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Effects of FDS mesh resolution on growth phase

smoke production

Haavard Boehmer, P.E.

Jason Floyd, PhD

Michael Ferreira, P.E.

Overview

• Practical smoke control configuration

• Different cell sizes

• Different fire growth methods

Overview

• Series of simple FDS v6.2 simulations− Cell size (7.5 – 30 cm [3 – 12 in.])− Fire growth mechanism

• Evaluated output in growth phase− HRR and smoke production (plume mass flow)

Overview

Fire Dynamics Simulator (FDS)

• Developed, managed and release by NIST

• Open source software

• Extensive verification and validation

Assumptions

• Fast, t-square fire growth

• 1,000 kW/m2 heat release rate per unit area

• 3,240 kW fire (divisible to all cell sizes)

• Well ventilated fire (domain sides open)

FDS Domain

• 8 x 8 x 15 m domain

• Open sides

• Fire in center

• Concrete ceiling8 m

8 m

15 m

Cell Size

• D*/δx evaluate fire size vs cell size

• Value of 5 – 10 recommended

Cell Size D*/δx

7.5 cm 20.3

15 cm 10.2

30 cm 5.1

𝐷𝐷∗ =�̇�𝑄

𝜌𝜌∞𝑐𝑐𝑝𝑝𝑇𝑇∞ 𝑔𝑔

𝛿𝛿x = cell size

Fire Growth

• Three different types of fire growth:− Ramp: Fire grows across whole area− Spread: Fire spreads from center− Vent spread: Expanding overlapping vents

Fire Growth: Ramp

• Fire occurs over the full area of the vent

• Unrealistically low HRRPUA

• Unrealistic plume shape and dynamics

Fire Growth: Spreading Fire

• Fire starts in the center cell

• Constant HRRPUA (more realistic)

• Circular spread outward, per:

SPREAD_RATE =𝛼𝛼𝜋𝜋

1𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻

Fire Growth: Spreading Fire

t1 t2

Fire Growth: Vent Spread

• Overlapping vents in expanding circle

• Constant HRRPUA (more realistic)

• Complex setup

Fire Growth: Vent Spread

t1 t2

Fire Growth - Compared

Ramp Spread Vent Spread

At 2 min

FDS Run MatrixRun Name Fire Growth Cell Size

Run 1Ramp

7.5 cmRun 2 15 cmRun 3 30 cmRun 4

Spread7.5 cm

Run 5 15 cmRun 6 30 cmRun 7

Vent Spread7.5 cm

Run 8 15 cmRun 9 30 cmSprk 1

Vent Spread7.5 cm

Sprk 2 15 cmSprk 3 30 cm

Results

Three parameters were evaluated:

• HRR with different fire growth

• HRR with different cell sizes

• Mass flow with different cell sizes

Results – HRR vs Growth

0

500

1,000

1,500

2,000

2,500

3,000

3,500

0 30 60 90 120 150 180 210 240 270 300 330 360

Heat

Rel

ease

Rat

e (k

W)

Time (s)

HRR With Three Growth Configurations (15 cm Cells)

Ramp

Spread

Ventspread

Results – HRR vs Cells Size

0

500

1,000

1,500

2,000

2,500

3,000

3,500

0 30 60 90 120 150 180 210 240 270 300 330 360

Heat

Rel

ease

Rat

e (kW

)

Time (s)

HRR With Three Cell Sizes

7.5 cm

15 cm

30 cm

(Vent spread fire growth)

HRR - Summary

• Ramp type yield small differences in HRR curve, likely insignificant.

• Different cell sizes gave no difference in HRR, as expected.

0

10

20

30

40

50

60

70

0 30 60 90 120 150 180 210 240

Mas

s Fo

w (k

g/s)

Time (s)

Mass Flow at 12 m With Three Cell Sizes

7.5 cm

15 cm

30 cm

Results – Plume Mass Flow

(Vent spread fire growth)

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

0 30 60 90 120 150 180

Mas

s (kg

)

Time (s)

Total Mass Flow at 12 m With Three Cell Sizes

7.5 cm

15 cm

30 cm

Results – Plume Mass Flow

Results – Plume Mass Flow

• Difference in total mass flow at 12 m after 2 minutes

7 cm > 15 cm 15 cm > 30 cm 7 cm > 30 cm

∆ mass-86.6 kg -277.0 kg -363.6 kg

-8% -27% -36%

∆ time-6.0 s -18.0 s -24.0 s

-5% -15% -20%

Results – Plume Mass Flow

• Mass flow differences develop in first two minutes

• Lower resolution: − Reduced total mass to upper layer − Less conservative

Results – Plume Mass Flow

• Going from 15 cm to 30 cm cells:− Decrease mass to upper layer by 27% − Decrease time to fill upper layer by 15%− Time difference not likely significant for

occupant egress considerations

Sprinklers & Detectors

• Vent spread method, using three cell sizes

• Ceiling placed at 5 m above fire

Sprinklers & Detectors

Sprinklers & Detectors

0

10

20

30

40

50

60

70

0 30 60 90 120 150 180 210 240 270 300

Link T

empe

ratu

re (˚

C)

