FIRE MODELING Marc L. Janssens, Ph.D.

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FIRE MODELINGMarc L. Janssens, Ph.D.

• Fire Protection Engineering Symposium
• Embassy Suites Hotel
• Portland, OR
• November 7-8, 2002

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FIRE MODELINGOutline

• Introduction
• What is a Fire Model?
• Types of Fire Models
• Uses of Fire Models
• Selection of a Computer Fire Model
• Evaluating the Predictive Capability
• Compartment Fire Models
• Zone Models
• Field Models

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FIRE MODELINGIntroduction

• Performance-based codes permit/encourage use of
  modern tools, such as computer fire models
• ?Acceptable? models must be suitable for the
  intended use, well documented, and verified
• ASTM developed guidelines to facilitate model
  selection and input/validation data gathering
• E 1355 - Evaluation
• E 1472 - Documentation
• E 1591 - Data
• E 1895 - Uses and Limitations

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FIRE MODELINGWhat is a Fire Model?

• ASTM E 176 defines fire model as ?A physical
  representation or set of mathematical equations
  that approximately simulate the dynamics of
  burning and associated processes.?
• Complexity of mathematical models varies widely
• Computer fire model Computer program that
  numerically solves a set of mathematical
  equations which approximately simulate the
  dynamics of burning and other fire processes for
  a set of user-specified input variables that
  describe the geometry, configuration, materials
  involved, ...

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FIRE MODELINGTypes of Fire Models

• Compartment fire models
• Usually predict effects of user-specified fire
• Zone models (mass, energy)
• Field models (mass, momentum, energy)
• Extensions to model fire and smoke spread
• Materials and system response models
• Sprinkler and detector activation
• Ignition of exterior claddings
• Load-bearing capacity of structural elements
• Egress models

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FIRE MODELINGUses of Fire Models

• Two main applications
• Reconstruction and analysis of fires
• Fire-safe design of (part of) a structure
• Need to supplement model calculations
• Models are approximate simulations of nature
• Models should be used as part of a tool kit
• Real-scale data are always needed
• Models also require small-scale data

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FIRE MODELINGModel Selection

• Consult surveys Friedman, Janssens, Sullivan
• Selection criteria
• Physical and chemical processes simulated
• Cost (software, hardware, user qualifications)
• Source code availability
• Predictive capability

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EVALUATING THE PREDICTIVE CAPABILITYFour Steps
in ASTM E 1355

• Define scenarios and review model documentation
• Validate the theoretical basis and assumptions
• Verify the mathematical and numerical robustness
• Evaluate, i.e., quantify uncertainty and accuracy

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EVALUATING THE PREDICTIVE CAPABILITYDefine
Scenarios and Review Documentation

• Is the model suitable for the intended use?
• ASTM E 176 defines fire scenario as
• ?Detailed description of conditions, including
  environmental, of one or more stages from before
  ignition to the completion of combustion in an
  actual fire, or simulation.?
• ASTM E 1472 ?Guide for Documenting Computer
  Software for Fire Models?

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EVALUATING THE PREDICTIVE CAPABILITYValidation
and Verification

• Validation How well do the equations represent
  the physics of the problem?
• Should be done by an independent expert
• Verification Are the equations coded and solved
  in a correct manner?
• Comparison to analytical solutions
• Check of computer source code
• Accuracy and convergence of numerical solutions

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EVALUATING THE PREDICTIVE CAPABILITYEvaluation
(1 of 2)

• Comparison of model output to experimental data
• Evaluation implies validation and verification
• Types of evaluation blind, specified, or open
• Accuracy and uncertainty of fire models
• Model uncertainty
• Primarily uncertainty of input data
• Sensitivity analysis ? critical parameters
• Experimental uncertainty
• Full-scale data generally accepted
• Round robins (ASTM E 691, ISO 5725)

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EVALUATING THE PREDICTIVE CAPABILITYEvaluation
(2 of 2)

• Consistency of model output and experimental data
• Difficult to determine how well curves agree

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COMPARTMENT FIRE MODELSIntroduction

