Burning characteristics of premixed sprays and gas-liquid coburning mixtures

COMBUSTION A N D FLAME 74:39-51 (1988)

39

Burning Characteristics of Premixed Sprays and Gas-Liquid Coburning Mixtures KAZUYOSHI NAKABE, YUKIO MIZUTANI, and TOMOYUKI HIRAO Department of Mechanical Engineering, Osaka University 2-1 Yamadaoka, Suita, Osaka 565, Japan

and SATOSHI TANIMURA Mitsubishi Heavy Industries, Ltd.

A premixed spray burner was carefully designed for the observation of stable turbulent flames propagating in a droplet cloud suspended in a stream of air or for a gaseous fuel-air mixture. Various methods were examined to determine the most suitable reference surface of a spray flame, corresponding to its burning velocity. Difficulties arose in finding the reference flame surface, which governs the burning velocity of sprays, because the spray flames were usually thick and unsteady. It was found that the surface on which the OH radical emission had a peak intensity was more suitable as a reference flame surface than the surfaces determined by either direct or schlieren photography. The qualitative features of our burning velocity data for kerosene sprays agree with those of other investigators; that is, we found a linear dependence on the fuel-to-air ratio and an inverse proportionality to the Sauter mean diameter. Concerning coburning, there exists an optimum kerosene-to-propane mass flow ratio and an optimum mean droplet diameter that maximizes the burning velocity. Instantaneous flame images in OH and C2 radical bands seem to suggest that group combustion occurs in a spray flame.

NOMENCLATURE

h Pf r ST O u' AX kp O 0

Sauter mean diameter of droplets height from the tip of the pilot burner atomizing air pressure radial distance from the burner axis apparent burning velocity time-averaged velocity of gas phase initial turbulence intensity of gas phase half-value width of interference filter transmission peak wave length of interference filter angle between the reference surface of a flame and a streamline angle between the reference surface of a flame and the flame axis

Copyright © 1988 by The Combustion Institute Published by Elsevier Science Publishing Co., Inc. 52 Vanderbilt Avenue, New York, NY 10017

~fg propane-to-air mass flow ratio ~fl kerosene-to-air mass flow ratio INTRODUCTION Spray combustion is frequently used in industrial combustion devices. Fundamental studies of spray combustion have been conducted for many years, and many detailed and extended reviews have been published [1--4]. In gaseous fuel combustion, the burning velocity is the most important factor governing the flame stability of a burner, the specific combustion load of a chamber, and so forth, and many experimental data on laminar or turbulent burning velocities have been published. Burning velocity

0010-2180/88/$03.50

40 data are equally important for spray combustion, as it has been reported that the front position of a flame stabilized in a spray injected coaxially into a hot vitiated air stream is governed by the flame propagation mechanism rather than by the ignition mechanism [5]. Furthermore, as gaseous and liquid fuel coburning schemes have recently become more common, burning velocity data for gas-liquid coburning systems are also important. Burning velocity data are extremely scarce for spray combustion, however, and only a few studies have been published: Mizutani and Nishimoto [6], Polymeropoulos and Das [7], Ballal and Lefebvre [8], and Myers and Lefebvre [91. As for gas-liquid coburning cases, data for practical use are more scarce, and only two studies have appeared: Browning and Krall [10], and Mizutani and Nakajima [11]. The burning velocity of a spray is significantly affected by turbulence [6], and spray flames for practical use are inherently associated with turbulent flow. So, attention has been focused on turbulent spray flames. If a turbulent spray flame is observed by high-speed photography, a continuous flame zone is not seen; rather, discrete flame lumps appear randomly in spots. Although there is some doubt that the concept of burning velocity is applicable to discrete flames, it is expected that the concept of apparent burning velocity defined in a time-averaged way will be practical use for spray flames as well as for turbulent premixed flames from gaseous fuel, because a fairly clear and continuous flame front is seen for both kinds of flames if observed in a time-averaged way. The most likely reasons why burning velocity data are so scarce for spray combustion, in contrast to gaseous fuel combustion, are as follows: 1. It is difficult to produce a premixed spray stream with a sufficient cross-sectional uniformity and with sufficiently small fluctuations in velocity and concentration. 2. Because spray droplets tend to move differently than the surrounding gas, the concept of equivalent one-dimensional flame does not apply. Therefore, it is difficult to define a universal burning velocity. 3. It is difficult to determine the reference surface

