ELSEVIER Fire Safety Journal 36 (2001) 55-72

www.elsevier.com/locate/firesaf

An apparatus for screening fire suppression

efficiency of dispersed liquid agents*

Jiann C. Yang*, Michelle K. Donnelly, Nikki C. Prive,

William L. Grosshandler

Building and Fire Research Laboratory, National Institute of Standards and Technology, 100 Bureau Drive,

Stop 8651, Gaithersburg, MD 20899-8651, USA

Received 16 September 1999; received in revised form 10 July 2000; accepted 17 July 2000

Abstract

The design, construction, demonstration, and operation of a bench-scale device capable of

comparison screening the fire suppression efficiency of liquid agents are described in this paper.

The apparatus is based on a well-characterized flame, a means to facilitate the introduction of

liquid agents, and a way to generate liquid droplets. A porous cylinder in a counterflow

diffusion configuration is used. A small-scale vertical wind tunnel, which allows for the delivery

of a uniform flow of oxidizer to the burner and also assists in the delivery of liquid agent

droplets to the flame, is used for the flow facility. Droplets are generated by a small

glass nebulizer. The performance of the screening apparatus was evaluated using several

liquid fire suppressants with different thermophysical properties. A test protocol is also

proposed. Published by Elsevier Science Ltd.

Keywords: Water additives; Droplets; Diffusion flames; Fire suppression

1. Introduction

The recent ban on halon 1301 (CF 3Br) production (as a result of its ozone depleting

potential) has resulted in extensive search for its replacements and alternatives. The

applications of fire suppression efficiency screening methods constitute an important

aspect of this search process because good screening methods can facilitate the

"'Official contribution of the National Institute of Standards and Technology not subject to copyright in

the United States.

* Corresponding author. Tel.: +1-301-975-6662; fax: + 1-301-975-4052.

E-mail address: jiann.yang@nist.gov (J.e. Yang).

0379-7112/01/$-see front matter Published by Elsevier Science Ltd.

PH: S 0 3 7 9 - 7 1 1 2 ( 0 0) 0 0 0 4 2 - 4
56 J.c. Yang et at. / Fire Safety Journal 36 (2001) 55-72

identification, comparison, and selection of potential candidates for halon replacement. Most of the current methods for fire suppression efficiency screening (e.g., cup

burners) are designed for evaluating fire suppressing agents that can be delivered in

the form of vapor. Potential uses of liquid agents as replacements have been recently

proposed in several applications (e.g.,shipboard machinery spaces, engine compartments in armored vehicles).Therefore, there is a need for the development of a reliable

screening method for liquid agents that can be delivered in droplet form. The objective

ofthis work is to design, construct, and demonstrate a laboratory-scale apparatus that

can perform the screening of liquid agents in a well-controlled experimental setting.

The design of the apparatus is based on a well-characterized flame, a means to

facilitate the introduction of small amounts of liquid agents, and a way to generate

liquid droplets that can be entrained into the flame. The device can also be used to

screen gaseous fire suppressants. In principle, the apparatus can be employed to

screen powders by incorporating a powder delivery system in lieu of a liquid droplet

generator.

In the literature, the experimental non-premixed configurations used for agent

screening applications include: (1) counterflow cylindrical burners; (2) counterflow

flat-flame burners; and (3) cup burners.

A porous cylindrical burner in a counterflow configuration has been extensively

used to study flame structure [1-4] and flame extinction using inert gases [5], halons

[6], and powders [6]. In such a configuration, the burner with fuel being injected

uniformly from its surface is placed in a uniform oxidizer flow, and a diffusion flame is

formed in the forward stagnation region of the burner. There are many advantages

associated with the use of a counterflow cylindrical burner. The fuel and the oxidizer

flows can be independently adjusted, if required. The flame is laminar, two-dimensional, and very stable in the forward stagnation region. The geometry of the burner

and the flow field allow for relatively simple analysis of the forward stagnation region

[7-13]. Both wake and enveloped flames can be easily maintained over a wide range

of fuel and oxidizer flows. The flame is easily observed, and critical stages such as the

blow-off limit (abrupt transition from an enveloped flame to a wake flame) can be

ascertained with ease and high reproducibility. The flame front can be easily accessed

by intrusive [2,3] or non-intrusive [8,13] probing techniques; thus, enabling detailed

studies of flame structure, if desired. However, clogging of the porous burner surface

due to soot deposition and/or condensed-phase suppressant may be a disadvantage if

the burner is operated for a long period of time; cleaning or replacing the clogged

burner may be required [6].

Another counterflow geometry, which involves the use of two opposed vertical

ducts separating at a distance with fuel and oxidizer counterflowing toward each

other to establish a flat flame, has recently been used for extinction studies with solid

aerosols [14,15] and two-phase droplet spray flames [16, and references therein].

