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Some Observations on the Shape of

Impinging Diffusion Flames

NATL INST OF STANDARDS & TECH R.I.C.

A1 11 02631 166

Kokkala, M. A/Some observations on the s QC100 .use NO.87-3505 1987 V19 C.1 NBS-P

M. A. Kokkala

Fire Technology Laboratory

Technical Research Center of Finland

02150 Espoo, Finland

and

W. J. Rinkinen

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Fire Research

Gaithersburg, MD 02899

January 1987

U.S. DEPARTMENT OF COMMERCE

natiomal bureau OF STANDARDS

QC

100

. 1)56

87-3505

1987

c. 2

NBSIR 87-3505

SOME OBSERVATIONS ON THE SHAPE OF

IMPINGING DIFFUSION FLAMES

M. A. Kokkala

Fire Technology Laboratory

Technical Research Center of Finland

02150 Espoo, Finland

and

W. J. Rinkinen

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Fire Research

Gaithersburg, MD 02899

January 1987

U.S. DEPARTMENT OF COMMERCE. Malcolm Baldrige. Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Director
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TABLE OF CONTENTS

Page

List of Figures Iv

Foreward v

Abstract 1

1. INTRODUCTION 1

2. EXPERIMENTAL 3

3. CHARACTERISTICS OF PROPANE AND NATURAL GAS FLAMES 5

3.1 Propane Flames 5

3.2 Natural Gas Flames 7

4. DISCUSSION 7

3. CONCLUSIONS 10

6. REFERENCES 10

-ill-
LIST OF FIGURES

Page

Figure 1. (a) Schematic view of the experimental set-up with

the burner far from the ceiling 12

(b) Details of the diffusion burner 12

(c) Close up of the stagnation region 12

Figure 2. The shape of the propane diffusion flames as a function of the heat release rate Q and the ceiling

height H 13

Figure 3. The shape of the natural gas diffusion flames as a function of the heat release rate Q and the celling

height H 13

Figure 4. The free flame height (broken line) and the sum of

ceiling height H and the celling flame diameter 2Hj^

(different symbols) as a function of the nominal

heat release rate of the propane diffusion flames 14

Figure 5. Complete rings of a propane flame with H = 100 mm and

Q

= 7.6 kW (1/60 s exposure) 15

Figure 6. Broken rings of a propane flame with H = 100 mm and

Q

= 11 kW (1/60 s exposure) 15

Figure 7. Stable daisy flower shape of a propane flame with

H = 20 mm and Q

= 40 kW (a flash photograph) 16

Figure 8. Swinging daisy flower of a propane flame with

H = 20 mm and Q

= 7.6 kW (1/60 s exposure) 16

Figure 9. Cellular daisy flower of a propane flame with

H = 20 mm and Q

= 11 kW (1/60 s exposure) 17

Figure 10. Stable disc of a natural gas flame with H = 50 mm

and Q

= 10.5 kW 17

Figure 11. Cellular daisy flower of a natural gas flame with

H = 20 mm and Q

= 10.5 kW 18

-iv-
FOREWARD

The work described in this report was carried out at the Center for Fire

Research of the National Bureau of Standards as a part of a project on wall

and ceiling fire dynamics.

The authors thank various members of the staff of the Center for Fire

Research for inspiring discussions, and especially Dr. James Quintiere, who

suggested this study.

The financial support by the Technical Research Center of Finland, by the

National Bureau of Standards, and by the United States Educational Foundation

in Finland is gratefully acknowledged.

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SOME OBSERVATIONS ON THE SHAPE OF IMPINGING DIFFUSION FLAMES

M.A. Kokkala and W.J. Rinkinen

Abstract

The structure of impinging diffusion flames of propane and natural gas on

a ceiling is described. In these experiments, a burner with a diameter of

64 mm was used. The spacing between the burner surface and the horizontal

ceiling was varied in the range of 15 mm - 100 mm and the nominal heat release

rate in the range of 2.9 kW - 11 kW. New kinds of regular flame shapes were

observed: complete rings, broken rings, laminar disc, daisy flower, cellular

daisy flower. Color photographs of the various flame shapes are presented.

Combustion processes causing the instabilities are discussed qualitatively,

and the differences between propane and natural gas flames are identified.

1 . INTRODUCTION

Regular flame shapes are interesting for two different reasons.

