Heat Transfer to I~ Sprinkler g 4"
Sprinkler Link |~” - On? No |" =4o
Convective
Y
/| Enthalpy .
Heat Transfer to

Radiative
Energy

Prescribed

Yes

Fuel Elements

Ignition?

Burning?

1, An overview of the calculation scheme.



PAGE 1 OF 2

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Reduced-Scale Experiments to Characterize the Suppression of Rack-Storage Commodity Fires

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A series of experiments are described that investigate the burning and water suppression of rack-storage
commodity fires. The objective of the research reported here is to support the NIST Industrial Fire Simulator
(IFS), a computational fluid dynamics model. The IFS model predicts fire growth, sprinkler activation, and
suppression by water. The model requires appropriate and implementable sub-grid algorithms that adequately
represent the full-scale heat and mass transfer that occurs in a warehouse fire.

Several types of experiments were conducted. These include ignition measurements using the LIFT apparatus and
the cone calorimeter, and heat release rate measurements using oxygen consumption calorimetry. All of these
measurements were made with and without water application.

KEY WORDS (MAXIMUM OF 9; 28 CHARACTERS AND SPACES EACH; SEPARATE WITH SEMICOLONS; ALPHABETIC ORDER; CAPITALIZE ONLY PROPER NAMES)
burning rate; computer models; fire growth; fire suppression; ignition; sprinklers

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PAGE 2 OF 2


to the fuel density, fuel specific heat, ignition temperature, heat of combustion, total burning rate,
heat of pyrolysis, and average water application rate. Yu et al. [1994] showed that the coefficient
k can be correlated as a linear function of the water application rate for a particular period (240 s)
from the time of water application. The values of k varied for the two types of commodities
tested using Yu’s model.

Unfortunately, Yu’s model cannot be directly applied in the FDS calculation. Yu’s model is a
global suppression model that predicts the effect of the water application rate on the total heat
release rate, but it does not predict the local heat release rate, nor the heat release rate without
water application. The FDS model requires a burning rate algorithm applicable to all times, with
and without water. This information is provided by the reduced-scale experiments described
below. Although Yu’s model (Eq. 10) is not directly applicable to the FDS methodology, we
seek a functional form for the local heat release that is analogous to Eq. 10, which relates the
local heat release rate with the local water application rate. It was originally hoped that this
could be achieved using information from experiments using single or multi-cell samples of the
Group A Plastic Commodity in the cone calorimeter or a mini-furniture calorimeter (50 kW
maximum), but as discussed earlier, those experiments do not adequately replicate the local
thermal environment of a single cell within the full-scale burning commodity. Thus, a larger
scale experiment was designed.

Apparatus and Procedure

The experiments were performed at Underwriters Laboratories. In total, 19 experiments were
conducted. The entire fuel/nozzle arrangement was located below a large exhaust hood that was
instrumented to measure the heat release rate using oxygen consumption calorimetry [ASTM E
1354]. The methodology is appropriate for a burning sample undergoing water suppression
[Dlugogorski, 1994].

The fuel and nozzle arrangement used in these experiments is shown in Fig. 12. It consisted of
four boxes of the commodity. The boxes were stacked two high. The two stacks were positioned
15 cm from each other, creating a flue that had the same separation as that used in the full-scale
tests. A water applicator was positioned above the boxes to deliver a uniform water flux directly
on top of the boxes. The applicator was supported such that its four nozzles were 30 cm above
the tops of the boxes. Four nozzles were positioned symmetrically about the center of the flue.
The nozzles were arranged on the corners of a square with 0.6 m sides, providing water coverage
ofa 1.49 m’ area. The nozzles were connected to a 8 cm diameter water line that was insulated
by approximately 1 cm of ceramic fiber insulation. Several nozzle sizes were used, depending

on the desired water flux. Table 4 lists the average water application flux (Mm. a) and the time of

water application. The water application time was varied from 30 s to 200 s, whereas the water
flux was varied from 0.03 kg/m*-s to 0.66 kg/m?-s. These flux values covered the full range of
interest. For example, Yu et al. [1994] found that water suppression was achieved with a flux of
0.077 kg/m?-s, 0.108 kg/m’-s, and 0,139 kg/m?-s in two, three, and four tier fires burning the
commodity, respectively.

12


Heat Release Rate (kW)

i a ne re oe
External Flux = 50 kWim?
Total integrated Heat Release = 1.47; 1. 45 MJ 4

Exposed Area = 0.011025 mm

Jit

2
4
0
e) 4100 200 300 400 500 600
Time (s)

5. The measured heat release rates from two experiments burning single
cells of the Group A Plastic Commodity in the cone calorimeter.

22


flowing over vertical surfaces of the commodity. Section 7 considers the ignition of wet
corrugated paper. Section 8 describes the effect of water on the heat release rate of the
commodity. The experimental apparatus, procedure, and results are described in each section.
Section 9 consists of a brief summary of our findings. Acknowledgements and references are
cited at the end of the report.

2. Group A Plastic Commodity

The fuel selected for testing was the Group A Plastic Commodity’, obtained from Underwriters
Laboratories [Sheppard, 1998]. The fire protection industry commonly uses this fuel to test the
effectiveness of water sprinklers and other fire protection devices [Sheppard, 1998]. It isa
multi-component fuel composed of compartmentalized unexpanded polystyrene cups in
corrugated paper cartons. Eight cartons loaded on a wood pallet comprise one pallet-load of the
Commodity. Each carton is 0.53 m wide by 0.53 m deep by 0.53 m high. Vertical and horizontal
cardboard dividers are used to form the

125 cells ina 5 x 5 x 5 array. Within each cell is one polystyrene cup. Each cup weighs 36 g and
has a 0.45 L capacity. Table 1 lists the mass and the exposed surface area of the constituents of
one (conditioned) palette of the commodity obtained from Underwriter Laboratories (UL). The
uncertainty listed in Table 1 is an estimate of the combined standard uncertainty (0), which is
dominated by instrument reading accuracy. The mass of the constituents of the Group A
Commodity reported by Factory Mutual (Yu et al., 1994) differs by as much as 20% from the
values shown in Table 1. All materials used in the experiments described in this report were
obtained from UL. Table 1 shows that the corrugated paper has the largest exposed surface of
the pallet components, and its burning dominates the early fire behavior. Observation indicates
that first the outer shell burns and that soon thereafter, the polystyrene cups become involved.
They begin to deform and eventually ignite. Subsequently, the cups melt and drip, and burning
proceeds to the next cell.