K. NAKABE ET AL. of a flame corresponding to its burning velocity because the time-averaged flame zone is thick and has an unsteady and complicated structure due to the locally statistical flame propagation mechanism. 4. It is difficult to set the flow rate of fuel and the mean diameter of droplets simultaneously and independently because of the complex operational characteristics of atomizing systems. It is also difficult to set the intensity and scale of turbulence independently of the other parameters using a turbulence-generating grid or a similar device. Therefore, the experimental conditions are restricted. However, if at least the first and third problems are overcome, the determination of an apparent burning velocity becomes possible even for turbulent spray flames. In the present study the traditional methods of burning velocity measurement using the direct or schlieren photographs have been reexamined, focusing attention on the solution of problems one and three listed above, and the method to determine the reference surface of a spray flame using a timeaveraged OH radical self-emission image has been proposed. Then, the appropriateness of the determination of burning velocity using an OH radical self-emission image was confirmed by comparing the self-emission image with the time-averaged temperature and composition fields. The structure of premixed spray flames, based on self-emission images of OH and C2 radicals, has also been examined. To obtain self-emission images of OH and C2 radicals, the flame images were viewed through an interference optical filter, intensified and visualized with an image intensifier, and then processed after being inverted into a video signal with a CCD camera [12]. Although the radical concentrations are not obtained by this method, unlike the planarbeam, laser-induced fluorescence technique (PLIF) [13-15], it has been reported that the zone in which OH radical self-emission appears corresponds well to the reaction zone [16]. Therefore, it is expected that the time-averaged reactionstrength pattern can be visualized by a rather simple apparatus. When, in addition, an axisymmetrical flame is treated, it is not difficult to invert

BURNING CHARACTERISTICS OF SPRAY FLAMES

41 1.Blower 2.Surgetank 5.Fuel tank 6.Fuel pump 8.Flow meter ~17 9'Ta~i°nm~lUied

the self-emission image into the cross-sectional brightness pattern so that no active measurement to induce fluorescence is particularly needed. The data of Browning and Krall [10] and Mizutani and Nakajima [11] concerning the burning velocities of gas-liquid coburning mixtures were obtained in special situations with weak turbulence. The coburning velocities under conditions of stronger turbulence, or for droplets having a larger mean diameter, are still open to question. Therefore, the burning velocities and flame structure of gas-liquid coburning mixtures have been also investigated by the present techniques.

14.Hz bottle 16.Pilot burner 17.Combustion tube 18.Window 20.C3FEbottte 73~1z~'~'/igto;if ice

D E S C R I P T I O N S OF E X P E R I M E N T

Various attempts were made to produce steady, premixed, spray (droplet-cloud) streams with cross-sectional uniformity and with slight fluctuations in velocity and concentration. An apparatus was developed that used an air blast atomizer (Spraying Systems Company, Japan, Type 2A), shown in Fig. 1, that produced dense droplet clouds with a mean droplet diameter of several tens of microns. The liquid fuel (kerosene) atomized with the airblast atomizer [9] was mixed with air or a propane-air mixture and issued from the premixed spray port [13] (52.7 mm i.d.), surrounded by a coaxial air stream, into the combustion tube [17] (80.9 mm i.d.). A plain quartz window [18] was installed in the combustion tube for optical observation. An inverted-cone-shaped flame was stabilized by a hydrogen diffusion flame from the pilot burner [16] (10.0 mm o.d.). The mass flow rate of kerosene in the premixed spray was determined by collecting droplets on a cotton-wool filter with a weak suction at the exit of the port. The size distribution of droplets was measured by the trace method using a magnesiumoxide-coated glass slide [17]. The Sauter mean diameter, d, was computed from the NukiyamaTanasawa size-distribution function [ 18] best fitted to the measured size distribution, because the mean diameter of droplets calculated directly from the raw data showed a noticeably poor reproducibility due to occasional huge droplets. Typical examples of this calculation process are

Fig. 1. Premixedsprayburnersystem. shown in Fig. 2. The curves in this figure represent the Nukiyama-Tanasawa function best fitted to the experimentally obtained histograms. The velocity and turbulence intensity of the premixed spray were measured using a hot-wire anemometer (KANOMAX,CTA 1011) with the liquid fuel turned off but with atomizing air injected. The flame temperature was determined with a suction pyrometer that had a Pt/Pt-13% Rh thermocouple of 100 /zm diameter, and an alumina

E -.=5.0

Z4.o

3°11 211 ~ I.