Transition from one shape to another offers a good check point for combustion

models. In addition, the often symmetrical and colorful flames are

esthetically attractive, and may inspire the minds of curious researchers.

Very often the regular shapes appear because of an instability in the

flame front. In premixed flames the instabilities occur near the lean or rich

concentration limits of flame propagation. The earliest observations of this

phenomenon date back to the 19th century, but the real pioneering work was

done by Markstein in the 1940's and the 1950's [5,6]. Markstein studied the

cellular structure of the flames of various hydrocarbon/air mixtures. He

- 1 -
found, e*g«, that the size of the flame cells Is a function of the molecular

weight of the hydrocarbon.

Mltanl and Williams later studied the cellular structure of the flames of

hydrogen-oxygen-nitrogen mixtures [7]. They found that the cell size depends

on the ratio of the concentrations of hydrogen and oxgyen. A remarkable

change in the cell size occurs approximately at the point where the concentration of hydrogen is twice the concentration of oxygen.

Recently, several researchers have studied the structure of premixed

flames in stagnation point flow [3,9,11]. Ishizuka and Law [3] studied the

extinction and stability of stretched premixed propane and methane flames.

They used both a counterflow burner and a stagnation flow burner with a water

cooled wall. Depending on the concentrations and the flow or heat loss conditions, they could identify a variety of different flame configurations:

cellular flames, star-shaped flames, groove-shaped flames, and vibrating

f lames

Two possible causes for the cellular and other regular flame structures

have been suggested: hydrodynamic instabilities or thermo-diffusive

instabilities. The former is seldom observed, but the later is regarded as

the cause of most of the cellular structures [10]. The key parameter in the

therrao-dif fusional model is the Lewis number, Le, of the mixture. The

critical Lewis number Le^ is of the order of 1 . If Le > Le^ the flame is

stable and if Le < Le the flame is unstable.

c

-2-
Cellular diffusion flames have been observed In systems In which the fuel

flows out of a downward facing surface [2,4,8]. DeRls and Orloff [2]

systematically studied the Instability of a downward facing diffusion flame.

They observed a transition from a laminar to a cellular flame when the

Rayleigh number of the system exceeds a critical value of about 2400. The

characteristic length In the Rayleigh number Is the separation between the

reaction zone and the solid surface. They also found that the cell size

depends on the molecular weight of the fuel but their cells were about twice

as big as those In Marksteln's premixed flames.

The observations described In this report were made while the authors

were studying the heat transfer to and the Ignition of a horizontal surface by

an Impinging flame. This report Is kept on a very descriptive level; a more

thorough study would require a much more detailed instrumentation. Because

the apparatus was not designed for this kind of study and because the main

goal of the work was con5)letely different, there is evident lack of systemacy.

This paper is, however, written to inspire those working on basic combustion

research to study these phenomena, which to our knowledge have never been

observed before and which, we think, can result in much new and valuable

information.

2 . EXPERIMENTAL

The experimental apparatus is shown in Fig. 1. The burner is a cone

filled with 6 mm glass beads up to about 5 mm below the 64 mm diameter upper

surface, which is covered with a 16 mesh screen.

- 3-
The burner is placed under a calcium silicate (Marinite) ceiling. At the

center of the ceiling board there are three 0.4 m diameter plates with a total

thickness of 38 mm. Outside the center there is only one 13 mm thick plate.

The total diameter of the unconfined ceiling is 1 m.

A raised floor plate of the same diameter as the ceiling is located 1 m

under the ceiling and 0.6 m above the floor of the room. The effect of the

floor plate is believed to be negligible.

Several layers of screen surround the apparatus to danq) disturbances in

the flow of room air. Above the apparatus is a hood to collect all combustion

products. The suxtion of the hood disturbs the flow around the apparatus.

However, this does not seem to affect the flames described here.

In each run the mass flow of gas was kept constant. Radiation from the

flame slowly heats up the glass beads. Heat exchange between the beads and

the through-flowing gas raises the temperature of the gas during the course of

the run, thereby continuously changing the flow velocity. With a lower gas

flow rate (smaller flame) the effect of the radiation feedback is stronger.

The flow rate is measured with a standard variable area rotameter. The

nominal heat release rate is calculated assuming a heat of combustion of

46.4 MJ/kg for propane and 50.0 MJ/kg for natural gas.

-4-
3. CHARACTERISTICS OF PROPANE AND NATURAL GAS FLAMES

In the following we call the different flame shapes rings, broken rings,

laminar disc, daisy flower, and cellular daisy flower. The meaning of these

descriptive words is best understood by looking at the photographs shown in

Figures 5 to 12 at the end of the report.

