Computer Models

1
Computer Models
2
INTRODUCTION

• In the previous lesson, you were introduced to
  mathematical fire modeling in the form of simple
  algorithms that could be done on a calculator.
• Each equation only considered a portion of the
  whole fire scenario.
• Computer models enable you to look at a number
  parameters that affect the fire and how they
  interact.
• Computer models enable you to see how the fire
  effects on a building can change over time.
• Remember, even though the numerics of the
  computer models are more complex than the simple
  equations, they still are only providing
  estimations.

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INTRODUCTION

• Whether you are working with a fire modeler on a
  case or defending against a fire modeler, it is
  important to understand the capabilities and
  limitations of the models.
• The basic types of computer based fire models
  include Zone Models and Field Models.
• In the simplest form a zone model divides a
  single room into a hot gas layer and a cool
  gas layer.
• ltltstick room with hot gas layer and cool lower
  layergtgt

4
INTRODUCTION

• The laws of conservation of mass and conservation
  of energy are taken on the hot gas layer.
• In other words, a mass and energy balance needs
  to be accounted for on the hot gas layer.
• So the energy input to the hot gas layer from the
  fire must equal the energy in the hot gas layer
  plus energy losses caused by heating the ceiling
  and walls of the room and any losses of energy
  flowing out of open doors or windows.

5
INTRODUCTION

• The same is true of the mass added to hot gas
  layer by the burning fuel.
• A two-layer zone model is well suited to modeling
  a rectangular compartment with a smooth, flat
  horizontal ceiling.
• Effectively, the two-zone model has one
  computational cell, for each room modeled.
• ltltstick room with mass flow and energy flow
  arrows.gtgt

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INTRODUCTION

• Zone models do not account for changes in
  pressure caused by a fire.
• Zone models do not predict the growth and
  development of a fire, the fire growth must be
  input into the model.
• The fire is considered as a point source.
• A point source is a source of thermal energy that
  has no relation to the size or shape of the
  flames that would compose the real fire in a
  room.
• Zone models use a quasi-steady assumption,
  which means that a change in the fire input at
  the source, results in an instantaneous change to
  conditions in the hot gas layer.
• No transportation time is considered.

7
INTRODUCTION

• Only one characteristic temperature will be
  predicted for the hot gas layer.
• In reality, the temperature near the ceiling will
  be significantly hotter than the temperature near
  the lower edge of the hot gas layer.
• ltltgraphic temp gradient vs single tempgtgt

8
INTRODUCTION

• A field model or computational fluid dynamics
  model (CFD) has the capability of modeling
  compartments or a building of various shapes and
  sizes since the model can partition a room or
  building into thousands or hundreds of thousands
  of computational cells.
• In addition, to using the laws of conservation of
  mass and energy, CFD models also use the laws of
  conservation of momentum and species to provide a
  more realistic and detailed description of the
  movement of fire gases and what they contain.
• A gas temperature and velocity will be predicted
  for each computational cell at very small time
  intervals yielding significant quantities of
  predicted values.
• Hence a field model has significantly greater
  resolution and detail than a zone model.

9
INTRODUCTION

• The down side to having a number that generates
  thousands of numbers every fraction of second is
  that it is extremely hard to evaluate the data.
• The only means of communicating the results until
  recently has been in the form of other equations,
  graphs or very large spreadsheets.
• Now, these models are usually used with another
  program that can turn the numbers into a
  graphical output for analysis.
• The visualization programs typically use
  isotherms and vectors to describe model output in
  the same manner as temperature and wind data is
  displayed on weather maps during the evening
  news.
• This makes the data much easier to view as a
  picture and interpret.

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A Sample of Public Domain Computer Fire Models

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A Sample of Public Domain Computer Fire Models

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Public Domain Computer Fire Models

• The above chart presents a cross representation
  of computer-based fire models ranging from the
  simplest model (DETACT-QS) to the most complex
  (FDS/Smokeview).
• A list of the values that the model can predict
  as well as a partial list of limitations is given
  for each model to give the investigator a sense
  of the capabilities of the models and the types
  of limitations each model has.
• Further information about each of the models can
  be found in the technical documentation for each
  of these models.
• The investigator should ask for these documents
  when a prosecution or defense based on a fire
  model is presented.

