Modeling Fire Spread in Large Compartments using Computational Fluid Dynamics

Abstract

Architectural design of buildings in the 21st century has emphasized large open spaces, atriums, high ceilings, and glazed facades. Traditional design fires developed for small enclosures assume uniform burning throughout the compartment and may not apply to these modern building layouts. In contrast, large compartment fires burn over a limited area and move across a floor system over time resulting in non-uniform and transient heating [1]. Recent advances have introduced simplified models for spreading (i.e., “traveling”) fires in large compartments, but the models have not been experimentally validated and only account for 1D fire spread. The purpose of the work presented in this dissertation is to (1) improve thermal and structural modeling of large compartments during traveling fire events and (2) to use these improved modeling techniques to characterize the range of possible thermal environments that a compartment can experience during a traveling fire. The goal of this work is to aid engineers in designing effective fire protection for large compartments. In order to model non-uniform temperatures and realistic fire dynamics, computational fluid dynamics (CFD) simulations of fire are utilized throughout this dissertation. The CFD fire simulations are coupled with finite element (FE) models to study the thermo-mechanical structural fire response. In the existing literature, a wide range of parameters are used when determining the boundary condition at the structure-fire interface. This dissertation investigates the various methods to identify best practices while balancing accuracy and computational expense. A novel fire spread model for large compartments using a transient flux-time product model for ignition was formulated to model realistic traveling fire behavior. The fire (modeled using CFD) begins with the assumed ignition of an object. The fire is allowed to spread realistically from object to object through the use of a transient flux-time product ignition model. Burning objects are represented as equivalent burners to reduce computational expense. The fire spread model for large compartments is validated using experimental data from NIST [2]. A parametric study of fire spread was performed using the transient flux-time product ignition model to characterize the behavior of fire spread in large compartments, and to quantify the range of thermal environments a structure can experience when subjected to traveling fires. Compartment characteristics and fire inputs were varied such as the ignition location of the fire, ventilation conditions, heat release rate of the objects, and fuel load density. The results of this study are compared to the simplistic traveling fire model (TFM) to determine if current models are adequate for predicting the thermal environment in large compartments. The work in this dissertation shows that fire spread can be accurately modeled using computational fluid dynamics through the implementation of a transient flux-time product ignition model, and that the response of structural members to localized burning can be accurately modeled through the use of an appropriate boundary condition at the structure-fire interface. The parametric fire spread study showed that ventilation conditions and the fuel load density had a significant impact on the thermal environment in the compartment. Additionally, when this detailed model was compared to TFM, it was concluded that large temperature gradients occur through the width of the compartment that may be significant for structural response. Also, TFM under-predicts far field temperatures given certain compartment conditions.