A study of premixed turbulent flames by scattered light

A STUDY OF P R E M I X E D TURBULENT FLAMES BY SCATTERED LIGHT G. C. WILLIAMS, H. C. HOTTEL, AND R. N. GURNITZ*

Massachusetts Institute of Tech~,ology, Cambridge, Massachusetts A nonflow-disturbing, light-scattering technique for continuous measurements of both mean and fluctuating values of temperature has been developed that is capable of high-frequency response. The technique was used to study turbulent atmospheric premixed flames of natural gas and air. Thermally stable, submicron, magnesium oxide particles were introduced steadily into a premixed unburned gas-air stream. Since the combustion occurred under conditions of turbulent convective transport at constant pressure with no molal change and no dilution with external fluid, the particle concentration at any point in the flame was inversely proportional to the absolute temperature. The concentration was determined from photomultiplier tube measurements of mercury-arc light scattered at 90 degrees from within a volume approximately 0.1-inch diameter by 0.1-inch long. The technique did measure temperature and temperature fluctuations and can be a powerful tool for the study of premixed turbulent flames. Analysis of the fluctuations gave information directly relatable to the speeds, amplitudes, and wavelengths associated with turbulent combustion. Flame shapes determined from high-speed photographs were consistent with interpretations based on the light-scattering measurements. Under the conditions studied, the influence of upstream turbulence on flame structure was negligible except for its action as a triggering mechanism for downstream flame turbulence. Demonstrations are given of the use of the technique both for measurements of local average volnmetrie burning rates within the flame brush and for measurements of gross volumetric burning rates.

Introduction The objective of this work was to develop and employ a technique for the quantitative study of the effects of upstream turbulence on premixed gaseous flames. Most previous studies were photographic. Normal slow-speed photography has shown that turbulence changes the timeaverage contour of a flame from a smooth laminar surface to a diffuse one, while at the same time decreasing the flame length) High-speed sehlieren photographs have revealed sharp density gradients within the burning zone. TM More quantitative studies were made with ionization probes, 4 but were somewhat limited, since the probe disturbs the flame.

Experimental Technique This study was based on the use of an optical probe for measuring concentration in a flame * Presently California.

at

Rocketdyne,

Canoga

Park,

containing stable flow-following submicron particles of concentration known in tt~e cold stream. Under the experimental conditions of turbulent convective transport, constant pressure, no molal change, and no dilution the absolute temperature at any point in the flame was inversely proportional to the particle concentration. The technique, initially proposed by Rosenswcig9, has the advantages of high-frequency response, no flow disturbance, and continuous measureinent at a "point" in space. The magnesium oxide particles employed were generated continuously by the controlled combustion of magnesium ribbons. The products were fed with negligible holdup into a stream of Cambridge city gas (93 percent methane) and air, thence through a calming chamber and nozzle with a 16:1 area contraction ratio. The resulting low-turbulence stream then passed through a biplane grid and duet. This stream of known and adjustable turbulence was burned in the ambient. The stabilizer employed was a 3-in.-diameter wire ring that added little turbulence to the stream so that, in the absence of turbulencegenerating grids, the flame was laminar. In the

1081

1082

TURBULENT FLAMES

regions of interest, no dilution of nozzle fluid with external fluid occurred. For the particle concentrations used, the light scattered in a given volume was proportional to the particle concentration. A 0.1-inch-diameter, collimated beam of dc mercury-arc light was passed through the flow field. The light scattered at 90 degrees by the magnesium particles in a 0.1 inch length along the beam was measured by a focused photomuttiplier tube. A narrow-bandpass optical filter reduced the light from the flame and passed essentially only the 5461 ,~ light from the arc. The electronic circuit response was linear to 10 kc. Further description of the technique appears in Ref. 2.

Results and Discussion

The mean temperatures determined by the light-scattering technique were found to be in agreement with those determined with a fine wire thermocouple.

