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The influence of fuel properties on drop-size distribution and combustion in an oil spray
Twenty-first Symposium (International) on Combustion/The Combustion Institute, 1086/pp. 557-566
THE INFLUENCE OF FUEL PROPERTIES ON DROP-SIZE DISTRIBUTION AND COMBUSTION IN AN OIL SPRAY A. BREICqA DE LA ROSA, A. SOBIESIAK, AND T.A. BRZUSTOWSKI
Thermal Engineering Group Department of Mechanical Engineering University of Waterloo Waterloo, Ontario, Canada N2L 3G1 The paper describes an experimental study of the effect of differences in fuel properties on oil atomized and burned under identical conditions. This was done with No. 2 and No. 6 oil sprayed from the same air-blast atomizer with the same atomization parameters. The local droplet-size distributions were measured throughout the cold sprays using the Malvern Particle Sizer. It was found that the more viscous heavy fuel systematically produced a droplet size range about double that of the lighter fuel for the same atomization parameters. The sprays of both fuels had large droplets at the periphery, but those of No. 2 oil had predominantly fine droplets in the core, in contrast to the sprays of No. 6 oil which had some relatively large drops in that region. The sprays of the lighter oil burned predominantly in vapour diffusion flames within the boundaries of the jet of atomizing air. The heavier oil burned very differently. In general, the large droplets had sufficient momentum to penetrate through the atomizing air jet and disperse widely. Many droplets appeared to burn individually, and many were quenched before burning out. The peak temperatures measured in the flames of the heavier oil were about 200K lower than for the lighter oil. Radiation measurements also confirmed that the heavy oil generally did not burn completely. These differences in flame characteristics are attributed to the different droplet-size and velocity distributions and to the different evaporation histories of the droplets, all of which reflect the different fuel properties. Data on the spatial variation of the droplet-size distributions in the cold spray of the two fuels are important in explaining the observed combustion behaviour.
Introduction It has long b e e n k n o w n that the droplet-size distribution (DSD) in the spray is i m p o r t a n t in d e t e r m i n i n g m a n y characteristics of the flame of atomized liquid fuel. Until recently, the bulk of research o n the atomization of liquid fuels did n o t involve the m e a s u r e m e n t of the actual DSD. Instead, a n a p p r o p r i a t e m e a n d i a m e t e r was taken as the proxy for the whole distribution. I n most c o m b u s t i o n work, that proxy was the Sauter M e a n D i a m e t e r (SMD or Ds2), the d i a m e t e r of the d r o p l e t which has the same ratio o f surface area to v o l u m e as the whole spray. T h e SMD could be m e a s u r e d directly a n d simply by a light o b s c u r a t i o n technique ~, which u n f o r t u n a t e l y h a d n o spatial resolution. Nevertheless, m e a s u r e m e n t s o f this k i n d have led to the evaluation o f m a n y atomizers of d i f f e r e n t types in terms of correlations b e t w e e n the SMD a n d the relevant d i m e n s i o n s of the
atomizer, the flow p a r a m e t e r s , a n d fluid properties. Many such correlations have b e e n tabulated 2"3, Some of their shortcomings have b e e n p o i n t e d o u t previously in n u m e r i c a l 4 a n d e x p e r i m e n t a l 5 studies. T h e recent d e v e l o p m e n t of laser optical i n s t r u m e n t s which c o n v e n i e n t l y p r o d u c e reasonable r e p r e s e n t a t i o n s o f local droplet-size distributions has b e e n a m a j o r advance. It is now possible to get a good idea of the spatial variation of the DSD, which is far m o r e informative about the processes o c c u r r i n g t h r o u g h out the spray v o l u m e t h a n any single global DSD or even the spatial variation o f a m e a n diameter. This research was the first step in a p r o g r a m to study the possibility o f c o m p e n s a t i n g for variable fuel properties by controlling the atomization parameters. Specifically, the spatial variation o f the DSD o f a steady u n c o n f i n e d oil spray p r o d u c e d f r o m a given nozzle was mea-
557
558
PRACTICAL COMBUSTION DEVICES TABLE I Physico-Chemical Properties of the Fuel Oils Studied at 298K and 101.3kPa
fuel #2 oil #4 oil #6 oil kerosene
M kg/kmol
p kg/m 3
Tb K
L kJ/kg
202 240 315 154
869 885 941 820
534 575 660 460
355 354 346 251
v 10 6m2/s ~ 4.0 16.1 1126 2.1
I*
31.7 27.5 19,8 41.9
T 10 3N/m
C kJ/kgK
Q MJ/kg
27.1 29.4 31.8 25.0
1.845 1.809 1.745 2.090
44.600 44.429 41.924 43.500
*at 15.5 ~ sured in cold flow for a wide range of atomization parameters. T h e b u r n i n g of this spray was subsequently studied for exactly the same values of the parameters. Extreme variations in fuel properties were obtained by using two different fuel oils, No. 2 and No. 6. The atomization parameters varied were fuel temperature, fuel flow, atomizing air temperature and pressure, the ratio of the mass flows of fuel and atomizing air, and the air pressure--hence velocity. In addition to D S D measurements, the flames were examined by direct photography and color schlieren flow visualization. Measurements were also made of the time-mean temperature field, total flame radiation, and the m o m e n t u m field of the atomizing air stream and of the flame of the corresponding spray.