3.1 Propane Flames

The experiments with propane cover roughly the region for which the

spacing H is between 15 mm and 100 mm and the nominal heat release rate Q

between 4 kW and 11 kW. A rough sketch of the "phase diagram" of the flame

shapes in the H-Q plane is shown in Fig. 2. The location of the phase

boundaries is very approximate, because the data points, shown with a

different symbol for each shape, do not allow us to locate the boundaries very

accurately.

With H = 100 mm and small Q the ceiling flames are concentric rings (see

Fig. 5). As the heat output, i.e., the gas flow is increased the complete

rings begin to break into shorter arcs, but the ring pattern still remains

clear (see Fig. 6). The origin of the rings appears to be the pulsation of

the flame, and the frequency of pulsation is within the resolution of our

measurements the same as that of the corresponding free flame.

Raising the burner closer to the ceiling makes the necking of the flame

disappear, and the flame diameter is at every height larger than the burner

diameter. At H = 50 mm and small Q the flame is a disc with the burner in the

-5-
center. Hardly any combustion takes place in the space between the burner and

the ceiling. The diameter of the flame stays constant, and no periodic

fluctuations are seen. At the perimeter of the disc the luminosity of the

flame smoothly decreases and the perimeter looks like a red ornament around

the yellow disc.

With an even smaller H the shape of the flame becomes dramatically

different. The flame is stable, but the flame front is broken into several

rays (see Fig. 7). The flame looks like a daisy flower or a star fish. With

small gas flow rates the daisy flower keeps its shape for tens of seconds.

With higher flow rates the rays swing like the legs of a star fish in water

(see Fig. 8). The lifetime of an individual ray may be up to 30 s. Sometimes

two adjacent rays combine into a new one and sometimes a new ray is born when

two adjacent rays drift too far apart. The number of rays is 15 ± 1 and that

does not seem to depend on the spacing H or the gas flow rate.

With higher flow rates the rays of the daisy flower break into cells

traveling outwards along the rays (see Fig. 9). There are as many rays in the

cellular daisy flower as in the complete daisy flower. The cellular daisy

flower also appears as the gas flow of the laminar disc flame is increased.

The transition from a disc to a cellular daisy flower is not sudden, and at

some points of the H-Q plane the shape depends on the temperature of the

ceiling surface (heat loss to the ceiling). For example, for H = 50 mm the

transition from a disc to a cellular daisy flower occurs somewhere between 6

and 10 kW. Flames in this intermediate region have features of both types of

flames. The cellular daisy flower is also found with very small values of H

when the radial flow velocity of the fuel from the spacing becomes so high

that it creates an additional instability in the flow field.

-6 -
3.2 Natural Gas Flames

For natural gas the phase diagram in the H-Q plane shown in Fig, 3 looks

very different from that of propane. As in the case of propane, when the Q

vertical part of the flame is necking and pulsating, the ceiling flames have

the characteristic ring shape. When H is decreased or Q increased a laminar

disc appears. The region in the H-Q plane, where the disc appears is much

wider here. For example, with H = 50 mm and Q

= 10.5 kW the flame is still an

almost perfect disc with a diameter of about 400 mm (see Fig. 10), Under the

same conditions propane forms a cellular daisy flower. With natural gas we

did not find the perfect daisy flower shape. However, when H is decreased and

the gas flow or Q increased the disc breaks into a shape which very much

resembles the cellular daisy flower of the propane flame (see Fig. 11). The

number of rays is now 10 ± 1, i.e., definitely less than the number of rays

for the propane flames.

4. DISCUSSION

When trying to explain why these various flame shapes appear we should

consider all the main processes of combustion: macroscopic flow dynamics,

diffusion of reactants, reaction rates, and heat transfer processes. We have

not tried to use any quantitative combustion models to explain the phenomena.

Therefore, much of the following discussion is speculation based on physical

reasoning and on ideas presented in the literature in connection with related

problems.

