Enclosure Fire Dynamics
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Chapter 1: Introduction
Chapter 2: Qualitative description of enclosure fires
Chapter 3: Energy release rates
Chapter 4: Plumes and flames
Chapter 5: Pressure and vent flows
Chapter 6: Gas temperatures
(Chapter 7: Heat transfer)
Chapter 8: Smoke filling
(Chapter 9: Products of combustion)
Chapter 10: Computer modeling
Content
Classification of fire models
 Computer fire models
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Zone models (Example CFAST)
 CFD models (Example FDS, Fire
Dynamics Simulator)
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Egress models
 Application of fire models
 Conclusions
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Before we start…
• Learning theory vs. learning software
– It is better to learn zone model or CFD theory
• You can then learn how to use any zone model
– If you just learn about one program, that is all you
know!
• Two hours of lecture does not make a fire
modeler
– To become good takes much practice
– The technical reference for CFAST alone is 250
pages
Probabilistic and
deterministic fire models
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Nearly all fire models used in practice
are deterministic models, and we will
concentrate on those.
Probabilistic models provide quantitative
measure of the probability of occurrence
– Attempt to provide the most probable
(range of) answers
– Associated with risk analysis and
reliability engineering
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We will not talk more about probabilistic
models
Deterministic models
• Deterministic = to determine, figure out.
Same input always gives same output
• Model fire physics such as:
– plume flow
– generation of heat, smoke, etc.
– heat transfer
– other fluid flows
• Simulate transport of smoke and heat in
enclosures
Specialized deterministic models
• Heat transfer
• Structural response
• Glass breakage
• Detector/sprinkler actuation
• Evacuation/egress
• Hydraulic/water supply
• Explosion venting
Classification of fire models
• Type: deterministic or probabilistic
• Complexity: single equation to
hundreds of thousands
• Fire type: steady state, quasi-steady,
transient
• Applications: pre-fire, post-fire
We will look at two main types
of deterministic fire models
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Zone models
– Tend to be specialized for fire
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Computational fluid dynamics models
(CFD)
– You may have seen an older term for this
type of model called “field models”
– Many CFD models used for fire are
general purpose computer codes
- Design airplanes, pumps, ships, etc.
Content
Classification of fire models
 Computer fire models

Zone models (Example CFAST)
 CFD models (Example FDS, Fire
Dynamics Simulator)
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Egress models
 Application of fire models
 Conclusions
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Zone models
Divides enclosure into a “small” number of
zones (usually two zones):
Upper layer, lower layer
– Plume, ceiling jet
– Boundary
– Objects (fuels)
Usual zone model
assumptions
• Typically, two zone models:
– Homogeneous: uniform, well-mixed
– No (or little) mixing between layers
– Combustion in upper layer may not be allowed
– Regular compartment shape (a box)
– Implies local effects are unimportant or nonexistent
(at least from a practical point of view), =>
Modeling global effects
• Sacrifice some accuracy for:
– Ease of use and setup
– Short computation time
Two zone model OK?
How about this case?
Different zone models
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HAZARD I
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WPI/Harvard fire code
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One of the first models to include flame spread
(couple radiation to solid heating)
FIRST version of code has 6 plume models
BRANZFIRE
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CFAST combined with tenability and egress
models
Hazard analysis package
FAST/CFAST etc…
From New Zealand, multi-room zone model,
comprehensive, flame spread
30+ different models available
Currently supported or
commonly used zone models
• ASET - US
• BRI2 - Japan
• CFAST -US
• CFIRE-X - Germany/Norway collaboration
• CiFi - France
• COMPBURN - US
• COMPF2 -US
• DSLAYV - Sweden
• FIRST (HARVARD V) - US
• FISBA -France
• MAGIC -France
• NRCC 1 and 2 (a component of FIRECAM) - Canada
• RADISM -UK
• RVENT - Norway
• Sfire - Sweden
CFAST programs
• CFAST is the fire model
– FAST, FASTLite, FireCAD, FireWalk
and all of the other programs are
basically data editors
– They all run CFAST
• CFAST is an acronym for the
Consolidate model of Fire Growth and
Smoke Transport.
CFAST
Transient calculation of smoke and
fire gas spread throughout a number
of compartments based on a user
defined fire (18 rooms possible)
 Download for free at www.fire.nist.gov
 See latest editions of Users Manual
and Technical Reference from
www.fire.nist.gov
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CFAST has features similar to
most zone models
• Multi-room fire model
• Input: room description, what’s
burning, etc
• Output: heat release rates, fluxes,
temperatures, flows, species (gas and
smoke),
• Consequences: smoke detector
operation, heat detector (including
sprinkler) operation
Control volumes are written
for each compartment
CV 1
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Conservation of
mass and
conservation of
energy
equations written
for each control
volume
,
g Tg
mg
mp
me
W=v
W=0
m f (fuel)
CV 2
Choosing input
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Choose scenarios
– Geometrical aspects (rectangular rooms, which
door/window is open, etc), define a few
scenarios
– Fire scenario (where does ignition occur, what
will the fire growth be, what is the maximum heat
release), define a few scenarios
– People and egress (how many people, what type
of occupancy, where are exits), define a few
scenarios
This will usually lead to many scenarios, but these
will generally be reduced to a handful
Typical apartment layout
What about irregular rooms?
Sub-models in zone models
• Provide source terms to conservation
equations
– Mass of fuel consumed
– Heat transfer to walls
– Plume entrainment
– Vent flows
• These are treated as “source terms”
for the conservation equations
Conservation equations
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Conservation of mass
dm n
dm
  mj 
 mg  me  m f  0
dt j 1
dt
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Conservation of species (fuel, O2, products)
n
dYi
m
+ m
 j  Yi , j - Yi  = y i m
 f -m
 i, loss
dt j=1
net out
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Conservation of energy
Vc p
n
dP
V
 c p  m j  T j  Tg   m react H eff  qloss
dt
dt
j 1
dTg
Types of plumes in CFAST
Types of vent flows
• Horizontal flow (doors, windows , ...)
• Vertical flow (holes in ceilings/floors)
• Forced flow (mechanical ventilation)
• The pressure in the enclosure is uniform
with respect to the energy equation
– Hydrostatic pressure differences lead to
vent flows
• Generally follow the vent flow equations
we derived in Chapter 5
Zone model heat transfer
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Typically 1-D conduction through walls
Different materials permitted for walls,
ceiling and floor
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Can also have different layers of materials
(up to 3 layers in CFAST)
CFAST uses constant material properties
Bounding surfaces and upper layer are gray
Lower layer is transparent in wall/layer
interchange calculations
Flame radiates to the upper layer and to
walls
No radiation interchange between rooms
Example, temperature prediction
Advantages of zone models
• Easy to learn and use software
– Can generate results for a number of scenarios in
a short time
• Important for design work
• Most fire deaths result from smoke movement
outside the compartment of origin
– Especially at NIST, zone models have been
developed to describe the movement of smoke
away from the fire toward occupants
• Real time calculations
– Allows many combinations to be investigated
What zone models do not do
• Fire models (as a rule) DO NOT model fire
– HRR not accelerated (increased) due to conditions
inside compartment
– Difficult to account for increase in HRR as other
fuels become involved (HRR under predicted)
– (Exception: Harvard, BRANZFIRE, etc.)
• Fire models DO predict the effects of a
userspecified fire
– User specified fire typically based on
understanding of fire dynamics
– Most zone models should simply be called smoke
filling models
Content
Classification of fire models
 Computer fire models

