Experimental Study to Evaluate Smoke Stratification and Layer Height in

Highly Ventilated Compartments

Jason Huczek

a, Marc Janssens

a, Kentaro Onaka

b, Stephen Turner

c

a SwRI, 6220 Culebra Road, San Antonio, TX 78238, USA

b TEPCO, 1 Chome-1-2 Uchisaiwaicho, Chiyoda, Tokyo 100-0011, Japan

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

ABSTRACT

Southwest Research Institute (SwRI) recently conducted a research program focused on the

effect of mechanical ventilation on smoke layer height for two geometric configurations

(corridor and tee) and two fire sources. The primary objective of the research was to define

performance metrics related to smoke layer height for relevant fire scenarios in order to specify a

smoke extraction system suitable for a specific configuration in a building.

A test enclosure was constructed, which consisted of a first floor that was 3 m wide, 10-m long

and 3-m tall, and a second floor that was 3-m wide, 4-m long and 4-m tall. The first and second

floor compartments were connected by a single hatch measuring 1 × 1-m. Mechanical exhaust

was connected to the second floor compartment and exhaust flow rates between 0 and 1.11 m3/s

were evaluated in the tests. Thermocouple trees, each outfitted with seven thermocouples at

various elevations, were positioned at three locations in the test enclosure. The smoke layer

height was calculated based on the vertical temperature measurements at the three locations. It

was also estimated based on video documentation of visibility of an array of exit signs at the

same nominal locations. These measurements are discussed and compared in detail in this paper.

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.

The FDS validation guide [1] indicates that, on average, the model slightly over-predicts the

smoke layer depth (layer depth = ceiling height minus layer height) by only 3% (bias δ ≈ 1.03),

with an uncertainty of ± 5% (relative standard deviation σ�� ≈ 0.05).

However, these estimates are based for a large part on a comparison between model predictions

and layer height measurements in naturally vented compartments. Moreover, the flow rates in

the mechanically vented experiments were relatively low (1 – 10 air changes per hour). The

building owner therefore decided to conduct a series of fourteen full-scale experiments to expand

the validation range of the model to include high ventilation rates (15 – 29 air changes per hour).

FULL-SCALE EXPERIMENTAL DESIGN

Fire Source Development

Two different fire sources were specified by the client. The specified fire sizes had heat release

rate (HRRs) of 106 kW (simulating a “small cable fire”) and 317 kW (simulating a “class A

combustible fire”).

Each fire source consisted of a propane burner with the center removed and through that opening

a liquid spray of toluene was injected into the propane flame. A series of 25 tests were conducted

to refine the two fire sources in terms of the propane/toluene ratio, nozzle size, etc. and a final

eight tests were conducted to obtain repeatable soot yield and heat release rate values for each

fire source.

Table 1 summarizes the final parameters of the fire sources and Table 2 provides a few selected

photographs of the test setup and burners.

Table 1. Fire Source Parameters

Fire HRR

(kW)

Soot

Yield

(g/g)

Relative

Smoke

Production

Rate

(m2/MJ)

Effective

Heat of

Combustion

(kJ/g)

Radiative

Fraction

(-)

Propane

Mix

Fraction

(%)

Toluene

Mix

Fraction

(%)

106 0.0456 9.88 39.9 0.41 27.1 72.9

317 0.0140 2.62 46.6 0.33 92.2 7.8

Table 2. Fire Source Testing – Selected Photographs.

Fire

HRR

(kW)

Overall Test Setup Close-Up View of Fire

Source

Close-Up View of Test in

Progress

106

317

For the 106-kW fire source, the spray nozzle used to discharge toluene was a Hago, 2.00 GPH,

80º, Hollow Cone. The discharge pressure was 9.6 bar and the mass flow rate was 1.91 g/s. For

the 317-kW fire source, the spray nozzle used to discharge toluene was a Monarch, 0.65 GPH,

90º, Hollow Cone. The discharge pressure was 8.6 bar and the mass flow rate was 0.55 g/s.

For the 106-kW fire source, the propane mass flow rate was 0.71 g/s and the heat release rate

was 33 kW. For the 317-kW fire source, the propane mass flow rate was 6.46 g/s and the heat

release rate was 300 kW.

A more detailed discussion of the ignition source characterization can be found in [2].

Experimental Configuration

The experiments were performed in two configurations, referred to as “corridor” and “tee” and

shown in Figures 1 and 2, respectively.

Figure 1. Corridor Configuration

Figure 2. Tee Configuration

Instrumentation

Three measurement stations were positioned downstream from the fire source location. At each

station, there was a thermocouple tree (Type K, 20 AWG wire) and two sets of exits signs

(illuminated and non-illuminated) at two heights, which were viewed by cameras through glass

windows. The cameras were set up to view the exit signs as well as the length of the corridor(s).