Table 1. Components of one pallet load of the Commodity procured from UL.

Component Mass Exposed Surface Exposed Vertical Surface in Flue
(kg) (m’) (m’)

Corrugated Paper: Shell 8+0.4 520.1 140.05

Corrugated Paper: Internal | 12+0.4 0 0

Polystyrene Cups 36+0.1 0 0

Wood Pallet 2441 0.5+0.02 0.1+0.01

Total 80+1.9 5.5+0.1 1.1+0.06

* Certain trade names and company products are mentioned in the text to specify adequately the
experimental procedure and equipment used or to identify types of currently available
commercial products. In no case does such identification imply recommendation or endorsement
by the National Institute of Standards and Technology, nor does it imply that the products are
necessarily the best available for the purpose.



k, (1/s)

0.00
-0.01
-0.02
-0.03
-0.04
-0.05
-0.06
-0.07

-0.08

nn Oe ne a

F 7 8 k1 = m1 * Mw"

E Value Error

L . m1 -1.0209 | 0.12342

E : Chisq | 0.0042866 NA

E . 0.62753 NA
.

br e

FE, Fe Se ee OS OO | po a

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07

" 2
M, (Kg/m*)

16. The value of k; as a function of the water application rate (M",) for Tests 3-19.

33


Time to Ignition (s)

140

120

100

80

60

40

20

“oT Tt 7
E LIFT q
‘- ° cone J
' LIFT non-piloted q
L oo Yq
L ° 1
- i) 4
[ ° ]
L ° e J
[ © e ]

ee a J
ie) 20 40 60 80

Flux (kWim?)

2. The time to flaming ignition of a conditioned sample of
corrugated paper as a function of the incident heat flux.

100


500 TT TT
[ | —-*— Heat Release Rate
_~ [ | —C— Radiometer #1 q
® = 400 || —o— Radiometer #2 q
get
og
Sez
oO = 4
ga 300}
oe
VO r 4
xs 200 f
Qa E
£2 1005
td
04 A po a
ie} 50 100 150 200 250 300 350

Time (s)

8. The measured total radiative loss (Q, ) and the actual heat release rate (0, ) during burning
of a single box of commodity.

25


1.00
0.80

¥ 0.60
2 0.40
0.20

0.00

Time to Ignition (s)

ot an
]
“| 0.042435 q
0.017252
0.8062 4
- — .
— e
= 2 — 4
b L e 4
[ .
va — 71
n n i n n 1 1 1
10 20

30

4, The ratio of the critical to the external (or incident) flux as a function of time to ignition.

21


k, (1/s)

0.0020, me]
[ —— k, = 0.001485 - 0.020468 M," R=0.60 |
L e |
0.0015 + J
0.0010 + |
[ |
0.0005 |} J
0.0000 berate
0.00 0.01 002 003 0.04 005 006 007

" 2
M, (kg/m*)

17. The value of k as a function of the water application rate (/",) for Tests 3-19.

34


11. References

ASTM E-1321-90, Standard Test Method for Determining Material Ignition and Flame Spread
Properties, American Society for Testing and Materials, Philadelphia, PA, 1990.

ASTM E-1354-94, Standard Test Method for Heat and Visible Smoke Release for Materials and
Products Using an Oxygen Consumption Calorimeter, American Society for Testing and
Materials, Philadelphia, PA, 1994.

Babrauskas, V., Appendix A, Fire Protection Handbook (Ed.: A. Cote), is edition, National Fire
Protection Association, Quincy MA, 1997.

Dlugogorski, B.Z., Mawhinney, J.R., and Duc, V.H., Proc. 4th Int. Sym. on Fire Safety Science,
877-888, 1994.

Enright, P.A. and Fleishman, C.M., Fire Tech., 35, 153-169 (1999).
Hamins, A., Kashiwagi, T., Gore, J., and Klassen, M., Combust. Flame, 86, 229-236, (1991).
McGrattan, K., Baum, H.R., Rehm, R.G., Fire Safety Journal, 30, 161-178 (1998a).

McGrattan, K., Hamins, A., Stroup, D., Sprinkler, Smoke, Heat Vent, Draft Curtain Interaction —
Large Scale Experiments and Model Development, NIST Technical Report NISTIR 6196-1,
National Institute of Standards and Technology, Gaithersburg, MD, 1998b.

McGrattan, K., Baum, H.R., Rehm, R.G., Hamins, A., and Forney, G.P., Fire Dynamics
Simulator-Technical Reference Manual, NIST Intemal Report NISTIR 6467, National Institute
of Standards and Technology, Gaithersburg, MD, January 2000a.

McGrattan, K., and Forney, G.P., Fire Dynamics Simulator-User’s Manual, NIST Internal
Report NISTIR 6469, National Institute of Standards and Technology, Gaithersburg, MD,
January 2000b.

Modak, A., Combust. Flame, 29, 177-192 (1977).

Ohlemiller, T.J., and Villa, K.M., Material Flammability Test Assessment for the Space Station
Freedom, Technical Report NISTIR 4591, National Institute of Standards and Technilkgy,
Gaithersburg, MD 20899, 1991.