-7 -
The ring flames are probably a hydrodynamic instability in the flame

arising from the pulsation of the vertical part of the flame. The rings under

the celling are essentially similar vortex rings such as those found in free

pool fires. The break-up of the rings with higher flow rates is probably

caused by Irregular disturbances in the flow field. The characteristic

frequency of both complete and broken rings was found to be practically the

same as that for the corresponding free flame. In the free pool the frequency

is known to depend only on the diameter of the pool [1].

In the natural gas flame it was found that after the ceiling became hot,

i.e., when the fuel in the quench layer became hot and light enough, a transition from the ring shape to the more stable disc shape occurred. For example,

in one experiment in which the gas flow was kept constant, the ring shape

existed several minutes in the beginning, but then within a period of about 15

seconds the flame turned to a stable laminar disc. During the transition the

surface temperature of the ceiling was still below 400®C. Natural gas at

400°C is still about 30% heavier than the combustion products at 1200“C in the

reaction zone.

No satisfactory explanation for the formation of the daisy flower shape

has been found. The mechanisms involved in this kind of a problem are

probably similar to those for the formation of cellular instabilities in

premixed flames. Such instabilities are believed to be caused by heat

transfer and diffusion. Cellular instabilities have never been observed in

rich methane flames. The cellular daisy flower of the natural gas flame may

therefore be caused entirely by the heavier impurities. The jetting of the

fuel from the burner with a radial velocity of the order of 0.2 m/s may also

-8-
affect the flame shape. The flame is, however, a real diffusion flame,

because the low flow rate is not able to entrain enough of air to make an

assumption of a semi -premixed flame plausible.

The stability of the daisy flower flame may be enhanced by the fact that

the sector of the ceiling surface under which a flame ray is once born becomes

immediately hotter than the adjacent sectors making the hot gases flow along

the same direction more easily thereafter.

The number of rays in the flower shaped flames depends on the molecular

weight of the fuel. This assumption was verified by burning a pool of heptane

close to the ceiling. With time dependent and uncontrolled burning rate we

found more than twenty rays. The effect of the molecular weight is consistent

with Markstein's observations [5].

For turbulent impinging flames the flame length along an unconfined

ceiling approximately obeys the simple relationship, H + 2 = Hp, where Hp

is the free flame height and the radius of the ceiling flame (both measured

as an average flame length) [12]. Figure 4 shows the sum H = 2 as a

function of the nominal heat release rate for various kinds of flames. The

values well below Hp are probably a sign of incomplete combustion.

The incomplete combustion is also indicated by increased soot production.

In the daisy flower shaped flame angular variation of smoke production was

observed. Close to the edge of the ceiling the flow of smoke formed clearly

visible stripes. It was not possile to determine whether more smoke came from

the rays or from between them.

-9-
5.

CONCLUSIONS

The observations described in this paper are far from sufficient to

characterize the instability phenomena occurring in impinging diffusion

flames. Some, but by no means all, of the physical and chemical processes are

understood qualitatively, but a quantitative understanding is lacking. A more

systematic study under better controlled conditions is definitely needed.

6.

REFERENCES

1. Byram, G.M. and Nelson, Jr., R.M., The modeling of pulsating fires. Fire

Techn. 6 (1970) 2, pp. 102-110.

2. de Ris, J. and Orloff, L., The role of buoyancy direction and radiation

in turbulent diffusion flames on surfaces. Fifteenth Symposium

(International) on Combustion, Pittsburgh, 1975, The Combustion

Institute, pp. 175-182.

3. Ishizuka, S. and Law, C.K., An experimental study on extinction and

stability of stretched premixed flames. Nineteenth Symposium

(International) on Combustion, Pittsburgh, 1982, The Combustion

Institute, pp. 327-325.

4. Kashiwagi, T. and Newman, D.L., Flame spread over an inclined thin fuel

surface. Comb. Flame 26 (1976), pp. 163-177.

5. Markstein, G.H., Instability phenomena in combustion waves. Fourth

Symposium (International) on Combustion, Baltimore, 1953, The Williams & Williams Co., pp. 44-59.

6. Markstein, G.H. and Somers, L.M., Cellular flame structure and vibratory

flame movement in n-butane-me thane mixtures. Fourth Symposium

(International) on Combustion, Baltimore, 1953, The Williams & Williams

Co., pp. 527-535.

7. Mitani, T. and Williams, F.A., Studies of cellular flames in hydrogenoxygen-nitrogen mixtures. Comb. Flame 39 (1980), pp. 169-190.