General Characteristics of Fluct~ations Typical oscilloscope traces of the fluctuations in the burning zone are shown in Fig. 1 (a). It is noted that the probe indicated either a fully burned condition, a fully unburned condition, or rapid transitions from fully burned to fully unburned. Thus, the burning zone was composed of

Fra. I. Typical oscillograph patterns of temperature fluctuations. Upper: 0.05-see duration; Lower: 0.005-sec duration.

PREMIXED TURBULENT FLAMES

•

25

b.I

i

I

5

t0

I

1083

I

20


wuJ t~l.a.P

15

~m

IO

mo

< hl

0 0

20

15

25

COLD FLOW V E L O C I T Y I FPS 2.0

200

tat "!" O ~.5 z

J.J

1.0

150

,,,

IO0

a.

oql w

z o u w

I

|

I

l

i,d

0.5

.J

z

50

a.

oi0

I

I

I

I

20

40

60

80

VOLUME

PERCENT

(RADIAL

I00

BURNED

TRAVERSE)

0

0

i

I

,

I

20

40

60

80

VOLUME

PERCENT

( RADIAL

100

BURNED

TRAVERSE)

FIG. 2. Typical fluctuation characterization measurements. rapidly oscillating regions of fully burned and fully unburned gases. Oscilloscope pictures [-Fig. l(b)-] with ten-times-higher time resolution than shown in Fig. 1 (a) reveal a measurable transition time of passage from the fully burned to the fully unburned condition. The thickness of a laminar f a m e front under the experimental conditions was much smaller than the width of the scatter volume; thus, the ratio of the probe size to the transition time was approximately equal to the velocity with which the turbulent flame fronts crossed the scattering volmne. This interface velocity is hereafter referred to as the transition speed. The ~verage spatial scales of the fluctuations were taken to be the product of the transition

speed and the duration of the pulse in either the fully burned or fully unburned state. Finally the time-averaged volume fraction burned at a "point" in the flame was obtained by computing an average temperature excess above the unburned gas and dividing this by the temperature difference between fully burned and fully unburned gas.

Experimental Conditions The experiments were designed to demonstrate the effects of flow rate, upstream turbulent scale, upstream turbulent intensity, and position in the flame on the characteristics of the observable fluctuations in the turbulent burning zone. For

1084

TURBULENT FLAMES

each vertical position and upstream flow condition, data were taken as a function of radial position. The cold stream velocity was varied from 9.9 to 18.7 fps, its turbulent intensity from 1.2 to 5.0 percent, and its stream scale of turbulence from 0.03 to 0.14 in. The mixture ratio was held approximately stoichiometric. The lightscattering data were recorded in analog form on F - M magnetic tape, digitally sampled at a rate of 40,080 per second, and retaped in digital form compatible with the IBM 7094. Computer programs were written to calculate the quantities discussed above.

consisting of wrinkled travelling laminar waves dividing the fully burned from the fully unburned gases. The mean transition speeds averaged over all radial positions (height and radius of probe position varied) were found to be about equal to the cold-flow nozzle velocity [-Fig. 2 (a)7. Significantly, Markstein, 6 in his studies of the interaction of flow disturbances and laminar wave fronts, found that a disturbance put into a laminar flame by a vibrating wire travels at the unburned gas velocity, independent of disturbance amplitude and frequency. This was later confirmed by Petersen and Emmons s in their work on disturbed laminar flames. The scale of the burned fluctuation versus time-average volume percent burned increased steadily along a radius from regions of low burnedness to high burnedness, undergoing a

The lVrinkled-Flame Model The data obtained were found to be consistent with a wrinkled-flame model--i.e., reaction zones

VOLUME 0 2.0

PERCENT U N B U R N E D

I0

20

:50

40

50

60

70

80

90

I

I

I

I

I

I

I

I

I

CON D I T I O N S

1.5

VELOCITY

(COLD)

2.0

0

TURBULENT INTENSITY (COLD) = 2.8% T U R B U L E N T SCALE (COLD) = 0 . 0 6 INCHES

-

I00

(/3

-I.5

: 15.8 FPS

PROBE H E I G H T ABOVE STABILIZER

h0

"1" C.) Z m a.