Although counterflow flat-flame burners possess all the operational merits of

a counterflow cylindrical burner mentioned above, the operation of these burners is

elaborate.

Cup burners, which are widely used for screening gaseous agents, have also

been employed recently to study the suppression efficiencies of condensed-phase

~; I I
J.c. Yang et al. / Fire Safety Journal 36 (2001) 55-72

Table I

Comparison of the operational characteristics of the three screening apparatusa

57

Co-flow Counterflow Counterflow

cup burner flat-flame cylindrical

burner burner

Gaseous agent screening applications x xx

tion xxx

xx xxxx xxx

xx xx

xxx xx

x xx

x xx

xxx xxxxxx xx

xxx xxx

x xxx

x xxxx

xx xxxx

x xx

Impossible Impossible x

a Degree of difficulty: x - simple; xx - difficult; xxx - very difficult.

agents [17,18]; however, the presence of the cup makes the introduction and characterization of liquid agents difficult. In addition, the global strain rate of a pool flame

established at the cup is not well defined.

Table 1 lists a comparison of the various operational characteristics among the

three screening apparatus: the cup burner, the counterflow flat flame burner, and the

counterflow cylindrical burner. Since the operation of a counterflow cylindrical

burner is less complicated than that of a counterflow flat-flame burner and the

experimental configuration facilitates the introduction of condensed-phase agents

into the oxidizer stream, we make use of such a burner in our liquid agent screening

apparatus.

2. Apparatus

There are three major elements in the apparatus: (1) a wind tunnel; (2) a porous

cylindrical burner; and (3)a droplet generator. These three components will be briefly

described in the following; detailed descriptions and the operation of the apparatus

are provided in Ref. [19].

2.1. Wind tunnel

The wind tunnel is used to provide uniform oxidizer flow to the porous cylindrical

burner at a low turbulence intensity and to facilitate the delivery of liquid agent

droplets to the flame for testing. The wind tunnel is open-circuit (non-recirculating)
58 J.c. Yang et at. / Fire Safety Journal 36 (2001) 55-72

and is oriented vertically upwards. A schematic of the tunnel is shown in Fig. 1. The

total length of the tunnel from the entrance of the diffuser to the exit of the test section

is approximately 1.2m. The tunnel, except the test section, is made of clear polycarbonate for visual observation of droplet transport toward the burner.

The test section has a cross-sectional area of 10cm x 10cm and a length of 20 em. It

is made of black-anodized aluminum with three borosilicate observation windows

Droplet generator

/UContraction section

Settling chamber

Screen + holder

Honeycomb + holder

130 cm

x-v traverse

Diffuser

Fig. 1. A schematic representation of the wind tunnel.

li;1 I II,
J.c. Yang et al. / Fire Safety Journal 36 (2001) 55-72 59

mounted flush against the three walls of the test section. The combustion products

from the burner are vented to an exhaust hood.

The wind tunnel is mounted on a x-y-z traverse mechanism which is used to

position the burner with respect to the stationary optical set-up for droplet characterization at various locations near the burner.

The velocity profile in the test section obtained using a pitot tube was relatively flat

«0.5% variation) except in the region near the walls (boundary layer thickness less

than 1cm). The combined standard uncertainty (uJ in the velocity measurement is

2cm/s. Due to the limited frequency response of the pitot tube, the turbulence

intensity level was not measured; however, the observation of a very stable laminar

flame zone in the forward stagnation region of the burner provided a qualitative

indication of low turbulence intensity. The volumetric flow rates are calculated using

the measured average air velocities (Vo) and the cross-sectional area of the test section.

2.2. Porous cylindrical burner

The design ofthe burner is based on several important criteria. The burner has to be

robust, easily built, installed, and operated, and able to generate reliable screen test

data.

The burner is a replaceable porous (20!lm pores) sintered stainless steel standard

!" UNF threaded cup filter with a length L of 3.18cm, an inner diameter of 1.12cm,

and an outer diameter D of 1.58cm. The advantage of this burner design over those

used in the past is that burner replacement can be easily performed if partial or

complete clogging of the porous burner surface occurs due to the deposition of soot

particles or residue from liquid agents containing dissolved solids. The burner is

screwed onto an extended water-cooled insert through which fuel is injected. The

water is used to cool the burner to prevent damage to the porous surface structure and

the fuel (propane) to prevent fuel pyrolysis prior to its ejection through the porous

surface. A cut-away view of the burner interior is shown in Fig. 2.

The burner, together with the insert, does not span the entire test section of the wind

tunnel. A cylindrical brass rod (same diameter as the burner) with internal water

T

1.58 em

1

Cooling water inlet

(Outlet is on the other side of the insert)

3.18 em -1

Fig. 2. A cut-away view of the burner insert.
60 J.c. Yang el al. / Fire Salely Journal 36 (2001) 55-72

cooling is inserted from the opposite wall and is used as an extension so that the

burner assembly can be treated as a single cylinder across the test section.