Experimental Procedure The fuels used were No. 2 (blended) and No. 6 (residual) fuel oils. Table 1 shows some of the important properties of these fuels, with two others included for comparison, Since these fuels are not pure compounds, the molecular weight, the latent heat, and the "mean average" boiling point (the arithmetic mean of the mole-fraction weighted b.p and the cube of, the volume-fraction weighted cube root of the b.p.) are nominal values inferred from the specific gravity and the distillation curve 6. The D S D was measured with a Malvern Particle Sizer, Model 3600D. This is a line-ofsight optical i n s t r u m e n t in which the signal is produced by small-angle forward scattering of light from a laser beam (2 roW, He-Ne) traversing the spray. T h e radial distribution of the scattered light intensity is measured with a concentric-ring detector array, digitized and converted to a distribution of volume fraction over droplet diameters with software based on Fraunhofer diffraction theory 7'8'9'1~ The software also provides for a curve-fit of the measured distribution to one of several common distribution functions. The Rosin-Ram-
mler distribution was chosen for this work, and its parameters X a n d N were used as the basis for comparing the sprays. In what follows, the D S D is the distribution of droplet volume over the droplet diameter space. It is defined by Eq. 1 below. In conventional notation, 1 - V = e x p [ - ( D / X ) x]
from which it follows that DSD ~ dV/dD = N X - x D N-1 e x p [ - (D/X) x]
( 1)
where D is the droplet diameter, V is the volume fraction of liquid occurring in drops of diameter less than D, X is the droplet diameter such that (1 - l/e) of the liquid volume resides in smaller droplets, and N is a dimensionless width parameter. Data are often presented as lOOdV/dD in units of Ixm-1 or of volume % per p,m. The nozzle used in this work was a small commercial Delavan air-blast atomizer, type Siphon SNA 30609-11 rated at 2 gph (7.6 l/h). In this nozzle, air is blasted at the liquid from inclined converging ports on the periphery of the liquid feed tube about 1 mm upstream of the 1.20mm i.d. discharge orifice. The nozzle was mounted horizontally on a rig which could be traversed horizontally and vertically. D S D measurements were made with the laser beam horizontal and normal to the nozzle axis, and crossing it at a downstream distance Z of 50, 100, 150, 200, and 250 m m from the tip of the nozzle. Measurements were also made with the centre of the laser beam at a distance R of 10, 20, 30, and 40 m m below the axis of the nozzle at Z of 50, 100, and 150 mm. These measurements are called axial and radial, respectively. Fig. 1 shows the co-ordinates. Differences in fuel properties and the limitations of the Malvern i n s t r u m e n t with respect to multiple scattering effects 15,a6A7 were considered in arriving at the combinations of experimental conditions investigated. These are
COMBUSTION OF A PRACTICAL OIL SPRAY
Fro. I. The co-ordinates used in reporting the data. presented in detaiI by Brefia de la Rosa is. The conditions selected for discussion in this paper are shown in the figure captions. The DSD were measured no closer to the nozzle than 50 mm. in order to work with ohscuration levels below 50%. Many workers 19'2~ have now shown that the Malvern Particle Sizer is accurate u n d e r those conditions. In the combustion experiments, the flames of No. 2 fuel oil were stable but the flames of No. 6 fuel oil had to be stabilized. Two small hydrogen diffusion flames located at R -- ---15 mm from the nozzle axis at Z = 2 mm downstream from the nozzle tip, with a total H2 flow of 1.05 x 10 -3 g/s (of the order of 0.1% of the mass flow of oil), provided the necessary stabilization. The i.d. of the hydrogen burners was 0.75 mm and the length of each hydrogen diffusion flame was about 15 mm. Radiation from the whole flame was measured with a Kipp & Zonen thermopile (model CA-l). The flame was treated as a point source and the measurements were made at a distance of S = 2.45 m from the centre of the flame. Measurements at varying S for several sets of experimental conditions showed that the incident radiant heat flux varied as S -z, verifying the applicability of the point-source model in this work. The results of the radiation measurements were expressed in terms of q~, the fraction of heat release radiated. It is a derived quantity, based on the assumption that all of the fuel is b u r n e d and given by
q"r = (mfgrQ/4"lrSZ)cosO
(2)
where q"r is the radiant heat flux incident on the thermopile, mf is the mass flow of fuel, Q is the standard heating value of the fuel per unit mass, and 0 is the angle between the normal to
559
the surface of the thermopile and the line of sight from the centre of the flame. The temperature measurements were made with an uncoated Pt/Pt - 10%Rh thermocoupie, with a wire diameter of 125 p.m. This size was used because thermocouples of 50 and 75 p~m wire proved not to be sufficiently robust for this work. They were not corrected for radiation. The reasons for these decisions were very similar to those recently described in detail24'25. T h e temperatures reported here are long-time mean values. T h e measurements of the m o m e n t u m flux were made with a fivehole water-cooled pitot probe 9.5 m m in diameter. Because of its large size and because of fouling by droplets, the probe was not used close to the nozzle. T h e use of a five-hole probe appeared appropriate because of the strongly divergent nature of the flow. The design and calibration of the probe followed conventional procedures 26.
Some Special Features of the DSD Measurement It is important to note the following features of the DSD measurements which affect the interpretation of the results: - - T h e Malvern Particle Sizer is a line-of-sight instrument that measures the DSD of the droplets which reside within the sampling volume during a sweep of the scattered light distribution; a period of 30 ms in the instrument used. The sampling volume is defined by the intersection of the laser beam (9mm dia.) and the spray. T h e beam is normal to the nozzle axis and falls along a horizontal chord in the circular cross-section of the spray. The data labelled "axial distributions" in the figures refer to the chord which passes through the atomizer axis at varying distance Z downstream of the injector face. The "radial distributions" refer to the DSD of droplets captured in a beam whose axis lies a distance R below the nozzle centreline. The DSD are presented without deconvolution of the line-of-sight measurements. Clearly, the DSD for any R includes the contribution of droplets flowing through the spray crosssection only at radii equal to or greater than R-4.5 mm. - - E a c h of the curves shown is the RosinRammler distribution fitted to the data by the software of the Malvern instrument based on the fraction of spray volume found in each of 15 size ranges. T h e symbols on these curves are not data points. They serve only to identify their parameters listed in the legend.
560
PRACTICAL COMBUSTION DEVICES
- - T h e measurement has a sensitivity threshold, imposed by the software. The relative contribution of any size range which falls below 0.1% of the total is totally ignored. T h e size ranges are always the same for a given lens, but the n u m b e r of ranges used in the curve-fitting depends on how many contribute more than 0.1% of the total volume of the spray. - - T h e DSD fitted to the data in each measurement is normalized by the total volume of liquid recognized at that location. As a consequence of these features of the measurement, the results are in the form of the closest-fit Rosin-Rammler distributions which represent about 99.8% of the total volume of liquid in each m e a s u r e m e n t volume. These distributions do not track the evolution of the spray measured upstream. Given a spray in which there are few very large droplets, the measurement will reveal these droplets in increasing proportion as the total volume of liquid decreases by the evaporation of the smallest droplets first. U n d e r certain circumstances, this procedure could falsely suggest the occurrence of coalescence even in very dilute sprays. If the spray is not homogeneous, there is a radial variation of the DSD which is only approximately given by the variation of the DSD with R. T h e error is greatest for the smallest values of R, since the measurement volume then contains droplets which are located at the edges of the spray as well as those in the core. Further from the axis, the location of the droplets measured becomes defined with increased precision as R becomes large relative to 4.5 ram.