Sheppard. D.T., Heptane Burner Tests and Commodity Tests, International Fire Sprinkler, Heat
& Smoke Vent, Draft Curtain, Fire Test Project, National Fire Protection Research Foundation
(NFPRF) Technical Report, Batterymarch Park, Quincy, MA, April 1998.

Tewarson, A., Prediction of Fire Properties of Materials, National Bureau of Standards Report #
NBS-GCR-86-521, December 1986.


The cone calorimeter results for burning of PS and corrugated paper are summarized in Table 3,
which lists the values of H, taken from the literature, the measured effective heat of combustion
(%aH,), and estimates of y, determined by dividing the latter by the former. Because H, values
for corrugated paper are not available, data for cellulose was used, as listed in Table 3. This is
considered a reasonable approximation because paper products are mainly composed of
cellulose. Estimates of y, for the box constituents were determined from the ratio of the
measured effective heats of combustion (x. H,) to the literature values of H,. The value of x, for
PS is within 6% of that reported by Tewarson [1987]. The estimate of y. 1. in Table 3 for the
Commodity was determined as a mass-based average of the results for the constituents. The
uncertainty listed in Table 3 is an estimate of the combined standard uncertainty (20) based on
repeat experiments and previous uncertainty analysis of heat release rate calculations in the cone
calorimeter [Enright and Fleishman, 1999]. The total mass of one box of the Plastic Commodity
is 7 kg (see Table 1). The results in Table 3 on 7.4, for each of the commodity’s components
imply that the total integrated heat release rate from one box of the commodity should be
approximately 161 MJ. Eight boxes form a palette, so one palette of the commodity has a
potential energy of ~1.3 GJ or a potential energy density of 1.1 GJ/m’.

5. Effective Radiative Fraction

The radiative loss fraction from a flame (x,) is defined as the ratio of the rate of radiative energy
emitted to the surroundings ( Q, ), to the idealized heat release rate (7 -H.):

%= Q,/ Gi He) (3)
Substituting Eq. 2 into Eq. 3 leads to the following:
w= ta 0,2, (4)

where x, and Q,, are defined in Section 4. We define an effective radiative fraction (7;a) as equal
to the ratio of the radiative fraction and the combustion efficiency (y,):

Ya = Ae! Ya= O, 1 On (5)

The value of ¥,. can be thought of as the ratio of the rate of radiative energy emitted to the actual
heat release rate. This is exactly the input required by the FDS fire model, which explicitly

partitions the actual heat release rate (O,, ) between the rate of radiative energy loss ( QO, ) and the
rate of sensible enthalpy convected by the fire plume (9, ). In other words, the sum of the rates
of radiative and convected energy is the actual heat release rate (O,, ):

0, = 2, + Q, ©

An effective sensible enthalpy fraction (yp,) can be defined, which is equal to the ratio of the


——— Representative
-O— Test 1
—-©— Test 2

1400

1200

1000

TO]

800

Q (kW)

600

400

200

Sr “hatter

0 100 200 300 400 500
Time (s)

14. The measured heat release rate without water application (O°) measured in Tests 1 and 2.

31



60 pe} Nee a a
horizontal configuration
10 cm x 10 cm sample

a

Ignition Time (s)

idealized behavior
4. uniform water application
2. complete water evaporation before ignition

10

pee be tere da ti

0 DO
0.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014

|

Water (g/cm?)

11. The ignition time as a function of the mass of water applied per unit sample area for an
incident flux of 20 kW/m? and 25 kW/m’.

28


longer preheat times than piloted ignition. In this study, piloted ignition is assumed to be
representative of the warehouse fire problem.

Figure 3 shows the reciprocal of the time to ignition as a function of the external flux for the
same data as shown in Fig. 2. The critical flux (g_,,, ) is found by fitting a straight line to the data
and noting at what flux the time to ignition becomes infinite. [Ohlemiller and Villa, 1991]. From
Fig. 3, the critical flux was determined to be approximately 14.5 kW/m?. This value was then
used to characterize the thermal properties of the corrugated paper sample. In Figure 4, the ratio
of the critical flux to the external flux is plotted as a function of the time to ignition. Such a plot
allows interpretation of a sample’s flammability parameters in terms of a thermally thin analysis
[Ohlemiller and Villa, 1991]. A fit to the data in the form shown in Fig. 4 yields an exponential
term whose coefficient is equal to the ratio of an effective heat transfer coefficient, h, to the
lumped thermal parameter, c,p8, which is the product of the effective specific heat, density, and
thickness of the material. Using the methodology outlined by Ohlemiller and Villa [1991], the
ignition temperature was estimated as 370°C and the value of h was estimated as 0.042 kW/m?-
K. Heat flux measurements during moderate-scale fire tests indicate that fluxes as high as 90
kW/m? to 110 kW/m? occur on the vertical box surfaces. Therefore, in Fig. 4, only data for
fluxes greater than 40 kW/m? were used to determine cpp, leading to a value of 0.98 kJ/m?-K.
The results in Fig. 4 show that the high flux data are adequately fit using this approach.
Treatment of the material as thermally thick also yields an adequate fit, but for reasons of
computational efficiency and simplicity, the thermally-thin approach was used.

4. Heat Release Rate of a Single Cell and its Components

A series of reduced-scale experiments was undertaken to characterize the effective caloric value
of a single cell from the commodity and its constituents. The cone calorimeter was used to
determine the effective heat of combustion of samples of the polystyrene cup, corrugated paper,
and a single cell sample of the Group A Plastic Commodity. A cell consisted of a single
polystyrene cup surrounded on six sides by corrugated paper. These experiments yield insight
into the burning character of the commodity at full-scale, which is composed of multiple cells
(see Section 2 for a description of the commodity).

In a calorimeter, the measured or actual heat release rate (Q”, ) is equal to the product of the mass

burning rate (7) and the evolved heat per unit mass of fuel (Hm). The parameter Hm is also
known as the effective heat of combustion. It should be noted that the measured heat release rate
is not equal to the product of the mass burning rate, # (in units of kg/s), and the idealized heat
of combustion per unit mass of fuel, H, (in units of MJ/kg).