= 3.17INCHES

I.,IJ J ,,~

_z -I.0

5

s

1.1.1 Z n," :C)

z

-0.5

0.5

0

I

0

I0

20

30

VOLUME

40

50

60

PERCENT

70

80

90

BURNED

FIG. 3. Variation of scale with volume percent burned and unburned.

0 I00

Z :C)

PREMIXED TURBULENT FLAMES

1085

TABLE I Observed wave velocities and rms deviations from mean wave velocities

Cold-flow velocity, fps

Cold-flow turb. scale, in.

Cold-flow turb. int., % of coldflow vel.

Observed ave. wave velocity, fps

See Note (1)

Base conditions

13.8 13.8

0.06 0.06

2.8 2.8

17.3 14.2

0.32 0.33

Effect of scale

13.8 13.8

0.03 0. l i

2.8 2.8

14.3 15.9

0.27 0.31

Effect, of intensity

13.8 13.8

0.06 0.06

1.2 5.0

14.9 14.1

0.33 0.33

Effect of flowrate

9.9 11.7 18.7

0.06 0.06 0.06

2.8 2.8 2.8

10.0 12.5 20.4

0.26 0.26 0.36

Case

Note (1) : Average of the values from all radial positions of the rms deviations from the mean wave velocity at each radial position divided by the mean wave velocity at each radial position. rapid increase as combustion was colnpletcd ~Fig. 2(b)]. Curves of fluctuation frequency versus volume percent burned reached maxima in the central regions of the flame brushes, positions corresponding to the most probable positions of the wrinkled flame fronts [-Fig. 2 (eli. Figure 3 presents typical curves of burned fluctuation scale versus volume percent burned and unburned fluctuation scale versus volume percent unburned. In all eases, the relative position of the two curves was the same, with all crossover points occurring at 40 to 60 volume percent burned. This is also consistent with the wrinkled flame hypothesis. Consider a sinusoidal flame front [-Fig. 4 (a)]. As suggested by Karlovitz, a portions of the flame front will eventually intersect each other and the originally sinusoidal wave will, ill time, tend towards a wave with sharp peaks toward the burned gas and smooth troughs toward the unburned gas. Figure 4(b) shows what form a more random wave train might take in a turbulent flame. First, it can be seen that the average scale of the unburned gas at a radial position corresponding, for example, to 10 volume percent unburned is smaller than the average scale of the burned gas at a radial position corresponding to 10 volume percent burned. Second, at a given burnedness, the number of burned pulses per second must be equal to the number of unburned pulses per second. Therefore, at 50 volume percent burned or unburned, the average burned and unburned scales must be equal. Finally, the average scale of the unburned pulses at 90 volume percent unburned is greater

than the average scale of the burned lmlses at 90 volume percent burned. An analogous argument can be used to account for the asymmetry ill Fig. 2 (el, a plot of pulses per second versus volume percent burned. Referring again to Fig. 4(b), it is seen, for example, that at 10 volume percent burned the number of pulses per second that would pass the probe is less than the number at 90 volume percent burned. Figure 5 shows a I nfillisecond photograph (together with a longer duration photograph for comparison) taken without smoke or arc lamp. i t should be noted that the flame structure appears continuous, in line with the wrinkledflame hypothesis. No eddies are evident, and the previously discussed peaks toward the burned gas and smooth troughs toward the unburned gas are clearly visible. The measured transition velocities for the 13.8-fps data varied linearly with volume pet'cent burned. A least-squares analysis of the same data gave a slope of 0.039 4-0.011 fps/volulne percent burned (99 percent confidence limit). That corresponded to a transition-velocity change of about 40 percent as the probe swept radially outward fl'om 0 to 100 volume percent burned, consistent with higher wave velocity on the burned than on the unburned side, and consistent with the wrinkled-flame hypothesis. Referring to Fig. 6(a), it is noted that the unburned gas in the region "a-b", for example, passes through the wave and thus accelerates. Therefore, the section of wave "b-c-d" is in a