The side and downstream 1800 portions of the burner surface are coated with a thin

layer of high-temperature resistant black paint to prevent fuel ejection into the wake

region. The high-pressure drop across the porous sintered surface assures a very

uniform fuel flow over the burner surface.

2.3. Droplet generator

A small glass nebulizer is employed in the screening apparatus to generate a fine

mist of droplets. This type of nebulizer has found applications in inductively-coupled

plasma (ICP) atomic emission spectroscopy and is commercially available. A schematic of the nebulizer is shown in Fig. 3. Aerodynamic break-up of a liquid stream

issued from the capillary by high-velocity air causes the formation of a fine mist of

droplets. Because of the large opening (;:;:;:100f..lm)of the capillary, the nebulizer can be

used with a wide range ofliquids, including those with a relatively high salt concentration. The large capillary opening makes the nebulizer less prone to clogging. Fluid is

fed to the nebulizer by a small, programmable syringe pump. Air is supplied to the

shell of the nebulizer by a mass-flow controller. The resulting mist is entrained

upwards toward the flame by the air flowing in the tunnel. The atomizing air flow is

set at 0.25l/min, which is the highest flow that can be used without disturbing

the flame at the burner. Because of this limit, the atomization efficiency (for all the

fluids reported herein) of the nebulizer may deteriorate when the liquid delivery

Capillary

40mm

Air input (0.25 Umin)

(sidearm)

25mm

I

Liquid (sample)

input

Fig. 3. A schematic representation of the nebulizer.

1«1 I II,'
J.C. Yang et at. / Fire Safety Journal 36 (2001) 55- 72 61

rate is increased beyond 1.3 cm3/min; that is, larger droplets are generated that may

not be easily entrained upward by the air flow in the tunnel.

The nebulizer is mounted in the settling chamber of the wind tunnel and is

approximately 42 cm upstream of the burner. The presence of the nebulizer in the

wind tunnel does not create any significant perturbation or blockage effect on the

oxidizer flow field near the burner because the velocity profile at the test section and

the flame characteristics do not change with or without the presence of the nebulizer

in the flow stream. In addition, deposition of droplets on the wall of the contraction

section was not visually observed.

3. Results and discussion

3.1. Burner characterization

For a given burner diameter, there are only two important parameters, fuel ejection

velocity (Vr) and air velocity (Vo) in the wind tunnel, that govern the performance of

the burner [1,2].

Under certain flow conditions, a thin, laminar, two-dimensional blue flame

is established at a distance in front of the cylinder surface. An example is given in

Fig. 4(a). As the fuel ejection velocity is decreased or the air velocity is increased, the

flame slowly approaches the cylinder surface, and eventually the flame is abruptly

blown off from the stagnation region, and a wake flame, an example of which is shown

in Fig. 4(b), is established. Conversely, with increasing fuel velocity or decreasing air

velocity, the flame zone gradually moves away from the cylinder surface, and eventually a laminar two-dimensional flame can no longer be sustained.

When the air velocity is very small and the fuel velocity is large, the flame zone

becomes thicker, and an inner luminous yellow zone and an outer blue zone appear.

When the air velocity is very large and reaches a critical value, the flame can never be

stabilized, irrespective of the fuel ejection velocity.

Fig. 5 shows the various flame stability regions of the burner obtained from the test

facility. Each data point on the upper curve was obtained by maintaining a fixed fuel

flow and increasing the air flow until blow-off occurred. The fuel ejection velocity is

calculated by dividing the fuel volumetric flow by the available fuel ejection area of

the burner surface, which is equal to nDL/2. The regions below and above the curve

correspond to the existence of a stable enveloped blue flame and a wake flame,

respectively. There is a critical air velocity above which a stable enveloped flame can

no longer be established, irrespective of the fuel flow. This critical blow-off velocity

depends on fuel type and burner diameter [1]. Each data point on the lower curve was

obtained by increasing the fuel ejection rate at a fixed oxidizer flow until a luminous

yellow zone appeared. The conditions below this curve represent the existence of

a yellow luminous zone.

For our proposed liquid screening applications, the fuel flow is always fixed at

2l/min, which corresponds to an ejection velocity of 4.2 cm/s. The selection of 21/min

is partly to eliminate the effect offuel flow on blow-off velocity. In addition, higher fuel
62 J.c. Yang et al. / Fire Safety Journal 36 (2001) 55-72

Fig. 4. Photographs of an enveloped flame (a) and a wake flame (b).

injection velocity reduces heat loss to the burner [1]. However, a higher fuel injection

velocity requires higher air velocity to achieve a stable blue flame (see Fig. 5). The

advantage of having a higher operating air flow is that it facilitates the droplet

transport to the flame. It also enables the use of larger droplets without having them

settled out by gravity. The disadvantage is that a higher air flow results in a higher

global strain rate of the flame, which may not be representative of a fire.