-

=79pro 01

40

L

8~:

'~-'-"~-~

160

0.02 a : 28 }Jrn

d prn

Fig. 2. Sizedistributionof droplets(U = 5.5 ms-' and u' = 1.3 ms ]).

42

K. NAKABE ET AL.

shield tube 5.0 mm i.d. This pyrometer was carefully calibrated in a flame of gaseous fuel. The hot junction of the thermocouple was coated with silicon oxide to eliminate the effects of surface reactions. The composition of the burning gas was determined by sampling it with a water-cooled sonic sampling probe made of stainless steel, having a suction hole 0.3 mm in diameter, and then analyzing it with a gas chromatograph or a chemiluminescence NOx analyzer. The optical system used to obtain the emission images of OH and C2 radicals is shown in Fig. 3. The self-emission of the OH radicals was passed through a metallic interference filter with a peak wavelength, Xp, of 309.0 nm and a half-value width, AX, of 17.0 nm of transmission (Vacuum Optics Corporation, Japan, IF-W), and through a colored-glass filter (Toshiba, UVD33S) to cut off the first transmission band for using the second harmonic band, and focused onto the photocathode of an image intensifier of electrostatic-field focusing type (Hamamatsu, V1389P) by a UV Nikkor lens (F4.5 a n d f = 105 ram). The projected image was reinforced and shifted in wavelength to an image in the visible band, then converted into a video signal by a CCD camera (NEC, TI-21), and stored in the frame memory of an image processor (ADS, IP-2). The uneven sensitivity and image distortion peculiar to this system were corrected using a uniform brightness board and a grid chart, respectively. For the emission images of the C2

radical, an interference filter with Xp = 516.5 nm and AX = 17.0 nm was used. The self-emission image of the C2 radicals was superimposed on the emission image of soot and particulates because the light emission from soot and particulates was much stronger than that from the C2 radicals and was continuous in wavelength [19]. The time-averaged self-emission image of the OH radicals was obtained by averaging over 64 frame images recorded in the memory at 4-s intervals (i.e., 256 s in total). This time interval consisted of image scanning (1/30 s), storage into the image processor, data transfer to a personal computer (NEC, PC9801UV2), and operation. In addition, the image was inverted into the intensity distribution of radical emission in a cross-section involving the flame axis. Instantaneous self-emission images of OH and C2 radicals were taken with a mechanical shutter (COPAL, SV: the minimum exposure measured was about 1/600 s) placed in front of the optical system. The schlieren images of flames were obtained using long-exposure and frame-averaging techniques. In the former, a mercury-vapor lamp was used as the light source and the slow shutter built into the camera was utilized, whereas in the latter, a stroboscope was used as the light source, the portion of the optical system described above following the CCD camera and the entire image processing system were utilized.

Flame Mechanical shutter

Video siqnal

tens

l

GPIB

Monitor

x 2 frame Image processor

Personal computer

Fig. 3. Optical system for obtaining self-emission images of OH and C2 radicals.

BURNING CHARACTERISTICS OF SPRAY FLAMES DISCUSSIONS ON THE REFERENCE SURFACE OF A PREMIXED SPRAY FLAME The direct photograph of a spray flame taken with an exposure of 1/15 s, and a frame from the highspeed photographs of a spray flame taken at a speed of 5000 frames per second (the exposure is about 100 ns) are shown in Fig. 4. In the long exposure photograph (1/15 s) it appears that a continuous, though somewhat rough, flame surface exists. In the short exposure photograph (100 ns), on the other hand, a continuous flame front cannot be observed, but the flame seems to be a cluster of discrete flame lumps that flow away downstream. This picture suggests that spray combustion has two features that are different from the gaseous fuel combustion. One is the locally statistical flame propagation mechanism caused by the nonuniform fuel distribution, and the other is the streamwise combustion processes of droplets independent of the flame's propagation.

