Numerical studies of fire and sprinkler induced air flows in atria

Abstract

The objective of this thesis is to study the interaction between sprinkler water spray and fire-induced air flow in atria with the application of Computational Fluid Dynamics (CFD). This involves numerical simulations on the two-phase, three-dimensional, compressible, turbulent flow. The studies can be divided into two sections: studies on the liquid phase (sprinkler water spray) and studies on the gaseous phase (smoke generated by atrium fire). For the sprinkler system, the sprinkler water spray is considered as a group of water droplets with variable values of initial diameter and velocity component. It is very important to understand the properties of sprinkler water spray, which include water droplet size distribution and trajectory of water droplets. A new droplet size distribution functions for water spray, known as Chow-Yin distribution function, is proposed to describe the droplet size distribution. It is derived from earlier expressions obtained from information theory. The parameters concerned can be measured more easily. Results predicted using the new function are compared with experimental data and fairly good agreement is found. The trajectories of water droplets discharged from an atrium sprinkler head are studied by solving the equation of motion, with the velocity components of droplets expressed in analytical form. With the Chow-Yin droplet distribution function, the shape of the sprinkler water sprays is calculated. The result is useful in studying the interaction between sprinkler and fire-induced air flow. Furthermore, another important parameter - water volume density rate was determined with the volume of water received at the floor level being calculated. For the gaseous phase, fire field model/CFD is applied to study the fire-induced air flow in atrium. The self-developed software CY-TEAM and the commercial software CFX4 are selected to carry out the studies in this thesis. Among the field models, the two-equation k-庰 turbulence model is widely used and is applied here for its robustness and simplicity. Tuning the parameters concerned would give better results. In this thesis, the diffusion coefficient and Prandt number of k-庰 turbulence model are tuned. Numerical results are compared with the experimental data reported in the literature. Plume expressions proposed in the literature are assessed and it is confirmed that the plume expressions are good enough for use as design equations. Applying fire field modeling in simulating fire-induced air flow in atria would require specification of free boundary conditions. Improper description of the free boundary conditions would give very different results. Three free boundary conditions commonly used are tested. Results predicted from all these different geometry, conditions and software are compared. It is concluded that free boundary condition should be specified carefully hi simulating fire-induced air flow with CFD. With the application of CFD, testing is done on scaling laws for compartment fires. Scale modeling studies are useful in understanding the smoke mowement pattern, but scaling parameters have to be selected carefully. Two sets of scaling parameters on temperature and air speed proposed in the literature are evaluated by CY-TEAM. The predicted results are applied to criticize these two sets of scaling parameters. With sprinklers commonly installed in the high headroom atrium of a building, it is doubtful whether those sprinkler heads can be activated in case of fires. The actuation time of sprinkler heads located at the atrium ceiling is estimated by a fire field model. Comparison with some experiments reported in the literature is made. Satisfactory agreement between the predicted and measured results is found. Finally, numerical studies on the atrium fire environment with the sprinkler-water spray interaction are carried out by using CFX. Models for both steady state and time-dependent situations are developed. Results predicted with the above theoretical model are limited, but still they can give fire phenomena helpful to the design of sprinkler system installed in atrium buildings.