Results
and
Discussion
Figures 2 and 3 illustrate the effect of different fuel properties, primarily viscosity and volatility, on the axial variation of the DSD. Figure 2 shows the effect of fuel temperature for No. 2 oil when Tf rises from 298 to 348 K, i.e.: from 0.56 to 0.65 of the mean average boiling point. (Note that the vertical scale in the lower part covers twice the range in the u p p e r part.) The corresponding decrease of the liquid viscosity is from 3.4 • 10 -~ to 1.2 x 10 -3 kg/(m s). These data were taken at the highest ratio of the mass flow of atomizing air to the mass flow of fuel, a/f = 2.0. T h e sprays are coarser at lower values of a/f, but the trends are the same. Two effects are evident. First, X decreases by about 40%, Secondly, the variation of the DSD
AXIAL DISTRIBUTIONS FUEL OIL NO. 2 Po=60kPa,
TQ=298K
a/f =2.0, mf =.509/s 1.5-9 ~'~
1.2 ]
J o >
O.9-
,-,
o.+-
0
0.~-
T, = 298K
7 f
\~ 1k
%
( X,N ) ; Z (r,,~) o = (526.1.6) 50
,
o =(51.2.;.5) 100 ,+.= (.~.4. ;.5) ;50 \%,
SO
100 DIAMETER,
+ = +e.+. ;.6)
150
2oo
200
,u,m
s.oE
T+ = 348 K
(X,N) ; Z(mm) 50 o =(~o.4. ,5) ;oo + =(~2.o. ;.6) 15o
:i 2.0 ~ , ._i
I///-%~k
>~
IF/
"0
'~ ~,
!
o
~ = (29.4. ~.5)
~ 1.0
'
~
k
~
+ = (35.0, 1.5) x = (39.6, 1.5)
200 250
o 0.o . . . . .
~ . , ."#''~'~.~.-~.-v-~. f . . . . ,50 100 150 DIAMETER . /../,m
200
FIG. 2. Axial variation of measured droplet-size distributions in a cold spray of No. 2 oil: top set of curves for oil temperature of 298K; lower set for 348K. The constant parameters are T~ = 298K, m/= 0.5 g/s, and a/f = 2.0. Note that the curves are the Rosin-Rammler distributions fitted to the data by the software of the Malvern Analyzer. The symbols on these curves serve only to identify them. Note that the vertical scale in the lower portion of the figure has been compressed by a factor of 2. with Z is quite different. At the lower temperature, the DSD are remarkably similar at Z from 50 to 250 mm. At the higher temperature, the proportion of volume represented by the smaller droplets decreases significantly with distance, because of the more rapid evaporation of the small droplets of the hotter oil. Similar effects for No. 6 oil are shown in Fig. 3. Here, the variation in fuel temperature is from 323K to 348K, or from 0.49 to 0.53 of the mean average boiling point, and the effect of volatility is not as p r o n o u n c e d as in Fig. 2. T h e decrease in the liquid viscosity is from 180 • 10 -~ to 40 x 10 -~ kg/(m s) and the decrease i n X is about 20%. A comparison of the lower sets of curves in Figs. 2 and 3 clearly shows the effect of fuel
COMBUSTION
OF A PRACTICAL
OIL
AXIAL DISTRIBUTIONS
a/f
: 2.0,
K
Po:60kPa,
m f = . 5 0 g/~s
s.0-
a / f = 2.0, / ~ [ ~ ~ !
1.5.
E
:t. ..~ j o>
Tf : 323 K
Z=50
0.9
0
E :3.
0.3-
50
I00
150
200 ,~m
300
Z = 100 m m
( X,N ) ;
T~ = 348 K
o
1.2-
~ ( X , N ) ; Z(~m)
j >0
0.9-
o = (52.4, 1.5) o = (64.9, 1.5)
t-~
0.6-
~, = (79.1, 1,4) + = (81.3, 1.4) x = (84.7, 1.3)
0
0.3-
100 150 200 250
100 DIAMETER,
i
200 ~m
,
"!"
,
,
~ / ) ~ +
1.0
o.o
50
100
R (~) ($0.4, 1,5) 0 (40.7, 1.6) 10 (61.1, 2.0) 20 (77.5, 2.5) 30
150
200
3.0Z = 150 men
~
.
( x,N ) ; R(ram) o : (32.0, 1.6) 0 o = (37.8, 1.5) 10 = (49.8, 1.5) 20 + : (72.5, 1.8) 30
+
2.0-
1.0. . . .
0
50
D= o= z~= +:
2.0
0
n
1.5-
0.(1
.j
200
3.0-
~ § 100 DIAMETER,
mm
( X.N ) : R (mm) D = (29.4, 1.5) 0 o = (37.3, 2.1) 10 = (53.9, 3.8) 20
3"0~
( X , N ) : Z(mm) o = (62.4, 1.5) 80 o = (81.7, 1.4) 100 = (92.9, 1.4) 150 + = (94.9, 1.4) 200 x = (98.5, 1.3) 250
0,0
E :k -~.
T1= 348K m I = .50 g/s
4.0
1.2-
o,6-
0
561
RADIAL DISTRIBUTIONS FUEL OIL NO. 2
FUEL OIL NO. 6 P Q = 6 0 kPa, T a = 2 9 8
SPRAY
x ~ .
;/
x=(94930)
40
?- i
300
F[e,. 3. As in Fig. 2, but for No. 6 oil at 323K (top) and 348K (bottom). Other parameters as in Fig. 2. properties, since all other conditions are the same. For No. 6 oil, the liquid viscosity is 33 times greater than for No. 2 oil. The main difference between the two sprays is the relatively greater number of large droplets on the axis of the spray of the more viscous oil, with X greater by about a factor of 2. Figures 4 and 5 show the variation of the DSD with R at increasing downstream distances. The data in these two figures were obtained under the same conditions as those in the lower parts of Figs. 2 and 3. T w o features of these data are evident. One is the close similarity of the DSD in the outer region of the spray for both fuels. There is a high concentration of relatively large droplets at the periphery of each spray. The other is again the difference between the DSD in the cores of the two sprays. The spray of the lighter oil has a core of small droplets; the spray o f the heavy oil has many more large droplets in the core, including some of the largest in the spray. The data of Figs. 2 to 5 could be condensed by showing only the variation of the RosinRammler parameters X and N. However, Figs.
0.0 50
100 DIAMETER ,
150
200
/~rn
Fie,. 4. Radial variation of the DSD for No. 2 oil at 348K. Other parameters as in Fig. 2. 2 to 5 also show what would be involved in any attempt to correlate these X and N with the atomization parameters and fuel properties. For each set of parameters and properties, X and N also depend on location in the spray. Correlations of X and N would, therefore, have to be very specific not only to the atomizer used but also to the aerodynamics of the environment in which it operates. Because of their lack of generality, no attempt to develop such correlations has been made in this study. The measurement of local DSD has revealed the existence of large droplets at the edges of the flame. These droplets are very important in determining the burning characteristics of the spray. Flame photographs and schlieren flow visualization have shown that the flame of No. 2 oil under the conditions of Fig. 4 looks very much like a gaseous diffusion flame surrounded by some burning droplets quite close to the nozzle. These droplets either burn up very quickly or are entrained into the luminous flame and the surrounding layer of hot gas. No
562
PRACTICAL COMBUSTION DEVICES RADIAL DISTRIBUTIONS FUEL OIL NO. 6 Pa = 60 kPo, T ~ = 348 K a / f = 2.0, in, = . 5 0 g / s
3o
"]
,.o4 I I 0
/%<+%
/ i~\
z=50 mm
\
:
50
100
150
,o
200
E
J 0>
e.o ~
/ A \
1 4
~U-. _ ~
/ ~
\
= = (6..9.1.5)
o
\~
o: c,o.~.,.,~
,o
~=
20
~+/<
(72.7,
2,8)
,o
0
I00
50
200
150
1.5 Z = 150
1.2-
]~+
mm
"~x
( X,N ) ', R (mr'n)
\
o
0.////+77
:~
o0)~/~ ........ 50
I00
DIAMETER
150 ,
200
,u.m
Fro. 5. As in Fig. 4, but tot No. 6 oil.
