Proceedings of the

5th ASMElJSME Joint Thermal Engineering Conference

March 15-19, 1999 San Diego, California

AJTE99-6148

FIRE SUPPRESSION EFFICIENCYSCREENING USING A COUNTERFLOW CYLINDRICAL BURNER

JIANN c. YANG, MICHELLE K. DONNELLY, NIKKE PRlVE, AND WILLIAM L. GROSSHANDLER

Building and Fire Research Laboratory

National Institute of Standards and Technology

Gaithersburg, Maryland 20899. U.S.A.

tel 301-975-6662; FAX 301-975-4052; jiann.yang@nist.gov

Keywords: Halon Replacement, Agent Screening, Counterflow burner

ABSTRACT

The design and validation of a counterflow cylindrical

burner for fire suppression efficiency screening are described.

The stability limits of the burner were mapped using various

fuel (propane) and oxidizer (air) flows. The stability envelopes

compared favorably with those reported in the literature. The

apparatus was characterized using inert gases (argon, helium,

and nitrogen), and the relative fire suppression efficiency

ranking of these three gases was found to be commensurate

with that from cup-burner tests. For liquid suppression

experiments, a piezoelectric droplet generator was used to form

droplets ( < 100 ~m). Water was used as a representative liquid

suppressant to study the feasibility of using such a burner for

screening liquid agents. Extinction was facilitated with the

addition of water droplets, and the effect of water became more

pronounced when its application rate was increased.

Suppression experiments using water with and without nitrogen

dilution in the oxidizer stream were also performed. Flame

extinction due to the combined effect of water and nitrogen

dilution was demonstrated.

O.S

• • Propane 0.03 Us 0.4 ~ •

• No water application

0

With O.OOSgls of water

OJ [

~N

• •...

• 0.2

0 •

8 0.1 ~

, 4 I 8 •

, 0.0 'oc~

0

100 200 400 6 300 soo 700

(2 Vo I R)blow-ojJ

Figure A-1 Mass fraction of nitrogen added In air as a

function of stagnation velocity gradient at blow-off

This paper was produced in the course of work by the U.S. Government and hence is in the public domain.
Figure 1 Cross sectional view of burner

1971) or non-intrusive (Dreier et al., 1986; Sick et ai., 1990)

probing techniques, thus enabling detailed studies of flame

structure.

EXPERIMENTAL APPARATUS

Burner

The design of the burner is based on several important

criteria. The burner has to be robust, easily built, installed, and

operated, and able to generate reliable screening test data.

The burner is a replaceable porous (20 J.1m)sintered

stainless steel standard Y:z" UNF threaded cup filter with a

length of 3.18 em (1.25"), an i.d. of 1.12 em (0.44"), and an

o.d. (D) of 1.58 cm (0.625"). 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. The burner is screwed onto an

extended insert through which fuel is injected into the interior

of the porous filter and double-pass cooling water runs. The

cooling water is used to cool the burner (to prevent damage to

Fuel

-. Cooling

water

Sintered Insert stainless steel

the porous surface strucfure) and the fuel (to prevent fuel

pyrolysis prior to its ejection through the porous surface). A

cross sectional schematic of the burner interior is shown in

Figure ].

The burner, together with the insert, does not

completely span the entire test section of the wind tunnel. A

cylindrical brass rod (same diameter as the burner) with internal

water 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 rearward 1800 of the burner surface is coated with

a thin layer of high-temperature resistant black paint in order 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, thus the fuel ejection velocity

(VjJ is ca]culated by dividing the fuel volumetric flow rate by

the available fuel ejection area of the burner surface.

Propane is used as fuel, and its flow rate is regulated

by a mass-flow controller which is controlled by a personal

computer. A bubble flow meter was used to calibrate the massflow controller.

INTRODUCTION

The recent ban on halon 1301 (CF3Br) production (as a

result of its ozone depleting potential) has resulted in extensive

search for its replacements and alternatives. Potential uses of

liquid agents as replacements have been recently proposed in

several applications (e.g., shipboard machinery spaces, engine

compartments in annored vehicles). The applications of fire

suppression efficiency screening methods constitute an

important aspect of this search process because good screening

methods can facilitate the identification, comparison, and

selection of potential candidates for halon replacement. Most

of the current screening methods (e.g., cup burners) are

designed for evaluating fire suppressing agents that are

delivered in the form of vapor although they have been

occasionally used for evaluating condensed-phase agents (e.g.,

sodium bicarbonate powder, Grosshandler et al., 1994).

