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LNG POOL FIRE MODELING

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Title: LNG POOL FIRE MODELING


1
LNG POOL FIRE MODELING
  • Background The MTB Model and the Need for
    Better Modeling Methods
  • View Factor Models LNGFIRE as an Example
  • Theoretical Fire Models The FDS Computational
    Fluid Dynamics Model as an Example

2
EXERCISE 2
  • What determines the radiant (thermal) energy you
    receive from a liquid pool fire?

3
ALPHABET SOUP
  • MTB (Materials Transportation Bureau, U. S. DOT
  • RSPA (Research and Special Programs
    Administration, U. S. DOT)
  • PHMSA (Pipeline and Hazardous Materials Safety
    Administration, U. S. DOT)

4
THE MTB MODEL FOR POOL FIRES
  • Promulgated into 49 CFR 193.2057 in 1980s
  • Resulted from Review of 1971 NFPA, U. S. Bureau
    of Mines, AGA-Sponsored, ESSO, U. S. Coast
    Guard-Sponsored Work
  • d f (A)0.5
  • where
  • d exclusion distance measured perpendicular
    to flame surface to target
  • A horizontal area of impoundment
  • f offsite classification factor based on
    radiant flux limit.

5
(No Transcript)
6
THE MTB MODEL (Cont.)
  • T Tilt angle 45o (always)
  • L Flame Length reduces to 3 D (always)
  • D Equivalent Diameter (rectangular as well
    as circular impoundments)
  • f

7
INCIDENT FLUX LIMITS
Abbreviated definitions from 49 CFR
193.2057(1980).
8
CRITICISMS OF THE MTB MODEL
  • L and T Fixed Specifications are Unsupported
  • Point Source Energy Model (used for calculating
    target energy from flame surface and f factors)
    is Inferior to Full Flame Surface Representation
    (cylinder or parallelepiped)
  • Average Maximum (black body) Surface Emissive
    Power Specification - Estimated 142.0 kW/m2
    (45,000 Btu/hr ft2) Is Not Consistent with Data
  • Full technical discussion in LNGFIRE A
    Thermal Radiation Model for LNG Fires, Gas
    Research Institute, GRI-89/0178, June 29, 1990.

9
CRITICISMS OF THE MTB MODEL (Cont.)
  • Surface Emissive Power is Not Constant Varies
    Exponentially With Flame Thickness
  • Flame Length Varies With Burning Rate (and,
    secondarily, wind speed) Slight Differences for
    Equilateral Pools and Elongated Trenches
  • Flame Tilt Angle Varies With Respect to Wind
    Speed and Dimensions (size and shape) of
    Impoundment
  • Flame Drag May be Important Varies With Wind
    Speed
  • Elongated Trenches
  • Radiation Attenuation Due to Water Vapor.

10
LNGFIRE (1989)
  • Currently Referenced Model in 49 CFR 193.2057
  • Resulted from Several Years of Effort to Resolve
    MTB Model Criticisms, Including Need to Model
    Elongated Trenches
  • Key Research
  • Coast Guard (view factors)
  • Shell (surface emissive powers)
  • British Gas (correlations of flame length, tilt,
    and drag)
  • GRI-ADL/British Gas (trench fires)

11
LNGFIRE SUMMARY
  • Model Type Semi-Empirical
  • Basic Equation
  • q F t e qs
  • where
  • q Incident radiant heat flux at the
    target (kW/m2)
  • F Geometric view factor from flame
    surface to the target (non-
    dimensional)
  • t Transmissivity of the atmosphere to
    thermal energy (0 to 1)
  • e Average emissivity of the flame ()
  • qs Maximum effective black body
    radiation of the flame (kW/m2)
  • eqs Surface Emissive Power (kW/m2)

12
VIEW FACTOR CONSIDERATIONS
Integration
FdA1?A2 1 / p A2? cosß1 cosß2 dA2/r2
Piecewise
13
VIEW FACTOR REQUIRED COVERAGE
  • Vertical and Horizontal Targets
  • Targets in the Flame Shadow
  • Elevated Flame Bases Relative to Target
  • Elevated Targets Relative to Flame Bases

14
FLAME LENGTH CALCULATION
  • Lf/D 42 (m / ?a v(gD))0.61
  • where
  • Lf Flame Length (m)
  • D Pool Diameter (m)
  • g Gravitational Acceleration (m/s2)
  • m Mass Burning Rate (kg/m2s)
  • ?a Ambient Air Density (kg/m3)
  • Calculation for circular pool.