The camera locations are depicted in Figure 3.

In the hatch corridor, two types of optical density measurements were taken. A set of seven

commercial beam detectors (System Sensor Model 1224S) and four laser detectors (Edmund

Optics components) were installed to measure optical density, and to a lesser extent, smoke layer

height, in each experiment. The layout of each thermocouple tree as well as the beam and laser

detectors is depicted in Figure 4.

Figure 3. Camera Port Locations (Plan View)

Figure 4. Instrumentation Layout (Elevation View)

The lasers were transmitted across the nominal 10 meters of the hatch corridor and the intensity

was measured with a photodiode receiver that converts intensity to voltage.

The laser sources are red in color and have a wavelength of 635 nm ± 5 nm. The photodiode

receivers are 5-mm diameter Si photodiode detectors with an operating wavelength of 0.1 –

1.1 µm and responsivity of 0.5 × 108 / 10

7 Volts/Watt (depending on gain position).

An in-situ, end-to-end calibration of the laser detector system was completed and the results are

shown in Figure 5. The transmittance was measured after placing neutral density filters ranging

from 0.5 to 3.0 OD at each laser beam height. The percent error between average measurements

and neutral density filters is provided in the table and ranges from approximately 4 to 14%, but is

very low over the range of interest for the compartment test measurements.

Figure 5. Calibration of Laser Detector System in Hatch Corridor (10-m Pathlength)

Compartment Test Matrix

Table 3 provides the details of the compartment test matrix.

Table 3. Compartment Test Matrix

Test ID HRR

(kW) Configuration

Flow

m3/s (ACH*)

Discussion

Preliminary Tests

1 106 Corridor 0.56 (15) Initial Tests to checkout

instrumentation 2 317 Corridor 0.56 (15)

S-1 106 Corridor 0-0.83

(0-22)

0 m3/s,0.28 m

3/s, 0.56 m

3/s,

0.83 m3/s, 7-10 minutes each

S-2 106 Corridor 0.83 (22)

0.56 (15)

Picked two flows from S-1, 15

minutes each

S-3 317 Corridor 0.56-1.11

(15-29)

0.56 m3/s, 0.83 m

3/s, 1.11 m

3/s,

10 minutes each

S-4 317 Corridor 1.11 (29)

0.83 (22)

Pick two flows from S-3, 15

minutes each

Final Tests

FS-1 106 Corridor

0.83 (22)

0.56 (15)

Cable fires, 106 kW

Corridor and Tee

Ventilation based on S-2, two

flows, 15 minutes each

FS-2 106 Corridor

FS-3 106 Corridor

FS-4 106 Tee

FS-5 106 Tee

FS-6 106 Tee

FS-7 317 Tee 1.11 (29)

0.83 (22)

Class A fires, 317 kW, Tee

Ventilation based on S-4, two

flows, 15 minutes each

FS-8 317 Tee

FS-9 317 Tee

FS-10 106 Tee 0.83 (22)

0.56 (15) Rerun of FS-6

*ACH=air changes per hour, based on total volume of test structure

FULL-SCALE TEST RESULTS

Table 4 provides a summary of the results for a few of the final tests (FS-1, FS-4, and FS7),

including temperatures and optical densities measured at 12.5 minutes after flow is set for each

test. Tables 5-7 provide selected graphical results for test FS-1, FS-4 and FS7, respectively.

Table 4. Summary of Selected Final Test Results (FS-1, FS-4 and FS-7).

Test

ID

Flow

m3/s

(ACH)

Amb.

Temp.

(̊C)

Tree Temp, 2.7m

(̊C)

Duct

Temp

Optical Density at

height in meters

Tr 1 Tr 2 Tr 3 ̊C 2.3 1.9 1.5 1.1

FS-1 0.83 (22)

20.2 29.2 63.0 114.7 49.3 1.97 1.07 0.3 1.22

0.56 (15) 68.6 84.2 128.5 58.3 3.40 1.76 1.23 1.57

FS-4 0.83 (22)

21.6 33.4 77.1 104.4 57.9 1.87 0.98 0.73 0.50

0.56 (15) 76.4 97.7 124.8 69.3 2.14 1.21 1.02 0.85

FS-7 1.11 (29)

14.9 139.6 164.4 208.5 126.9 1.38 0.50 0.56 0.32

0.83 (22) 171.6 207.1 252.3 144.9 1.66 0.72 0.60 0.39

ACH=Air Changes per Hour, Amb=Ambient, Temp=Temperature, Tr=Tree

Table 5. Test FS-1 Results

Table 6. Test FS-4 Results

Table 7. Test FS-7 Results

A few general test observations can be made from the data. At higher ventilation rates, a clear

layer was observed in both corridors. At lower ventilation rates, a clear layer was observed in

the fire corridor, but mixing was observed in the hatch corridor.