O,, = tit Hoy 0)
These parameters are related, however, through the combustion efficiency (72), which is defined
as the ratio of H,, to H.:

Ya = Hn/H. = O01.) He (2)


Because the cartons are constructed of corrugated paper, their burning behavior is affected by the
directionality of the corrugation. The thickness of the corrugated paper sheet is approximately 4
mm, although observation indicates that it is often compressed to as little as 1 mm. For all
experiments with the sheet oriented in a vertical direction, the direction of the ribs or flues was
maintained vertically, as is the case for the sides of the manufactured box.

3. Ignition

Flame spread along the surface of the carton is controlled by the local incident heat flux and the
thermal properties of the corrugated paper. A search of the fire literature indicates that
information on the ignition of corrugated paper has not been previously documented. In any case,
the thermal properties of one type of corrugated paper may be very different than other types,
due to differences in physical properties such as its internal structure.

Apparatus and Procedure

The time to sustained flaming was measured using a sample of the outer shell of the commodity
oriented in a vertical configuration in the LIFT apparatus. The LIFT test and apparatus has been
described in detail previously [ASTM E 1321]. Similar experiments were conducted using the
cone calorimeter [ASTM E 1354]. Standard test methods were used. As in all experiments, the
samples were conditioned before testing at standard conditions: 50% relative humidity and a
temperature of 20°C. The incident heat flux at the sample surface was measured with a total
heat flux gauge and allowed to equilibrate to a steady value. Unless otherwise noted, a small
pilot flame or electric spark was used as an ignition source. The sample was placed in front of
the radiant source, covered by a thick marinate board. The board was removed and the time to
sustained flaming was measured using a stopwatch.

Results and Discussion

As the conditioned sample of corrugated paper was exposed to an incident flux, it became darker
until a black char layer formed on the surface. Figure 2 shows results from the LIFT experiments
for the time to flaming ignition as a function of the incident heat flux. Figure 2 shows that the
time to ignition increased as the incident flux decreased until a critical flux was obtained below
which sustained flaming was not obtained. For flux levels less than 14 kW/m’, the sample
smoldered, but flaming ignition did not occur before the entire sample smoldered away. For flux
levels less than 20 kW/m’, the sample first smoldered and only later was a sustained flame
observed. Unfortunately, flux levels in the LIFT device are limited. For moderate to high flux
levels (>40 kW/m’), the cone calorimeter was used.

During observation of small, moderate and full-scale experiments using the Plastic A
Commodity, a large number of flaming brands were observed. This implies that the normal
mechanism for ignition in the warehouse fires of interest is piloted, although non-piloted ignition
could occur for example during the phenomena of aisle jumping in an actual warehouse fire. To
study the effect of the pilot flame on ignition, a number of non-piloted ignition experiments were
also conducted in the LIFT. Figure 2 shows that non-piloted ignition required significantly


Yu, H.Z., Lee, J.L., Kung, H.C., Proc. 4th Int. Sym. on Fire Safety Science, 901-912, 1994.


Table 2. Mass, Mass Percent, and Heat Release Rate per Mass of Oxygen for Components
of a Single Box of the Commodity.

Component Mass Mass Ht.”
(kg) Percent (MJ/kg of O2)
Corrugated Paper 2.5+0.1 * 3641 13.19
Polystyrene Cups 4.5+0.01 6441 13.61
Average per box 7.0+0.1 100 13.46

A. Measurements in bold.
B. Babrauskas, 1997.
C. Data for cellulose [Babrauskas, 1997] - data not available for corrugated paper.

Results and Discussion

Figure 5 shows the measured heat release rate during two experiments, when single cells of
Group A Plastic Commodity were burned in the cone calorimeter. The results in the figure
indicate that the measurements were reproducible until ~150 s, with larger variations thereafter.
The combined standard uncertainty (2c) in the heat release rate is approximately +5% based on
previous studies [Enright and Fleishman, 1999]. Differences in the curves in Fig. 5 are attributed
to the stochastic nature of fire phenomena. In Fig. 5, the corrugated paper ignited after
approximately 10 s of exposure to the 50 kW/m’ incident flux, As the front face burned, the heat
release rate rose rapidly. After the front face burned away, the corrugated paper sides of the box
began to burn and the PS cup began to melt. The presence of the PS cup moderated the burning
process, shielding the rear portions of the cell from exposure to radiative flux. After
approximately 40 s, the PS cup ignited and started to burn. Large amounts of smoke and
radiation were then emitted by the fire. The heat release rate increased, leveled-off, and then
gradually decreased. Typically, several grams of PS (from the cup) dripped over the metal frame
in fine strands and large drips. Figure 6 shows the effective heat of combustion (%2 H. or Hm),
for the same data as shown in Fig. 5. The data in Fig. 6 are consistent with the description of the
burning process of the Group A Plastic Commodity. The value of x. H- was initially small (10
MJ/kg to 15 MJ/kg), which is characteristic of the burning of corrugated paper (see Table 3).
Later in the experiment, y, H- increased to ~27 MJ/kg, a value between the H, values for PS and
corrugated paper listed in Table 3.

Table 3. Heat of Combustion of the Constituents of the Commodity.

Component Ae Ya He Ya
(Mi/kg) (Mi/kg)

Corrugated Paper 17° 1422 0.82+0.12

Polystyrene Cups 40 2622 0.65+0.05

Average per box 32 2342 0.71+0.06

A. Babrauskas, 1997.
B. Data for cellulose; data is not available for corrugated paper.



SO PT TT

35 [ External Flux = 50 kWim? o
30 E Ha

FE ©

E ° EF
25 E ( Sp
20

15

chose e teria tipi tira tiiiitiie

10
QO Test #17489

Effective Heat of Combustion (MJ/kg)

O. Test #17490

2 po a)
0 100 200 300 400 500 600 700

time

6. The effective heat of combustion (72 H,), which is equal to the ratio of the measured heat
release rates to the sample mass loss rate for the same data shown in Fig. 5.

