Pool Fire Model - Present

8/13/2019 Pool Fire Model - Present

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LNG POOL FIRE MODELING

Background: The MTB Model and the Needfor Better Modeling Methods

View Factor Models: LNGFIRE as an Example

Theoretical Fire Models: The FDSComputational Fluid Dynamics Model as an

Example

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EXERCISE #2

What determines the radiant (thermal) energy

you receive from a liquid pool fire?

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ALPHABET SOUP

MTB (Materials Transportation Bureau, U. S.DOT

RSPA (Research and Special ProgramsAdministration, U. S. DOT)

PHMSA (Pipeline and Hazardous MaterialsSafety Administration, U. S. DOT)

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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.

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THE MTB MODEL (Cont.)

= Tilt angle = 45o(always)

L = Flame Length = reduces to 3 D (always)

D = Equivalent Diameter (rectangular as wellas circular impoundments)

f :f Incident Flux

Btu/hr ft2Incident Flux

kW/m3

3 1,600 5.05

1.6 4,000 12.62

1.1 6,700 21.14

0.8 10,000 31.54

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INCIDENT FLUX LIMITS

Off Site Description at Target* Incident Flux, Btu/hr ft2 Incident Flux, kW/m2

Outdoor areas occupied by 20 or more persons

during normal use, such as beaches, playgrounds,

outdoor theaters, other recreation areas or other

places of public assembly

1,600 5.05

Buildings that are used for residences, or occupied by

20 or more persons during normal use

4,000 12.62

Public streets, highways, and mainlines of railroads 6,700 21.14

Other structures, or if closer to [its normal angle to

the flame surface], the right-of-way of the facility

10,000 31.54

*Abbreviated definitions from 49 CFR 193.2057(1980).

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CRITICISMS OF THE MTB MODEL*

L and Fixed Specifications are Unsupported

Point Source Energy Model (used for calculatingtarget energy from flame surface and f factors) is

Inferior to Full Flame Surface Representation(cylinder or parallelepiped)

Average Maximum (black body) Surface EmissivePower Specification - Estimated142.0 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 ResearchInstitute, GRI-89/0178, June 29, 1990.

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CRITICISMS OF THE MTB MODEL(Cont.)

Surface Emissive Power is Not Constant; VariesExponentially With Flame Thickness

Flame Length Varies With Burning Rate (and,

secondarily, wind speed); Slight Differences forEquilateral Pools and Elongated Trenches

Flame Tilt Angle Varies With Respect to Wind Speedand Dimensions (size and shape) of Impoundment

Flame Drag May be Important; Varies With WindSpeed

Elongated Trenches

Radiation Attenuation Due to Water Vapor.9


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LNGFIRE (1989)

Currently Referenced Model in 49 CFR193.2057

Resulted from Several Years of Effort to ResolveMTB 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)

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LNGFIRE SUMMARY

Model Type: Semi-Empirical

Basic Equationq = F qs

where:

q = Incident radiant heat flux at the target (kW/m2)

F = Geometric view factor from flame surface to the target (non-

dimensional)

= Transmissivity of the atmosphere to thermal energy (0 to 1)

= Average emissivity of the flame (%)

qs = Maximum effective black body radiation of the flame (kW/m2)

qs= Surface Emissive Power (kW/m2)

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VIEW FACTOR CONSIDERATIONS

Piecewise:

Integration:

FdA1A2 = 1 / A2 cos1cos2 dA2/r2

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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

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FLAME LENGTH CALCULATION*

Lf/D = 42 (m / a(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.