15
FLAME TILT CALCULATION
  • cos ? 1 / vU for U 1
  • cos ? 1 for U 1
  • where
  • U U / Uc
  • U Wind Velocity (m/s)
  • Uc Characteristic Velocity (m gD /
    ?v)1/3
  • m Mass Burning Rate (kg/m2s)
  • ?v LNG Vapor Density (kg/m3)
  • Calculation for circular pool.

16
FLAME DRAG CALCULATION
  • DR (D D)/D 1.5 (Fr)0.069
  • where
  • DR Drag Ratio (Drag Distance/Diameter)
  • D Pool Diameter (m)
  • D Extension of the Flame Base Beyond
    Pool Edge
  • Fr Froude Number u2 / gD
  • Calculation for circular pool.

17
BURNING RATE CALCULATION
  • m 0.11 1 exp (-0.46D)
  • or
  • m 0.11 kg/m2s
  • where
  • m LNG Burning Rate (kg / m2s)
  • D Pool Diameter (m)
  • Calculation for circular pool.

18
ATMOSPHERIC TRANSMISSIVITY
  • t 1 - aw - ac awac
  • where
  • aw Absorptivity of Water Vapor
  • ac Absorptivity of Carbon Dioxide

19
FLAME SURFACE EMISSIVE POWER
  • e qs 190 (1 - e-0.3Df)
  • where
  • e Flame Emissivity
  • qs Maximum Effective Black Body Radiation
    Emissive Power (kW/m2)
  • Df Flame Thickness (m)

20
HYPOTHETICAL ZONED FLAME
21
SURFACE EMISSIVE POWERS AS MEASURED
GRI-ADL/British Gas trench fire tests, Test 8,
side view
22
MONTOIR 35 METER POOL FIRES (1987)
Test 1
Test 2
Test 3
23
SURFACE EMISSIVE POWER DATA AND CURVE FIT FOR
LNGFIRE EQUATION
24
LNGFIRE VALIDATION AND MTB MODEL COMPARISON,
DOWNWIND
GRI-ADL/British Gas trench fire tests, Test 4
25
LNGFIRE VALIDATION AND MTB MODEL COMPARISON,
CROSSWIND
GRI-ADL/British Gas trench fire tests, Test 4
26
OTHER REGULATORY MODELS
  • FIRES2 British Gas/Advantica
  • CORE Gaz de France
  • Model Comparison Results to Montoir 35m Scale
    Pool Fire Scenarios, Including Experiments
  • The conditions calculations corresponding to
    the Montoir experiments lead to a rather good
    agreement,with relative differences being 10 to
    30 for crosswind and downwind, respectively.
  • Debernardy, J. L., Perroux, J. M., Nedelka, D.
    Comparison of LNG Fire Radiation Calculation
    Codes, Gaz de France, 1992.

27
BUT DO SEMI-EMPIRICAL VIEW FACTOR MODELS MEET ALL
NEEDS?
  • Irregular Shapes Unconfined Spreading, Flow
    Barriers
  • Interaction with Fire Control Measures
  • Structures in Flames and Their Interaction with
    Fire Dynamics (e.g., presence of a tank shell)
  • Smoke Shielding
  • Transient Behaviors

