On the Design of an Apparatus for Screening Liquid Fire 

Suppressants*†

Jiann C. Yang, Michelle K. Donnelly, Nikki Privé, and William L. Grosshandler 

Building and Fire Research Laboratory 

National Institute of Standards and Technology 

Gaithersburg, MD 20899 U.S.A. 

ABSTRACT

This paper discusses the design and use of an apparatus for evaluating the effectiveness of 

liquid fire suppression agents as possible replacements for halon 1301. The apparatus consists of a 

porous cylindrical burner located in the test section of a vertical wind tunnel and a droplet generation 

system. Liquid droplets are injected into the flow stream and entrained upward toward the burner for 

evaluation. The suppression effectiveness is ranked based on the conditions at the transition from one 

mode of flame (enveloped) to the other (wake) established at the burner for a given fluid application 

rate. The apparatus is currently being used to test a variety of fluids. 

INTRODUCTION 

The recent ban on the production of halon 1301 (CF3Br), an ozone depleting substance, has 

resulted in extensive search for replacements and alternatives1,2. 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 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, the development of a reliable screening method for liquid agents that are delivered in 

droplet form is needed. As a part of the U.S. Department of Defense’s Next Generation Fire 

Suppression Technology Program (NGP), we are tasked to design, construct, and demonstrate a 

laboratory-scale apparatus that can perform the screening of liquid agents in a well-controlled 

experimental setting. Although the primary function of the apparatus is to screen liquid agents, the 

experimental facility is designed to be flexible enough to accommodate the screening of gaseous 

agents as well as solid particulates (with the addition of a powder delivery system), if desired. The 

liquid agent screening apparatus consists of a vertical wind tunnel, a burner, and a droplet generation 

device. The design and development of these components will be discussed in detail in the following 

corresponding sections. 

EXPERIMENTAL APPARATUS 

 

* This research is part of the Department of Defense’s Next Generation Fire Suppression Technology Program, 

(co-) funded by the DoD Strategic Environmental Research and Development Program. 

†

 Contribution from the National Institute of Standards and Technology, not subject to copyright in the United 

States.
Wind Tunnel

A small-scale vertically upward open-circuit wind tunnel is used to provide uniform oxidizer flow to 

the cylindrical burner. Air is supplied to the tunnel via a frequency-controlled blower. 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. A schematic 

of the tunnel is shown in Figure 1. The total length of the tunnel from the entrance of the diffuser to 

the exit of the test section is approximately 1.2 m. The tunnel, except the test section, is made of 

clear polycarbonate and polymethyl methacrylate for visual observation of droplet transport toward 

the burner. The test section, with a cross section of 10 cm x 10 cm, is made of anodized aluminum 

with three observation windows. The burner is mounted across the test section, and the droplet 

generator is located in the settling section of the wind tunnel. 

Test section

Thermal insulation

Contraction section

Transition flange

Settling chamber

Screen + holder

Honeycomb + holder

Diffuser

Transition duct

X-Y traverse

Flexible

aluminum ductwork

Screw

jack

Aluminum plate

Aluminum plate

T-slotted aluminum

structural extrusion

Burner

Figure 1. A schematic of the wind tunnel.

Burner

A cylindrical burner operated in a counterflow configuration is selected. This type of burner has been 

extensively used in the past to study flame extinction and suppression using inert gases3

, halons4, and 

powders4 (sodium bicarbonate and Purple K) due to the ease of maintaining a stable flame over a wide 

range of fuel and oxidizer flows and of introducing condensed phase materials in the carrier (air) 

stream. 

The burner is a replaceable porous (20 μm pores) sintered stainless steel standard threaded cup filter 

with a length of 3.18 cm, an I.D. of 1.12 cm, and an O.D. of 1.58 cm. 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 

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 (to prevent fuel pyrolysis prior to its ejection 

through the porous surface). A cut-away schematic of the burner interior is shown in Figure 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 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 downstream 180E portion of the burner surface is coated with a thin layer of hightemperature 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. Propane is used as fuel, and its flow rate is regulated by a mass-flow controller. 