individual droplet combustion is seen outside the bulk of the luminous flame. The behaviour is very different tbr the heavier oil. The direct flame photographs (exposure of 1/4000 s) shown as Figs. fi(a) and (b) illustrate the difference. Both photographs are to the same scale, and correspond to the conditions of Figs. 4 and 5, respectively. T h e flame of No. 6 oil is much less uniformly luminous. Many small flames, associated with large single droplets or clusters of droplets, are clearly visible over its entire length. In fact, the whole flame is nmch less uniformly luminous than the flame of the lighter oil and appears to have a small-scale structure. This interpretation is reinforced by schlieren and schlieren/direct photographs which show, in addition, that many droplets remain outside the hot gas layer s u r r o u n d i n g the flame. It would appear, therefore, that the lighter oil is atomized into a finer spray, evaporates more quickly, and b u r n s predominantly as a single diffusion flame of the vapour formed in the core of the spray. T h e heavier oil atomized
u n d e r exactly the same conditions forms a coarser spray, evaporates more slowly, and burns in what seems to be a large n u m b e r of small diffusion flames, some of which are far removed from the bulk of the luminous region. The appearance of this flame strongly suggests that many of the large droplets are b u r n i n g individually. The measurements of flame radiation produced unexpected results, but these can be explained in terms of the different flame structures of the two fuels. Radiation from flames of No. 2 oil behaved similarly to that from diffusion flames of hydrocarbon gases. For the whole set of fuel and air temperatures, 't r ranged from about 34% for the lowest a/f ratio to a value about 10% at the highest all ratio. These results were quite independent of the air and fuel temperatures. The results for No. 6 oil were very different. They were very sensitive to both the air and the fuel temperatures. At the conditions of Figs. 4 and 5, xI* was about half of that for the lighter oil. The reason was incomplete combustion. Many large droplets of the heavier oil were quenched when only partly burned, and some at the outer edges of the spray did not b u r n at all. However, the calculation of 9 from Eq.2 was on the assumption that combustion was complete. At lower fuel and air temperatures, No. 6 oil b u r n e d even more poorly and xI* was only a few percent, and at 298K it would not burn at all. The flame temperatures of the two fuels were also very different. Figs. 7 and 8 show the mean temperatures measured for the two fuels u n d e r similar conditions. In this case, 9 was about 10% for both fuels. The temperature profiles for the lighter oil look like those of a horizontal gaseous diffusion flame showing the effects of buoyancy, with a peak value of 2000K at Z = 200 mm. (The thermocouple had melted at smaller values of Z.) For the heavier oil, the highest temperature measured at these conditions was 1600K, and the profiles close to the nozzle showed that there was little b u r n i n g in the core. The highest temperature measured in the flame of No. 6 oil u n d e r any conditions was only 1800K. A final result of the measurements was a very simple characterization of the flow field of the spray flame of the lighter oil. Profiles of the axial (horizontal) c o m p o n e n t of velocity were calculated from the measured profiles of mom e n t u m flux and temperature for the b u r n i n g spray whose temperature profiles are shown in Fig. 7, and compared with the corresponding far-field velocity profiles of the jet of atomization air alone (Re=5 x 104). It turned out that the two sets of profiles were similar in both
FIG. 6. exposure FIG. 6. exposure
(a) P h o t o g r a p h o f the b u r n i n g spray o f No. 2 oil p r o d u c e d under the same conditions as in Fig. 4; 0.25 ms; to same scale as in Fig. 6(b). (b) Photograph o f the b u r n i n g spray o f No. 6 oil produced under the same conditions as in Fig. 5; 0.25 ms; to same scale as in Fig. 6(a). 563
564
PRACTICAL COMBUSTION DEVICES TEMPERATURE PRORLES
T E M P E R A T U R E PROFILES
FUEL OIL NO. 2 P==96kPa, o/f
2000
FUEL OIL NO. 6 T==373K
= 2.0,
P==96kPa,
T~, = 3 4 8 K Z = 400
1500
0
ooooooo=
~ ~
I
ao
. . . .
I
. . . .
I
. . . .
o
1000
o
9
~
" . . . .
Z = 3 0 0 Inn"=
1500-
~ o
500
Tf=348K
mm
ooooo~176
1000
T==575K
a/f=2.0,
2000-
[
. . . .
I
. . . .
I
. . . .
o ~
o o
500
9 oo=
o
0
I
2000
v ~176176176176
Z --- 3 0 0 m m
Z = 250 mm
1500 9
9
~
=
@
500
i ....
i ....
i ....
O
1000
Q=~
~ ....
QQOOOQaO@OOOO
1 ....
i ....
i ....
i
O
0
2000
~ 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6Z = 2 5 0 m m
1500.
Oo
o~
1000
v ~
DOOODO@DOO
.i
Z = 200 mm
OO
2
@@O D
~176
500. 0 2000.
....
i ....
i ....
i ....
] ....
i ....
i ....
i 2000i
oo~
1500
Z = 200 mm ~
0
-100
-50
. . . .
I
0
. . . .
~ ....
~ ....
~ ....
~ ....
i
Z = 150 m m ~ " o
500~ I
, ~ ....
oo~ 1 7 6 1 7 6 o1 7=6 1 7 6 1 7 6
~ . . . .
~ ,',',
1000i
~
=
....
1500 j
1000 500.
@
I'---
I
. . . .
50 r" ~
I
. . . .
100
I
150
. . . .
I
200
. . . .
I
250
~
0 2000-
~ J ....
T ....
i ....
i ....
i ....
r'Rrtl o~0==oo~176
o o o o
10 O0 i
0
. . . .
i
-50
. . . .
i
0
. . . .
i
Measurements o f the spatial variation of the droplet-size distributions in the cold sprays of liquid fuels provide new information which can be used together with more traditional techniques to explain the structure and behaviour of their flames. Using such measurements, it
. . . .
50 r
Conclusions
o
o I
500 -100
shape and width, but the peak velocities in the flame were greater by a factor between 2 and 3 than those in the air j e t at the same value of Z. (These velocities c o r r e s p o n d e d to pressures of the o r d e r of several P a below ambient measured in the flame.) Similar observations have been reported in similar circumstances 27'2s with slightly higher velocities as a result of buoyancy in a vertical flame. T h e effect o f buoyancy in the horizontal flame in this study was to deflect the velocity profiles upward. Figure 6(b) clearly shows that the same description does not apply to the heavier oil. The large droplets at the periphery of the spray of No. 6 oil evaporate slowly and have sufficient m o m e n t u m to penetrate through the shear layer of the atomizing air jet.
