Numerical Modeling of Compartment Effects in Benchmark Compartment Fire Configurations

The focus of this two-year project is on computational fluid dynamics-based (CFD-based) modeling of both gas burner and upholstered furniture fires. Using the Fire Dynamics Simulator (FDS), maintained by the National Institute of Standards and Technology (NIST), the project team aims to simulate a series of UL free-burn and compartment fire experiments that varied fuels, fuel location, and ventilation. 

The computational work is separated into two phases, each consisting of a series of tasks. Phase I focuses on performing simulations with the official version of FDS (v.6.7.7). The Phase I tasks assess the treatment of: 

• thermal radiation from gaseous species and soot particles; 
• soot formation and oxidation; 
• pyrolysis in upholstered furniture; 
• and flame extinction due to under-ventilation. 

Particular attention is given to the effects of spatial resolution in the gas phase solver as well as angular resolution in the thermal radiation solver. Through detailed comparisons between experimental data and simulation results, the objective of this first series of tasks is to define best practices for CFD-based fire model practitioners.

Building on the results obtained in Phase I,  Phase II extends the computational work to the exploration of alternative models to be implemented into the FDS source code. This work focuses on the following topics: 

• description of thermal radiation from gaseous species using a Weighted-Sum-of-Grey-Gases model;
• description of soot formation and oxidation using a semi-empirical model based on measurements of the smoke point height;
• prediction of the convective heat flux at solid surfaces. 

Through detailed comparisons between experimental data and simulation results, the objective of this second series of tasks is to increase the validation space of FDS.

Led by three co-principal investigators (PIs), Arnaud Trouvé of the University of Maryland (UMD), Bart Merci of Ghent University and Tuan Ngo of University of Melbourne, the Numerical Modeling of Compartment Effects in Benchmark Compartment Fire Configurations study is the second collaborative research initiative stemming from the Underwriters Laboratories’ Fire Safety Research Institute (FSRI) partnership with the International Fire Safety Consortium.  

Context

CFD-based fire models are a vital engineering tool for projects aimed at evaluating fire protection engineering systems, designing enhanced fire protection engineering solutions, or performing forensic analyses of fires. There is a general need in the fire research literature to evaluate the ability of CFD-based fire models to correctly describe compartment fire effects, i.e., the modification in the air entrainment process due to the presence of solid walls, the modification of the thermal feedback due to the presence of an upper layer, and the effect of possible air vitiation due to recirculating upper layer gases in under-ventilated fire configurations. While the fire modeling community has made significant progress in recent years in the validation of fire models in free-burn configurations, there is a continued need to extend this work to the case of compartment fires. 

Objectives

The general objective of this study is to evaluate the performance of current fire modeling capabilities in the simulation of compartment fires with a particular focus on the Fire Dynamics Simulator (FDS) developed by the National Institute of Standards and Technology (NIST). This project aims to:

• identify best modeling options in FDS to simulate radiation heat transfer (from gaseous combustion products and from soot particles), flame extinction, soot production, and fuel production due to pyrolysis taking place inside solid flammable objects; 
• identify best practices for FDS practitioners in terms of spatial resolution in the gas phase solver and angular resolution in the radiation solver; 
• evaluate novel modeling concepts for the description of gas radiation, soot production and soot radiation, and wall functions proposed to estimate the convective heat flux at burning surfaces. 

In addition, this work seeks to advance understanding of complex coupled phenomena, including flow fields and heat transfer, as they occur in compartment fires, to aid in interpreting and complement experimental data.