NATIONAL BUREAU OF STANDARDS REPORT

5790

A TECHNIQUE FOR STUDY

OF

FLAME QUENCHING PROPERTIES OF SALTS

by

T. G. Lee

<NB^>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT MBS REPORT

1002-30-^873 February lb, 1958 5790

A TECHNIQUE FOR STUDY

OF

FLAME QUENCHING PROPERTIES OF SALTS

by

T « G • Le e

for

Bureau of Ships

Department of the Navy

Code 538

Index Number NS-183-001

IMPORTANT NOTICE

NATIONAL BUREAU OF STAN

Intended for use within the Co

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listing of this Report, either In the Office of the director. Natlc however, by the Government eg

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Approved for public release by the

Director of the National Institute of , n u obtained In writing from

Standards and Technology (NIST) «ch permission is not needed, on October 9, 2015.

,f •*•**» "l,k"

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

A TECHNIQUE FOR STUDY OF FLAME QUENCHING PROPERTIES

OF SALTS

ABSTRACT

A 100 mesh stainless steel screen coated with a thin

(3 to 5 microns) deposit of crystalline salt, was held

horizontally inside a long b-8 mm ID pyrex tube to

intercept the propagating flame front of a heptane-air

mixture. Under certain conditions this interaction

resulted in quenching of the flame by the screen. For

flame front velocities in the region of 3,000 cm/sec the

quenching efficiency of bare, potassium bicarbonate,

potassium oxalate monohydrate, and rubidium iodide

coated screens appeared to be the same. Quenching

depended on flame front velocity and could be explained

by thermal considerations alone.

At a flame front velocity of about 5,000 cm/sec

however, potassium oxalate monohydrate quenched readily

whereas the others did not. Interpretation of streak

photographs seemed to show that quenching was caused by

the vapor phase of the salt at distances up to 15 mm above

the screen. This indicates that quenching ascribable to

chemical influence becomes effective at this velocity.

INTRODUCTION

The use of dry chemical powder, mainly NaHC03 , as

a fire extinguishing agent has become increasingly common

in the past 5 years. The exact mechanism of extinguishment

involved is not completely understood. A study of the

mechanism of interaction between powder material and flame

would provide valuable aid in the search for better

extinguishing agents.

At present the general chain-breaking mechanism, to

a limited extent at least, is applicable to both the

inert and thermally unstable powders. The theory was

developed and shown to be in good agreement with data based

on wall quenching and quenching distance studies /I //2 //37.

It attributes the loss of energy and/or radicals (energy

and/or radical chain need not be specified) of the reacting

gases to collisions with the cold surfaces of solids.

- 2-

Thus flame extinction would result when the rate of chainbreaking (deactivation of chain carrier or energy transfer)

processes exceed that of chain-branching. Similarly,

the initial cold and large surface area of the powder

particles would precipitate chain-breaking in the fire.

However, two questions may be raised immediately /*+_/.

(1) To what extent is the chain-breaking

efficiency of the surfaces controlled by

temperature and/or by the chemical nature

of the salt?

(2) Is chain-breaking by the powder a surface

reaction or a gas phase reaction?

One of the aims of this study is to develop a method

in which experimental data may be obtained for solution of

the above problems. Any hypothesis that may be formulated

on the mechanism of flame extinction by the use of powder,

must also be consistent with the following three generalized

observations which are based on qualitative and available

quantitative data:

(1) The minimum concentration required for extinction

and suppression of flames is determined by the

total surface area of a given salt regardless

of particle_size., or weight for 5 to 100 micron

particles Z5_//^_/»

(2) Some salts are more effective than others as

measured by minimum weight concentration

required for extinction Z^+_//7_/*

(3) Generally, salts of alkali metals and especially

those that decompose below 200°C are among.

the most effective sdlts reported Z5_/Z8_/»

This report will describe a method and give some

preliminary results on comparison of the extinction

efficiencies of salts and the effect of flame front

velocity on the efficiency. This method means close

control of variables such as fuel type, fuel-air ratio,

pressure, temperature of both the mixture and salts as

well as salt concentration. Although the ultimate usefulness

of a powdered salt depends primarily on its effectiveness

against hydrocarbon diffusion flames, such flames are not

particularly suited for the study of extinction mechanisms.