23


NISTIR 6439

REDUCED-SCALE EXPERIMENTS TO
CHARACTERIZE THE SUPPRESSION
OF RACK-STORAGE COMMODITY
FIRES

Anthony Hamins
Kevin McGrattan

Building and Fire Research Laboratory
Gaithersburg, MD 20899

NUST

United States Department of Commerce
Technology Administration
National Institute of Standards and Technology


Results and Discussion

Figure 8 shows the measured radiative loss (Q, ) from the two radiometers and the measured
heat release rate (O,, ) during the first 300 s of burning of a single box of the Plastic Commodity.
The combined standard uncertainty (20) in Q, is estimated as approximately +10%. As

mentioned previously, the combined standard uncertainty (20) in Q,, is approximately +5%.
During the experiment shown in Fig. 8, the data acquisition system commenced to acquire
background signal at 0 s. The propane igniter flames were initiated at 60 s and terminated 60 s

later, 120 s into the measurement. The values of Q, (Eq. 5) calculated from the two radiometer

measurements tracked each other very closely. Using Eq. 5 and averaging the Q, data in Fig. 8,
the time varying values for y;, were determined and are shown in Fig. 9. The measurement of ra
was meaningful after 120 s, when the propane igniter flames were terminated. For early times in
the experiment, before the PS cups ignite, the value of y;a is rather small (~0.2). As the cups
burned more vigorously, the fire became more luminous and the value of y,, increased until it
reached a value of nearly 0.44. Repeat experiments yielded nearly identical results. The
combined standard uncertainty (20) in ¥,q is estimated as approximately +15%.

6. Water Flow on the Commodity

In this Section, the impact of water on the ignition and burning rate of the fuel is investigated.
First, measurements were made on the speed of water sliding down a vertical surface. This is
important information because the FDS model must track sprinkler droplets, both in the air and
on the surface of the commodity.

Apparatus and Procedure

A 1.5 mm thick copper plate (25 cm wide by 35 cm long) was oriented vertically. A9 mm
diameter tube with 1 mm diameter holes every 1 cm was used to introduce the water flow into a
triangular trough with an adjustable gap to assure uniform water application over the entire width
of the plate. A cloth was periodically swept across the face of the plate, creating a uniform flow
over the width of the plate. This was necessary because surface tension effects caused breakup
of the water flow into rivulets after a period of 10 s to 20 s. The rate of water flowing down the
vertical surface was controlled with a regulating valve. A small amount of water-soluble white
dye was periodically added to the water flow by a spray bottle to increase contrast with the plate
and facilitate tracking the water’s descent. A video camera recorded the flow.

Results and Discussion

A series of volumetric flows was tested in the range from 7 ml/s to 23 ml/s. Figure 10 shows the
measured water speed as a function of the volumetric water flow normalized by the width (25



enthalpy convected by the plume to the actual heat release rate:
Xpa = Q, / 0, 7)
Dividing Eq. 6 by O,, leads to:

1 = Yra + Xpa (8)

Information regarding the combustion efficiency is not explicitly needed by the model. The value
of xr for the commodity was estimated by measuring Q, and Q,, (see Eq. 5). The value of Xpa

was then calculated using Eq. 8.
Apparatus and Procedure

The radiative flux was measured using a single location measurement with total heat flux gauges
[Hamins et al., 1991; Modak, 1977]. The gauges had a wide view angle (150°) and were coated
with a thin layer of spectrally flat, high absorbtivity paint. Figure 7 is a schematic drawing of the
experimental set-up. Both gauges were positioned approximately 2.6 m (5 box lengths) away
from the fire, beyond the footprint of the exhaust hood. This was done to avoid convective heat
transfer to the gauges and to ensure that the assumption of an isotropic radiation field could be
applied. The gauges were separated by 2 m with one of the gauges positioned directly in front of
the box and the second located 45° away, as seen in Fig. 7. Both gauges were positioned 0.9 m
above the ground. The gauges were calibrated using a high intensity lamp and a gauge traceable
to NIST standards.

Four line-igniters were used to ignite all four sides of the corrugated box simultaneously as seen
in Fig. 7. The igniters provided 0.8 (L/s) of propane, yielding an idealized heat release rate of 65
kW distributed about the four sides of the box. The flames engulfed the commodity and caused
rapid ignition uniformly about the box. Several seconds after establishing the propane igniter
flames, the flame height began to grow as the cartons began to burn. At the same time, the box
surface blackened. The propane igniters were turned off 60 s after ignition. The experiment was
terminated after 300 s.

Assuming isotropic radiation, the radiative flux from the fire to the surroundings was calculated
as:
O,=4nR’ g, @)

where q; is the radiative flux (kW/m?) measured at a distance R from the box center (2.6 m). The

value of %,a was determined using Eq. 5. The actual heat release rate (Q,, ) was measured using
oxygen consumption calorimetry in the NIST furniture calorimeter [ASTM E 1354].


—o— Test 3

-G— Test 4

1400 ;— mr —o— Test 5
--x-- Test 6

1200 |- -+-- Test 7

r a— Test 8

1000 |- -e- Test9

~ 5 -m— Test 10
= 800 --#-- Test 11
= f a Test 12
‘S600 F —*— Test 13
E -O— Test 14

400 E -S— Test 15

- -®- - Test 16

200 r --- Test 17

0 hate. 7 P k +— Test 18

-50 ---- Test 19

Time (s)

13. The measured heat release rate with water application (Q") measured in Tests 3-19.

30



Exhaust hood
and calorimeter

6
dwar 5 4495,
nozzles 0.15 m Jom

i Tom
a

2 propane 4 boxes of
line igniters plastic A commodity

12. The fuel and nozzle arrangement for Tests 1-19.

29


The value of x, cannot be directly measured, but it can be inferred through knowledge of H,,

measurements of Q” and yi, and use of Eq. 2. The value of H, is known for pure fuels (see
Table 3) and can be calculated for multicomponent fuels where the relative mass percentages are
known, such as for the commodity (see below).