Time (s)

Sprinkler Temperature With Three Cell Sizes

7.5 cm15 cm30 cmT_Act

Sprinklers & Detectors

Sprinkler activation times

• Coarser cells more conservative

Cell Size Activation Time

7.5 cm 196 s

15 cm 202 s

30 cm 244 s

Conclusions – Ramp Type

• Little difference in HRR curve between three fire growth methods

• Insignificant for smoke control evaluations

• Vent spread most accurate, recommended

Conclusions – Cell Size

• No significant difference in HRR curve

• Mass flow: potentially significant− 8% and 27% for each cell size step in this case− Smoke production reduced with coarser cells− Non-conservative results!

Conclusions - Sprinklers

• Potentially significant impact

• Difference of 8 s and 46 s in this case

• Systematic study could further quantify

• Coarser cells is more conservative

Questions

For More Information:

Haavard Boehmer, P.E.

410-737-8677

hboehmer@jensenhughes.com

The Design FireSelecting Fire Characteristics for a CFD

Model

Adam Edwards

David Stacy

Presentation Overview

• Importance of Fire Characteristics

• Real Life Applications

• Fire Characteristic Analysis (FDS)

• Fire Scenario Considerations

Fire Characteristics

• Frequency of Performance Based Designs (PBD) − Atrium Smoke Control− Smoke Protected Seating− Malls

• Limited full scale test data available

• Architectural and “pretty” interior design

• “Preset” values

Why is it Important Now?

Gymnasium Design FireReal Life Application

“Noises Off-Backstage Set” by Keven T. Houle

“London Boat Show 2013” by Andrew Havis

Convention Center Design FireReal Life Application

Corporate Atrium Design FireReal Life Application

FDS Analysis• A base geometry of an atrium was selected to evaluate the effect

various fire characteristics have on tenable conditions.

• Design parameters to be used as constants included: − The Exhaust Rate (60,000 CFM)− Natural Makeup Air (400 sq. ft.)− Design Fire Size (5,000 kW)

• Five different design fire characteristics were used as variables− 100% Wood (cellulose product)− 75% Wood/25% Polyurethane Foam− 50% Wood/50% Polyurethane Foam− 25% Wood/75% Polyurethane Foam− 100% Polyurethane Foam

Atrium Geometry• 6 story circular atrium

with a clearstory

• Four exhaust fans (15,000 CFM each) located at top of clearstory

• Four 100 sq. ft. doors used as natural makeup air locations (spaced evenly around Level 1)

Geometry Rendering

Atrium Geometry• Each floor is approximately

8,000 sq. ft. with a 700 sq. ft. center opening

• Two rooms on either side of opening at each level, which resist the passage of smoke (depicted as red areas)

Typical Floor Plan

Walkable Area

Open to Below

Diagnostic Devices• Visibility, Carbon Monoxide,

Temperature slice files included 6 ft. above each level, as well as in –x and –y coordinates.

• Spot gas-phase devices of the same also included on each level.

• Visibility Conditions are of primary concern

Typical Device Layout

Rendering of Slices

Fire Characteristics• Fuel load compositions consisted of various percentage (by mass) of

wood (pine) and flexible polyurethane foam (GM23).

Ref: 4th Edition of the SFPE Handbook, Table 3-4.16

Fire Characteristics• Five different fuel load compositions were considered to

demonstrate how varying the fuel load effects tenability in the atrium.

Ref: 4th Edition of the SFPE Handbook, Table 3-4.16

Visibility Slice ImagesTruncated >10m

100 Seconds

Run A

Run D

Run B

Run C

Run E

Visibility Slice ImagesTruncated >10m

200 Seconds

Run A

Run D

Run B

Run C

Run E

Visibility Slice ImagesTruncated >10m

300 Seconds

Run A

Run D

Run B

Run C

Run E

Visibility Slice ImagesTruncated >10m

400 Seconds

Run A

Run D

Run B

Run C

Run E

Visibility Slice ImagesTruncated >10m

500 Seconds

Run A

Run D

Run B

Run C

Run E

Visibility Slice ImagesTruncated >10m

600 Seconds

Run A

Run D

Run B

Run C

Run E

Level 6 Visibility (m) vs. Time (sec)

Level 5 Visibility (m) vs. Time (sec)

Level 4 Visibility (m) vs. Time (sec)

Fire Considerations

• Typical, Severe, or Worse Case

• Uses of the Space

• Sprinklers

• Architectural Geometry

• Modern vs. Legacy

Questions?