3.18 cm

Cooling water

Fuel (propane)

1.58 cm Cooling water

Fuel Brass insert

Sintered stainless steel filter

High-temperature paint

Figure 2. A cut-away view of the burner and its insert.

Droplet Generation Device

In the early development stage of the screening apparatus5

, a piezoelectric droplet generator was used 

to create liquid droplets (< 100 μm) from the controlled breakup of a liquid jet emerging from a 

sapphire orifice. However, clogging of the orifice constantly plagued the continuous operation of the 

piezoelectric droplet generator, aggravated by liquids with high loading of dissolved salts. Only 

distilled and de-ionized water and a few very dilute aqueous solutions have been successfully tested 

with the droplet generator6. A glass nebulizer is currently 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. A schematic of the nebulizer is shown in Figure 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 differences in the droplet formation mechanisms, 

a relatively large opening (~ 100 μm) of the capillary in the nebulizer, compared to the sapphire 

orifice (~ 30 μm), can be used with a wide range of liquids, including those with a relatively high salt 

concentration. 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 air flow is set at 0.25 l/min, which is the 

highest flow that can be used without disturbing the flame at the burner. The resulting mist is 

entrained upwards toward the flame by the air flowing in the tunnel. 

RESULTS AND DISCUSSION

For a given burner size, there are only two important parameters, fuel ejection velocity and air 

velocity in the wind tunnel, that govern the performance of the burner. Under certain flow conditions, a 

thin, laminar, two-dimensional blue flame is established at a distance from the forward stagnation region 

of the burner. An example is given in Figure 4(a). If the air velocity is increased with the fuel ejection 

velocity fixed, the flame slowly approaches the burner surface, and eventually the flame is abruptly 

blown off from the stagnation region, and a wake flame, an example of which is shown in Figure 4(b), is 

established behind the burner. Figure 5 shows the operational boundary for the current burner. In the 

figure, each data point was obtained by maintaining a fixed fuel flow and increasing the air flow until 

blow-off occurred. The regions below and above the curve correspond to the existence of an enveloped 
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 velocity depends on fuel 

type and burner diameter7

. For our screening applications, the fuel flow is always fixed at 2 l/min, 

which corresponds to an ejection velocity of 4.2 cm/s. The transition from enveloped to wake flames, 

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. 

40 mm

25 mm

Capillary

Liquid (sample)

input

Air input (0.25 l/min)

(sidearm)

Figure 3. A schematic of an ICP nebulizer. 

To demonstrate its flexibility and versatility, the apparatus was first used to screen three inert gases 

(argon, helium, and nitrogen) which were gradually added in the oxidizer stream until blow-off 

occurred. The relative fire suppression efficiency ranking of these three gases was found to be 

commensurate with that obtained from the cup-burner tests5

. 

 

(a) (b)

Figure 4. (a) An enveloped flame and (b) a wake flame.

For screening of liquid fire suppressants, the air flow (with fixed liquid application rate and fuel flow) 

is gradually increased until blow-off occurs. Potassium lactate solutions were used as test fluids to 

evaluate the screening apparatus because these fluids have been found in preliminary testing to 

exhibit better fire suppression effectiveness than pure water8

. Figure 6 shows the air velocity at blowoff as a function of fluid application rate for water, 30 % (by mass) potassium lactate, and 60 % (by 

mass) potassium lactate. Several salient points are noted in the figure. As the fluid application rate 

increases, the air velocity at blow-off decreases. For a given application rate, the blow-off velocity 

for water is higher than the two potassium acetate solutions, indicating water is not as effective as the 

potassium lactate solutions. Based on the average blow-off velocities at 0.3 ml/min application rate, 

the 30 % solution is only about 1.2 times (~ 170 cm/s vs. ~ 200 cm/s) more effective than water, 
whereas the 60 % solution is about 1.5 times (~ 140 cm/s vs. ~ 200 cm/s) more effective than water. 