(a)

Long exposure

(i/15s)

43

Because the long-exposure flame image shown in Fig. 4a gives the furthest upstream envelope of the domain in which flame lumps exist, estimates of the flame angle O, the angle between the reference surface of the flame and a streamline, will be too large compared to the time-averaged value, if the reference surface is determined from this picture. Figure 5 shows the flame images obtained by various methods, where the chain line at the left edge of each picture represents the flame axis. Figure 5a shows a schlieren image taken with 1/ 15-s exposure. It is extremely obscure, and no information on the flame structure is obtained. In addition, the shape of flame image is altered depending on the depth at which the knife edge cuts the beam in the schlieren optical system and on the depth of printing. Figure 5b shows a schlieren image obtained by averaging instantaneous flame images over 64 frames; the broken line in the figure represents the contour of the reference surface of the flame. Although the flame contour is seen rather clearly in this case. it i~ ~till

(b) Short exposure

(ca. lOOns)

Fig. 4. Direct photographs of spray flames with long or short exposures (O = 4.3 ms- 1, U ' : 0.98 ms-~, On = 0.050 and d = 47 #m).

44

K. NAKABE ET AL.

(a) Schlieren image

(b) Schlieren image

(c) Cross-sectional

with exposure

averaged over

emission image

of 1/15s

64 frames

of OH radical

Fig. 5. Variouskinds of flame imagesof premixedsprays(0 = 4.3 ms-% u' = 0.98 ms-~, = 0.050andd = 47#m). questionable whether the apparent flame contour can be taken as the reference surface as the local probability of appearance of flame lumps is not visualized in the figure. Figure 5c shows the crosssectional distribution of emission intensity of the OH radicals inverted from an emission image, where the domains with intensity less than 20%, 40%, 60%, 80%, and 100% of the maximum value are indicated by the brightness of the area. The locus of the position having the maximum intensity in each horizontal cross section is shown by the solid line, and this is regarded as the surface where the most violent reaction is occurring, that is, the reference surface of the flame. The broken line in Fig. 5c corresponds to the broken line in Fig. 5b. As anticipated, the frame-averaged schlieren image gives a larger flame angle because it visualizes the upstream envelope of flame lumps. RELATIONS BETWEEN SELF-EMISSION IMAGE OF OH RADICAL A N D DISTRIBUTION PATTERNS OF TEMPERATURE A N D COMPOSITION OF GAS

The patterns of the isointensity lines of OH radical emission in a vertical cross section, isotherms, and

isoconcentration lines for various components of gas (in volume fraction) are shown in Fig. 6 for a gas-liquid coburning system using propane and kerosene. The axial and radial distances, h and r, respectively, are taken as the ordinate and the abscissa, with the origin located at the center of the pilot burner port. A comparison of Fig. 6a and Fig. 6b indicates that the zone of peak emission intensity from the OH radicals coincides well with the zone of peak density of isotherms (that is, zone of steepest temperature gradient). Temperature data are lacking in the region above 1600°C due to the fusion of the thermocouple. Moreover, a comparison of Fig. 6a with Figs. 6c-e shows that CO, CO2, and 02 have concentrations of 0.5-1.0%, 0.5-2.0%, and about 18%, respectively, in the zone of peak emission intensity from the OH radicals. This confirms that intense combustion reaction occurs in this zone. The central region (r < 7 mm) is under the influence of the pilot flame, so it is omitted in the present comparison. These facts guarantee that, in a time-averaged sense, the zone of peak OH emission intensity represents the combustion zone of the most active reaction, which governs the burning velocity. The flame front determined from a long exposure picture (Fig. 4a) or a schlieren image (Fig.

BURNING CHARACTERISTICS OF SPRAY FLAMES

45 ,o

30 ,o

E

~<,o< f,, ~

.5

:°ol

t

10

0 (a)

~.o

.

OH

,0 OH

r

20

mm

radical

30

'~b'2~'3b' r

(b)

mm

Temperature

'

0 r

(c)

~'0'2'0'

mm

r

CO

(d)

0 ',b'2b

mm

r

C0 2

0

i0

mm

(e)

20 r

(f)

02

mm

NO

X

Fig. 6. Distribution patterns of the emission intensity of O H radicals, the temperature and the composition of gas for a gas-liquid coburning flame (0 = 5.5 ms ~, u' = 1.16 ms ~, ~n = 0.020, ~b/g = 0.045 and d = 37 #m).