1086

TURBULENT FLAMES 0 w OZ wn

~

ne(II ~Z end

<1:

a ,,J

I--I-ZZ WW O0 nell: W~

cO

,.o~ Z D rc~O DZ m D

~no n-

O0 Z Z W W O o n-nW w a. 0. O O

~J

~c

1 5

D

D00 ~J

DO

.C

>

o

CD

Z C~n"

nn

w~ OO LOL0

<1

I-Zw

o~ t~l n--

I,, o,

]

9

PREMIXED TURBULENT FLAMES

1087

FIG. 5. Photographs of a turbulent flame. Left: 1 msec; right: 40 msec. Uc = 13.8 fps; (ul/U)~ = 0.028; L~ = 0.06 in. faster moving stream than is section "d-e". Thus, it would be expected that the section "b-c-d" would travel faster than the section"d-e". The consequence of this, as far as wave shape is concerned, is shown in Fig. 6 (b). There, the wave is shown at positions 1 to 5 wavelengths downstream. A linear increase in propagation velocity from the base to the vertex of the wave was assumed, with the latter traveling at 1.4 times the speed of the former. One might, therefore, expect the waves to lengthen as shown. However, the "tails" burn in towards each other and thus the shaded portions become part of the fully burned region. Hence, no long "tails" are observed in photographs such as shown in Fig. 5. All data obtained therefore appear to be consistent with a wrinkled-flame model. Thus, for TABLE

the present system, the measured transition speeds may be interpreted as wave velocities. The measured scale at 50 volume percent burned may be interpreted as one-half the wavelength. The width of the flame brush as measured from data on volume percent burned versus radial position may be interpreted as twice the wave amplitude.

Effeds of Upstream Flow Conditions The average wave velocity was found to be determined solely by the cold-flow velocity, independent of all other upstream conditions. This is demonstrated in Table I and was shown in Fig. 2(a). The root-mean-square deviation from the mean wave velocity at a given radial II

Comparison of present experimental conditions with those of Karlovitz

System Base diameter of flame Turbulence generation Turbulence intensity (cold flow) Turbulent scale (cold flow) Vertical position of probe Flow velocity

Karlovitz Stoichiometric natural gas-air 1.25 in. Developed pipe flow Approx. 3% at centerline Approx. 0.17 in. at centerline 0.95 diameters downstream Approx. 15.5 fps

Present study Approx. stoichiometric natural gas-air 2.80 in. Grids Uniform at 0.00 in. Uniform at 0.06 in. 1.13 diameters downstream 11.7, 13.8, and 18.7 fps

TURBULENT FLAMES

1088

POSITION WAVELENGTHS DOW

4

d

r ,.I]"

Z O O

2

0

Fro. 6. Effect of wrinkled surface on velocity and subsequent effect on wave growth pattern. position was about 31 percent of the measured wave velocity for all radial positions and eases investigated, independent of all upstream conditions. This is also shown in Table I. The distribution of wave velocities at a radial position appears, therefore, to be sealed to the cold-flow velocity. The measured wavelengths and amplitudes within the flame brush were found to be essentially independent, of the upstream turbulent conditions investigated. This is shown in Fig. 7. It was therefore concluded that, except for serving as a triggering mechanism for downstream flame turbulence, upstream turbulence was without influence on the flame structure. The wavelengths were found to increase with

increasing vertical position. This is shown in Fig. 8 (a). The wave alnplitudes showed a similar increase [see Fig. 8 (b)], a finding consistent with linear hydrodynamic stability analyses carried out by previous investigatorsP ,6 The wave amplitude was defined as one-half the width of the turbule~t brush measured along a normal to the 1-volmne-pereent-burned locus.

Mean Burning Rates Radial variation of mean volume percent burned provided the basic data from which local volumetric burning rates could be estimated.

