COMPUTER FIRE MODELING

1
COMPUTER FIRE MODELING

• Principals, Applications, and Validation

2
Overview
4

• Principals of modeling
• Zone Models
• Assumptions
• Governing Equations
• Zone Model Example (CFAST)
• CFD Models
• Assumptions
• Governing Equations
• CFD Model Example (FDS)
• Need for verification and validation

3
Principals of Modeling2
8

• Probabilistic
• Based on statistics and risk assessments
• Network Model (Decision Tree)
• Statistical Modeling (Risk Assessment)
• Simulation Models
• Deterministic
• Utilize known inputs to compute a specific
  result (Computer Fire Model)
• Same output achieved with same input

4
Deterministic Models for Life Safety2
2

• Detector/Sprinkler Activations
• DETACT
• People Movement in Evacuations
• Course
• Node/Arc
• EVACNET
• ERM
• EXITT
• Fine
• Grid
• SIMULEX
• EXODUS

5
Deterministic Models for Life Safety Continued2
3

• Building Thermal Element Response
• SAFIR
• VULCAN
• Hydraulic Calculations
• SprinkCAD
• HASS
• (Hydraulic Analyzer of Sprinkler Systems)
• Transport of Smoke and Heat in Enclosures
• CFAST
• (Consolidated Fire And Smoke Transport Model)
• NIST FDS (Fire Dynamics Simulator)

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Enclosure Fire Models
2

• Zone Model
• CFAST (Consolidated Fire And Smoke Transport
  Model)
• CFD (Computational Fluid Dynamics)
• NIST FDS (Fire Dynamics Simulator)

7
Enclosure Fire Models Continued
6

• Scientific Tool
• All dimensions are metric
• Output is only as good as input
• Dont try this at home!
• Unlike most computer programs theuser must be
  informed in the underlying operation of the
  program
• Utilized in the research industry and educational
  realm
• Still requires the scientific process

8
Zone Model
4

• Single Enclosure
• Known fire size
• Will not generate smoke/heat transport times.

9
Zone Model Continued
1

• Conservation equations are applied to each zone.

10
Zone Model Continued
5

• Assumptions
• Each zone contains only ideal gas with a constant
  molecular weight and specific heat. Cp and Cv.
• Mass transport/exchange occurring at free
  boundariesis due to difference in pressure or
  shear mixing effects.
• Combustion is modeled as a source of mass and
  energy.
• Fire plume is assumed to be instantly at the
  ceiling, and time to transport mass horizontally
  and vertically is ignored.

11
Zone Model Continued
6

• Assumptions
• Room contents are ignored with regards to their
  mass or heat sink capacity.
• Horizontal cross section is constant with respect
  to area.
• Pressure is considered uniform, the stack effect
  is neglected.
• Mass flow into the plume is turbulent and
  linearly proportionate to velocity.
• Fluid frictional effects are ignored at the
  boundaries.

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Conservation Equations
3

• Conservation of Mass

13
Conservation Equations
1

• Conservation of Species

14
Conservation Equations
14

• Conservation of Energy

15
Vent Flow
1
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Zone Model Example
4

• CFAST(Consolidated Fire And Smoke Transport
  Model)
• Two Zone Model
• Most Current Version 6.0
• Utilizes a Windows Based Input Screen
• Developed and maintained by the Building Fire and
  Research Lab at the National Institute of
  Standards and Technology

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Zone Model Example
3

• CFAST cont.
• (Consolidated Fire And Smoke Transport Model)
• Solves the conservation equations discusses for
  each zone
• Outputs values both visually and numerically

18
Zone Model Example Continued
3

• Enclosure
• Room 15m x 15m x 3m
• Vents
• Double Door 1.5m x 1.4m
• Fire
• T2 Fire Growth reaching 1 MW _at_ 300s

19
Zone Model Example Continued

• Output Smoke View

20
Zone Model Example Continued

• Output Excel (Normal)

21
Zone Model Example Continued

• Output Excel (WALL)

22
Zone Model Example Continued

• Output Excel (Flow)

23
Zone Model Example Continued

• Output Excel (Flow) cont.

24
Zone Model ExampleContinued

• Output Excel (Species)

25
Zone Model Example Continued

• Output Excel (Species) cont.

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CFD Model5
3

• Area of interest divided into grids
• Each grid is its own control volume as in Zone
  Model
• Capable of more advanced analysis
• Combustion
• Phase Change
• Multi Phase Flow
• Chemical Reactions

27
CFD Model Continued
3

• NIST FDS
• (Fire Dynamics Simulator)
• Computational fluid dynamics model for
  fire-driven fluid flow.4
• Solves a modified version of the Navier-Stokes
  equations appropriate for low-speed,
  thermally-driven flow with an emphasis on smoke
  and heat transport from fires.4

28
CFD Model Continued
5

• Governing Equations
• Conservation of Mass
• Conservation of Momentum
• Transport of Sensible Enthalpy
• Equation of State for a Perfect Gas

29
CFD Model Continued
5

• Conservation of Mass (Zone)

Conservation of Mass (CFD)
u Velocity Vector (u,v,w)
30
CFD Model Continued
4

• Conservation of Species (Zone)

Mixture Fraction Combustion Model (CFD)

• A Single-Step, Instantaneous Reaction

31
CFD Model Continued
3

• Combustion Model5

• Mixture Fraction - combination of the mass
  fractions of the fuel and the carbon-carrying
  combustion products

Function of space and time
32
CFD Model Continued
4

• Combustion Model5

• Stoichiometric mixture fraction

• Assume combustion occurs instantaneously
• Fuel and oxygen cannot coexist

33
CFD Model Continued
4

• Combustion Model5

• A Single-Step Reaction with location extinction

• Single-step adaptation eliminates assumption that
  oxygen and fuel cannot co-exist

• No Burn O2 and Fuel mix but do not combust.