A diffusion flame burning on the surface of a liquid fuel

for example, lacks homogeneity both in structure and in

types of reaction. If powder is applied to such flames,
‘

'
-3-

additional aerodynamic complications, such as varying

transit time of powder particle through the flame, etc.,

are encountered.

To avoid some of these objections, a flame burning

premixed gas and air was propagated through a tube. A

screen (60 to 100 mesh) or a series of screens coated with

a thin and uniform layer (3 thick) of the salt to be

studied was held horizontally in the upper part of the

tube to intercept the advancing flame front. Thus the

mesh and wire diameter of the screen were used to simulate

the salt concentration. This type of salt concentration

model was readily standardized.

In addition, the temperature of the salt (plus screen)

as well as the fuel could be controlled. Streak photography

using Tri-X as well as Super Anscochrome color film was

used to measure flame front velocity and record the

result of interaction near the screen area.

EXPERIMENTAL APPARATUS

Fig, 1 is a schematic drawing of the apparatus. The

pyrex tube was 4,8 cm ID and 56 cm long. It could be

easily removed from the straight joint at the bottom for

cleaning. Near the top of the tube were three protrusions

which acted as a twist lock to hold a flat aluminum ring.

Three stainless steel rods connecting rigidly to this ring

extended 15 cm below to support a screen holder. The

screen holder consisted of two sections of very thin wall

tubing that were machined to give a tight fit. The screen,

usually 100 mesh, was held firmly by these two sections when

pressed together. The top of the tube was ground flat in

order that a plastic plate, coated with vacuum grease,

might be used as an air-tight seal during evacuation of

the system.

Two tungsten electrodes, separated by a 5 mm gap

were located at the bottom of the tube. The outside leads

of the electrodes were connected to a 10 KV ignition

transformer. A 35 mm, 10 in, long film strip , revolving drum

camera, with a f/2 lens was focused near the central

axis of the flame tube. The field of view covered 5 cm

above and below the screen. The film was used to record

flame front velocity following a standard technique /9_/«

It also revealed ionization, flame front oscillation,

and extinction phenomenon. The speed of the built-in

motor in the drum camera was varied to suit flame velocity

conditions for each run.

-4-

A separate electronic circuit was arranged to provide

for a timing marker light operating at 1000 cps. It also

controlled synchronization of ignition and camera shutter

release. An ionization probe, placed near the top of

the tube, was used to detect flame passage when the

camera was not being used.

The central section of the apparatus consisted of a

manifold connected to condenser traps, vacuum pump,

manometer, fuel bulb, drying tube and storage plenum.

A mixing fan inside the plenum was used for agitating

the gas to insure proper mixing. It was driven by a

magnetic stirrer at the bottom.

EXPERIMENTAL PROCEDURES

The data and procedures reported in this paper are

confined to the quenching of premixed n-heptane-air

flames of 1.9 to 2.2 vol. percent heptane (both sides of

the flammability peak) at atmospheric pressure and room

temperature. The quenching tests were made by intercepting the flame front with the salt coated, 100 mesh

screen. The wire diameter of the screen was • 114 mm and

the width of opening 0.l4 mm, which gives an open area of

about 30 percent. The fuel-air ratio was determined by the

partial pressure method. Vaporized heptane was first slowly

metered into the evacuated plenum to a predetermined pressure.

Air, which was introduced later, was supplied from a compressed air cylinder in order to assure a consistant and

known moisture concentration. The mixture in the plenum

was charged to slightly higher than atmospheric pressure

in order to fill the evacuated flame tube to exactly

atmospheric pressure. The tube was closed by a thin

polyethylene film backed up by a 1/8 in. plastic cover.

After evacuation and subsequent charging, the plastic

cover was removed leaving the film, film was easily

blown away by a slight pressure rise in the tube. Thus

the tube may be considered an open tube.