5a, b) appears in a zone of extremely lowtemperature and low CO concentration, and so is inappropriate as the reference surface. Therefore, the measurement of burning velocity hereafter is wholly to be made using a cross-sectional image of OH radical self-emission. The pattern of isoconcentration lines for NOx is shown in Fig. 6f for a coburning case. A comparison of this figure with the distribution patterns of OH radical emission (Fig. 6a), gas temperature (Fig. 6b) and the concentrations of other components (Figs. 6c-e) shows that NOx is mostly produced downstream of the zone of active combustion reaction. B U R N I N G V E L O C I T I E S OF P R E M I X E D KEROSENE SPRAYS The apparent burning velocities, Srs, of premixed sprays of kerosene are shown in Figs. 7 and 8 for a time-averaged gas-phase velocity, O, of 5.5 ms -1. The apparent burning velocity is defined as

St= 0 sin0,

the Sauter mean diameter, d, of droplets (with a margin of __5/~m). It can be seen from this figure that Sr increases linearly as (~yt, the kerosene-toair-mass flow ratio, is increased or d is decreased. Notice that the initial turbulence intensity, u ' , varies along a line with a = constant, because the pressure, Pf, of atomizing air varies following the combination of ~bft and d. The turbulent burning velocities of propane are also drawn in chain line in Fig. 7. The upper and lower chain lines are for atomizing air pressure Pys equal to 0.30 MPa and 0.10 MPa, respectively. (Measured points are plotted in Fig. 9.) The atomizing air was injected to keep the initial turbulence intensity at the same level as that of spray flames. These data are within the range of reference [20], though a little lower. We noticed 5.0

----

Propane-Air

o a=6ojJm £

4.0

6.,//

~,

~q~" (,,9,;" ,.~-"

,b~"////rb'__..'.////~ ~?'~q)~/~'~.

A d=70 jJrn

(1)

where 0 is the angle between the reference surface and the axis of the flame. Although the streamline appear to bend slightly outward in Fig. 4a, the difficulty in measuring the flame angle, O, between the reference surface and a streamline has compelled us to adopt O. The parameter in Fig. 7 is the nominal value of

/ /

/ /

.,,..

/ /

/ / /

~.o

0

.

.;'+" /i/

If ~ I I 0.01 0.02 0.03

V4YJ,/

/ i

I I I 0.04 0.05 0.06 0.07

~fl.

kglkg

Fig. 7. Apparent burning velocities of premixed kerosene sprays, I (O = 5.5 ms i).

46

K. NAKABE ET AL. 5.0 ¸

~,~' .~,'

4.0

MPa kg/kg um

[3 0.!0 0.0 • - - ' E3.0 2o

',~6Z7

u "z7

E3. 0 u5 2.0

_/

,:,

1.0

n ¢~f1=0.04kg/kg n ~ft=O.05 kg/kg • ~fl=O.06 kglkg

0

I I I I I I 0.005 0010 0.015 0.020 0025 0.03(

////

lid

zx 0.30 o . o . - - . • 0.10 0.02 41 • 0.30 0.02 28

1.0

I

0

l/IJm

Fig. 8. Apparent burning velocities of premixed kerosene

I

0.02

I

I

I

0.04 ¢~t.t+ ¢~fg kglkg

I

0.06

Fig. 9. Effects of gas-liquid coburning on the apparent burning velocity (O = 5.5 ms-l).

sprays, II (I.7 = 5.5 ms-t).

that the burning velocities of kerosene spray are smaller than those of propane except for t7 = 70 /.tm. An empirical relation,

ST= (6800/dl)(@ft--0.012)(u ')1.15

(2)

is given in reference [6] for the burning velocity of premixed kerosene sprays. The values of ST calculated from this relation are given by the broken lines in Fig. 7. Although these values agree qualitatively with the present data, they are considerably larger. This may be partly because the experiments in reference [6] were carried out with a violent fluctuation of liquid concentration, and partly because the flame angle O was determined on the long-exposure direct photographs of a flame. The data in Fig. 7 are replotted in Fig. 8, taking d - I as the abscissa. Again, the broken lines in Fig. 8 represent the values calculated from Equation 2. Our comments on Fig. 7 also apply to this figure. The present data also agree qualitatively with their counterparts in reference [9]. BURNING VELOCITIES OF GAS-LIQUID COBURNING MIXTURES OF PROPANE AND KEROSENE Figure 9 shows the variation in the appararent burning velocity when a fraction of the propane in a propane-air mixture is replaced by the same amount of kerosene spray, with the initial turbu-