Apparatus and Procedure

The cone calorimeter has been described in detail previously [ASTM E 1354]. Three types of
samples were tested. In the first experiment, a flat circular piece (5 cm diameter) of polystyrene
(PS) extracted from the bottom of a single polystyrene cup was placed in a standard holder and
mounted horizontally in the cone calorimeter. In the second experiment, a flat square piece of
conditioned corrugated paper (10 cm x 10 cm) was positioned in a manner similar to the PS. In
the third experiment, a single inverted (bottom-side up) polystyrene cup was placed within a
corrugated paper cell (10cm x 10 cm x 10 cm). This cube was placed within a metal housing that
had only the front face open. A 10 mm gap was provided between the paper sides of the cell and
the metal housing. The front surface of the cell was placed at the standard distance (2.5 cm)
from the cone heater (which was positioned vertically).

The heat release rate of the burning cell measured in the cone calorimeter cannot be extrapolated
to large-scale (multiple cells stacked vertically). This is because the large-scale local
environment of a single cell cannot be simulated using the cone calorimeter. For example,
during the single cell experiments in the cone calorimeter, the external flux was incident only
onto the front face of the sample. While this arrangement may adequately represent the early
burning behavior of the outer cell at full-scale, at later times, convective and radiative heating
from below will influence the full-scale burning. A further limitation of a single cell cone
calorimeter experiment is that the applied flux on the sample is too low. Full-scale experiments
in two, three and four tier configurations showed that the heat flux on the vertical surfaces of the
cardboard boxes was typically 90 kW/m? to 110 kW/m’. The inconel heater in the NIST cone
calorimeter is restricted to a maximum flux of 75 kW/m’.

Table 2 lists the mass, mass percent, and the heat of combustion per unit mass of oxygen
consumed (H,/ro) for a single box of the commodity and its constituents. The parameter H, was
defined above and r, is the mass-based stoichiometric ratio of fuel to air. For the calculation of
the heat release rates, the value of H,/ry was assumed to be a constant throughout the experiment,
with a value equal to 13.46 MJ/(kg of O2 consumed). This represents an average of the mass-
weighted values of polystyrene and corrugated paper (see Table 2). This value is only slightly
different (<3%) from the value of 13.1 MJ/kg of O2 assigned typically to uncharacterized fuels. .
The uncertainty listed in Table 2 is an estimate of the combined standard uncertainty (0), which
is dominated by instrument reading accuracy.


titig (s* )

0.12

ee a el
0.10 ° j
0.08 . J
0.06 F- 4
0.04 L critical flux 7q
0.02 7 | 4
0.00 [ ae ee es ee ee A fe oe oe Cee te |
i) 10 20 30 40 50
Flux (kWim?)

3. The data from Fig. 2 replotted to determine the critical flux.

20


1.0

0.8

0.6

0.4

0.2

0.0

i
[ |
[ |
[ igniter off ]
L en” 4

|
Co
ie} 50 100 150 200 250 300 350

Time (s)

9. The calculated value of x,, based on the data shown in Fig. 8.

26


1. Introduction

This report describes a number of reduced-scale experiments supporting the development of the
NIST computational fire model known as the Fire Dynamics Simulator (FDS). The FDS is a
computational fluid dynamics (CFD) model that calculates fire growth and spread in a large
enclosure. A central challenge for the FDS model is to represent fire phenomena like ignition,
flame spread, and water suppression in a way that preserves what is known about the underlying
physics, but is tractable computationally. The objective of the work described here is to provide
measurements from appropriate reduced-scale experiments that can be used within the various
fire related FDS sub-models.

An overview of the FDS calculation scheme is shown in Fig. 1. Fire growth is a process that
couples gas phase heat release and condensed phase mass loss in a positive feedback loop. The
model includes convective and radiative heat transfer to the fuel surface. Ignition is achieved
when the surface temperature exceeds a prescribed threshold. Once ignition is achieved,
Lagrangian particles, here referred to as thermal elements, are emitted from the fuel surface and
release heat at a prescribed rate. The thermal elements generate a buoyancy-induced flow field,
which in turn transports the elements. The energy released by a thermal element is partitioned
into convective and radiative components. The convective component heats the gas, and the
radiative component heats surrounding surfaces. The total heat release rate of the fire is the sum
of the heat release rates of the thermal elements. Sprinklers are activated mainly by convective
heat transfer from the buoyant hot gases. In the model, water from a sprinkler impacts both

ignition and the local heat release rate (denoted asq’ in Fig. 1). These interactions are the focus

of this investigation. In particular, we quantify the effect of water application on (1) the time
required to achieve ignition of the unburned commodity and (2) the heat release rate of the
burning commodity.

The spatial resolution of the model is limited by the size of the domain and the calculation time.
For the application of interest, a single cell of the computational grid is on the order of 10 cm.
Phenomena that occur at smaller scales cannot be modeled directly, but must be described
through sub-models and empirical correlations. The FDS model requires information on the
thermal properties of the fuel, including the ignition temperature, and the local burning rate with
and without water application. It is preferable to obtain this information from experiment rather
than through more detailed sub-models because it saves on computational costs, minimizes the
propagation of compound errors, and because the underlying physics is not well understood. A
detailed discussion of the thermal element model and its application to fire phenomena can be
found in McGrattan et al. [1998a; b]. Details regarding the FDS computational scheme as
applied to the rack-storage problem can be found in the companion reports [2000a; b].