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Zone Models

• DETACT-QS - DETACT-QS (DETector ACTuation - Quasi
  Steady)
• A program for calculating the actuation time of
  thermal devices below unconfined ceilings.
• It can be used to predict the actuation time of
  fixed temperature heat detectors and sprinkler
  heads subject to a user specified fire.
• DETACT-QS assumes that the thermal device is
  located in a relatively large area, i.e. only the
  fire ceiling flow heats the device and there is
  no heating from the hot gas layer in the room.

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Zone Models

• DETACT-QS - DETACT-QS (DETector ACTuation - Quasi
  Steady)
• The required program inputs are the height of the
  ceiling above the fuel, the distance of the
  thermal device from the axis of the fire, the
  actuation temperature of the thermal device, the
  response time index (RTI) for the device, and the
  rate of heat release of the fire.
• The program outputs are the ceiling gas
  temperature and the device temperature both as a
  function of time and the time required for device
  actuation.
• ltschematic of inputsgt ltRTI chart with discussiongt
  
• ltltbased on schematic add table of inputs and
  animate output i.e. activation timegtgt

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Zone Models

• ASET-B - ASET-B (Available Safe Egress Time -
  BASIC)
• A program for calculating the temperature and
  position of the hot smoke layer in a single room
  with closed doors and windows.
• ASET-B is a compact easy to run program, which
  solves the same equations as ASET.
• The required program inputs are a heat loss
  fraction, the height of the fire, the room
  ceiling height, the room floor area, the maximum
  time for the simulation, and the rate of heat
  release of the fire.

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Zone Models

• ASET-B - ASET-B (Available Safe Egress Time -
  BASIC)
• the maximum time for the simulation, and the rate
  of heat release of the fire.
• The program outputs are the temperature and
  thickness of the hot smoke layer as a function of
  time.
• ltltadd pictures on how an investigator would
  determine input such as height of fire/fuelgtgt
• ltlt using stick room, add inputs and animate
  output i.e. layer height and Temperaturegtgt

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Zone Models

• CFAST CFAST (Consolidated model of Fire growth
  And Smoke Transport)
• A multi-room zone model that predicts the effect
  of a specified fire on temperatures, various gas
  concentrations and smoke layer heights within a
  structure.
• CFAST can accommodate up to 31 compartments with
  multiple opening between the rooms and the
  outside.

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Zone Models

• CFAST CFAST (Consolidated model of Fire growth
  And Smoke Transport)
• The require inputs include
• geometry of the rooms, doorways, windows and
  other vents
• the thermal properties of the ceiling, walls, and
  floors
• the fire as a mass loss rate or heat release
  rate
• the generation rates of products of combustion.
• CFAST outputs include
• a characteristic temperature,
• thickness,
• species concentration of the hot upper layer
• cool lower layer in each compartment.

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Zone Models

• FPE Tool/Fire Simulator
• FPETool is a set of engineering equations useful
  in estimating potential fire hazard and the
  response of the space and fire protection systems
  to the developing hazard.
• Version 3.2 incorporates an estimate of smoke
  conditions developing within a room receiving
  steady-state smoke leakage from an adjacent
  space.
• Estimates of human viability resulting from
  exposure to developing conditions within the room
  are calculated based upon the smoke temperature
  and toxicity.

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Computational Fluid Dynamics Models

• Fire Dynamics Simulator
• Fire Dynamics Simulator or FDS is a computational
  fluid dynamics (CFD) fire model using large eddy
  simulation (LES) techniques.
• Inputs required by FDS include
• the geometry of the structure,
• the computational cell size,
• the location of the ignition source,
• the energy release of the ignition source,
• thermal properties of walls, ceiling, floors and
  furnishings,
• the size, location, and timing of door and window
  openings to the outside which critically
  influence fire growth and spread.
• FDS utilizes material properties of the
  furnishings, walls, floors, and ceilings to
  compute fire growth and spread.

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Computational Fluid Dynamics Models

• Fire Dynamics Simulator
• FDS utilizes material properties of the
  furnishings, walls, floors, and ceilings to
  compute fire growth and spread.
• Based on the laws of conservation of mass,
  momentum, species and energy the model tracks the
  generation and movement of fire gases.
• FDS computes the density, velocity, temperature,
  pressure and species concentration of the gas in
  each cell.
• FDS has been demonstrated to predict the thermal
  conditions resulting from a compartment fire.
• ltltsample input and outputgtgt

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Computational Fluid Dynamics Models

• SMOKEVIEW
• Smokeview is a scientific visualization program
  that was developed to display the results of a
  FDS model computation.
• Smokeview produces animations or snapshots of FDS
  results.
• ltltsample output from demo pagegtgt
• A new feature of Smokeview allows the viewing of
  FDS output in 3-dimensional animations.
• An iso-surface is a three dimensional version of
  a contour elevation often found on topographic
  maps.