lence intensity kept constant. This can be done by turning kerosene on and off while the atomizing air is injected at a constant pressure Pf. It is known that the turbulence intensity in the cold flow field depends almost solely on Pf. In addition, according to Browning and Krall [10], kerosene vapors and propane have almost the same burning velocity. The apparent burning velocity increases from [] to • for Pf = 0.10 MPa, and from & to • for Pf = 0,30 MPa due to the change from a pure combustion mode to gas-liquid coburning. The mean diameter, iT, of droplets is 41 #m and 28 t~m, respectively, for pressures of 0.10 MPa and 0.30 MPa. It can be seen from this figure that the coburning effects are more prominent for a smaller overall fuel-to-air ratio or for a smaller mean diameter of droplets. Figure 10 shows the apparent burning velocities of gas-liquid coburning mixtures for an overall fuel-to-air ratio (ckft + ~t)fg) of 0.060 kg/kg. The mass fraction of kerosene in fuel is taken as the abscissa in Fig. 10a, whereas the reciprocal, a?-1, of mean droplet diameter is taken as the abscissa in Fig. 10b. These figures indicate that the burning velocity increases as the mass fraction of kerosene in fuel is increased or as the mean droplet diameter is decreased. Again, the initial turbulence intensity varies along the curves of constant mean diameter in Fig. 10a or along the curves of constant mass fraction of kerosene in Fig. 10b. This is because the pressure of the atomizing air varies if either the

s;

BURNING CHARACTERISTICS OF SPRAY FLAMES

47

m,+~=o.o 6 _

4.

•

•

3.0~'0 d=30 ~m 2.0

1.0 0

[]

• d=40jJm D d=50 um • a=60 jJm I

I

10

20

(a)

....

I

•

Reduced to s t a n d a r d i n i t i a l turbulence intensity

I

I

I

30 40 50 60 100 ¢~t/(m,+m~)

I

I

I

70

80

90

Effect o f Kerosene mass f r a c t i o n

5.01

4. 0

.....

-

/

~

%

in

100

fuel"

Reduce to standard initial turbulence intensity ~

t

~

E3I0

,+~)

2,0

Z~

35 %

•

50 %

1.0

[] ~7

~t+¢>~=0.06 0 I I I 0.01 0.02

I lid

60 *1. 100 */. I I I 0.03 0.04 l/jJm

(b) Effect of mean droplet diameter

Fig. 10. Apparent burning velocities of gas-liquid coburning mixtures of propane and kerosene (O = 5.5 ms-l and ~/t + ¢;g = 0.060). injection rate of kerosene or the mean droplet diameter is changed with the other parameter kept constant, which causes the variation of initial turbulence intensity. Therefore, an attempt was made to convert the burning velocity for an arbitrary value of Pf into the one for the standard value, 0.15 MPa, utilizing the turbulent burning velocity data of propane-air mixtures. It has been found that, if only atomizing air issues from the injection nozzle for pure propane combustion (~fg = 0 . 0 6 0 ) , and if the value of P / i s changed, then the burning velocity varies within the range indicated by an arrow on the ordinate. In that case the observed burning velocity can be converted into the one for the standard Pf value, assuming that the ratio of the burning velocities for arbitrary Pf value to the one for the standard value is the same as for pure propane combustion. The results are indicated by the broken lines in Fig. 10.

If experiments could be executed with the initial turbulence intensity kept constant, and if the results were close to the broken lines drawn in Fig. 10, it would be noticed from Fig. 10a that for every value of a there exists a specific value of mass fraction of kerosene for which the burning velocity is a maximum. The position of maximum burning velocity on a constant a line shifts toward smaller kerosene mass fraction as the value of a value is decreased. In addition, Fig. 10b shows that for a constant mass fraction of kerosene, the apparent burning velocity increases as the mean diameter decreases and that the maximum value seems to occur around a --- 30 #m. DISCUSSIONS ON FLAME STRUCTURE The distribution pattern of the emission intensity of OH radicals is compared with that of tempera-

48

K. NAKABE ET AL.

E5°

~c

%%°:" t / ~

/Hm.J I~00

~:3e 2c

//#iT

1c ,o

2o

r mm

3o

( a ) OH radical

o

"C ~

200

' ,'o'2b'3b' r mm

(b) Temperature

Fig. 11. Distribution patterns of the emission intensity of OH radicals and of the temperature for a propane flame ( O = 5.5 ms -j , u ' = 1.16 ms -I and ¢fg = 0.065).

ture in Figs. 11 and 12 for a pure propane flame and for a pure kerosene spray flame, respectively, for the same fuel-to-air ratio. In this case, the kerosene spray had a considerably larger mean diameter of droplets. As mentioned previously, a fairly good correspondence is seen between the OH emission and the temperature. As for the distribution of temperature, the intervals between isotherms are wider for the pure spray flame (see Fig. 12b) than for the pure propane flame (see Fig. lib) or for the gas-liquid coburning flame (see Fig. 6b), so that the temperature gradient in the axial direction is considerably more gentle in the