28
THEORETICAL MODELS FOR POOL FIRES FDS
  • FDS Fire Dynamics Simulator (Version 4, 2004),
    Developed and Supported by the U. S. National
    Institute of Standards and Technology (NIST)
  • Under Development for 25 Years
  • Computational Fluid Dynamics (CFD) Model for
    Low-Speed Fire-Driven Flow Emphasizing Heat
    Transport and Smoke
  • Time-Dependent ,3-D Spatially Computed
    Differencing Solutions Approximating the Partial
    Differential Navier-Stokes Equations for
    Conservation of Mass, Momentum, and Energy

29
FDS APPLICATIONS
  • Low-Speed Transport of Heat and Combustion
    Products from Fires (Thermal Radiation Computed
    Using a Finite Volume Technique Within the 3-D
    Grid)
  • Radiative and Convective Heat Transfer Between
    Gas and Solid Surfaces
  • Pyrolysis
  • Flame Spread and Fire Growth
  • Interactions with Fire Suppression and Detection
    Systems

30
FDS MODEL RESULTS
  • Within the Fire Plume and Surrounding Air
  • Gas Temperature, Velocity, Concentration by
    Species, and Density
  • Smoke Concentration and Visibility
  • Pressure
  • Heat Release Rate per Unit Volume
  • Mixture Fraction
  • Water Droplet Mass per Unit Volume
  • Impingement on Solid Surfaces
  • Surface and Interior Temperatures
  • Radiative and convective Heat Flux
  • Burning Rate
  • Others, Including Global Quantities

31
FDS GENERAL MODEL STRUCTURE
  • Hydrodynamics Model, Including Navier-Stokes
    Approximation Differencing Equations and
    Turbulence
  • Large Eddy Simulation (LES) Course Grids
  • Direct Numerical Simulation (DNS) Fine Grids
  • Combustion Model, Based on Scalar Quantity
    Mixture Fraction
  • Radiation Transport Model, Based on Finite Volume
    Method (FVM) Including 100 Discrete Transport
    Angles
  • Geometry (Gridding) Model for One or More
    Rectilinear Grids
  • Boundary Condition Definitions, Assessed as
    Thermal as well as Physical Boundaries for
    Controlling Heat and Mass Transfer
  • Fire Target Response Models, Including Sprinkler
    and Detectors, and Water Sprays (Lagrangian
    Droplets)

32
FUNDAMENTAL CONSERVATION EQUATIONS
33
SIMPLIED EQUATIONS USED IN FDS
34
VALIDATION FOR POOL FIRES
  • Historical Development on Unconfined Fires
  • Since 2000 and Revision Code
  • Methane Pool Fire 1 m diameter (Xin,et. al.,
    2002)
  • Methane and Methanol Pools (Hostikka, et. al.,
    2002)
  • Heptane Pools (Hietaniemi, et. al., 2004)

35
COMPUTER REQUIREMENTS
  • Recommended Minimum
  • Windows-Based PC Running 1 GHz Pentium III, with
    512 MB RAM
  • 1 GB Storage per Average Large Simulation
  • But Really - The Faster (and Bigger), the Better

36
DOCUMENTATION
  • FDS
  • SMOKEVIEW
  • Website http//www.fire.nist.gov/fds/

37
GENERAL STEPS FOR SETTING UP FDS RUNS
  • Input Files
  • Setting Time Limits
  • Defining Computational Domain (i.e., Grid Mesh)
  • Defining Boundary Conditions
  • Defining Fire Conditions Via Combustion
    Parameters
  • Defining Obstructions, Mitigation Systems
  • Running
  • Monitoring Progress
  • Error Statements
  • Output Files
  • Point Measurements Within the Domain
  • Animated Planar Slices, Boundary Quantities,
    Isosurfaces (SMOKEVIEW)
  • Static Data Files

38
POTENTIAL APPLICATIONS OF CFD TO LNG FIRES
  • Addressing Limitation of View Factor Models
  • Non-Regulatory Cases
  • Complex, Progressive Failures
  • Complex Consequence Analysis
  • Analysis of Hazard Mitigation Measures
  • Phenomena Other Than Pool Fires?
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