• In the first part we discussed many phenomena of
  fire dynamics
• Compartment fire models automate calculations
  based on equations that describe these phenomena
• Compartment fire models do not usually calculate
  the fire itself but estimate the consequences of
  a fire specified by the user
• Compartment fire models provide approximate
  solutions to physical problems and their output
  is only valid for a specific range of conditions

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COMPARTMENT FIRE MODELS Zone Fire Models

• Zone models approximate the conditions in a room
  as two uniform gas layers with a source
• Hot smoke layer beneath the ceiling
• Cool layer of air above the floor
• Zone models solve the conservation equations and
  calculate P, T, V, and composition for each zone
• Zone models have been around since the early
  1980s
• Commonly used zone models are ASET, FIRM, CFAST,
  and COMPF2

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ZONE FIRE MODELSASET and FIRM
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ZONE FIRE MODELSCFAST and COMPF2

• CFAST
• Consolidated Fire And Smoke Transport model
• Developed by Jones et al. at NIST
• Capability of simulating fires in multi-room
  structures
• More sophisticated heat transfer calculations
• Many other features (corridor smoke flow, HVAC,
  etc.)
• COMPF2
• Single-zone post-flashover fire model
• Developed by Babrauskas at NIST more than 20
  years ago

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COMPARTMENT FIRE MODELS Zone Model Example (1 of
3)

• Single room with furniture
• Journal of Research of the National Institute of
  Standards and Technology, July-August 1991
• Data available from NIST in the form of ?Fire
  Data Management System? (FDMS) ASCII data files
• Test room
• 2.26 x 3.94 x 2.31 m
• Various door and window configurations
• Loveseat (F31) or armchair (F21), 3 tests each

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COMPARTMENT FIRE MODELS Zone Model Example (2 of
3)

• Example Test 1 F31 loveseat in room with window
• Generate energy release rate file with HRR-VB
• Use Babrauskas model (thermoplastic fabric, PU
  foam, wood frame, ornate shape, mass 40 kg, ?Hc
  18 MJ/kg)
• Simulate fire using FIRM-VB
• A 8.90 m2
• H 2.31 m
• Wv 2.00 m
• Zb 0.87 m
• Zt 2.00 m

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COMPARTMENT FIRE MODELS Zone Model Example (3 of
3)
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COMPARTMENT FIRE MODELS Field Fire Models

• Field models attempt to predict conditions at
  every point in the domain of interest
• Field models are based on computational fluid
  dynamics (CFD) codes first developed in the 1980s
• The domain of interest is subdivided into
  hundreds of thousands of small control volumes or
  nodes
• The model solves the conservation equations for
  the temperature, velocity, composition etc. in
  each node
• Fire Dynamics Simulator (FDS) developed by the
  NIST is quickly becoming the most popular field
  fire model

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COMPARTMENT FIRE MODELS Field Model Example (1
of 11)

• Townhouse fire of May 30, 1999 at 3146 Cherry
  Street, Washington DC
• Two firefighters died and one firefighter
  sustained serious burn injuries
• NIST performed CFD calculations using FDS and
  Smokeview based on input from three sources
• Reconstruction committee
• Site investigation by NIST staff on June 3, 1999
• Material properties in FDS database
• NIST also simulated slightly different scenario

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COMPARTMENT FIRE MODELS Field Model Example (2
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COMPARTMENT FIRE MODELS Field Model Example (3
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COMPARTMENT FIRE MODELS Field Model Example (4
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COMPARTMENT FIRE MODELS Field Model Example (5
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COMPARTMENT FIRE MODELS Field Model Example (6
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COMPARTMENT FIRE MODELS Field Model Example (7
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COMPARTMENT FIRE MODELS Field Model Example (8
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COMPARTMENT FIRE MODELS Field Model Example (9
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COMPARTMENT FIRE MODELS Field Model Example (10
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COMPARTMENT FIRE MODELS Field Model Example (11
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FIRE MODELINGReferences and Web Sites

• References
• Janssens, M., An Introduction to Mathematical
  Fire Modeling, Technomic/CRC, 2000
• Cox, G., Combustion Fundamentals of Fire,
  Academic Press, 1995
• SFPE Handbook of Fire Protection Engineering,
  NFPA, 2002
• Web Sites
• http//fire.nist.gov
• http//wtc.nist.gov
• http//www.astm.org