CFD modeling of Compartment Fires (compartment : One Room or Space)

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CFD modeling of Compartment Fires(compartment
One Room or Space)

• 2003.04.08

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Fire

• Uncontrolled flame
• Subsonic area
• Multi-phase flow
• Chemical reacting flow
• Radiation
• Combustion
• Kinetics

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Fire Safety Science

• To minimize fire loss in mills In 19 century
• 1950 delayed
• 19802000 growth
• Based on turbulence and combustion
• Unified model difficulties
• Prescriptive based design ?
• Performance-based Fire safety Regulation

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Example Diagram(SmartFire S/W)
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Typical Compartment Fire

• Ignition
• Growth
• Flashover
• Fully developed fire
• Decay

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Schematic of Compartment Fire
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Ignition (main modeling for prevention)

• Flame, Spark Spontaneous reaction of fuel (
  heat accumulation )
• Fuel Pyrolysis
• Heat release to Gaseous volatiles
• Burning rate
• Smoldering (backdraft with fresh air supply)
• Incident net flux
• Pyrolysis model ( PDE, cone calorimeter )

http//www.firetactics.com
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Growth Spread (removal of ignition factor)

• Fire growth depends on type of fuel, oxygen and
  other factor
• Buoyancy
• Upper hot smoke layer toxic gas
• Unpredictness
• Convective radiative heat transfer
• Turbulent flow environment

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Flashover (structure design)

• Rapid growth to fully developed fire
• Thermal instability
• Strong radiation
• (smoke layer ? combustible materials)
• Failure of structure
• Fire impact

http//www.firetactics.com
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Fully developed fire

• The rate of heat release Maximum
• 700 1200 celcius
• Extinction limit
• Computational coupling

Decay ( extinguishment )

• Diminishment of energy release rate
• Water spray ( gas, solid and dispersed liquid)

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Fire Field Model

• RANS ( Reynolds-Averaged Naviers-Stokes
  Equation )

• LES ( Large Eddy Simulation )

Prescribed Fuel Release Rate
Gas Solid fuel coupling
Suppression (Gas Dispersed Liquid Flow)
No Suppression
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1st Modeling ( Zone modeling)

• Basic conservation equation
• Robustness sophistication
• Validation of reliability
• Speed by algorithm
• Parameter (limited information)
• Preliminary knowledge about flow
• Not tractable with exact accuracy
• ( ex. flashover )

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2nd Field modeling

• Classification according to turbulence phenomena
• 1) RANS Reynolds Averaged Navier-Stokes
• 2) LES Large Eddy Simulation
• Coupling Gas phase the solid combustible fuel
• Suppression

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RANS 1
Group 1 Group 2
Simulation Only Gas Flow Solid Gas
Relation of solid phase Decoupled from calculation coupled
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RANS 2

• Object time mean flow acquisition
• Key
• - Reynolds Stress tensor
• - Turbulent scalar fluxes
• Eddy viscosity hypothesis
• k - e model
• Density variation Favre averaged conservation

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Flow field Turbulence Modeling
FLOW

• Equilibrium between production and dissipation
• Buoyancy effects
• Prandtl / Richardson number
• Wall flow
• - Surface roughness
• - Low Re. Number

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Combustion Modeling
Combustion

• Fast Chemistry Conserved Scalar Approach
• Eddy Break-UP Model
• Finite Reaction Rate Model
• Detailed Chemistry

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Conserved Scalar field
Combustion

• Fast chemistry
• Irreversible reaction or equilibrium
• PDF ( Probability Density Function )
• Chemical Kinetics
• Laminar Flamelet consideration ( similar to
  counterflow flame)

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Detailed Chemistry
Combustion

• Toxic substance
• - CO, Nox, HCN asphyxiants, narcotics
• Empirical results
• Flamelet model prediction

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Radiation
Combustion

• Dominant mode of heat transfer
• Radiation heat transfer
• gtgt convective heat transfer
• Emission / absorption
• Scattering in / out
• Computational skill
• 1) Flux
• 2) Discrete Transfer

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Radiation Computation Method media
Combustion

• 1) Flux Method
• - Angular variation of radiant intensity
• 2) Discrete Transfer Method
• - Ray trace
• 3) Media
• - Soot Gas ( Soot 95 )
• 4) Non-gray Gas Modeling
• - Narrow Band Statistical
• - Exponential wide Band

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Soot Generation Mode
Combustion

• Control the rate of fire spread
• Main property
• Volume fraction , Number density
• Soot growth by surface reaction and coagulation ,
  aggoleration

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Numerical Techniques

• Pressure Correction Scheme Spatial Derivative
• Velocity Pressure Simple algorithm
• model
• Recent discretization scheme TVD scheme (Total
  Variation Diminishing scheme)

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Application of the Conventional RANS Model

• Benchmark problem (Tunnel Compartment Fire)
• Prescribed heat / fuel release rate
• (quasi steady burning rate)
• Buoyancy / Stress production Richardson
• Buoyancy / Viscous stress Grashof
• Treatment of solid phase Heat Conduction
  Convection and radiation
• Unburned area / pyrolysis
• Trench effect

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Trench angle
19 degree
20 degree
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RANS Limits Performance

• 1) Flow field temperature prediction
• - Uncertainties near flame base
• 2) Combustion, radiation and flame spread
• predictions
• 3) Limits
• - Shortage of Experimental results
• - Low openess of internal details of codes

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Extinguishment

• Eulerian Lagrangian Model
  ( water sprinkler model )
• Drag, pressure gradient, inertia, basset force,
  gravity
• Basset force (past movement of particle)
• Magnus effect ( Rotation of particle )
• Fire plume water spray interaction

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Sprinkler Mechanism

• Heat Extraction
• - Pyrolysis reaction
• - Flammable volatiles
• Oxygen displacement
• - dilution of gas phase
• Blocking of radiant heat ( water curtain )
• - Attenuation of thermal feedback
• Water delivery
• Penetration of plume

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LES ( Large Eddy Simulation )

• Sub grid scale (SGS) sub set Modeling
• RANS? LES in fire modeling