Use of FDS to Comply with Performance-Based Requirements

Based on Smoke Layer HeightMarc Janssensa, Jason Huczeka, Kentaro Onakab, Stephen Turnerc

a SwRI, 6220 Culebra Road, San Antonio, TX 78238, USAb TEPCO, 1 Chome-1-2 Uchisaiwaicho, Chiyoda, Tokyo 100-0011, Japan

c Leidos, Inc., 301 Laboratory Road, Oak Ridge, TN 37830, USA

Summary Outline

• Introduction• Calculating HGL Height from Vertical Temperature Profiles• FDS Uncertainty Statistics• Full-Scale Experiments (Motivation, Setup, Test Matrix and Results)• FDS Predictive Capability of Layer Height• Proposal for Demonstrating Compliance with AHJ Requirement• Conclusions• Acknowledgements/Questions

Introduction

• To meet the fire safety requirements in an industrial corridor-like structure, the building owner had to ensure that, in the event of a fire, the smoke layer will not descend below 1.5 m above the floor.

• To comply with this requirement, the building owner decided to provide mechanical ventilation through a ceiling hatch at one end of the structure and use Fire Dynamics Simulator (FDS) to determine the extraction rate that would be needed to meet the performance-based requirement.

HGL Height from Temperature Profile

• The primary assumption of compartment fire zone models is that gases stratify in two uniform layers with temperatures Tℓ and Tu.

• To compare zone model output with room test measurements, the data must be recast to be consistent with the two-layer assumption.

• Janssens and Tran developed a method to determine “equivalent” Tℓ. Tu and HGL height values from measured temperature profiles.

• FDS uses a slight variation to calculate Tℓ. Tu and HGL height.

Example: Test FS1 at 12 min 30 s• From TC Data

• Tℓ = 25.6 °C• Tu = 88.6 °C• zint = 1.93 m

• Calculated by FDS• Tℓ = 22.5 °C• Tu = 92.6 °C• zint = 1.75 m0.0

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ght (

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Test FS1 TC Tree 3

Test FS1 Two-Layer

FDS FS1 TC Tree 3

FDS FS1 Two-Layer

HGL Heightfrom TC Data

FDSHGL

Height

FDS Uncertainty Statistics

• Example from FDS validation guide: HGL temperature rise for fires with forced ventilation.

• Slope of solid red line is equal to the bias, δ (1.22).

• Dashed red lines show model uncertainty, ±2�σM (0.20).

Full-Scale Experiments: Motivation

• The FDS validation guide indicates that, on average, the model slightly over-predicts the smoke layer depth by only 3%, with an uncertainty of ± 5%.

• However, these estimates are primarily based on a comparison between HGL height predictions and measurements in naturally vented compartments.

• Moreover, the flow rates in the mechanically vented experiments included in the uncertainty analysis were relatively low (1 – 10 ACH).

• The building owner therefore decided to conduct 14 full-scale experiments to expand the model validation range to high ventilation rates (15 – 29 ACH).

Full-Scale Experiments: Test Setup

Corridor Configuration Tee Configuration

Full-Scale Experiments: Test Matrix

Test(s) Configuration Fire Size (kW)

Fire Height

(m) Exhaust Flow Rates

(m3/s) ACH

S1 Corridor 106 0.9 0-0.28-0.56-0.83 (8 min ea.) 0-36

S2 Corridor 106 0.9 0.83-0.56 (15 min ea.) 36-24

S3 Corridor 317 0.9 0.56-0.83-1.11 (10 min ea.) 24-47

S4 Corridor 317 0.9 1.11-0.83 (15 min ea.) 47-36

FS1-FS3 Corridor 106 0.9 0.83-0.56 (15 min ea.) 36-24

FS4-FS5 Tee 106 0.9 0.83-0.56 (15 min ea.) 52-35

FS6, FS10 Tee 106 0.6 0.83-0.56 (15 min ea.) 52-35

FS7-FS8 Tee 317 0.9 1.11-0.83 (15 min ea.) 70-52

FS9 Tee 317 0.6 1.11-0.83 (15 min ea.) 70-52

Full-Scale Experiments: Test Results

• The HGL height was calculated based on temperature measurements at the three thermocouple tree locations.

• The smoke layer height was estimated from visual observations at the same nominal locations based on video footage of the exit signs at each location.

• At higher ventilation rates, a clear layer was observed in both corridors.• At lower ventilation rates, considerable mixing made it difficult to determine

the smoke layer height in the hatch corridor.• Detailed results can be found in a companion paper presented here.

FDS Simulations

• FDS 6 was used to simulate the 14 tests that were conducted.• The FDS installation was verified based on the results for a test with forced

ventilation (10 ACH) discussed in the FDS validation guide.• The grid spacing was 0.2 m, except in the vicinity of the fire and the inlet vent

where a finer 0.1 m mesh was used.• D*/δx was 3.9 and 6.1 for the 106 kW and 317 kW fires, respectively.• A grid sensitivity study was performed to justify the spacing for the 106 kW fire.• Some normalized parameters were outside the NUREG-1824 validation range.

FDS Meshes

Corridor Configuration Tee Configuration

FDS Predictive Capability of HGL Height

• δ = 1.11 and �σM = 0.06 based on 44 observations under steady-state conditions.

• Three reasons were identified that could explain why δ and �σM are higher than the values reported in the FDS validation guide (δ = 1.03 and �σM = 0.05).

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icte

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GL

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th (m

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Measured HGL Depth (m)

FDS HGL Height vs. Observed SL Height

• The differences between the FDS HGL depths and the observed smoke layer depths are even larger than the differences between the FDS and measured HGL depths.

• The experimental error is also much higher.

Proposed Approach for Compliance (1)

• Adjusting the FDS HGL depth for the thermal bias of 1.11 still results in a generally conservative estimate of the observed smoke layer depth.

• This is confirmed by the frequency distribution on the next slide, which shows that the adjusted FDS layer depth is smaller than the observed smoke layer depth for only 3 of the 44 observations (two borderline cases, and Test S1 in which layer mixing in the hatch corridor was the most intense).

• This would imply that the probability that the thermal bias adjustment to the FDS HGL prediction leads to a non-conservative estimate of the smoke layer depth is approximately 7% (or less).

Proposed Approach for Compliance (2)

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FDS HGL Depth - Observed Smoke Layer Depth (m)

Relative FrequencyCumulative

Steps to Show Compliance

• Use FDS to calculate the layer height as a function of time for the specified design fire(s) in the compartment, and the mechanical ventilation rate being proposed.

• Adjust the FDS HGL height to account for model bias, i.e., divide by 1.11.• If the adjusted HGL height remains at or above 1.5 m prior to fire brigade intervention,

the design is acceptable.• If the adjusted HGL layer height drops below 1.5 m prior to fire brigade intervention,

either (1) repeat the previous steps with a higher ventilation rate until the design is acceptable, or (2) show that smoke development is relatively low and visibility through the smoke at and below 1.5 m will not exceed an AHJ-established threshold.

Conclusions

• Fourteen full-scale mechanically vented full-scale fire tests were conducted to validate FDS for predicting the HGL depth in room fires with high ventilation rates.

• The bias and uncertainty of FDS in predicting HGL depth determined from this work is slightly higher than the values reported in the FDS validation guide.

• FDS is likely to significantly over-predict smoke layer height.• Implications for egress modeling and manual fire suppression of the systematic

difference between HGL and visible smoke layer heights will be discussed at Interflam 2016.

Acknowledgement

The presenter would like to express their gratitude to TEPCO for the financial support that made the work described in this paper possible.

Thank You!

Questions?