P R E M I X E D T U P B U LEN T FLAMES For mean conditions, a local average rate of 27 X 106 B t u / ( c u ft) (hr), 240 X 106 kcal/m a hr was found at a position corresponding to 50 mass percent burned. From mean-temperature traverses taken at a number of vertical positions, the loci of 1 and 99 volume percent burnedness were determined. From this flame contour, the gross

(/) hi "r Z

20f 1.5

.J r

laa z

volumetric burning ]'ate was estimated to be 14 X 106 B t u / ( c u ft)(hr). For comparison, the volumetric burning rate for a laminar flame of the same composition and 0.2 mm thick would be about 42 X 10r B t u / ( c u ft) (hr). A more detailed diseussion of these results may be found in Ref. 2. 2.0 03 w

c~

..t-

A : 0.0:3 IN.

z

0 = 0 . 0 6 IN. : 0 . 1 4 IN.

d

A= 1.5% 0 =2.8%

1.5

rl : 5.0%

G

_1

1.O

u o3 Q w z n-

0.5

l.O

0.5

lID

[] 0.0 0

I

I

I

I

25

50

"75

I00

0.0

~ I . ~ _ TW ICE WAVE AMPLITUDE ~

I00

~0 I.-

1

-

Z b.I fJ n,,

I.l.I 0.. I.=.1

40-

J

20-

I

I

I00

o

i /~

60-

=E

I

./

0 = o,I

//.~

0

tit Z

80

D. Z tat r

60

_

50

I00

75

PERCENT

BURNED

TW ICE WAVE AMPLITUDE

/

f

I

I I

I

,N.

,_~ J O

-

I - I NCH

RELATIVE RADIAL POSITION

I 40 -

I

A= 1.3% O= 2.8%

[ I I

n=

20 -

"'

0 '~

VOLUME

~

i

80-

25

0

VOLUME P E R C E N T BURNED

o ILl Z

1089

Jr I

5.0%

A I -INCH ="J ~RELATIVE R A D I A L POSITION ~

Fro. 7. Effect of cold-flow t.urbuleace characteristics on wavelengths and amplitudes.

1090 {D tll "t-

5[

TURBULENT FLAMES

VERTICAL POSITION INCHES ABOVE STABI LIZER :

O Z

u2 _1

[] 4 . 6 7 INCHES 0 3.17 INCHES A 1.67 INCHES

1.0

0

0 W Z

0.5 LIJ (.9 n.W

o 0

I0

I

I

20

50

1.5-

VOLUME

I

I

I

I

~

I

~

~

~

70

80

90

PERCENT

I i00

BURNED

03 uJ -rL) Z

u2

1.0

F._1 O. 0.5

-

o

taJ PROBE 1/10 INCH'--'-~

0~ 0

I

l

I

I

2

3

VERTICAL

POSITION,

,.

I

4

I 5

INCHES

FIG. 8. Growth of wavelength and amplitude with vertical position in flame.

Comparison with Other Data Another quantitative study on fluctuations in a premixed turbulent flame was that of Karlovitz.4 In his experiments, an unshielded negatively charged probe was inserted into the flame brush, with probe axis transverse to the cold-flow direction. With suitable electronics, a signal was obtained that indicated whether the probe was in contact with the high-ion concentration in the combustion wave. He measured, as a function of radial position, the fluctuation frequency and the

percentage of time the probe contacted the combustion wave. For continuous waves, the latter is equivalent to the time-averaged volume percent burned at the probe tip. Karlovitz's experimental conditions are compared with those of this study in Table I I and a comparison of his data with the present d a t a appears in Fig. 9. Even though neither Karlovitz's data nor his flow conditions correspond exactly to those of the present study, their similarity demonstrates that both techniques probably measured the same kind of fluctuation frequency and volume percent burned.

PIIEMIXED TUllBULENT FLAMES

SYMBOL 360

- -

SOURCE OF DATA

[~, ,/~ 0

PRESENT STUDY PRESENT STUDY P R E S E N TSTUDY PRESENT STUDY K ARLOVITZ

1091

COLD FLOW VEL. I 1.7 13.8 13.8 I 8,7

FPS FPS FPS FPS 1 5 . 5 FPS

320

280

240 r~ Z

0 0 ILl (/)

200

n,' ILl 12. f,/) ILl 0') _1 E) (3.

160 f

0

120

80

40

O L-._._L__._L 0 I0 20

J 30

40

50

60

70

80

VOLUME PERCENT BURNED FIG. 9. Comparison of present data with those of Karlovitz.