To assess the burner performance due to burner-to-burner variation, blow-off

experiments were performed using four different burners. Based on two independent

repeated observations, the overall coefficient of variation in the measurements of the

air velocities at blow-off is 2%.

11,
J.c. Yang et al. / Fire Safety Journal 36 (2001) 55-72

300

63

250

~ 200

E

.£.

Z- '0 150

o

Qj

>

~ 100

50

o

o

0

Burner #1 • Wake flame

#2

0 ~o Burner #1 F~•

cJfJi

o~o

•

cP Stable blue 0 enveloped flame ()

cs·

08

o •

0 000

o 00 Enveloped flame with 80 luminous yellow zone

1 234 5

Fuel ejection velocity (cm/s)

Fig. 5. Flame stability curve.

6

3.2. The characterization of the droplet generator

Since the droplets are propelled upwards, there are several limitations to the droplet

sizesthat can be used in the experiments. The first limitation is that for a given burner

size, there is a maximum (critical) air flow (as discussed previously) above which the

experiments cannot be performed because an enveloped flame cannot be initially

established. However, this limitation can be easily alleviated by using a larger burner

which results in a higher critical blow-off velocity [1]. The second limitation is that the

droplet generator is located in the settling chamber where the air velocity is low

because of the large cross-sectional area of the chamber. If the initial droplet velocity

is small, it is likely that the droplet (depending on its initial size) will not be entrained

upwards by the low-speed air, and the droplet will eventually fall down due to gravity

before reaching the test section. The desirable droplet size for the experiments can be

estimated by using the equation of motion for a droplet [20]. The calculated results

demonstrate that a 60/lm droplet can easily be entrained upwards by the existing air

flow in the settling chamber.

An Aerometrics1 two-component phase Doppler particle analyzer (PDPA) with

a Doppler signal analyzer (DSA)was used to measure droplet size and velocity distributions. The measurements were made at several positions near the droplet generation

device and near the burner to assessthe uniformity of the small dispersed droplet spray.

1 Certain commercial products are identified in this paper in order to specify adequately the equipment

used. Such identification does not imply recommendation by the National Institute of Standards and

Technology, nor does it imply that this equipment is the best available for the purpose.
64 J.c. Yang et al. / Fire Safety Journal 36 (200f) 55-72

PDP A measurements of the nebulizer spray were taken on the centerline, 2 cm

downstream of the nebulizer exit. Air was supplied to the nebulizer at 0.251/min, and

de-ionized water was used as the calibration liquid. The liquid flow was varied from

0.3 to 1.2cm3/min. Fig. 6 is an example of one set of results taken at a liquid flow of

0.5cm3/min. The nebulizer creates droplets with a range of diameters, as evident in the

diameter histogram in the figure. In the measurements, the nebulizer was pointed

upward in the opposite direction of the vertical velocity prescribed by the orientation

of the PDPA transmitter. In Fig. 6, Channels 1 and 2 represent the vertical and

horizontal droplet velocity components, respectively.

PDP A measurements were also taken on the centerline at the burner location. Since

the experiment protocol requires the blower air speed to increase until blow-off

occurs, it is necessary to determine if such an increase could result in secondary

disintegration of the droplets due to increasing aerodynamic forces on the droplets.

Droplet size measurements were taken at blower air speeds of III and 179cm/s. The

change in blower air speed within the range for the experiments was found to have

a negligible effect on the diameter of the droplets which reached the burner. The

Sauter mean diameter, defined as the ratio of spray droplet volume to droplet surface

area, is in the range of 25- 351lm for all air velocities and water application rates.

There is a slight tendency for the droplet diameter to increase with liquid delivery rate.

The standard uncertainty (based on repeated measurements) is 21lm.

Droplet size measurements were also performed by moving the nebulizer to different off-center locations in the settling chamber to account for possible misalignments,

and this was found to have no effect on the droplet size near the burner.

The droplet number densities were also measured at four locations (1 cm upstream

of the burner): (1) centerline, (2) 0.5cm off centerline, (3) 1.0cm off centerline, and (4)

1.5cm off centerline. There was a ~ 40% variation in the measurements, with the

highest number density on the centerline and the lowest at 1.5cm off centerline.