Understanding Wall and Corner Effects Using the Fire

Dynamics Simulator

Francisco Joglar P.E., PhD

The Problem Fires that are located near boundaries experience a

asymmetric air entrainment and a force imbalance on the plume: Tends to push the flames against the boundary, and May increase plume temperatures.

Existing guidance for implementation in performance based regulation is limited. Requires broad assumptions

Existing Technical Approach Fires adjacent to walls or corners are

treated with the “Image” or “Mirror”.

The fire location factor parameter represents the number of reflections (symmetry) necessary to match the configuration to an open configuration.

Fires within 2-3 ft of the wall or corner are considered to be affected by the wall(s).

Existing Experimental Data FM/SNL

Wall and corner results used in validation exercises, large fires (500-2000 kW)

Results include room heat up effects

Hasemi and Tokunaga Doubling the heat release rate as

outlined in the image method resulted in an overestimation of the flame height on the order of 30%

− Smaller fires (~60kW)

Technical Approach

Simulations for wall and corner configurations in FDS Currently 95 simulations evaluated Various heat release rates Various fire diameters Various distances from wall and corners Non dimensional parameters within validation ranges Additional verification and sensitivity cases as necessary

Technical Approach

Data analysis and correlation to fire dynamics key parameters Use Heskestad’s correlation to the plume temperature Compare results to Heskestad’s correlation and solve for the

fire location factor Correlate the fire location factor to key non dimensional

parameters

The Experimental Matrix• Heat Release Rate, Q

− Relevant range of fire sizes for fire PRA

• Distance from Wall Surface, L− Demonstrate the effect of various distances

• Fire Diameter, D− Evaluate the sensitivity of the result to varying diameter

• Height above Fire Base, H− The relevant parameter for plume temperature

FDS Simulations Configuration 6 m x 6 m x 6 m Concrete walls, floor Open ceiling and remaining sides

Open Configuration No walls

Used as a baseline or control simulation for comparison of results

5 simulations performed, one for each heat release rate considered

Wall Configuration Fire near one wall

Fire placed at various distances from the wall

With current guidance, a value of 2 location factor for fires near a wall is recommended

39 total simulations performed

Corner Configuration Fire near two walls

Fire placed at various distances from the corner

With current guidance, a value of 4 location factor for fires near a corner is recommended

44 total simulations performed

Results

∞

Results for fire location factor (kF)

Only fires very near the corner are affected by the corner (higher temperatures)

The effect decreases rapidly as the fire is moved away from the corner.

No apparent effect on temperatures from single wall surface.

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

0 1 2 3 4 5 6 7k F

Distance from Wall Surface (ft.)

Corner Results Wall Results Open Results

Results – Wall Configuration

• Normalized by L/D*

• Different Fire Sizes Shown

• There is no apparent difference between an open configuration and a wall configuration

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 5 6 7 8

k F

L/D*

40 kW 78 kW 250 kW 500 kW 1000 kW Base

∞

Updated Implementation - Wall All wall configurations can be

treated as an open plume.

(kF = 1.0)

This has large implications for Fire PRA: Objects adjacent to walls can be

treated identically with objects in the open (reduced complexity)

The ZOI is reduced when the location factor is 1.0 relative to 2.0 (reduced risk) 0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

0 0.5 1 1.5 2

k F

Distance from wall (m)

MeanLower Model UncertaintyUpper Model UncertaintyLocation Factor, kF=1

∞

Results – Corner Configuration• Normalized by L/D*

• Different Fire Sizes Shown

• There is a significant impact associated with fires very near corners

• The average result is very near the recommended value of 4 in the corner

• The impact of the corner disappears at approximately one fire diameter of separation from the corner

0

1

2

3

4

5

6

7

0 2 4 6 8

kF

L/D*

40 kW 78 kW 250 kW 500 kW 1000 kW Base

∞

Updated Implementation - Corner Ignition sources within 0.3 m

(1ft) of a corner should be treated with a location factor of 4.

Ignition sources between 0.3 –0.6 m (1 – 2 ft) of a corner should be treated with a location factor of 2.

All other ignition sources can be treated as an open plume. 0

1

2

3

4

5

6

7

0 0.5 1 1.5 2

k F

Distance from corner (m)

Lower Model Uncertainty MeanUpper Model Uncertainty Location Factor, kF=4Location Factor, kF=2 Location Factor, kF=1

∞

Conclusions Explored effects of wall and corner locations on fire plume

temperatures A single wall does not increase the plume temperature or ZOI A fire must be very near the corner in order for the plume temperature or

ZOI to increase The analysis is not sensitive to fire HRR, diameter, or height above the fire

Implications for future implementation. Fewer ignition sources require special treatment for wall or corner effects

(reduced complexity) Overall, the location factor can be reduced relative to existing guidance

(reduced risk)

Thank You National Fire Protection Association (NFPA)

For Your Sponsorship