Currently there is no apparatus designed specifically for

screening liquid agents; 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 of our

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 amount of liquid agents, and

a way to generate liquid droplets that can be entrained into the

flame. These individual components will be described in detail

in the following sections.

Since a porous cylindrical burner in a counterflow

configuration has been extensively used to study flame

structure (Tsuji and Yamaoka, ]967; 1969; 1971, Tsuji, 1982)

and flame extinction using inert gases (Ishizuka and Tsuji,

198]) and halons (Milne et ai, 1970), it is logical to extend the

application of such a burner configuration to liquid fire

suppressants. In our liquid agent screening apparatus, we make

use of such a burner wherein a diffusion flame is formed in the

forward stagnation region of the burner placed in a uniform

oxidizer flow, with fuel being ejected uniformly from the burner

surface. Other counterflow geometry has recently been used for

extinction studies with water aerosols (Fleming et ai, ]998).

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 (Dixon-Lewis et aL, 1984; Dreier et al., 1986;

Peters and Kee, 1987; Olson and T'ien, 1987; Dixon-Lewis and

Missaghi, 1988; Chen and Weng, 1990; Sick et al., 1990). 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 (Tsuji and Yamaoka, 1969;
Flow Facility

A small-scale vertically upward open-circuit wind

tunnel is used to provide oxidizer (air) flow to the burner. The

tunnel, except the test section, is made of clear polycarbonate

for visual observation of droplet transport toward the burner.

The tunnel consists of a blower, a diffuser, a flow straightener,

a contraction section, and a test section where the burner is

placed in cross-flow. This configuration not only allows for the

delivery of a uniform flow of oxidizer to the burner at a low

. turbulence intensity but also assists in the delivery of liquid

agent droplets to the flame. Figure 2 is a photograph of the

flow facility.

Figure 2 Photograph of flow facility

The air flow in the tunnel is provided by a variablespeed (frequency controlled) blower, whose outlet is connected

to the main part of the wind tunnel via a flexible aluminum tube

and a coupling to convert a circular cross section to a square.

The blower was calibrated using a pitot probe equipped with a

differential pressure transducer capable of measuring up to 133

Pa. In the measurements, the pitot-static tube was placed in the

location where the burner would be mounted. The velocity

profile obtained was relatively flat ( < ± 0.5 %) except in the

region near the walls (boundary layers). Due to the limited

frequency response of the pitot tube, the turbulent 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 turbulent

intensity. The volumetric flow rates are calculated using the

measured average velocities (Va) and the cross sectional area of

the test section.

The diffuser is 30 cm long, with an inlet cross

sectional area of 10 cm x 10 cm and an expansion ratio (based

on areas) of 1:9. The flow straightener consists of a

polycarbonate honeycomb with cell diameter of 0.32 cm

(0.125") and a 50 mesh stainless steel screen with 30.3 % open

area. The contraction section with a contraction ratio (based on

areas) of 9: 1 has an inlet cross sectional area of 30 cm x 30 cm

and is 30 cm long. A square flange, which was machined to

have a smooth transition passage to minimize flow separation,

is placed between the outlet of the contraction section and the

inlet of the test section. The test section has a cross sectional

area of 10 cm x 10 cm and a length of 20 em. It is made of

anodized aluminum with three PyrexGll observation windows

(12 cm x 7 cm x 0.64 cm) flushly mounted on the three walls of

the test section using high-temperature silicone RTV0. The

burner is inserted through the fourth wall. The combustion

products from the burner are vented to an exhaust hood.

Droplet Generator

One of the most important steps in the liquid

suppressant screening is to be able to deliver the liquid agent in

droplet form to the flame zone to cause suppression or

extinction. A droplet generator is used in the proposed

screening in lieu of a spray because all the droplets, in

principle, can be directed to the flame zone, thus minimizing

droplet loss to the wall of the flow facility caused by the fan-out

of a spray. In addition, the application of a spray may require

more agent to perform a test than using a droplet generator,

which may not be ideal or feasible for some potential new

liquid agents that can only be synthesized in minute quantity.