• Burn O2 and Fuel react but not necessarily
  completely

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CFD Model Continued
5

• Combustion Model5

• In order to utilize this model mixture
  fraction must be broken into two terms

35
CFD Model Continued
2

• Combustion Model5

• Instead of having transport equations for Z we
  now have two separate transport equations for Z1
  and Z2.

• Fuel and oxygen can co-exist however only react
  if the conditions permit.

36
CFD Model Continued
5

• Combustion Model5

• Reaction is the conversion of fuel to products or
  the conversion from Z1 to Z2.

• The HRR (Heat Release Rate) is determined through
  the equation

37
CFD Model Continued
5

• Combustion Model5

• Can also be further complicated where fuel stream
  may by diluted with an inert gas. (Ex. Nitrogen)

• Restate Z1 to Z2 keeping in mind the soot yield
  and CO yield remain fixed

38
CFD Model Continued
4

• Combustion Model5

• Being that the a species mass fraction is a
  linear combination of that species mixture
  fraction variables the following are true.

39
CFD Model Continued
5

• Combustion Model5

40
CFD Model Continued
8

• Combustion Model5

• Where the stoichiometric coefficients are defined
  as

41
CFD Model Continued
3

• Combustion Model5

• Import assumptions
• Z1 and Z2 do not imply a the rate of combustion
• Combustion is only assumed to occur in a single
  step

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CFD Model Example
4

• Enclosure
• Room 15m x 15m x 3m
• Grid
• 0.5m x 0.5m x 0.5 m
• Vents
• Double Door 1.5m x 1.4m
• Fire
• T2 Fire Growth reaching 1 MW _at_ 300s

43
CFD Model Example Continued

• Input

HEAD CHID'Presentation', TITLE'Sample Room for
Engineering Expo Presentation' / A 15m x 15m x 3
meter room with a vent 7 meter MESH
IJK32,30,30 XB-1.0,15.0,0.0,15.0,0.0,3.0 /
TIME T_END600. / SURF ID 'FIRE' HRRPUA
1054. COLOR 'RED' TAU_Q 300 /T2 fire
growth reaching 1054 at 300 seconds. SURF ID
'WALL' COLOR 'GRAY' / HOLE XB-0.5, 0.5,
6.0, 7.5, 0.0, 1.4 / VENT MB'XMIN',
SURF_ID'OPEN' / VENT XB7.0,8.0,7.0,8.0,0.0,0.0
, SURF_ID'FIRE' / OBST XB0.0, 0.0, 0.0, 15.0,
0.0, 3.0, SURF_ID'WALL' / TAIL /
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CFD Model Example Continued

• Output Smoke View

45
CFD Model Example Continued

• Output Excel (Hrr)

46
CRD Model Example Continued
10

• Output More User Input required for desired
  output
• Surface Temperatures
• Vent Flows
• Thermocouples
• Heat Flux Gages
• Temperature Gages
• Sprinklers
• Smoke Detectors
• Species Concentration
• And Many Others

47
Verification/Validation
5

• What is validation? 6
• Answering the following questions
• Does it accurately represent physical and
  chemical phenomena of interest?
• Was the Model appropriate for use in with the
  given scenario?

Check the physics!!
48
Verification/Validation
6

• What is Verification?6
• Answering the following questions
• What is the mathematical uncertainty associated
  with the model
• What is the mathematical uncertainty associated
  with the experiments?
• Is the difference between known experimental
  values and the models results explained by
  mathematical uncertainty?

Check the math!!
49
Verification/Validation
5

• Validation of Model7
• Validation is the responsibility of the user not
  the creator of the model.
• Can be done through the following
• Comparing the results to known experimental
  results
• Conducting scaled or full scale experiments to
  evaluate the results

50
Verification/Validation
7

• Verification of Model6
• Verification is the responsibility of the user
  not the creator of the model.
• Can be done through the following
• Checking the models calculations
• By hand or through the use of other models
• Comparing calculations to exact solutions
• Considering the sensitivity of parameters

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References

• DiNenno, Philip J. et.al., eds. SFPE Handbook of
  Fire Protection Engineering, Third Edition.
  Quincy National Fire Protection Association,
  2006.
• Karlsson, BjÖrn and James G. Quintiere. Enclosure
  Fire Dynamics. Boca Raton CRC Press LLC, 1999.
• Quintiere, James G. Fundamentals of Fire
  Phenomena. West Sussex Wiley, 2006.
• United States. National Institute of Standards
  and Technology. Fire Dynamics Simulator (Version
  5) Users Guide. Washington GPO, 2007.
• United States. National Institute of Standards
  and Technology. Fire Dynamics Simulator (Version
  5) Technical Reference Guide Volume 1
  Mathematical Model. Washington GPO, 2007.
• United States. National Institute of Standards
  and Technology. Fire Dynamics Simulator (Version
  5) Technical Reference Guide Volume 2
  Verification. Washington GPO, 2007.
• United States. National Institute of Standards
  and Technology. Fire Dynamics Simulator (Version
  5) Technical Reference Guide Volume 2
  Validation. Washington GPO, 2007.

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1
QUESTIONS ?

• Robin Zevotek
• Fire Protection Engineer
• CS Companies
• rzevotek_at_cscos.com