The techniques of depositing a thin and uniform layer

of salt on the 100 mesh stainless steel screen were

developed gradually. To obtain a coating, the screen

was first cleaned in distilled water and then rinsed in

the (water) solution of the salt of proper concentration.

After draining off the excess solution, it was held horizontally and exposed to a low velocity stream of dry air

from the bottom. The adhesion between the crystalline

deposit and the metal surface was good. Salt lost was

negligible after the explosion for all the salts tested.

The quantity of salt applied to the screen was about

-5-

5 x 10 gm/cm^. Quantities of salt in this range were

not visible to the unaided eye. Microscopic examination

shoved that the coating was between 3 thick. Since the

deposit was small, the difference of pressure drop across

a coated and uncoated screen was expected to be very low.

Fig. 2 is a photomicrograph of 100 mesh screen coated with

potassium oxalate monohydrate crystals.

RESULTS

Table 1 presents a summary of results on the flame

quenching properties of various salts at a flame front

velocity of about 3,000 cm/ sec. It shows the number of

runs at which flame velocity measurements were made.

Based on these runs, it gave the maximum velocity at which

the flame was quenched and the minimum velocity with which

it penetrated the screen. The flame velocities ranged

from 2600 to 3500 cm/sec for all runs. The results for

each salt were about evenly divided between flames which

were quenched and those which penetrated the screen. There

were no overlapping results for any one salt.

Error in measurement on the film record was estimated

at +50 cm/sec. The flame front velocity, which resulted

from movement of the burned and expanding gas column in

the tube, as well as the flame speed, was controlled by

the fuel-air ratio, the total pressure of the charge, and

other factors. Reproducibility of the velocity was in

the order of +6 percent.

Figures 3 and 4- show streak photographs of the flame

front quenched by the screen and those which penetrate the

screen respectively. The time marker spacing represents

an elapsed time of one millisecond. When the velocity was

slightly below critical, stabilization of the flame front

below the screen resulted, probably caused by the fuel

diffusing down from aboveXFig. 5) » On the other hand, when the

velocity was quite low (at 2800 cm/sec) quenching was

immediate.

The vibrations of the flame front above and below the

screen were caused by coupling between the sound and

combustion waves accentuated by the screen. Its

characteristic did not change much within the flame

velocity limits reported above. However, change of screen

position relative to the tube affected the vibration.

It appears from table 1 that there is very little,

if any, significant difference in quenching performance

between coated and non-coated screen or among the coated

- 6 -

screens themselves. This is contrary to the results

observed when the salts were compared by dispensing them

in powdered form into a diffusion flame. The difference _

in performance might be as large as a factor of five /10/.

Apparently, the chemical activity of the salts were nonoperative under the conditions of the current experiments.

Indeed, the evidence indicates only thermal quenching,

since all flames having velocities below 2800 cm/sec were

stopped. The critical flame front velocity and mesh width

of the screen used fell very close to the curve worked

out by Palmer on the basis of thermal considerations /ll/

for a bare screen.

HIGHER FLAME FRONT VELOCITY

When the flame front velocity was increased to

about 5,000 cm/sec, the quenching characteristic of one

of the salts appears to change. To obtain these changes,

a 40 mm ID pyrex tube *+5 cm long was used. Preliminary results

shows that at flame velocities of ^550, 5100? and 5300

cm/sec, a screen coated with potassium oxalate monohydrate

at a concentration used previously was able to quench the

flame. However, the mode of quenching was different from

that observed at 3200 cm/sec. Examination of the film

records appeared to indicate that decomposition and

ionization occurred above the screen. The flame was

quenched at some distance above the screen probably by the

salt in the vapor phase.

Fig. 6 is a streak photograph of the quenched flame

above the screen coated with potassium oxalate monohydrate

at a flame front speed of 4,550 cm/sec. Note the high

density region above the screen which may indicate

ionization.

With other salts, such as titanium potassium oxalate,

KHCO3 and NaHCO^, the screens were not effective in

quenching velocities in the region of 5? 000 cm/sec.

However this conclusion is based on limited data only.