At lower ventilation rates, multiple layers of smoke were observed: an area of darker smoke at

the top above approximately 2-2.5 m, a layer of lighter smoke with a depth of approximately 0.5-

1 m, and a clear layer below that elevation. This was consistently observed for all the tests.

The measured optical densities for the simulated cable fires were higher than for the simulated

class A combustible fires, as expected due to the higher soot yield for the former. The upper

layer gas temperature for the simulated cable fires were lower than for the simulated class A

fires, as expected due to the higher HRR for the latter.

Test Photograph Arrays and Exit Sign Comparisons

Still images from the cameras used during testing were collected for comparison to the FDS

visualizations in Smokeview. Figure 6 shows an example of this type of photograph array for

FS-1. Table 8 provides a comparison between FS-1 and FDS calculations for the same scenario

and Tables 9-10 provide comparison images between the FS-1 and FS-4 experiments and FDS

output. The details of the FDS modeling for this project are discussed in [3].

Figure 6. FS-1 Photo Array (Top: 0.83 m

3/s, Bottom: 0.56 m

3/s).

Table 8. FS-1 Visibility and FDS Results for Exit Sign Position No. 3

Parameter Test Series

FS1

FDS Result

(S2, FS1-FS3)

Estimated Smoke Layer Height (m) at 0.83 m3/s 2.0 1.6

Estimated Smoke Layer Height (m) at 0.56 m3/s 1.9 1.4

2.3-m Illuminated Exit Sign Visible at 0.83 m3/s? Yes OD = 0.505 1/m

2.3-m Reflective Exit Sign Visible at 0.83 m3/s? No OD = 0.505 1/m

2.3-m Illuminated Exit Sign Visible at 0.56 m3/s? No OD = 0.402 1/m

2.3-m Reflective Exit Sign Visible at 0.56 m3/s? No OD = 0.402 1/m

1.5-m Illuminated Exit Sign Visible at 0.83 m3/s? Yes OD = 0.023 1/m

1.5-m Reflective Exit Sign Visible at 0.83 m3/s? Yes OD = 0.023 1/m

1.5-m Illuminated Exit Sign Visible at 0.56 m3/s? Yes OD = 0.082 1/m

1.5-m Reflective Exit Sign Visible at 0.56 m3/s? Yes OD = 0.082 1/m

Table 9. FS-1 and FS-4 Visibility and FDS Results for Exit Sign Position No. 3

Test

ID

Test Result FDS Calculation

0.83 m3/s

FS-1

FS-4

Table 10. FS-1 and FS-4 Visibility and FDS Results for Exit Sign Position No. 3

Test

ID

Test Result FDS Calculation

0.56 m3/s

FS-1

FS-4

Comparison of Observed Smoke Layer Heights and FDS Calculations

Figure 7 shows an example of layer height comparisons between experiments and FDS for both

configurations and one of the fire sources. The first 900 seconds of each graph corresponds to an

exhaust rate of 0.83 m3/s and the second 900 seconds of each graph corresponds to an exhaust

rate of 0.56 m3/s.

Figure 7. Selected Smoke Layer Comparison Results.

CONCLUSIONS

The main conclusions from this research are as follows:

• The agreement between observed layer heights and calculated layer heights is better in

the fire compartment, than in the adjoining compartments, which are closer to the

mechanical exhaust extraction point. The mechanical exhaust introduces significant

mixing between the layers, which greatly impairs the ability to use the thermocouple tree

data to estimate the layer height close to the extraction point.

• The mixing effects appear to be more significant for the corridor configuration than the

tee configuration. This is likely due to the closer proximity to the exhaust hatch of the

fire in the tee configuration, as compared to the corridor configuration.

• The smoke layer height based on visual observations is generally and often significantly

higher than the HGL height determined from vertical temperature profile data.

ACKNOWLEDGEMENT

The authors would like to express their gratitude to TEPCO for the financial support that made

the work described in this paper possible.

REFERENCES

1. McGrattan, Kevin, et. al., “Fire Dynamics Simulator Technical Reference Guide Volume 3:

Validation”, NIST Publication 1018, National Institute of Standards and Technology,

September 30, 2014.

2. Huczek, J., et al., “Full-Scale Fire Tests to Obtain Optical Density Data for Validation of

Compartment Fire Models,” Interflam 2016, London, England, July 4-6, 2016.

3. Janssens, M., et al., “Use of FDS to Comply with Performance-Based Requirements Based

on Smoke Layer Height,” 11th

Conference on Performance-Based Codes and Fire Safety

Design Methods, Warsaw, Poland, May 23-25, 2016.