The remainder of this report is broken into a number of sections. Section 2 describes the
commodity in some detail including the thermal and flammability properties of its components.
Section 3 describes ignition measurements using the LIFT device and the cone calorimeter.
Section 4 describes heat release rate measurements using oxygen consumption calorimetry on the
individual components of the commodity (polystyrene, and corrugated paper) and on a single cell
within the commodity. Section 5 describes experiments designed to measure the effective
radiative fraction of the fire. Section 6 describes experiments to measure the speed of water


Table 4. Time and Rate of Water Application.

Experiment | Application Time Total Water Flow _| Average Water Application Flux
No. 6) kes)_[_(epm)_|_ke/m’-s) (epm/i)
il 380 0.98 15.5 0.66 0.97

2 470 0.57 9.0 0.38 0.56

3 65 0.41 6.5 0.28 0.41

4 106 0.41 6.5 0.28 0.41

5 115 0.11 1.8 0.074 0.11

6 122 Oil [18 0.074 0.11

7 150 0.079 1.3 0.053 0.08

8 93 0.11 1.8 0.074 0.11

9 93 0,21 3.3 0.14 0.20
10 110 0.21 3.3 0.14 0.20
i 205 21 33 0.14 ~ 0.20
12 116 0.16 2.5 0.11 0.16
13 63 0.16 2.5 0.11 0.16
14 64 0.28 4.5 0.19 0.28
15 71 0.079 1.3 0.053 0.08
16 62 0.047 0.9 0.032 0.05
17 104 0.047 | 0.9 0.032 0.05 |
18 58 0.079 1.3 0.053 0.08
19 30 0.079 1.3 0.053 0.08

The ignition source was a propane igniter that consisted of two parallel 12.5 mm diameter copper
tubes each 30 cm long. The two tubes were joined by a tee at one end. The tubes were separated
by a 12.5 cm gap. Along the top of each tube, 1 mm diameter holes were drilled every 1 cm. A
small torch was used to ignite the igniter, which was turned on at time zero and turned off

30s later. The propane flow through the igniter was approximately 0.5 L/s, yielding a 40 kW
fire that was nearly 50 cm in height. The igniter flames entirely covered the vertical sides of the
boxes in the flue space. The Commodity rapidly ignited (~5 s) and the flames rapidly grew,
reaching nearly 1.5 m just 10 s after ignition.

Results and Discussion

The heat release rate results of the suppression experiments for Tests 1-19 are shown in Fig. 13.
As expected, the heat release rates were very similar from test to test for the first 30 s of burning.
The heat release rates decreased at the time of water application (see Table 4). After an initial
rapid decrease at the time of water application, the heat release rates often increased slowly with
time. This behavior occurred somewhat in Tests 8, 9, 10, and 16, and more prominently in Tests
13-15 and 17-19. In these experiments, either small water fluxes were applied or the application
times were relatively late in the experiment


The values of a; and b2 are equal to -0.020 m’/kg-s and 0.0015 s", respectively, determined by
the best-fit line shown in Fig. 17. When t<to, then the values of the constants k2, a2, and bz are

defined as zero, insuring that g" is equal to g” in Eq. 11.

The functional form of Eq. 11 is appropriate for characterizing the heat release rate in the Fire
Dynamics Simulator. In the analysis, however, the water flux was assumed to be uniform. In
Tests 1-19, however, the water density was not uniform, but varied spatially. For this reason, the
coefficients in Eqs. 12 and 14 may need modification when implemented in the Fire Dynamics
Simulator, which is discussed in detail by McGrattan et al. [2000a;b].

9. Summary

A series of experiments was conducted to characterize the suppression of rack-storage
commodity fires burning the Group A Plastic Commodity. This work provides information that
adequately quantifies a number of important mass and heat transfer processes that occur in a rack
storage fire involving the Group A Plastic Commodity. The information derived here will be
used in the Fire Dynamics Simulator to predict the growth and suppression of a full-scale rack
storage fire [McGrattan, 2000a;b]. Details of that model can be found in Mcgrattan et al.
[2000a;b]. The efficacy of the results presented here should be judged in terms of the success of
that model in simulating the fire growth, spread, and water suppression in full-scale rack-storage
experiments.

10. Acknowledgements

Thanks are due to Steve DeGiovanni of the University of Maryland and Dr. Jukka Hietaniemi of
VTT Building Technology, Finland for many helpful discussions and for assistance with some of
the measurements. The authors are also grateful to Lauren DeLauter, Gary Rodarmel, Roy
McLane, Randy Shields, and Michael Smith of NIST and Dave Sheppard of U.L. for assistance
with the measurements.

15



Abstract

A series of reduced-scale experiments were conducted to investigate the burning and water
suppression of rack-storage commodity fires. The objective of the research reported here is to
support the NIST Fire Dynamics Simulator (FDS), a computational fluid dynamics model that
endeavors to predict fire growth, spread, sprinkler activation, and suppression by water in rack-
storage commodity fires. The model requires appropriate and implementable sub-grid algorithms
that adequately represent the full-scale heat and mass transfer that occurs in a warehouse fire
with rack-storage of standard commodities. In particular, this report describes experiments that
investigated the effect of water application on the time required to achieve ignition of the
unburned commodity and on the heat release rate of the buming commodity.

Several types of experiments were conducted. These include ignition measurements using the
LIFT apparatus and the cone calorimeter, and heat release rate measurements using oxygen
consumption calorimetry. All of these measurements were made with and without water
application. The data were analyzed using algorithms that were appropriate and readily
implementable in the FDS. A description of the experimental apparatus and procedures precedes
a description of the experimental results.


Box of plastic Propane

A commodity line igniters
Exhaust ee
hood *
—>
2.6m
‘,
Ms
t Radiometer 2

v

Radiometer 1

7. A plan-view drawing of the experimental arrangement for the measurement of the effective
radiative heat loss fraction of the commodity.