The droplet generator is based on the following wellestablished liquid jet break-up theory. According to Rayleigh's

analysis of the instability of capillary jets, the frequency f for

maximum instability is given by the following equation (Bayvel

and Orzechowski, 1993):

(1)

where Uj is the jet velocity and dj is the jet diameter. When the

jet is perturbed at this frequency, uniform sized droplets with

uniform spacing are formed. Rayleigh's analysis is based on an

inviscid liquid jet. Experimentally, mono-dispersed droplets

can be generated within a range of frequencies (Schneider and

Hendricks, 1964):

lCertain 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.
Depending on the droplet generator design, an extension of the

above frequency range has been reported (Warnica et al.,

1991). The droplet diameter, Dd, resulting from the controlled

jet break-up can be calculated from the jet velocity and the

imposed disturbance frequency using a simple mass balance,

assuming the droplet mass is equivalent to that of a jet cylinder

oflength Uj If and jet diameter dj.

(a) Enveloped flame (b) Wake flame

Figure 3 Examples of two flame modes

When the air velocity is very small and the fuel velocity

is large, the flame zone becomes thicker, and an inner luminous

yellow 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.

Figure 4 shows the various flame stability regions of the

burner obtained from the test facility. The abscissa is expressed

as 2VdR (R being the burner radius) because this term represents

the stagnation velocity gradient (Tsuji and Yamaoka, 1967) and

has the unit of strain rate (S-I). Each data point in solid circle was

obtained by increasing the fuel flow rate at a fiXed oxidizer flow

until a luminous yellow zone appeared. The conditions above

these data points represent the existence of a yellow luminous

zone. The data points in solid triangles represent the blow-off

limit. Each data point was obtained by increasing the oxidizer

flow at a fixed fuel ejection velocity until blow-off occurred. The

conditions below the blow-off limit indicate the existence of a

wake flame. However, the oxidizer flow will eventually reach a

limiting or critical value as the fuel injection velocity increases.

From Figure 4, this critical blow-off (2 Vol R) critical is- 615 S·I.

RESULTS AND DISCUSSION

Validation of Burner Performance

The first step to evaluate the test facility was to ensure

that the burner functioned properly. This was achieved by

mapping out the flame stability diagram of the burner and by

observing flame behavior consistent with that described in Tsuji

and Yamaoka (1967; 1969). There are two important parameters,

fuel ejection velocity (Vfi and oxidizer velocity (Vo) in the wind

tunnel, that govern the performance of the burner.

Under certain flow conditions, a thin, laminar, twodimensional blue flame is established at a distance in front of the

cylinder surface. An example is given in Fig. 3(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. 3(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 twodimensional flame can no longer be sustained.

(2)

(3)

u·J

< f < 35dj

where Q is the volumetric flow rate of the jet. For a given flow

rate, the droplet diameter can be varied by changing the

perturbation frequency.

In our experimental apparatus, a piezoelectric

transducer driven at a fiXed frequency (10 kHz) was used as a

means to control the break-up of a liquid jet to form uniform

droplets « 100 J.Lm).

The design of the droplet generator is similar to that

described in Yang et al. (1997). The droplet generator consists of

a liquid chamber which is connected to a reservoir, a bleed port

(for eliminating any air bubbles trapped inside the chamber

during priming), a 25 J.Lm sapphire orifice mounted on a set

screw, and a piezoelectric transducer. The initial jet emanating

from the orifice is obtained by pressurizing the liquid reservoir

with nitrogen. Jetting from the orifice can be achieved with very

low nitrogen pressurization (- 0.03 MPa). A 0.5 J.Lm filter is

used in the liquid feedline to minimize clogging of the orifice

opening due to potential foreign particulates in the liquid.

The droplet generator is located in the settling chamber

and is approximately 42 em upstream of the burner. The

presence of the droplet generator in the wind tunnel does not

create any significant perturbation or blockage effect on the

oxidizer flow field near the burner because the flame

characteristics do not change with or without the presence of the

droplet generator in the flow stream. Although unifonn size

droplets with unifonn spacing are observed several centimeters (-

10 cm) from the orifices as a result of controlled jet break-up, the

droplet behavior ·becomes random further downstream, which

may be due to the wake and drag effects on the droplets in the

stream. The air stream in the wind tunnel also facilitates the

dispersion of the single droplet stream into a small droplet cloud.