C:I .. IMAR]099IRUNOI

DATA ACQUISITION

Velocity 2 rn/s

59)

30

o

-13.0

73J

37

o

-]0.0

--

776

5(X)

64

!TI/S

Ikg

(m/s)2

on2

Ice

eels

cc!s/crn2

3:4]:07 = 19.0um

19,9 urn

20.7 um

22.7 urn

),()lE-6

7.95E+3

5.60E-H

9.97E-3

776

500

64

5~5

4g.50 see

pas = (0.00. 0.00. 0.(0)

Attempts

Valid

rk Valid

Corn::clcd

Run lime

IOMAR ]999

Arithmetic Mean (DIO)

Area Mean (D201

Volume Mean(030)

Sauter Mean (D32)

Probe An::a

NumhcrDensity

Vol. Flow Rate

Volume Flux

Analytical pue & Column ND

CHI Velocity Mean = -2.588 m/s

RMS = 2.154 m/s

CH2 Velocity Mean = -0.137 m/s

RMS = 1.076 mls

Vel l.W

Angle 176.98

Shear 1,OE-1

ceoe! 0.046

':lAI~"., , I 2 W ~ ~ ~ Diameter um

J~I"" I -6.5 0.0 6.5 13.0

:i,,,, I

-5.0 0.0 5.0 ]0.0

C

o

U

N

T

C

o

U

N

T

C

a

U

N

T

Fig. 6. Sample measurement of the mist from the nebulizer using the two-component PDPA at the

centerline location, 2cm downstream of the nebulizer exit.
Ie. Yang et al. / Fire Safety Journa/36 (2001) 55-72 65

Since the atomizing characteristics of the nebulizer depend on the physical properties of the fluids [21], different droplet size distributions may result when different test

fluids are used; this could complicate the interpretation of the screening results by

introducing the additional effect of droplet diameter. A series of measurements was

performed using the PDPA to determine the dependence of droplet size on the

physical properties of the test fluids. Several surrogate fluids (water, 30% and 45%

(mass fraction) potassium lactate, and 1 and 2 g/l sodium dodecyl sulfate (SDS))

were used to simulate variations in densities, viscosities, and surface tensions.

Table 2 lists some of their physical properties. Fig. 7 shows the PDPA measurement

results on the centerline, 2 em downstream of the nebulizer exit for liquid flows

Table 2

Physical properties of surrogate fluids at 20GC

Fluid Surface Viscosity· tensionb Density (g/cm3) ± 0.01

(g/scm) ± 0.001

(dyne/cm) ± I

72 0.0101.0066 0.0251.15

68 0.0381.2352 0.00950.9838 0.00930.96

"Measured using a Cannon® Glass Capillary Viscometer.

bMeasured using a DuNouy® Tensiometer (Model No. 70535, CSC-Scientific Co., Inc.).

I

•

Distilled water

0

1 g/L Sodium Dodecyl Sulfate 0

""

30 % K-Iactate 0

\l

2 g/L SDS

•

45 % K-Iactate ""

""

~ •

ij

0 (!)

\l ""

~

~

\l

, ~

•

\l

•

•

•

•

,

,

32

30 ~ 28

Q)

QjE

26 l\l '6t: Q) III 24 •... E

Q) :5 l\l en 22

20

18

0.2 Q3 Q4 Q5 Q6 Q7 0.8 0.9 1.0

Volumetric flow rate (cm3/min)

Fig. 7. Droplet diameter measurements of various fluids using the PDPA at the centerline location, 2 cm

downstream of the nebulizer exit.
66 J.C. Yang et al. / Fire Safety Journal 36 (2001) 55 - 72

between 0.3 and 0.9 cm3/min. In all cases, the Sauter mean diameters only vary

between 20 and 30 flm.

3.3. Screening of liquid agents

The transition from a stable enveloped blue flame to a wake flame, that is the air

velocity at blow-off, is used as a criterion for screening the fire suppression effectiveness of various fire suppressants; the higher the air blow-off velocity, the less

effective the fire suppressant.

There are two ways to perform the screen experiments: (1)increasing the air flow at ,

a fixed liquid agent application rate until blow-off occurs, and (2)increasing the liquid

agent application rate at a fixed air flow until blow-off occurs. The former was selected

because the procedure requires less agent and there is no need to correct for the lag

time from changing the syringe pump setting (to increase the liquid flow) to attaining

a steady liquid delivery rate.

The screening procedure is as follows. A blow-off experiment without agent is first

conducted to check the burner performance, followed by a blow-off experiment with

a fixed agent application rate. This process is shown schematically as the vertical line

in Fig. 8. The blow-off velocities are used to provide a relative ranking of various

liquid agents.

Several test fluids (water, skim milk, 30% sodium iodide, and 30 and 60% potassium lactate) have been used to evaluate the performance of the screening apparatus.