"1
Z = 100 mm
1500 j
FIG. 7. Temperature profiles in the flame of a spray of No. 2 oil. Tt- = 348K, To = 373K, r~ = 0.5 g/s, a/f= 2.0.
i ....
i
100 ,
. . . .
~
150
. . . .
t
200
. . . .
i
250
i'TIm
FIG. 8. Temperature profiles in the flame of a spray of No. 6 oil; conditions the same as in Fig. 7. was possible to explain the differences between the flames of two fuels with very different viscosity and volatility when they were sprayed from the same atomizer and with the same atomization parameters. It turns out that large differences in viscosity produce not only different mean droplet sizes, but also different spatial distributions of d r o p size and velocity. These were the main reasons for the differences in the flames. Comparisons o f the b u r n i n g of unconfined sprays undoubtedly exaggerate the difference between two fuels o f different volatility. With a long enough residence time in the presence o f hot walls, the combustion of the heavy oil would be more complete, with higher flame temperatures, and less quenching than was observed in this work. However, there is no reason to believe that the initial spray would be any different since it d e p e n d s mainly on the viscosity of the liquid, which depends only on its temperature. In a complementary study 18 to be r e p o r t e d elsewhere, a simulation showed that if their
COMBUSTION OF A PRACTICAL OIL SPRAY sprays have exactly the same initial DSD, a n d if o n the average they are exposed to the same e n v i r o n m e n t , No. 2 a n d No. 6 fuel oils b u r n essentially the same way, with very similar average droplet lifetimes.
Nomenclature
a/f ~ C D D32 DSD
L M m
N
Q q"r R r
S SMD T V
vz X Z 2:
ratio of atomizing air mass flow to fuel mass flow empirical m e a s u r e of specific gravity [141,5/(pf/PH20)60oF] -- 131.5 specific heat of the liquid, kJ/kgK d r o p l e t diameter, txm Sauter m e a n diameter, ixm =- dV/dD fraction o f the liquid volume per u n i t d r o p l e t d i a m e t e r at the d i a m e t e r D, Ixm -1 latent heat o f evaporation, kJ/kg molecular weight, kg/kmol mass flow, kg/s or g/s e x p o n e n t in R o s i n - R a m m l e r distribution, dimensionless s t a n d a r d heat o f combustion, MJ/kg incident r a d i a n t heat flux, W/m s radial position of c e n t r e of laser beam, mm radial coordinate, m m distance f r o m the c e n t r e of the flame to the thermopile, m Sauter m e a n diameter, Ixm temperature, K fraction of total liquid droplet volume c o m p o n e n t of velocity in the axial direction, m/s characteristic d i a m e t e r in Rosin-Rammler distribution, Ixm axial position of c e n t r e of laser beam, mm axial coordinate, m m fraction of heat of c o m b u s t i o n radiated angle density, kg/m 3 surface tension, N / m kinematic viscosity, m2/s
Subscripts a b f
atomizing air boiling point fuel Acknowledgements
The authors wish to thank Dr. J.L. Rapanotti for taking part in useful discussions of this work. The research was supported by an Individual Operating Grant from the Natural Sciences and Engineering Research Council of Canada to T.A. Brzustowski.
565
REFERENCES 1. GIFFEN, E. AND MURASZEW,A.: The Atomization of
Liquid Fuels, p.218, Wiley, 1953. 2. LEFEBVRZ,A.H.: Prog. Energy & Comb. Sci., 6, 233, (1980). 3. LEFEBVRE,A.H.: Gas Turbine Combustion, p. 433, McGraw-Hill, 1983. 4. DICKINSON, D.R. ANn MARSHALL, W.R. Jr.: A.I.Ch.E. Journal 14,541 (1968). 5. WITTIG, S., A1GNER, M.,
SAKBANI, KH. AND
SATTELMAYER,TH. : Optical Measurements of Droplet Size Distributions: Special Considerations in the Parameter Definition for Fuel Atomizers, paper 13, AGARD CP-353, Oct. 1983. 6. Technical Data Book, American Petroleum Inst., Vols. I and II, 1970. 7. SWITHENBANK,J., et al.: Progress in Astronautics and Aeronautics, Vol. 53 (B.T. Zinn, Ed.), A.I.A.A., p. 421 (1977). 8. HODKINSON, J.P. AND GREENLEAVES, I.: J. Opt. Soc. Amer., 53, 577 (1963). 9. STEVENSON,W.H.: Spray and Particulate Diagnostics in Combustion Systems: A Review of Optical Methods, Paper presented at the Central States Section, The Combustion Inst., West Lafayette, Ind., April 1978. 10. FZLTON,P.G.: Measurement of Particle~Droplet Size Distributions by a Loser Diffraction Technique, Paper presented at the 2nd European Symp. on Particle Characterization, Niirnberg, W. Germany, Sept. 1979. 11. RL'SCELLO,L.V. AND HIRLEMAN, E.D.: Determining Droplet Size Distributions of Sprays with a Photodiode Array, Paper WWS/C1-81-49, Fall Meeting, Western States Section, The Combustion, Inst., Arizona State U., Oct. 1981. 12. VAN DE HULST, H.C.: Light Scattering by Small Particles, Dover Publications, 1981. 13. BORN, M. AND WOLF, E.: Principles of Optics, Pergamon Press, Sixth ed., 1980. 14. FOWLES,G.R.: Introduction to Modern Optics, Holt, Rinehart, and Winston, Inc., 2nd ed., 1975. 15. NEGUS, C. AND AZZOPARDI, B.J.: The Malvern Particle Size Distribution Analyzer: Its Accuracy and Limitations, U.K. Atomic Energy Authority, Harwell, Report AERE-R 9075, Dec. 1978. 16. FELTO~,', P.G., HAMIDI, A.A., AXt) ALGAL, A.K.: Multiple Scattering Effects on Particle Sizing by Laser Diffraction, Report No. 431 HIC, Dept. of Chem. Eng. and Fuel Tech., U. of Sheffield, Aug. 1984. 17. FELTON, P.G., HAMIDI, A.A., AND AIGAL, A.K.: Measurement of Drop Size Distribution in Dense Sprays by Laser Diffraction, Paper presented at ICLASS-85 and pub, in the proc. Vol.2, p. IVA/4/1, Imperial College, London, July 1985. 18. BREI~'A DE LA ROSA, A.: The Influence of Drop Size Distribution on the Structure of Unconfined Oil Sprays, Ph.D. Thesis, Dept. of Mech. Eng., U. of Waterloo, 1986.