Zone models (Example CFAST)
 CFD models (Example FDS, Fire
Dynamics Simulator)
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Egress models
 Application of fire models
 Conclusions
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CFD (Computational Fluid
Dynamics)
• At some point in time, the number of equations and
complexity of the zone model reach a point where
it may be better to use a CFD code
– For example:
• Very large buildings or tall atria
• Detailed wall heat transfer calculations
• Radiation transfer for flame spread
• Sprinkler suppression
• As we will see, just because we are using a CFD
code does not mean we have the right (correct)
answer!
Computation fluid dynamics
(CFD) models
Solve conservation equations over a large
number of control volumes
• – Navier-Stokes equations
• – Field equations
• The big problem…
– Combustion time scales down to 10-3 – 10-6
seconds
– Length scales down to 10-6 m
– But our problem dimensions are in 10’s of
meters and minutes
– Computer resources not (yet) ready
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General transport equation
• φ is any one of the solution variables
(u,v,w, enthalpy, concentration)
• Linear partial differential equation
• Source can include combustion, radiation
etc.
Discretization
• Discretization = divide
the area of interest
(domain) into many
control volumes
• Type of discretization
can have a big impact
on how the equations
are solved
• Solving conservation
equations only at a
limited number of
points
CFD computational domain
with structured grid
Unstructured grid
Closure Models
Closure model = a simplification to
allow solution of the
conservation equations
– Discretization not small enough
for all fire physics
– Models sub-grid scale physics
– Computationally expensive
• Radiation transfer
– Radiation properties
• Combustion
– Soot production
• Turbulence
– This is the big one!
Reynolds stress turbulence
models
• Model Reynolds “apparent” stresses
• κ-ε turbulence model
– Strategy is to solve time averaged NavierStokes equations
– Conservation equations for turbulent kinetic
energy, κ, and dissipation of turbulent
kinetic energy, ε
– Very common in current codes
– There are a number of well documented
problems with this model
• Unfortunately many of them apply to fire
More detailed turbulence
models
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Large eddy simulation
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FDS (Fire Dynamics Simulater) is the CFD model
you will use in your computer lab, and FDS uses
Large eddy simulation
Solve a simplified form of Navier Stokes equations
Solves large eddies directly
Only model turbulence on scales less than grid
• Artificial fluid viscosity to dissipate remaining energy
• Things tend to be more uniform at this scale, thus
(somewhat) easier to model correctly
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Direct numerical simulation (give a few years)
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Grid (discretization) fine enough to calculate
important flow features
2-D calculations and low Re number 3-D
Finite Volume Radiation
Transfer
• Integrations for each direction and for all
wavelengths
– Computationally expensive
Computational domain extends
beyond the room of interest
Cross section showing gas velocities
Competent use of CFD models
• Characterization of Fire
– Heat source versus combustion
source
– Fire plume temperature and flame
height
• Grid design and solution convergence
• 2D versus 3D solution
• See Manuals and Best Practice
Guidelines now available
Summary on CFD
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Conservation equations based on first
principles
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Developed for wide range of problems
including:
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Offers a universal modeling tool
indoor air movement, smoke movement, flame
spread, fire resistance furnace, atmospheric
dispersion
More complex than simple engineering
correlations and zone models
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Increased chance for misuse
Input becoming much easier – so it is possible
to run CFD codes with no knowledge of CFD!
Summary on CFD
Powerful tool if in right hands
 Requires significant education
 Requires proper simulation of physical
and chemical processes with appropriate
initial and boundary conditions
 Still solving conservation equations at a
limited number of points
 Realistic treatment of fire source is
important
 Rapid implementation (use) in consulting
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Content
Classification of fire models
 Computer fire models