8. Orloff, L. and deRis, J., Modeling of ceiling fires. Thirteenth

Symposium (International) on Combustion, Pittsburgh, 1982, The Combustion

Institute, pp. 1541-1548.

9. Sato, J., Effects of Lewis number on extinction behavior of premixed

flames in a stagnation flow. Nineteenth Symposium (International) on Combustion, Pittsburgh, 1982, The Combustion Institute, pp. 1541-1548.

- 10-
10. Sivashinsky, G.J., Comb. Sci. Tech. 15 (1977), pp, 137-146.

11. Tsuji, H. and Yamaoka, I., Structure and extinction of near-limit flames

in a stagnation flow. Nineteenth Symposium (International) on Combustion, Pittsburgh, 1982, The Combustion Institute, pp. 1533-1540.

12. You, H.Z. and Faeth, G.M., Ceiling heat transfer during fire plume and

fire impingement. Fire and Mat. 3 (1979) 3, pp. 140-147.

- 11 -
Fig . 1 a) Schematic view of the experimental set-up with

the burner far from the ceiling.

b) Details of the diffusion burner.

c) Close-up of the stagnation region.

12
PROPANE

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laminar disc

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daisy ^ flower

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cellular daisy flower

5 .10 HEAT RELEASE RATE Q (kW) •

Fig. 2. The shape of the propane diffusion flames as

a function of the heat release rate Q and the

ceiling height H.

100

eEW

= 50

H

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t-H w 20

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HEAT RELEASE RATE Q (kW)

Fig. 3. The shape of the natural gas diffusion flames

a function of the heat release rate Q and the

ceiling height H

1 —I

' r NATURAL GAS

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rings

*^***“  **  . ^ *..|H •*

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daisy flowet

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HEAT RELEASE RATE Q (kW)

Fig. 4. The free flame height Hp (broken line) and the sum

of ceiling height H and the ceiling flame diameter

2Hp (different symbols) as a function of the

nominal heat release rate of the propane diffusion

flames

14
Fig. 5. Complete rings of a propane flame with H = 100 mm

and Q

= 7.6 kW (1/60 s exposure).

Fig. 6. Broken rings of a propane flame with H = 100 mm

and Q

= 11 kW (1/60 s exposure).

15
Fig 7 Stable daisy flower shape of a propane flame, with

H = 20 iron and 0 ^ 4.0 kW (a flash photograph).

Fig. 8. Swinging daisy

H = 2.0 mm and

flower of a Q

= 7.6 kW

propane flame with

(1/60 s exposure).

16
Fig. 9. Cellular daisy flower of a propane flame with

H = 20 mm and Q

= 11 kW (1/60 s exposure).

Fig. 10. Stable disc of a natural gas flame with H = 50 mm

and Q

= 10.5 kW.

17
Fig. 11. Cellular daisy flower of a natural gas

H = 20 ram and Q

= 10.5 kW.

18
NBS-114A (REV. 2-8C) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

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4. TITLE AND SUBTITLE

PUBLICATION OR

REPORT NO.

NBSIR-87/3505

2. Performing Organ. Report NoJ 3. Publication Date January ly87

Some Observations on the Shape of Impringing Diffusion Flames

5. AUTHOR(S)

M.A. Kokkala and W.J. Rinkinen

6. PERFORMING ORGANIZATION (If joint or other than N BS, see instructions) 7. Contract/Grant No.

National Bureau of Standards

Department of Commerce

Gaithersburg, MD 20899

8.

Type of Report & Period Covered

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, Stote, ZIP)

10,

SUPPLEMENTARY NOTES

Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200~word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here) The structure of impinging diffusion flames of propane and natural gas is described.

In these experiments, a burner with a diameter of 64 mm was used. The spacing between

the burner surface and the horizontal ceiling was varied in the range of 15 mm - 100 mm

and the nominal heat release rate in the range of 2.9 kW - 11 kW. New kinds of regular

flame shapes were observed: complete rings, broken rings, laminar disc, daisy flower,

cellular daisy flower. Colour photographs of the various flame shapes are presented.

Combustion processes causing the instabilities are discussed qualitatively, and the

differences between propane and natural gas flames are identified.

12. KEY WORDS fS/x to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

ceiling flame dynamics; cellular structured flames; impinging diffusion flames;

new regular flame shapes; unstable flame fronts

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PRINTED PAGES

Unlimited

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