Milk is known to be a fire suppressant [22J, sodium iodide was selected because it

Blow-off occurs

(without agent)

Experimental

pathway

Blow-off occurs

with high agent

application rate

Blow-off occurs

with low agent

application rate

Initial conditions

Stable blue

enveloped flame

Wake flame

Critical blow-off

velocity

Reference

blow-off velocity

4.2 em/s

Fuel injection velocity (cmls)

Fig. 8. A schematic representation illustrating the experimental procedure.

jll I
J.c. Yang et al. / Fire Safety Journal 36 (2001) 55-72 67

240

~ 2200 180

Ul E 160 2-~ 140 '0..QQ) 120 ~0 100

~ I 80 iD0 60

402

o

0.0

g Q

I § El 8 0 •

"

§ : T

T §

•

No agent

0 WaterT

60 % K-acetate

'V

30 Nal

•

Skim milk

0

30 % K-Iactate

• 60 %

0.2 0.4 0.6 0.8

Flow rate (cm3/min)

1.0

Fig. 9. Screening results using different types of fluids.

may be more effective than sodium bromide [23], and potassium lactate has been

demonstrated to be more effective than water [23]. Fig. 9 shows the screening results

using these test fluids. Each data point represents one test. For a given fluid, increasing

the liquid application rate decreases the blow-off velocity. As expected, a mass fraction

of 60% potassium lactate is more effective than 30% potassium lactate. Water is the

least effective when compared to skim milk, 30% sodium iodide, and 60% potassium

acetate. In this set of data, the coefficient of variation from run-to-run using the liquid

screening apparatus was estimated to be better than 20%.

There are many liquid delivery rates that one can use in the screening procedure

described above, and a reference delivery rate is needed to assess the fire suppression

effectiveness of various liquid agents in a consistent way and under conditions

commensurate with a fire. Since cup burners have been used for estimating gaseous

agent concentrations required to extinguish a fire, we have developed the following

protocol, which is based on the conditions commensurate with the cup-burner results

for nitrogen.

The average propane cup-burner value for nitrogen is 32% (mass fraction) [17].

This value corresponds to a blow-off velocity of :::::: 30 cm/s from the experimental

results obtained using the same porous cylindrical burner with nitrogen added to

the air stream [19]. At this velocity and a propane flow of 21/min, a flame cannot be

stabilized in the desired blue enveloped flame region (refer to Fig. 5). In addition, the

experimental protocol calls for increasing the air velocity (i.e., moving away from

30 cm/s) until blow-off at a fixed fluid delivery rate. To compare the results obtained

from the cylindrical burner to conditions commensurate with cup-burner results,

extrapolation to lower air velocity (strain rate) is required.
68 lC. Yang et al. / Fire Safety Journal 36 (2001) 55-72

Note that nitrogen is selected as a reference gas simply due to the availability of

suppression data for the cylindrical burner [19]. Similar reference blow-off velocities

will be obtained when the cup-burner results for other gases are used because at the

same low global strain rate in a counterflow flat-flame burner, the agent extinction

concentrations agree well with the measurements obtained from a cup burner [17].

Fig. 10 demonstrates the proposed extrapolation mechanism for obtaining the

reference liquid application rate. Note that the linear extrapolation process is an

approximation because the relationship between blow-off velocity and agent application rate may not be linear at low blow-off velocities. A blow-off air velocity without

fluid application is obtained, followed by a blow-off experiment with a given fluid

application rate (or a series of blow-off experiments with different application rates).

The fluid delivery rate at an air velocity of 30 cm/s is then deduced by linear extrapolation. Based on our experience, an application rate between 0.6 and 1cm3/min

appears to be appropriate, which is a compromise between minimizing the fluid

consumption for a test and attaining a blow-off velocity close to the reference blow-off

velocity of 30 cm/s.

Once the application rate corresponding to the reference blow-off velocity is

deduced, the reference mass flow rate of the liquid agent, magent, reb can be calculated

using the liquid density. The reference mass fraction of the liquid agent in the air

stream is then

magent. ref

-------;-- , Yagent, ref = m' agen t, ref + mair, ref

(1)

Blow-off with agent B

Blow-off without agent /

I

I

Reference blow-off velocity I ------------+--- I

I

I

I

Agent application rate (cm3/min)

Fig. 10. A schematic representation illustrating the extrapolation for obtaining agent application rates at

the reference blow-off velocity.

I,ll I
J.C Yang et at. I Fire Safety Journal 36 (2001) 55-72 69

where rnair. ref is the mass flow of air, calculated based on the cross-sectional area of

the test section and 30 cm/s.

Note that in writing Eq. (1),it is implicitly assumed that the droplets are homogeneously dispersed in the carrier phase (air). If the droplets are not homogeneously

dispersed across the total cross-sectional area, the calculated agent mass fraction will

be underestimated because rnair . ref is overestimated. The effective area can be considered as the effective coverage area of the mist in the test section. Depending on the

effective coverage area, a difference of a factor of two to three in the calculated liquid

mass fraction can result. By placing a filter paper over the exit of the test section for

a short duration with the wind tunnel operating (without the burner) and the

nebulizer with a dye added to the fluid, the droplet-impact (color) pattern on the filter

paper can be visualized and used as an indicator to determine the mist coverage area

in the test section. The color pattern, which is approximately circular, indicates

that the mist from the nebulizer completely covers the burner and its vicinity. The mist

coverage area was estimated to be ca. 40% of the total cross-sectional area of the

test section for all the fluids used in our screening tests. In addition, the droplet-impact

(color) pattern was concentrated in the center and gradually diffused outward. This

observation was in qualitative agreement with the droplet number density measurements using the PDPA.