566
PRACTICAL COMBUSTION DEVICES
19. HIRLEMAN,E.D.: On-Line Calibration Techniquefor
Laser Diffraction
Droplet
Sizing
Instruments,
A.S.M.E. Paper 83-GT-232, 1983. 20. Dot)ce, L.: Appl. Optics, 23, 2415 (1984). 21. HIRLEMAN, E.D., OECHSLE, V., AND CHIGIER, N.A.: Optical Eng. 23, 610 (1984). 22. HIRLEMAN, E.D.: Modeling of Multiple Scattering
Effects in Fraunhofer Diffraction Particle Size Analysis, Paper presented at the Canadian and Western States Sections, The Combustion Institute, 1986 Spring Technical Meeting, Banff, Alberta, April 1986. 23. GL'LDER, O.L., BILLINGHAM, R., AND CHELL1NGWORTH, F.W.: Intermittent Spray Characterization
and Spray Ignition at High Pressures and Temperature.s: Description of an Experimental Set-up, Paper
24. SMYTH, K.C., MILLER, J.H., DORFMANN, R.C., MALLARG, W.G., AND SANTORO, R.J.: Comb. and Flame 62, 157 (1985). 25. Arwva, A.M. AND WHITELAW, J.H.: Velocity,
Temperature, and Species Concentrations in Unconfined Kerosene Spray" Flames, A.S.M.E. Paper 81-WA/HT-47, 1981. 26. NOWACK, C.F.R., J. Phys. E: Scientific Instruments 3, 21 (1970). 27. CHmXER, N.A. AXD STYLES,A.C.: Sixteenth Symposium (International) on Combustion, p. 619, The Combustion Institute, 1977. 28. CHmlER, N.A. Axn STYLES, A.C.: EvaporationCombustion of Fuels, (J.T. Zung, Ed.), p. 111, Advances in Chemistry Series, #166, A.C.S., Washington, D.C., 1978.
presented at the Canadian and Western States Section, The Combustion Institute, 1986 Spring Technical Meeting, Banff, Alberta, April 1986.
COMMENTS C. R. Negus AERE Hatwell, UK. Our studies on a kerosene-fired furnace (similar in size, etc., to yours), using photography and phase-difference particle sizing, have all shown the droplets to lie predominantly within the lmninous flame front. However, your photographs show a considerable number of droplets outside the flame front. Do these droplets represent a significant fraction of the total fuel? What happens to these droplets? Is this situation common, or is your system poorly optimised? Author's Reply. The droplets lying outside the flame represented a small fraction of the fuel for No. 2 oil, and a larger fraction for No. 6 oil. In the case of the lighter oil, these droplets are eventually entrained into the flame and burned. In the case of the heavier oil, many of them fall out unburned. This situation is not common in furnaces. One
hopes that it is also rare in liquid incinerators, where just a few unburned large droplets evaporating downstream of the flame could mean the difference between acceptable and unacceptable performance. Our system was, of course, not optimized for complete combustion as in a furnace or combustor. On the contrary, it was designed to show the greatest possible effect of extreme variations of fuel properties on the atomization and combustion of a spray from a given nozzle operating under a given set of conditions. The lack of control over the air flow surrounding the spray and the absence of hot walls exaggerated the effects of initial spray momentum and fuel volatility. By showing the effects in an exaggerated form, our experiments have pointed the way to what has to be done to compensate for varying fuel properties in order to operate a real system in optimal fashion.
THE INFLUENCE OF FUEL PROPERTIES ON DROP-SIZE DISTRIBUTION AND COMBUSTION IN AN OIL SPRAY A. BREICqA DE LA ROSA, A. SOBIESIAK, AND T.A. BRZUSTOWSKI
Thermal Engineering Group Department of Mechanical Engineering University of Waterloo Waterloo, Ontario, Canada N2L 3G1 The paper describes an experimental study of the effect of differences in fuel properties on oil atomized and burned under identical conditions. This was done with No. 2 and No. 6 oil sprayed from the same air-blast atomizer with the same atomization parameters. The local droplet-size distributions were measured throughout the cold sprays using the Malvern Particle Sizer. It was found that the more viscous heavy fuel systematically produced a droplet size range about double that of the lighter fuel for the same atomization parameters. The sprays of both fuels had large droplets at the periphery, but those of No. 2 oil had predominantly fine droplets in the core, in contrast to the sprays of No. 6 oil which had some relatively large drops in that region. The sprays of the lighter oil burned predominantly in vapour diffusion flames within the boundaries of the jet of atomizing air. The heavier oil burned very differently. In general, the large droplets had sufficient momentum to penetrate through the atomizing air jet and disperse widely. Many droplets appeared to burn individually, and many were quenched before burning out. The peak temperatures measured in the flames of the heavier oil were about 200K lower than for the lighter oil. Radiation measurements also confirmed that the heavy oil generally did not burn completely. These differences in flame characteristics are attributed to the different droplet-size and velocity distributions and to the different evaporation histories of the droplets, all of which reflect the different fuel properties. Data on the spatial variation of the droplet-size distributions in the cold spray of the two fuels are important in explaining the observed combustion behaviour.
Introduction It has long b e e n k n o w n that the droplet-size distribution (DSD) in the spray is i m p o r t a n t in d e t e r m i n i n g m a n y characteristics of the flame of atomized liquid fuel. Until recently, the bulk of research o n the atomization of liquid fuels did n o t involve the m e a s u r e m e n t of the actual DSD. Instead, a n a p p r o p r i a t e m e a n d i a m e t e r was taken as the proxy for the whole distribution. I n most c o m b u s t i o n work, that proxy was the Sauter M e a n D i a m e t e r (SMD or Ds2), the d i a m e t e r of the d r o p l e t which has the same ratio o f surface area to v o l u m e as the whole spray. T h e SMD could be m e a s u r e d directly a n d simply by a light o b s c u r a t i o n technique ~, which u n f o r t u n a t e l y h a d n o spatial resolution. Nevertheless, m e a s u r e m e n t s o f this k i n d have led to the evaluation o f m a n y atomizers of d i f f e r e n t types in terms of correlations b e t w e e n the SMD a n d the relevant d i m e n s i o n s of the
atomizer, the flow p a r a m e t e r s , a n d fluid properties. Many such correlations have b e e n tabulated 2"3, Some of their shortcomings have b e e n p o i n t e d o u t previously in n u m e r i c a l 4 a n d e x p e r i m e n t a l 5 studies. T h e recent d e v e l o p m e n t of laser optical i n s t r u m e n t s which c o n v e n i e n t l y p r o d u c e reasonable r e p r e s e n t a t i o n s o f local droplet-size distributions has b e e n a m a j o r advance. It is now possible to get a good idea of the spatial variation of the DSD, which is far m o r e informative about the processes o c c u r r i n g t h r o u g h out the spray v o l u m e t h a n any single global DSD or even the spatial variation o f a m e a n diameter. This research was the first step in a p r o g r a m to study the possibility o f c o m p e n s a t i n g for variable fuel properties by controlling the atomization parameters. Specifically, the spatial variation o f the DSD o f a steady u n c o n f i n e d oil spray p r o d u c e d f r o m a given nozzle was mea-
557
558
PRACTICAL COMBUSTION DEVICES TABLE I Physico-Chemical Properties of the Fuel Oils Studied at 298K and 101.3kPa
fuel #2 oil #4 oil #6 oil kerosene
M kg/kmol
p kg/m 3
Tb K
L kJ/kg
202 240 315 154
869 885 941 820
534 575 660 460
355 354 346 251
v 10 6m2/s ~ 4.0 16.1 1126 2.1
I*
31.7 27.5 19,8 41.9
T 10 3N/m
C kJ/kgK
Q MJ/kg
27.1 29.4 31.8 25.0
1.845 1.809 1.745 2.090
44.600 44.429 41.924 43.500
*at 15.5 ~ sured in cold flow for a wide range of atomization parameters. T h e b u r n i n g of this spray was subsequently studied for exactly the same values of the parameters. Extreme variations in fuel properties were obtained by using two different fuel oils, No. 2 and No. 6. The atomization parameters varied were fuel temperature, fuel flow, atomizing air temperature and pressure, the ratio of the mass flows of fuel and atomizing air, and the air pressure--hence velocity. In addition to D S D measurements, the flames were examined by direct photography and color schlieren flow visualization. Measurements were also made of the time-mean temperature field, total flame radiation, and the m o m e n t u m field of the atomizing air stream and of the flame of the corresponding spray.