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Models for Human behavior (egress)/tenability

• While egress models are not fire models, they
  are used in conjunction with a fire model to
  determine how longer it would take for a given
  occupancy to be evacuated.
• For example, a modeler might use a model to show
  that it would take at least 5 minutes to evacuate
  a store.
• The modeler would use a zone model to determine
  how long it would take for the smoke layer to be
  5 ft above the floor.

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Models for Human behavior (egress)/tenability

• If the model showed that the smoke layer did not
  reach the 5 ft level until 8 minutes after alarm
  than it could be inferred that the occupant have
  sufficient time to safely escape, if they
  response rapidly to the fire alarm.
• A report published by the Society of Fire
  Protection Engineers, A Review of the
  Methodologies and Critical Appraisal of Computer
  Models Used in the Simulation of Evacuation from
  the Built Environment, by Gwynne and Galea,
  discusses 22 evacuation models.
• A few of the models are discussed here to provide
  a sense of what they do.

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Models for Human behavior (egress)/tenability

• EVACNET4 EVACNET4
• A node network based building evacuation model.
• The user identifies the initial location of
  occupants, then the models predicts an optimized
  time to evacuation of the building.
• This model has been used for high occupancy
  buildings such a stadiums, shopping malls etc.
• The model does not allow for different occupant
  travel speeds or behavior.
• ltltschematicgtgt

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Models for Human behavior (egress)/tenability

• EXIT89 EXIT89
• Designed to simulate the evacuation of large,
  high occupancy buildings, such as highrises, so
  that the movement of individuals can be tracked
  as they exit the building.
• EXIT89 can account for occupants with a range of
  travel speeds and capabilities, choice of routing
  options, travel up and down stairs, opposed flows
  and delays.
• The model can also examine the impact of smoke on
  evacuation given specified smoke blockages or
  from CFAST output.

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Models for Human behavior (egress)/tenability

• EXITT
• A part of the HAZARD suite of models developed by
  NIST.
• Its intended use is for residential dwellings
  with a limit of 12 rooms or nodes.
• The model follows individuals and the actions
  are determined by pre-defined criteria and
  decision rules based on location, gender,
  capabilities, status i.e. awake or sleeping at
  the time of alarm, etc.
• Based on the criteria for each person in the
  simulation, the occupants may investigate the
  fire, help others or directly evacuate the
  building.
• The model ends when all individuals have either
  have successly left the building or are trapped
  in the building by smoke.

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CONCLUSION

• In general models, physical or mathematical, can
  provide valuable insight into how a fire may have
  developed.
• However the model is only a simulation.
• The model output is dependent on a variety of
  input values such as material properties, times
  lines, geometry, and ventilation openings.
• Since perfect knowledge of every detail of the
  fire site, fuel load or fire timeline is never
  known, estimations are incorporated into the
  model.

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CONCLUSION

• For example, the estimation of the energy release
  rate of an initial source fire as a starting
  point for fire development and spread throughout
  the structure is a necessary part of re-creating
  this fire scenario.
• Another estimation used in this case, the plaster
  ceilings and walls of the structure were modeled
  with the thermal properties of gypsum board.
• These estimations and others used in this
  simulation are further described in Section 6 of
  this report.
• FDS The ability of the FDS model to accurately
  predict the temperature and velocity of fire
  gases has been previously evaluated by conducting
  experiments, both lab-scale and full-scale, and
  measuring quantities of interest.

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CONCLUSION

• For relatively simple fire driven flows, such as
  buoyant plumes and flows through doorways, FDS
  predictions are within the experimental
  uncertainty of the values measured in the
  experiments 2.
• For example, if a gas flow velocity is measured
  at 0.5 m/s with an experimental uncertainty of
  ?0.05 m/s, the FDS model gas flow velocity
  predictions were also in the range between 0.45
  m/s and 0.55 m/s.
• In large scale fire tests, FDS temperature
  predictions were found to be within 15 of the
  measured temperatures and the FDS heat release
  rates were predicted to within 20 of the
  measured values.
• Therefore the results are presented as ranges to
  address these uncertainties.