Zone models (Example CFAST)
 CFD models (Example FDS, Fire
Dynamics Simulator)
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Egress models
 Application of fire models
 Conclusions
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Egress models
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Predict egress (exit) time for occupants to
exit a structure
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May include occupant behavior
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Research shows different people respond
differently during a fire
Should include impact of fire
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Time = distance / walking speed
Results from fire model feed into egress
model
A few models have even been specifically
developed for ships, airplanes, tunnels
Predicting fire impact on people
is difficult
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People complicate things by moving
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Impact of smoke may change people’s
movement
While moving they are exposed to
different levels of smoke/heat
Example models
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EXITT
Evacnet
SIMULEX
EXODUS
Example: Simulex
Content
Classification of fire models
 Computer fire models

Zone models (Example CFAST)
 CFD models (Example FDS, Fire
Dynamics Simulator)
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Egress models
 Application of fire models
 Conclusions
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Application of fire models
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Post-Fire
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Pre-Fire
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Investigation
Reconstruction
Design
Analysis
Variances or equivalencies
During-Fire
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Predict possible growth of fire (very seldom)
Post fire analysis
• Generally looking at a number of
scenarios
– Ignition location
– Position of doors
– Status of windows
– Actual materials (and their properties)
– Location of contents and occupants
– Fire department activities
• Fire destroys evidence necessary for
modeling
Post-fire example: Göteborg
disco fire, 65 died
Post flashover burning
Pre fire analysis and design
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Many input parameters to vary:
Ventilation conditions
 Material properties
 Placement of fuel packages
 Initial ignition location
 Number and location of occupants
 Future changes in building
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Using fire models
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“Given” a scenario
Complexity depends on model being used
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Geometry
Develop input data set
• room dimensions
• door, window, other vent locations
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Physics and chemistry
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material properties
fuel characteristics
Most importantly, the (input) fire
Looking at the results
Do the predictions make sense?
 Anything with flame temperature
greater than 1300oC should be
examined closely
 Compare the predictions of flame,
flame height, and plume entrainment
with empirical correlations.
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Looking at the results
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There is no One answer.
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The user must give a range of likely
answers.
This means for every problem, the user must
do sensitivity analyses involving many runs.
Rule of thumb:
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Zone model, most simple problems: 15-30
runs
Zone model, more complicated 30-100 runs
Probably fewer, but still many for CFD
How good are your results?
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Uncertainty Analysis
Single point representation of a
distribution
 Accounting for “unknown randomness”
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Sensitivity Analysis
How much does the result change when
an input parameter value is changed?
 Ideally, check all “assumed” input
parameters, but practically difficult
 What are the significant assumed input
parameter values?
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Content
Classification of fire models
 Computer fire models

Zone models (Example CFAST)
 CFD models (Example FDS, Fire
Dynamics Simulator)
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Egress models
 Application of fire models
 Conclusions
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Conclusions
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Models supplement engineering judgment
They should never be used instead of
engineering judgment
 No one run of any model will give THE
correct answer!
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Many people assume that CFD and zone
models are easy to use
Reality is it takes a significant amount of
work to use such models properly
 So, they are actually easy to misuse
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Conclusions
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Rapid development in the building industry,
larger and more complex buildings, more
complex technologies, design and materials
New building regulations based on performance
requirements
Progress in the understanding in fire
phenomena, risk concepts and human
behaviour has been rapidly increasing
Many models available for simulating fires and
simulating movement of humans
But, there is a lack of education in Fire Safety
Engineering