The 60 % potassium lactate solution is about 1.2 times (~ 140 cm/s vs. ~ 170 cm/s) more effective 

than the 30 % solution. 

The tests reported in Ref. [8] using a small JP-8 pool fire and a commercial spray gun indicate that the 

60 % potassium lactate solution (applied at room temperature) is about four times more effective than 

water based on the average extinguishment times (2.4 s vs. 9 s) of the fires. However, the fires and 

the sprays used in Ref. [8] were not well controlled and characterized. 

Fuel ejection velocity (cm/s)

0123456

Air velocity (cm/s)

0

50

100

150

200

250

300

Burner #1

Burner #2

Fuel: Propane

Stable enveloped flame

Wake flame

Figure 5. Flame stability diagram of the burner. 

Our initial measurements of droplet size distributions at 2 cm above the nebulizer on the centerline 

using a Phase Doppler Particle Analyzer (PDPA) indicate that the Sauter mean droplet diameters 

(based on volume of the droplets divided by surface area of the droplets) vary only between 20 μm to 

30 μm, irrespective of application rates (0.3 ml/min to 0.9 ml/min) and the fluids used here; this 

implies that the droplet size (when different fluids are used) is not influencing the ranking of fire 

suppression effectiveness in our screening apparatus. Figure 6 also demonstrates the ability of the 

apparatus to perform screening using only a small amount of fluid (less than 15 ml); this requirement 

is critical because potential new liquid agents may be synthesized and available in minute quantity for 

testing.
Fluid application rate (ml/min)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

Air velocity at blow-off (cm/s)

0

25

50

75

100

125

150

175

200

225

250

H2O

30 % (w/w) K-lactate

60 % (w/w) K-lactate 

Fuel: Propane

Figure 6. Blow-off velocity as a function of application rate for three fluids.

CONCLUDING REMARKS

An apparatus for screening liquid fire suppressants has been developed. The device is easy to 

operate and only requires a small amount of fluid to perform a test. The apparatus can also be used 

for screening gaseous agents. In principle, the device could be employed to screen powder agents by 

incorporating a (yet-to-be-designed) powder delivery system. The apparatus is currently used to 

examine aqueous solutions with other additives and potential new liquid agents. The nebulizer will 

be further characterized by expanding the range of the thermophysical properties using water with 

surfactants and viscosity modifiers. The droplet sizes and number densities at various locations near 

the flame will be assessed for uniformity using a PDPA. 

REFERENCES

1. W.L. Grosshandler, R.G. Gann, and W.M. Pitts (eds.), NIST SP 861, U.S. Department of 

Commerce, Washington D.C., April 1994. 

2. R.G. Gann (ed.), NIST SP-890 (Vol. I & II), U.S. Department of Commerce, Washington 

D.C., November 1995.

3. S. Ishizuka and H. Tsuji, Eighteenth Symposium (International) on Combustion, The 

Combustion Institute, pp. 695-703, 1981. 

4. T.A. Milne, C.L. Green, and D.K. Benson, Combustion and Flame, 15, 255-264 (1970). 

5. J.C. Yang, M.K. Donnelly, N.C. Privé, and W.L. Grosshandler, Proceedings of Halon 

Options Technical Working Conference, Albuquerque, New Mexico, May 1998. 

6. J.C. Yang, M.K. Donnelly, N.C. Privé, and W.L. Grosshandler, Proceedings of the 5th

ASME/JSME Joint Thermal Engineering Conference, San Diego, California, March 1999. 

7. H. Tsuji and I. Yamaoka, Eleventh Symposium (International) on Combustion, The 

Combustion Institute, pp. 979-984, 1967. 

8. A.E. Finnerty, R.L. McGill, and W.A. Slack, ARL-TR-1138, U.S. Army Research 

Laboratory, Aberdeen Proving Ground, July 1996.