Further work, such as directly controlling the temperature

of the salt might provide additional information on the effect of such salts in quenching flames.

CONCLUSIONS

1. The results obtained by this method indicate its

limited usefulness in the study of variables that effect

the chemical quenching efficiency of salts.

-7-

2. The negligible difference in performance between

nncoated and coated screens at the 3,000 cm/sec region

show that with the fuels and quenching materials studied

the thermal quenching efficiency of all relatively cold

surfaces are about equal.

3. The ability of potassium oxalate monohydrate to quench

flames at velocities in the region of 5,000 cm/sec,

indicate that the chemical quenching factor becomes

operative. Decomposition and ionization of the salt as

interpreted from the film records, appears to be

necessary for chemical quenching which appeared to

occur in the vapor phase.

*+. Additional work is necessary to determine the

conditions under which chemical quenching will appear

in other salts.

REFERENCES

/!_/ Lewis, B. and Von Elbe, G: Combustion, Flames and

Explosions of Gases, Academic Press, Inc, (New

York) 1951*

/2_7 Friedman, Raymond: The Quenching of Laminar

Oxyhydrogen Flame by Solid Surfaces, Third

Symposium on Combustion and Flame and Explosion

Phenomena, The Williams & Wilkins Co. , Baltimore

(19^9).

j/3_7 Belles, F. E. and Berlad, L0 : Chain Breaking

and Branching in the Active-Particle Diffusion

Concept of Quenching, NACA Tech, Note 3*+-09(Feb. 1955)

&./ Friedman, R. and Levy J. : Survey of Fundamental

Knowledge of Machanism of Action of FlameExtinguishing Agents. WADC Tech, Rept. 56-568

(1957).

/5_7 Dolan, J# E, : The Suppression of Methane/Air

Ignition by Fine Powders. 6th Symposium on

Combustion, Reinhold Publishing Corp. , New York

0- 9570 . /6_/ Dyksman, A, J. : Chem. Weekbl. 12.1, 718 (193*+).

Z7_ 7 Dufraisse, C., LeBras, J. and German, M.

Extinguishing Powers of Powders in Flames and

Explosion. Compt. rend., 236 (1953)

.

/B_/ Fryburg, G«: Review of Literature Pertinent to

Fire Extinguishing Agent and to Basic Mechanisms

Involved in Their Action, NACA Tech, Note 2102

(1950).

/9_/ Ladenburg, R, W, , Lewis, R, Pease, R, N, , and

Taylor, H. S, Physical Measurement in Gas

Dynamics and Combustion. Princeton University

Press (195*0.

/10/ McCamy, C, S., and Lee, T* G. A Laboratory

Method of Evaluating the Extinction Efficiency of

Powders on Small Scale Fires. NBS Report 5200

(1957).

/II/ Palmer, K. N, , Private Communication,
.
Table 1

Quenching Characteristic of Coated and Non-coated Screen

Velocity cm/sec

No

Coating K2C 204/H20 Rbl KHCO3 NaHCO^

Maximum, Quenched 2800 3250 3250 3050 2950

Minimum, Penetrated 3150 3300 3300 3150 3000

Number of Runs 9 11 3 5 5

USCOMM-NBS-DC
' '"‘V .

-
VARIABLE

SPEED

DRUM

CAMERA

WITH

SOLENOID

OPERATED

SHUTTER v

FIG.ISCHEMATIC

DIAGRAM

OF

APPARATUS

Fig. 2 Photomicrograph of a 100 mesh screen with

thin crystalline coating. Light is reflected

from coatings on the wires.

Fig. 3 Streak photograph of flame front quenched

by a coated screen. Some ionized salt

particles are deflected by the screen.

^ STA, /jr Fig. 4 Flame front penetrating a coated screen.

Fig. 5 Flame stabilized below the screen. Note overlapping of events on film.

Fig. 6 Flame being quenched above the screen, probably by salt in the vapor phase.

Ft4

ME

U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks. Secretory

NATIONA I. BUREAU OF STANDARDS

A. V. Aslin, Director THE NATIONAL BUREAU OF STANDARDS

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