90

I00

1092

TURBULENT FLAMES Conclusions

1. A light-scattering technique has been developed for high-frequency response measurements of local temperatures in premixed turbulent flames. These measurements yield information about velocity and scale of temperature fluctuations in the flame brush. Consequently, the technique is valuable for studying turbulent combustion. 2. All of the data obtained with the above technique are consistent with a wrinkled flame model: a. The turbulent brush was found to consist of fully unburned regions. b. Measured wave velocities were found to equal the upstream cold-flow velocities. c. Curves of fluctuation frequency as a function of radial position reached a maxinmm in the central region of the turbulent brush. d. Curves of wavelength as a function of radial position were as would be expected from the wrinkled-flame model.

were found to ire'tease with increasing vertical position in the flame. 6. The light-scattering technique pernfitted measurement of both local and gross burning rates. ACKNOWLEDGMENTS

Part of the work was done at the TX-O and 7094 computer facilities at M.I.T. A portion of the work was supported by the U.S. Army I~esearch Office. REFERENCES 1. DAMKOHLER, G.:

46,

601

3. KARLOVITZ, B., ET AL.: Fourth Symposium

5. 6.

3. High-speed photographs of the flame indicated a wrinkled-flame structure characterized by cusps on the burned side of the flame. Such a flame shape was consistent with that deduced from the light-scattering measurements. 4. Measured wavelengths and amplitudes within the flame brush were essentially independent of the upstream conditions investigated. Thus, except for serving as a triggering mechanism for downstream flame turbulence, upstream turbulence is concluded to have negligible influence on flame structure under the conditions studied. 5. Both measured wavelengths and scales

Z. Elektrochem.

(1940), translated by J. Vanier, NACA TM ll0, 1947. 2. GURNITZ, R. N. : Development and Application of a Light Scattering Technique for the Study of Premixed Turbulent Flames, Ph. 1). thesis, M IT, Cambridge, Mass., 1966.

7.

8.

9.

(International) o,~ ('omb~lstio,~, p. 613, Williams and Wilkins, 1953. LANDAt', L.: Acta Physicochinl. UIISS 19, 77 (1944). MARKSTEIN, G. H.: Third Symposium on Combustion, Flame, and Explosion Phenomena, p. 162, Williams and Wilkins, 1948. MARKSTEIN, G. H.: "Experimental and Theoretical Studies of Flame-Front Stability", J. Aeron. Sci. 18, 199 (1951). PETERSEN, R. F. AND EMMONS, H. W.: Phys. Fluids 4, 456 (1961). ROSENSWEIG, ]~. E.: Measurement and Characterization of Turbulent Mixing, Sc.D. thesis, MIT, Cambridge, Mass., 1959.

10. WOHL, K., SHORE, L., VON ROSENBERG, H., AND WELL, C. W. : Fourth Symposium (D~ternational)

on Combustions, p. 620, Williams and Wilkins, 1953.

COMMENTS R. W. Bilger, University of Sydney. Have you computed the Kolmogoroff microscale for the turbulence in the unburned flow? It would seem that, if the microscale were less than the laminar flame thickness, the flanle, while still wrinkled, would be thicker and locally faster. G. C. Williams. This could be an inlportant point, for one would think that, in any turbulent flow, there would be some "eddies" smaller than the thickness of the lanfinar flame. Measurement of such an effect as mentioned by Bilger would require a probe that is smaller than the laminar flame thickness. Our probe size is smaller than the scale of the wrinkles, but larger than the laminar flame thickness. We did not determine the microscale.

T. Fufiwara, University of Tokyo. The local variation of the turbulent pulse shown in your slide is very interesting, since it may throw light Oil the essential mechanism of the generation of turbulence by flame. Have you ever tried to arrange your experimental results on one curve with dimensionless coordinates? If this can be done, it would indicate the important parameter for generation of turbulence by the flame. G. C. Williams. The fact that the measured wavelengths and amplitudes within the flame were essentially independent of the upstream conditions investigated precludes our correlating these quantities on dimensionless coordinates. Tile results do show that the ratio of the mean transition speed to the cold-flow velocity is a constant of value unity under the conditions studied.