out exploring the relationships between the heat release rate, flame structures as well as fire whirl development stages. Nevertheless, the mechanisms involved in the fire whirl combustion process, the stability of swirling structures, and its onset condition still remain unknown. Therefore, this study aims to characterize the stability of fire whirls and explore insight into the onset of a fire whirl in the indoor building environment, by establishing three small-scale fire whirl models using the large eddy simulation (LES) framework. Small-scale vertical shift model and the half cylinder model was utilized to investigate the formation of whirling motion, and its impact on flame shape and height. Furthermore, the composition of the vortex core due to asymmetric and symmetric air entrainment was characterized in these two models. Moreover, the fire whirl stability and the condition of onsetting fire whirls were investigated through the partly opened vertical lift model.
 
In terms of the modelling framework, this study also proofed that the prediction of the combustion phenomenon is highly influenced by the combustion models. The chemistry equilibrium model and the steady diffusion flamelet model was applied to predict a medium scale fixed frame fire whirl facility. The swirling vector refinement method and the maximum tangential velocity refinement method was utilized to identify instantaneous fire whirl vortex core. Through the comparison of two numerical results, the steady diffusion flamelet model could predict the turbulence suppression in fire whirls more accurately, the prediction of the Richardson number was close to the experimental measurement. Moreover, the fire whirl in the steady diffusion flamelet simulation was found to have a profound unsteadiness. Due to the intense rotating flow structure, it is worth noting that the time-averaged quantities in fire whirl simulation significantly deviated from the instantaneous one, and the steadiness of fire whirl was enhanced by the intensive vertical flow inside the vortex core.

Based on the present study of fire whirls, a new smoke removal system, the vortex flow driven ventilation system, was proposed and investigated in this thesis to improve the smoke exhaust efficiency in the large-scale atrium. Comprehensive investigations were performed to optimize the vortex flow ventilation system, and this smoke removal system significantly enhanced the smoke exhaust efficiency.

The main body of this thesis is composed of seven chapters. In the first two chapters, research background and a comprehensive literature review are summarised with highlighted research gaps found in the existing literature followed by the research methodology in Chapter 3. Main research contributions are demonstrated from Chapters 4 to 6. In Chapter 4, the fire whirls¿ swirling vortex and the impact of ventilation conditions on the formation and stability of fire whirls was investigated by performing CFD simulations of two small-scale fire whirl. Chapter 5 presents the numerical simulations of a medium-scale fire whirl utilizing the chemistry equilibrium subgrid-scale model and the steady diffusion flamelet subgrid-scale model. The maximum tangential velocity refinement method and the swirling vector refinement method were also accessed to identify the boundary of the vortex core. The unsteadiness and the transition process for the inclined flame to the fire whirl were also examined. Subsequently, the smoke exhaust in the large-scale atrium was investigated in Chapter 6. A new smoke removal system, which replies on vortex flow driven ventilation, was developed. All the contributions are concluded and highlighted in Chapter 7.