Experimental Procedure The fuels used were No. 2 (blended) and No. 6 (residual) fuel oils. Table 1 shows some of the important properties of these fuels, with two others included for comparison, Since these fuels are not pure compounds, the molecular weight, the latent heat, and the "mean average" boiling point (the arithmetic mean of the mole-fraction weighted b.p and the cube of, the volume-fraction weighted cube root of the b.p.) are nominal values inferred from the specific gravity and the distillation curve 6. The D S D was measured with a Malvern Particle Sizer, Model 3600D. This is a line-ofsight optical i n s t r u m e n t in which the signal is produced by small-angle forward scattering of light from a laser beam (2 roW, He-Ne) traversing the spray. T h e radial distribution of the scattered light intensity is measured with a concentric-ring detector array, digitized and converted to a distribution of volume fraction over droplet diameters with software based on Fraunhofer diffraction theory 7'8'9'1~ The software also provides for a curve-fit of the measured distribution to one of several common distribution functions. The Rosin-Ram-
mler distribution was chosen for this work, and its parameters X a n d N were used as the basis for comparing the sprays. In what follows, the D S D is the distribution of droplet volume over the droplet diameter space. It is defined by Eq. 1 below. In conventional notation, 1 - V = e x p [ - ( D / X ) x]
from which it follows that DSD ~ dV/dD = N X - x D N-1 e x p [ - (D/X) x]
( 1)
where D is the droplet diameter, V is the volume fraction of liquid occurring in drops of diameter less than D, X is the droplet diameter such that (1 - l/e) of the liquid volume resides in smaller droplets, and N is a dimensionless width parameter. Data are often presented as lOOdV/dD in units of Ixm-1 or of volume % per p,m. The nozzle used in this work was a small commercial Delavan air-blast atomizer, type Siphon SNA 30609-11 rated at 2 gph (7.6 l/h). In this nozzle, air is blasted at the liquid from inclined converging ports on the periphery of the liquid feed tube about 1 mm upstream of the 1.20mm i.d. discharge orifice. The nozzle was mounted horizontally on a rig which could be traversed horizontally and vertically. D S D measurements were made with the laser beam horizontal and normal to the nozzle axis, and crossing it at a downstream distance Z of 50, 100, 150, 200, and 250 m m from the tip of the nozzle. Measurements were also made with the centre of the laser beam at a distance R of 10, 20, 30, and 40 m m below the axis of the nozzle at Z of 50, 100, and 150 mm. These measurements are called axial and radial, respectively. Fig. 1 shows the co-ordinates. Differences in fuel properties and the limitations of the Malvern i n s t r u m e n t with respect to multiple scattering effects 15,a6A7 were considered in arriving at the combinations of experimental conditions investigated. These are
COMBUSTION OF A PRACTICAL OIL SPRAY
Fro. I. The co-ordinates used in reporting the data. presented in detaiI by Brefia de la Rosa is. The conditions selected for discussion in this paper are shown in the figure captions. The DSD were measured no closer to the nozzle than 50 mm. in order to work with ohscuration levels below 50%. Many workers 19'2~ have now shown that the Malvern Particle Sizer is accurate u n d e r those conditions. In the combustion experiments, the flames of No. 2 fuel oil were stable but the flames of No. 6 fuel oil had to be stabilized. Two small hydrogen diffusion flames located at R -- ---15 mm from the nozzle axis at Z = 2 mm downstream from the nozzle tip, with a total H2 flow of 1.05 x 10 -3 g/s (of the order of 0.1% of the mass flow of oil), provided the necessary stabilization. The i.d. of the hydrogen burners was 0.75 mm and the length of each hydrogen diffusion flame was about 15 mm. Radiation from the whole flame was measured with a Kipp & Zonen thermopile (model CA-l). The flame was treated as a point source and the measurements were made at a distance of S = 2.45 m from the centre of the flame. Measurements at varying S for several sets of experimental conditions showed that the incident radiant heat flux varied as S -z, verifying the applicability of the point-source model in this work. The results of the radiation measurements were expressed in terms of q~, the fraction of heat release radiated. It is a derived quantity, based on the assumption that all of the fuel is b u r n e d and given by
q"r = (mfgrQ/4"lrSZ)cosO
(2)
where q"r is the radiant heat flux incident on the thermopile, mf is the mass flow of fuel, Q is the standard heating value of the fuel per unit mass, and 0 is the angle between the normal to
559
the surface of the thermopile and the line of sight from the centre of the flame. The temperature measurements were made with an uncoated Pt/Pt - 10%Rh thermocoupie, with a wire diameter of 125 p.m. This size was used because thermocouples of 50 and 75 p~m wire proved not to be sufficiently robust for this work. They were not corrected for radiation. The reasons for these decisions were very similar to those recently described in detail24'25. T h e temperatures reported here are long-time mean values. T h e measurements of the m o m e n t u m flux were made with a fivehole water-cooled pitot probe 9.5 m m in diameter. Because of its large size and because of fouling by droplets, the probe was not used close to the nozzle. T h e use of a five-hole probe appeared appropriate because of the strongly divergent nature of the flow. The design and calibration of the probe followed conventional procedures 26.