Table 3 summarizes the calculations of the reference agent mass fraction in air

using the screening results from Fig. 9, and the proposed approach described

above without mist coverage area correction. Average values of the blow-off velocities

were used in the extrapolation. For cases where blow-off velocities at more than

one liquid application rates are available, linear regressions were used to extrapolate

the reference blow-off velocities. When data with one application rate were available, simple linear extrapolation was applied to obtain the reference blow-off

velocities.

The last column of Table 3 lists the ranking indices relative to water. For example,

the 60% K-acetate and K-lactate solutions are considered to be four times more

effective than water at the reference blow-off velocity. The ranking (60% K-lactate vs.

water) is consistent with the suppression results reported in Ref. [23] using a small

lP-S pool fire and a commercial spray gun.

Table 3

Calculated nominal agent mass fractions at reference blow-off air velocity of 30 cm/s

Agent

Vagenl, ref

Nominal Agent magent, density agent mwater,ref refmass (g/cm3) percent (g/s)at 20°C(%)

--- magent, ref

2.61.00 0.08 4.621.0

0.81.34 0.02 0.994.0

1.31.29 0.04 1.762.0

1.61.01 0.05 2.781.6

1.21.15 0.04 1.74 2.00.61.33 0.02 0.714.0
70 J.c. Yang et at. / Fire Safety Journal 36 (2001) 55-72

Irrespective of the uncertainty associated with the estimated nominal agent mass

concentration, water and the aqueous agents studied here are found to be more

effective than CF 3 Br, compared to the propane cup-burner value (mass fraction of

17% [17J) for CF 3Br. The computational study by Lentati and Chelliah [24J also

demonstrates that 20!lm water droplets are more effective in extinguishing an opposed-flow methane diffusion flame than CF 3Br, which is in qualitative agreement

with the results reported here.

Care should be exercised when interpreting the agent concentrations from the

screening results in Table 3, which were obtained using an idealized laboratory flame

and a droplet delivery system such that the transport of fine liquid droplets to the

flames is not a factor in determining the suppression effectiveness. In the case of real

fires, droplet entrainment and transport to the fire can significantly affect the liquid

agent mass concentration required to suppress a fire, especially in highly obstructed

enclosure fires.

4. Conclusions

An apparatus for screening liquid agents delivered in droplet form has been

developed. The performance of the apparatus has been characterized using fluids with

different thermophysical properties and fire suppression effectiveness. The apparatus

is robust and easy to operate. The droplet delivery system is designed to handle small

quantity of liquid sample; this requirement is critical because potential new liquid

agents may be synthesized and available in minute quantity for testing. For all the test

results reported here, 10cm3 of sample is needed to perform a rapid screen (with at

least one repeat). The apparatus can also be used to screen gaseous agents. When a

powder delivery system is integrated into the current apparatus, the facility, in

principle, can be employed to screen powder agents.

Acknowledgements

The work is supported by the Department of Defense's Next Generation Fire

Suppression Technology Program (NGP), funded by the DoD Strategic Environmental Research and Development Program (SERDP). Dr. Richard G. Gann is the

Technical Program Manager. The authors would also like to thank Drs. Marc

Rumminger and John Widmann for reading the manuscript and providing many

useful comments and the referees who performed a careful review and helped to .•

improve the contents of the paper.

References

[1] Tsuji H, Yamaoka 1.The counterflow diffusion flame in the forward stagnation region of a porous

cylinder. Eleventh International Symposium on Combustion, The Combustion Institute, Pittsburgh,

PA, 1967. p. 979-84.
J.c. Yang et a!. / Fire Safety Journal 36 (2001) 55-72 71

[2] Tsuji H, Yamaoka I. The structure of counterflow diffusion flames in the forward stagnation region of

a porous cylinder. Twelfth International Symposium on Combustion, The Combustion Institute,

Pittsburgh, PA, 1969. p. 997-1005.

[3] Tsuji H, Yamaoka I. Structure analysis of counterflow diffusion flames in the forward stagnation

region of a porous cylinder. Thirteenth International Symposium on Combustion, The Combustion

Institute, Pittsburgh, P A, 1971. p. 723-31.

[4] Tsuji H. Counterflow diffusion flames. Prog Energy Comb Sci 1982;8:93-119.

[5] Ishizuka S, Tsuji H. An experimental study of effect of inert gases on extinction of laminar diffusion

flames. Eighteenth International Symposium on Combustion, The Combustion Institute, Pittsburgh,

PA, 1981. p. 695-703.

