Advanced Fire Modeling

Summary

Advanced Fire Modeling lays the research foundation for engineered fire safety in commercial and residential buildings and within wildland-urban interface communities.  We develop and maintain the following computer models:

• Consolidated Fire and Smoke Transport (CFAST) - is a fast zone model that predicts the average thermal environment in the upper layer of a compartment.
• Fire Dynamics Simulator (FDS) - a high-fidelity fire model based on computational fluid dynamics.
• FireX - a research branch of FDS that runs on supercomputers.

These models excel at predicting smoke and heat transport in scenarios where the fire size is predefined.  The next research frontier lies in accurately modeling fire growth and flame spread using material properties measured at the bench scale.

Description

Figure 1. FDS simulation of ceiling smoke deposition

Credit: NIST

Objective
To develop and maintain robust, validated fire models—and the associated visualization and analysis tools—for

• performance-based fire safety,
• forensics (fire reconstruction), and
• fire research applications in the built and natural environments.

Technical Idea
Applications in fire protection engineering require a range of models, from fast operational models to high-resolution physics-based models.  To this end, the Advanced Fire Modeling project develops and maintains:

• CFAST (Consolidated model of Fire and Smoke Transport)
• FDS (Fire Dynamics Simulator) and Smokeview
• FireX (High-Performance Computing branch of FDS)

CFAST is a zone model.  Using empirical correlations and global mass and energy balances, CFAST provides an analysis of the thermal environment for multiply connected compartments (rooms in a hotel or high-rise building, for example) in a fire. Because it runs much faster than real time (usually within seconds), CFAST may be used for Monte Carlo-based probabilistic risk analysis or parametric studies potentially requiring thousands of model runs.  The Advanced Fire Modeling project keeps CFAST up to date with modern computer platforms and operating systems.

Since the early 2000s, the Fire Dynamics Simulator (FDS) and its companion visualization code, Smokeview, have been the industry standard computational fluid dynamics (CFD) model for performance-based fire safety applications.  An authority having jurisdiction (AHJ) may require FDS and Smokeview analysis to determine the suitability of a fire safety design for unique spaces such as airports, train stations, shopping malls, or stadiums, where prescriptive building codes are not applicable.  When an architect applies for an exemption from a building code, the AHJ often requires an FDS analysis to ensure that performance criteria for fire safety are still met by the proposed design.  FDS is the basis for much of the (multi-billion dollar) fire protection engineering industry in the U.S. and abroad.

Specialty applications such as forensics or advanced fire suppression strategies often push FDS beyond its basic capabilities.  The research frontier in fire science and ultimately in fire protection engineering is prediction of fire growth and flame spread behavior both in confined spaces and outdoor environments.  The ability to model these phenomena is fundamental to understanding forest fuels management and wildland-urban interface hazard mitigation methodologies as well as developing advanced fire detection and suppression strategies for residential and commercial buildings and the aerospace industry.

Advanced models of burning behavior require a shift in the way we think about the problem.  Basic fire models rely on a prescribed heat release rate or fire size.  In advanced models, we must consider the solid phase thermal decomposition or pyrolysis process and the subsequent burning and thermal feedback which sustain the fire---this is the key difference between "fire modeling" and "combustion modeling".  In the nonlinear, two-way coupling between the gas and solid phases modeling errors quickly amplify.  Whereas with prescribed fires the heat release was by far the most important factor, in fire growth prediction every matters.

To tackle this problem, we have been pushing FDS to ever higher resolutions, requiring more advanced computing hardware and algorithms.  Pushing the boundaries of computational science while maintaining an engineering level code is a daunting task, and it quickly became evident that an advanced development branch was necessary.  The FireX branch of FDS links with modern libraries for linear solvers and chemistry with GPU (graphics processing unit) capabilities. Think of FireX as the race car team to the FDS daily driver---advances in technology proven in FireX will eventually make their way into the engineering level FDS code.  At the same time, FireX can be used now for advanced research applications on leadership class supercomputers allowing unprecedented resolution for a broad range of applications from wildfire to backdraft simulations.

Research Plan
At the core of the success of the Advanced Fire Modeling project is a rigorous verification and validation process. We have developed a continuous integration framework that allows for code development, testing, and documentation with a distributed team of developers across multiple institutions.  The research priorities for the engineering codes are driven by user feedback from our discussion forum and direct interaction with our stakeholders.  A detailed research road map may be found here: FDS Road Map.

Major Accomplishments

• Fire Dynamics Simulator (FDS) is a large-eddy simulation (LES) code for low-speed flows, with an emphasis on smoke and heat transport from fires.  Smokeview (SMV) is a visualization program used to display the output of FDS and CFAST simulations.
• CFAST is a two-zone fire model capable of predicting the environment in a multi-compartment structure subjected to a fire. It calculates the time-evolving distribution of smoke and gaseous combustion products as well as the temperature throughout a building during a user-prescribed fire.
• The availability of robust and validated fire models makes many forensic and fire safety applications possible.  A few notable examples of the use of FDS in these areas are provided below:
  • The World Trade Center investigation (NIST)
  • NASA Kennedy Space Center (Rolf Jensen)
  • NASA Saffire flame spread experiments (NASA)
  • Opreyland USA, Nashville, TN (Schirmer Engineering)
  • Camp Swift, TX, prescribed burns (USFS, NIST)
  • WUI Hazard Mitigation experiments (NIST)