Development of 3D Heat Transfer and Pyrolysis

in FDS

Randall McDermott1, Chao Zhang1, Morgan Bruns2, Salah Benkorichi3

1National Institute of Standards and Technology, Gaithersburg, Maryland, USA2Virginia Military Institute, Lexington, Virginia, USA

3Omega Fire Engineering Ltd., Manchester, UK

Fire and Evacuation Modeling Technical Conference 2018

Oct 1-3, Gaithersburg, Maryland

Background and Motivation

• Structural analysis

• Lateral and downward flame spread

• Smoldering combustion

FEMTC 2018 3D Heat Transfer and Pyrolysis in FDS 2

Ohlemiller & Shields, 2008Choe, 2017 Huang et al., 2016

Previous Work

FEMTC 2018 3D Heat Transfer and Pyrolysis in FDS 3

• Andreas Vischer, Aachen, 2009 thesis

• Volker Hohm and Matthias Siemon, Building Materials, Solid Construction and Fire Protection (iBMB) at Technische UniverstätBraunschweig

• Gpyro, Chris Lautenberger, Reax Engineering

• Thermakin, Stas Stoliarov, U. Maryland

Integration into FDS master

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TEMPERATURE SLICE

Governing Equations

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Local deformation

affects heat flux

Computing Heat Flux

Fourier’s law:

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Input Parameters

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HT3D=T invokes 2-way coupling with gas phase

Heat Diffusion in Steel I-Beam

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Any mention of commercial products within this paper is for information only; it does not imply recommendation or endorsement by NIST.

Internal Heating in Sphere

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• Constant, uniform internal

heat generation

• Constant, ambient surface

temperature

0 0.02 0.04 0.06 0.08 0.1

Radial Distance (m)

20

30

40

50

60

Te

mp

era

ture

(°C

)

FDS6.7.0-515-g7416f3a-master

Analytical

FDS t=10 s

FDS t=20 s

FDS t=60 s

FDS t=120 s

FDS t=180 s

Density Definitions

1. Material

2. Bulk

3. Total solid

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Modeling Deformation

• Changes in composition generally cause contraction or expansion of material

• Some challenges in 3D:1. Mechanical constitutive relation

2. Advection term in conservation equations

3. Moving boundaries

• Simple solution: 1. Subgrid scale models of fluxes

2. Burn away—remove solid cells as cell density goes to zero

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Heat Flux with Local Deformation

• As material contracts (expands), distance between material points decreases (increases)

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Local Material Deformation

Two simple models:1. Isotropic:

2. Unidirectional:

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Compute new

densities

using old

temperatures

Compute solid

volumes

using new

densities

Compute new

temperatures

using heat

fluxes

Compute heat

fluxes on

solid volumes

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PMMA Slab: Mass Loss Rate and Thickness

0 100 200 300 400 500 600

Time (s)

0

0.01

0.02

0.03

0.04

ML

RP

UA

(kg

/s/m

2)

3D Pyrolysis (pyro3d_vs_pyro1d)

FDS6.7.0-515-g7416f3a-master

PYRO1D (no burn away)

PYRO3D (burn away)

FEMTC 2018 3D Heat Transfer and Pyrolysis in FDS 150 100 200 300 400 500 600

Time (s)

0

0.002

0.004

0.006

0.008

0.01

0.012

Th

ickn

ess (

m)

3D Pyrolysis (pyro3d_vs_pyro1d)

FDS6.7.0-515-g7416f3a-master

PYRO1D (no burn away)

PYRO3D (burn away)

Radiative Heat Transfer

• Many problems are thermally thick

• Diffusion approximation is extremely efficient in 3D

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Material Absorption Coefficient (1/m) Thermal Thickness for 1 cm slab

HDPE 1300 13

PMMA 2700 27

PA66 3920 39.2

Radiation Verification

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Exact solution from Modest,

Radiative Heat Transfer, 2nd

Edition.0 0.02 0.04 0.06 0.08 0.1

x (m)

0

200

400

600

800

Tem

pera

ture

(°C

)

HT3D Internal Radiation (ht3d_radiation)

FDS6.7.0-610-g8606b24-master

Exact =100 1/m

FDS =100 1/m Nx=10

FDS =100 1/m Nx=20

FDS =100 1/m Nx=40

Exact =2000 1/m

FDS =2000 1/m Nx=10

FDS =2000 1/m Nx=20

FDS =2000 1/m Nx=40

Pyrolysis Gas Transport

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?

Burn Away• 40 cm cubes

• Low density “foam”

• Compartment walls at 1100 °C

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3D Model 1D Model

0 5 10 15 20 25 30

Time (s)

0

0.5

1

1.5

Mass (

kg)

Pyrolyzed Mass (box_burn_away1)

FDS6.7.0-534-g33dc811-master

Ideal

PYRO1D (fuel)

PYRO3D (fuel)

PYRO3D w/ transport (fuel)

Solid Sub-Surface Heat Flux Model

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Closing remarks

• Development of an efficient 3D pyrolysis model is needed for reliable predictions of flame spread (work in progress)

• Subgrid-scale models of heat and mass fluxes were used to account for local deformation

• The resultant model has been verified and tested for several scenarios

• Next steps:• Anisotropy• Thin obstructions (coupling with 1D model?)• Porous media flow

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Acknowledgements

• FDS development team

• Simo Hostikka, Deepak Paudel

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