[6] Milne TA, Green CL, Benson DK. The use of the counterflow diffusion flame in studies of inhibition

effectiveness of gaseous and powdered agents. Comb Flame 1970;15:255-64.

[7] Dixon-Lewis G, David T, Gaskell PH, Fukutani S, Jinno H, Miller JA, Kee RJ, Smooke MD, Peters

N, Effelsberg E, Warnatz J, Behrendt F. Calculation of the structure and extinction limit of

a methane-air counterflow diffusion flame in the forward stagnation region of a porous cylinder.

Twentieth International Symposium on Combustion, The Combustion Institute, Pittsburgh, PA,

1984. p. 1893-904.

[8] Dreier T, Lange B, Wolfrum J, Zahn M, Behrendt F, Warnatz J. CARS measurements and computations of the structure of laminar stagnation-point methane-air counterflow diffusion flames. Twentyfirst International Symposium on Combustion, The Combustion Institute, Pittsburgh, PA, 1986.

p. 1729-36.

[9] Peters N, Kee RJ. The computation of stretched laminar methane-air diffusion flames using a reduced

four-step mechanism. Comb Flame 1987;68:17-29.

[10] Olson SL, T'ien JS. A theoretical analysis of the extinction limits of a methane-air opposed-jet

diffusion flame. Comb Flame 1987;70:161-70.

[II] Dixon-Lewis G, Missaghi M. Structure and extinction limits of counterflow diffusion flames of

hydrogen-nitrogen mixtures in air. Twenty-second International Symposium on Combustion, The

Combustion Institute, Pittsburgh, PA, 1988. p. 1461-70.

[12] Chen CH, Weng FB. Flame stabilization and blowoff over a porous cylinder. Combust Sci Technol

1990;73:427-46.

[13] Sick V, Arnold A, DieBel E, Dreier T, Ketterle W, Lange B, Wofrum J, Thiele KU, Behrendt F,

Warnatz J. Two-dimensional laser diagnostics and modeling of counterflow diffusion flames,

Twenty-third International Symposium on Combustion, The Combustion Institute, Pittsburgh, PA,

1990. p. 495-501.

[14] Fleming JW, Reed MD, Zegers EJP, Williams BA, Sheinson RS. Extinction studies of propane/air

counterflow diffusion flames: the effectiveness of aerosols. Halon Options Technical Working Conference, Albuquerque, New Mexico, USA, May 12-14,1998. p. 403-14.

[15] Chelliah HK, Krauss RH, Zhou H, Lentati AM. Characterization of physical, thermal, and chemical

contributions of sodium bicarbonate particles in extinguishing counterflow non premixed flames.

Proceedings of the 5th ASME/JSME Joint Thermal Engineering Conference, San Diego, California,

USA, March 1999.

[16] Li Sc. Spray Stagnation Flames. Prog Energy Comb Sci 1997;23:303-47.

[17] Grosshandler WL, Gann RG, Pitts WM. Evaluation of alternative in-flight fire suppressants for

full-scale testing in simulated aircraft engine nacelles and dry bays. NIST SP861, U.S. Department of

Commerce, Washington DC, April 1994.

[18] Shilling H, Dlugogorski BZ, Kennedy EM. Extinction of diffusion flames by ultrafine water mist

doped with metal chlorides. Proceedings of the Sixth Australasian Heat and Mass Transfer Conference, Sydney, Australia, December 9-12, 1996. p. 275-82.

[19] Yang JC, Donnelly MK, Prive NC, Grosshandler WL. Dispersed liquid agent fire suppression screen

apparatus. NISTIR 6319, U.S. Department of Commerce, Washington DC, July 1999.

[20] Yang JC, Donnelly MK, Prive NC, Grosshandler WL. Fire suppression efficiency screening using

a counterflow cylindrical burner. Proceedings of the 5th ASME/JSME Joint Thermal Engineering

Conference, San Diego, California, USA, March 1999.
72 J.c. Yang et al. / Fire Safety Journal 36 (2001) 55-72

[21J Bayvel L, Orzechowski Z. Liquid atomization. Washington DC: Taylor & Francis, 1993.

[22J Rossotti H. Fire. Oxford: Oxford University Press, 1993.

[23J Finnerty AE, McGrill RL, Slack WA. Water-based halon replacement sprays. ARL-TR-1138, U.S.

Army Research Laboratory, Aberdeen Proving Ground, Maryland, July 1996.

[24J Lentati AM, Chelliah HK. Physical, thermal, and chemical effects of fine-water droplets in extinguishing counterflow diffusion flames. Twenty-Seventh International Symposium on Combustion, The

Combustion Institute, Pittsburgh, PA, 1998. p. 2839-46.

~--

t

I, i I II