24


NISTIR 6439

REDUCED-SCALE EXPERIMENTS TO
CHARACTERIZE THE SUPPRESSION
OF RACK-STORAGE COMMODITY
FIRES

Anthony Hamins
Kevin McGrattan

Building and Fire Research Laboratory

National Institute of Standards and Technology
Gaithersburg, MD 20899

November 1999

U.S. Department of Commerce

William M. Daley, Secretary

Technology Administration

Dr. Cheryl L. Shavers, Under Secretary of Commerce for Technology
National Institute of Standards and Technology

Raymond G. Kammer, Director


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y= enilMo 2T wik Toe
10 Yaa ard
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08 Gag 9908 cy
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Test 14

Time (5)

10
08
g 08
04
02

00

10

08

06

ago

o4

02

0.0

°
Ye shtshmiPnr owe Test 15
Vee Ere} Fed
i] smn | sins é
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mg [esse ma
° 100 200 300 400 500 600
Time (s)
Fe eho wir AP NO?
van |
E meas |e oer Test 19
s2-[ antes | “7 res
[ma [0755 rm
(anaes ae)

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100 © 200-300
Time (s)

400

15. The normalized heat release rates (q"/q,) and fits using the form given by Eq. 11 for Tests
9, 10, 14, 15, 18, and 19.


BO TTT TT TT TT TT
60 - 4

ze | ig

5

2 40F 4

oO

ey

oO

> b 4
20 - 4
00 |

20 30 40 50 60 70 80 90

Water Flow/unit Width (mi/m-s)

10. The measured water speed and the calculated thickness as a function of the volumetric water
flow normalized by the width (=25 cm) of the plate surface.

27


cm) of the plate. The measurements showed that the average speed of the water along the vertical
surface was approximately 0.54 m/s (+0.03 m/s) with little influence of the total water flow on
the speed.

7. Effect of Water on the Ignition of a Commodity
Apparatus and Procedure

The time to sustained flaming due to application of water on the exposed surface of a
horizontally oriented corrugated paper sample was investigated using the cone calorimeter
[ASTM E 1354]. The sample was positioned horizontally to promote uniform water
evaporation and a uniform heat flux on the sample surface. The experiments were performed
only in the cone calorimeter, because the LIFT device requires vertical orientation of the sample.
The samples were conditioned before testing and the same standard test methods described in
Section 3 were used.

The sample was weighed before water application. Water was applied with a small brush and
allowed to soak into the sample for approximately two minutes. The sample mass was measured
just before testing to avoid significant water evaporation. The sample and cover were placed in
front of the radiant source and the time to ignition was measured after removing the cover.

Results

Figure 11 shows the results from experiments testing the effect of applied water in delaying the
time to flaming ignition of the corrugated paper sample. The figure presents the ignition time as
a function of the mass of water applied per unit area of cardboard for two incident fluxes: 20
kW/m? and 25 kW/m’. In the figure, idealized behavior is indicated by the solid lines and was
based on the assumptions that (1) the added water was completely evaporated before ignition and
(2) the incident heat flux, water application, and evaporation were uniform over the sample
surface. Figure 11 shows that the measured time to ignition appears to be adequately represented
by the idealized behavior. In summary, application of water on an intact portion of the
commodity can significantly delay or prevent ignition, depending on the incident heat flux and
the water application flux.

8. Suppression by Water
Yu et al. [1994] performed a series of large-scale fire suppression tests for the commodity. They

analyzed their results in terms of a global heat balance model, which assumed a form for the total
fire heat release rate after water application as:

Q(t) = Q, -expl-k(t-t.)] (10)

where Q(t) is the total heat release rate following the application of water, Q, is the total heat
release rate at the time of water application to, and k is a fire suppression parameter that is related

1


The heat release rate measurements are well-fit by the following functional form:
F (= 45 O[(exp(kilt-to)))+ ka[t-to]] qty

where q’ (t) is the local heat release rate per unit area, 4’ (t) is the heat release rate per unit area

without water application, and k; and ka are the local suppression coefficients (s'), which are
equal to zero for t<to. As in Eq. 10, ty is the time of water application. The form of Eq. 11 is
similar to the form of Yu’s correlation (Eq. 10), except there is an additional term involving k2,

which represents the observation that gq” (t) slowly increased with time after an initial rapid
decrease in the heat release rate.

The results of Tests 1 and 2 are shown in Fig. 14. In these tests, the water was not applied until
very late in the experiments (> 400 s). These results were used to define the profile of 47 (t),
which is the basis for all conclusions regarding the effect of the applied water flux. For that
reason, the data was smoothed. A constant value was used to represent q. for times greater than

400 s, as indicated by the line in Fig. 14. Following Yu et al., k; is taken as proportional to the
water application rate:

kj =a, M, (12)

where aj is a constant (m?/kg-s) and M]”, is the average water application mass per unit exposed
area of the commodity surface (kg/m’). The value of M" is defined as:

M),= Mj), A/UP (13)

where M uw is the average water flux at the box tops (kg/m?-s), A is the surface area of a single

box top (0.28 m’), Uis a reference speed (0.54 m/s) taken as the water cascade speed (see
Section 6 above), and P is the perimeter along the top of a single box unit (2.12 m). The average

water flux (M wn) is listed in Table 4 for each of the experiments. When M" is equal to zero,

q is equal to g;. Figure 15 shows the normalized heat release rates (q"/q, ) for a number of the
experiments, indicating that the form of Eq. 11 is appropriate. The other experiments are equally
well-fit. Figure 16 shows the value of k; as a function of the water application rate (M’). From

the figure, the best-fit value for a; (Eq. 12) is found to be approximately -1.0. The value of a; is
negative, because the water application decreased the heat release rate.

The value of kz (s"') was also related to the water application rate. Figure 17 shows kz as a
function of the water mass per unit area for all of the experiments listed in Table 4. A straight
line fit to the data yields:

ky =a) M" +by (14)

14