50

%0

/ l l l k / . L ~ . 'ooo

I / I / I / L . ~ 80o

o~

2o

///////~

600

Hill/ ~

1

0

Ill/I/

0

10

r

20

mm

(a) OH radical

~oo

T °C

0 'lb'2b r

~

200

rnm

(b) Temperature

Fig. 12. Distribution patterns of the emission intensity of OH radicals and of the temperature for a premixed kerosene spray flame ( 0 = 5.5 ms -~, u ' = 1.16 ms - l , ¢,f~ = 0.065 and d = 110 #m).

spray flame. This implies that the combustion process in a spray flame with a large mean droplet diameter is slower than in a gaseous flame or in a gas-liquid coburning flame, so that the large droplets of spray survive to the downstream region and, accordingly, the spray flame has a long tail. The small temperature gradients of a coarse spray (d = 110 /zm) cause its low burning velocities. However, the apparent burning velocities of fine sprays (d _<_ 70 ~m) exceed those of propane-air mixtures (see Figs. 7 and 8). This is probably due to the local statistical propagation mechanism that is inherently associated with spray flames (see Fig. 4b). Typical examples of the self-emission images of OH radicals in short exposure (about 1/600 s) are shown in Figs. 13a and b for a pure propane flame, and for a pure kerosene spray flame, respectively. A definite conclusion cannot be drawn because it is difficult to invert these images into vertical cross-sectional images due to their poor S I N ratio; however, it seems that a zone of strong emission from OH radicals extends through a rather thin layer in the outer flame zone of propane. This implies the existence of a continuous turbulent flame front. On the other hand, for the spray flame, bright regions survive to the inner downstream flame zone, forming the flame lumps as shown in Fig. 13b. This implies that a spray bums in the mode of group combustion of droplets [21, 22] where no continuous flame front exists, and that droplets move downstream, burning as single droplets or as a group of droplets accompanied with the vapor evaporated from the droplets. This agrees well with the distribution patterns of temperature shown in Figs, 11 and 12. It is conjectured that turbulence only enhances the discontinuity of spray flame fronts, because gaseous flame fronts are not discontinuous in the same initial turbulence intensity and fuel-to-air ratio. Figure 14 shows the flame images of short exposure taken under the same condition with Fig. 13, replacing the optical filter for OH radicals by the one for C2 radicals. Because a pure propane flame is nonluminous, we expect that Fig. 14a is close to the self-emission image of C2 radicals.

BURNING CHARACTERISTICS OF SPRAY FLAMES

49

50 E E

40

a.-

30 20 10 0

I

i

0

I

i

10

I

20 r

l

0

i

I

= 0.065)

i

10

rnm

(a) Propane flame (~fl

,

I

20 r

rnm

(b) Kerosene spray flame

(~fl

= 0.065

and a = ll0~m) Fig. 13. Selgemissionimagesof OH radical with sho~ exposure (0 - 5.5 ms-i and u' 1.16 ms i). Though its appearance is almost the same as Fig. 13a, a strong emission of C2 radicals is observed around the rim of the pilot flame burner. Figure 14b, on the other hand, is regarded as a particulate radiation image because, in a pure spray flame, the self-emission of C2 radicals is superimposed on the very strong radiation of luminous flame emitted from soot particles and particulates. This figure reconfirms that a premixed spray burns by group combustion of droplets. For a propane flame, the intensity of selfemission of C2 radicals is weakened rapidly as the equivalence ratio becomes less than unity, whereas, for a kerosene spray flame, the level of emission intensity of the flame lumps does not change much, even if the fuel-to-air mass ratio is decreased. This implies that, in the luminous regions where droplets burn in groups, fuel vapor having a high local concentration is burning with light emission from soot particles, even for a low fuel-to-air ratio.

CONCLUSION The premixed spray burner was carefully designed to provide the two-phase flow with lower concentration and velocity fluctuations and better crosssectional uniformity than previously achieved. Various methods were examined for determining the reference surface of a flame that governed the spray burning velocity. Then, the apparent burning velocities of premixed kerosene sprays and gas-liquid coburning mixtures of propane and kerosene, were measured, and some discussions were made on the structure of spray flames. The results are summarized as follows: 1. Among a direct photograph, a schlieren image of a flame, and an emission image of OH radical, the last is the most appropriate for determining the reference surface of a spray flame, which is inverted into a surface involving points with the maximum emission intensity in a vertical cross section containing the flame axis.