By adjusting the location of the droplet generator with respect to

the burner, droplet loss to the wind tunnel walls can be eliminated

or minimized because the resulting dispersed droplet cloud is

confmed to a very narrow region near the burner.

Distilled and de-ionized water was used as a

representative liquid agent to evaluate the feasibility of the

proposed screening apparatus. Experiments with nitrogen

addition to the air stream were also performed.
---------------

0.02 0.03 0.04 O.OS

-- Dd=25mm

Vo = 0.14 mls .... _.... Dd * 50 mm

Vd=Omls Dd~ 75mm

0.01

------

0.14

0.12 0.10 0.0& i 0.06

~

0.04

.0.02 0.00 .0.04 0.02

sectional area of the chamber. If the initial droplet velocity is 0.00

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. The desirable

droplet size for the experiments can be estimated by using the

equation of motion for a droplet.

where m is the droplet mass, Vd is the droplet velocity, t is the

time, ~o is air viscosity, h is the drag factor, g is the

gravitational acceleration, Po is air density, and Pd is droplet

density. For Re, ~ 105, the drag factor can be obtained from

(Crowe et aI., 1998)

I(s)

Figure 7 Droplet velocity as a function of time for

three Initial droplet diameters

fd =1+O.l5Re,0.687 + 0.0175

(1+425 xl04 Rer-U6) (5)

Using the above two equations, the worse-case

experimental design scenario, which would likely be Vd = 0

and Vo = 05 VO,Critical x (1/9), is examined. The assumption

that the burner is operating at half of the critical velocity is

made, and the air velocity at the settling chamber is one-ninth

of that at the test section (based on the 9: 1 contraction area

ratio). Figure 7 shows the calculated droplet velocity as a

function of time with three droplet sizes. The calculations were

obtained by using V 0, critical = 2.4 mls. Negative droplet

velocity indicates that the droplet is falling and cannot be

entrained upwards by the existing air flow. Since the orifice

size used in the experiments is 25 ~m, the droplets generated

should be less than 75 l1m from Eqs. (1) and (3). In addition,

the non-zero initial droplet velocity and the acceleration of the

air as it travels into the contraction section will facilitate the

droplet delivery into the burner.

Figure 8 shows the mass fraction of water added as a

function of 2Vo / R at blow-off. Extinction tests were

performed by gradually increasing the air flow until blow-off

occurred at a fixed water flow. The mass fraction of water was

calculated by

where Re ,_- Dd IVo - VdlPo

~o

mH,O

YHp = mH,O + mo

(6)

the flame with water droplet addition than without. For low

water droplet mass loading, higher air flow is required to cause

blow-off. Care should be exercised when higher oxidizer flow

is used because aerodynamic forces of the oxidizer may cause

secondary disintegration of the liquid droplets; this will

inadvertently change the droplet size distribution. The

maximum velocity that can be used to establish a stable

enveloped flame in our current burner is less than 2.5 mls

(based on the critical 2 V0/ R value of 615 s·\); therefore, given

the droplet size used, the aerodynamic effect on secondary

droplet break-up is expected to be insignificant (Bayvel and

Orzechowski, 1993).

The extinction test results for the addition of nitrogen in

the air stream with and without the presence of water droplets are

shown in Fig. 9. The combined effect of nitrogen and water on

blow-off is illustrated. With water addition, the blow-off

stagnation velocity gradient is higher than that with air diluted

with nitrogen. For a given nitrogen dilution, the blow-off

2.50-4

I"

Propane 0.03 Us 2.0e-4

1.5.-4

... 0 ... N ~ 1.0e-4

5.0e·5 0.0e+il

480

52084 GOO S 560 624

(2 Vol R)blo•••ojI

where mH,O is the mass flow rate of water and ';'0 is the mass

flow rate of oxidizer. As shown in Fig. 8, it is easier to blow-off

Figure 8 Mass fraction of water added in air as a

function of stagnation velocity gradient at biow-off

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