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FLAME TILT CALCULATION*

cos = 1 / U* 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.15


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FLAME DRAG CALCULATION*

DR = (D + D)/D = 1.5 (Fr)0.069

where:

DR = Drag Ratio (Drag Distance/Diameter)


D = Extension of the Flame Base Beyond Pool Edge

Fr = Froude Number = u2/ gD


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BURNING RATE CALCULATION

m = 0.11 [ 1 exp (-0.46D)]

or:

m = 0.11 kg/m2s

where:

m = LNG Burning Rate (kg / m2s)



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ATMOSPHERIC TRANSMISSIVITY

= 1 - w - c + wc

where:

w = Absorptivity of Water Vaporc = Absorptivity of Carbon Dioxide

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FLAME SURFACE EMISSIVE POWER

qs = 190 (1 - e-0.3Df)

where:

= Flame Emissivity

qs = Maximum Effective Black Body RadiationEmissive Power (kW/m2)

Df = Flame Thickness (m)

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HYPOTHETICAL ZONED FLAME

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SURFACE EMISSIVE POWERS AS

MEASURED*

*GRI-ADL/British Gas trench fire tests, Test #8, side view

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MONTOIR 35 METER POOL FIRES

(1987)

Test #1 Test #2

Test #3 22


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SURFACE EMISSIVE POWER DATA AND

CURVE FIT FOR LNGFIRE EQUATION

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LNGFIRE VALIDATION AND MTB

MODEL COMPARISON, DOWNWIND*

*GRI-ADL/British Gas trench fire tests, Test #4

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LNGFIRE VALIDATION AND MTB

MODEL COMPARISON, CROSSWIND*

*GRI-ADL/British Gas trench fire tests, Test #4

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OTHER REGULATORY MODELS

FIRES2British Gas/Advantica COREGaz de France

Model Comparison Results to Montoir 35mScale Pool Fire Scenarios, Including

Experiments:

The conditions [calculations] corresponding tothe Montoir experiments lead to a rather good

agreement,with relative differences being 10to 30% [for crosswind and downwind,

respectively].*

*Debernardy, J. L., Perroux, J. M., Nedelka, D. Comparison of LNG FireRadiation Calculation Codes, Gaz de France, 1992. 26


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BUT DO SEMI-EMPIRICAL VIEW

FACTOR MODELS MEET ALL NEEDS?

Irregular Shapes: Unconfined Spreading, FlowBarriers

Interaction with Fire Control Measures

Structures in Flames and Their Interaction withFire Dynamics (e.g., presence of a tank shell)

Smoke Shielding

Transient Behaviors

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THEORETICAL MODELS FOR POOL

FIRES: FDS

FDSFire Dynamics Simulator (Version 4, 2004),Developed and Supported by the U. S. NationalInstitute of Standards and Technology (NIST)

Under Development for 25 Years Computational Fluid Dynamics (CFD) Model for

Low-Speed Fire-Driven Flow Emphasizing HeatTransport and Smoke

Time-Dependent ,3-D Spatially ComputedDifferencing Solutions Approximating the PartialDifferential Navier-Stokes Equations forConservation of Mass, Momentum, and Energy

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FDS APPLICATIONS

Low-Speed Transport of Heat and CombustionProducts from Fires (Thermal RadiationComputed Using a Finite Volume TechniqueWithin the 3-D Grid)

Radiative and Convective Heat Transfer BetweenGas and Solid Surfaces

Pyrolysis

Flame Spread and Fire Growth

Interactions with Fire Suppression and DetectionSystems

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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

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FDS GENERAL MODEL STRUCTURE

Hydrodynamics Model, Including Navier-StokesApproximation 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 VolumeMethod (FVM) Including 100 Discrete Transport Angles

Geometry (Gridding) Model for One or More RectilinearGrids

Boundary Condition Definitions, Assessed as Thermal aswell as Physical Boundaries for Controlling Heat and MassTransfer

Fire Target Response Models, Including Sprinkler andDetectors, and Water Sprays (Lagrangian Droplets)

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FUNDAMENTAL CONSERVATION

EQUATIONS

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SIMPLIED EQUATIONS USED IN FDS

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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), theBetter

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GENERAL STEPS FOR SETTING UP


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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 37


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Documents

Pool Fire Model - Present