50

K. NAKABE ET AL.

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30 20 10

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0

i

I

i

10 r

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(a) Propane flame (~fl = 0.065)

I

I

I

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10 r

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(b) Kerosene spray flame (~fl = 0.065 and d = ll0um)

Fig. 14. Self-emissionimages of C2 radical with short exposure (O = 5.5 ms- ' and u' = 1.16 ms-J). 2. A fairly good correlation exists between the distribution pattern of the emission intensity of OH radicals and those of the temperature and the composition of gases. 3. The apparent burning velocity of a premixed kerosene spray increases linearly as the fuel-toair mass ratio is increased, and decreases inversely proportional to as the mean diameter of droplets is increased. 4. The present burning velocity data for premixed kerosene sprays agree qualitatively with those of reference [6], but the values of the former were about a half of the latter. 5. The promoting effect of gas-liquid coburning on the burning velocity becomes more prominent as the overall fuel-to-air ratio decreases or as the mean diameter of droplets becomes smaller. 6. For a gas-liquid coburning case, there exist optimum values of the kerosene fraction in the whole fuel and the mean diameter of droplets that maximize the apparent burning velocity.

7. A definite difference in the structure of flames from a gaseous fuel and from a premixed spray can be recognized using self-emission images with short exposure (about 1/600 s) of OH and C2 radicals.

This research was partially supported by the Grant-in-AM o f Scientific Research, Ministry of Education, Science and Culture, Japan, as well as by the Grant-in-AM of Scientific Research, General Sekiyu Research & Development Encouragement & Assistance Foundation. The authors wish to express their gratitude to Takatoshi Saeki, Yukihito Yoneda, and Tadao Ogawa, students o f Osaka University, for their cooperation in experiments.

REFERENCES

1. Williams,A. Combust. Flame21:l-31 (1973). 2. Faeth, G. M. Prog. Energy and Combust. Sci. 3:191224 (1977).

BURNING CHARACTERISTICS

OF SPRAY FLAMES

3.

Law, C. K. Prog. Energy and Combust. Sci. 8:171-201 (1982). 4. Sirignano, W. A. Prog. Energy and Combust. Sci. 9:291-322 (1983). 5. Mizutani, Y., et al. Sixteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, Pennsylvania, 1977, p. 631. 6. Mizutani, Y., and Nishimoto, T. Combust. Sci. Technol. 6:1-10 (1972). 7. Polymeropoulos, C. E., and Das, S. Combust. Flame 25:247-257 (1975). 8. Ballal, D. R., and Lefebvre, A. H. Eighteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, Pennsylvania, 1981, p. 321. 9. Myers, G. D., and Lefebvre, A. H. Combust. Flame 66:193-210 (1986). 10. Browning, J. A., and Krall, W. G. Fifth Symposium (International) on Combustion, Chapman & Hall, London, 1955, p. 159. I I. Mizutani, Y., and Nakajima, A. Combust. Flame 21:343-350 (1973). 12. Mizutani, Y., et al. Trans. JSME 52:1931-1939 (1986) [in Japanese]. 13. Crosley, D. R., and Dyer, M. J. Proceedings o f the

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51 International Conference on Lasers '82, STS Press, McLean, Virginia, 1983, p. 752. Kychakoff, G., et al. Applied Optics 23:704-712 (1984). Allen, M. G., and Hanson, R. K. Optical Engineering 25:1309-1311 (1986). Dyer, M. J., and Crosley, D. R. Proceedings o f the International Conference on Lasers "84, STS Press, McLean, Virginia, 1985, p. 211. May, K. R. J. Sci. Instru. 27:128-130 (1950). Nukiyama, S., and Tanasawa, Y. Trans. JSME 5:131135 (1939) [in Japanese]. Beretta, F., et al. Combust. Sci. Technol. 22:1-15 (1980). Andrews, G. E., et al. Combust. Flame 24:285-304 (1975). Chiu, H. H., and Liu, T. M. Combust. Sci. Technol. 17:127-142 (1977). Chiu, H. H., et al. Nineteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, Pennsylvania, 1983, p. 971.

Received 24 July 1987; revised 21 January 1988