Some Special Features of the DSD Measurement It is important to note the following features of the DSD measurements which affect the interpretation of the results: - - T h e Malvern Particle Sizer is a line-of-sight instrument that measures the DSD of the droplets which reside within the sampling volume during a sweep of the scattered light distribution; a period of 30 ms in the instrument used. The sampling volume is defined by the intersection of the laser beam (9mm dia.) and the spray. T h e beam is normal to the nozzle axis and falls along a horizontal chord in the circular cross-section of the spray. The data labelled "axial distributions" in the figures refer to the chord which passes through the atomizer axis at varying distance Z downstream of the injector face. The "radial distributions" refer to the DSD of droplets captured in a beam whose axis lies a distance R below the nozzle centreline. The DSD are presented without deconvolution of the line-of-sight measurements. Clearly, the DSD for any R includes the contribution of droplets flowing through the spray crosssection only at radii equal to or greater than R-4.5 mm. - - E a c h of the curves shown is the RosinRammler distribution fitted to the data by the software of the Malvern instrument based on the fraction of spray volume found in each of 15 size ranges. T h e symbols on these curves are not data points. They serve only to identify their parameters listed in the legend.
560
PRACTICAL COMBUSTION DEVICES
- - T h e measurement has a sensitivity threshold, imposed by the software. The relative contribution of any size range which falls below 0.1% of the total is totally ignored. T h e size ranges are always the same for a given lens, but the n u m b e r of ranges used in the curve-fitting depends on how many contribute more than 0.1% of the total volume of the spray. - - T h e DSD fitted to the data in each measurement is normalized by the total volume of liquid recognized at that location. As a consequence of these features of the measurement, the results are in the form of the closest-fit Rosin-Rammler distributions which represent about 99.8% of the total volume of liquid in each m e a s u r e m e n t volume. These distributions do not track the evolution of the spray measured upstream. Given a spray in which there are few very large droplets, the measurement will reveal these droplets in increasing proportion as the total volume of liquid decreases by the evaporation of the smallest droplets first. U n d e r certain circumstances, this procedure could falsely suggest the occurrence of coalescence even in very dilute sprays. If the spray is not homogeneous, there is a radial variation of the DSD which is only approximately given by the variation of the DSD with R. T h e error is greatest for the smallest values of R, since the measurement volume then contains droplets which are located at the edges of the spray as well as those in the core. Further from the axis, the location of the droplets measured becomes defined with increased precision as R becomes large relative to 4.5 ram.
Results
and
Discussion
Figures 2 and 3 illustrate the effect of different fuel properties, primarily viscosity and volatility, on the axial variation of the DSD. Figure 2 shows the effect of fuel temperature for No. 2 oil when Tf rises from 298 to 348 K, i.e.: from 0.56 to 0.65 of the mean average boiling point. (Note that the vertical scale in the lower part covers twice the range in the u p p e r part.) The corresponding decrease of the liquid viscosity is from 3.4 • 10 -~ to 1.2 x 10 -3 kg/(m s). These data were taken at the highest ratio of the mass flow of atomizing air to the mass flow of fuel, a/f = 2.0. T h e sprays are coarser at lower values of a/f, but the trends are the same. Two effects are evident. First, X decreases by about 40%, Secondly, the variation of the DSD
AXIAL DISTRIBUTIONS FUEL OIL NO. 2 Po=60kPa,
TQ=298K
a/f =2.0, mf =.509/s 1.5-9 ~'~
1.2 ]
J o >
O.9-
,-,
o.+-
0
0.~-
T, = 298K
7 f
\~ 1k
%
( X,N ) ; Z (r,,~) o = (526.1.6) 50
,
o =(51.2.;.5) 100 ,+.= (.~.4. ;.5) ;50 \%,
SO
100 DIAMETER,
+ = +e.+. ;.6)
150
2oo
200
,u,m
s.oE
T+ = 348 K
(X,N) ; Z(mm) 50 o =(~o.4. ,5) ;oo + =(~2.o. ;.6) 15o
:i 2.0 ~ , ._i
I///-%~k
>~
IF/
"0
'~ ~,
!
o
~ = (29.4. ~.5)
~ 1.0
'
~
k
~
+ = (35.0, 1.5) x = (39.6, 1.5)
200 250
o 0.o . . . . .
~ . , ."#''~'~.~.-~.-v-~. f . . . . ,50 100 150 DIAMETER . /../,m
200
FIG. 2. Axial variation of measured droplet-size distributions in a cold spray of No. 2 oil: top set of curves for oil temperature of 298K; lower set for 348K. The constant parameters are T~ = 298K, m/= 0.5 g/s, and a/f = 2.0. Note that the curves are the Rosin-Rammler distributions fitted to the data by the software of the Malvern Analyzer. The symbols on these curves serve only to identify them. Note that the vertical scale in the lower portion of the figure has been compressed by a factor of 2. with Z is quite different. At the lower temperature, the DSD are remarkably similar at Z from 50 to 250 mm. At the higher temperature, the proportion of volume represented by the smaller droplets decreases significantly with distance, because of the more rapid evaporation of the small droplets of the hotter oil. Similar effects for No. 6 oil are shown in Fig. 3. Here, the variation in fuel temperature is from 323K to 348K, or from 0.49 to 0.53 of the mean average boiling point, and the effect of volatility is not as p r o n o u n c e d as in Fig. 2. T h e decrease in the liquid viscosity is from 180 • 10 -~ to 40 x 10 -~ kg/(m s) and the decrease i n X is about 20%. A comparison of the lower sets of curves in Figs. 2 and 3 clearly shows the effect of fuel
COMBUSTION
OF A PRACTICAL
OIL
AXIAL DISTRIBUTIONS
a/f
: 2.0,
K
Po:60kPa,
m f = . 5 0 g/~s
s.0-
a / f = 2.0, / ~ [ ~ ~ !
1.5.
E
:t. ..~ j o>
Tf : 323 K
Z=50
0.9
0
E :3.
0.3-
50
I00
150
200 ,~m
300
Z = 100 m m
( X,N ) ;
T~ = 348 K
o
1.2-
~ ( X , N ) ; Z(~m)
j >0
0.9-
o = (52.4, 1.5) o = (64.9, 1.5)
t-~
0.6-
~, = (79.1, 1,4) + = (81.3, 1.4) x = (84.7, 1.3)
0
0.3-
100 150 200 250
100 DIAMETER,
i
200 ~m
,
"!"
,
,
~ / ) ~ +
1.0
o.o
50
100
R (~) ($0.4, 1,5) 0 (40.7, 1.6) 10 (61.1, 2.0) 20 (77.5, 2.5) 30
150
200
3.0Z = 150 men
~
.
( x,N ) ; R(ram) o : (32.0, 1.6) 0 o = (37.8, 1.5) 10 = (49.8, 1.5) 20 + : (72.5, 1.8) 30
+
2.0-
1.0. . . .
0
50
D= o= z~= +:
2.0
0
n
1.5-
0.(1
.j
200
3.0-
~ § 100 DIAMETER,
mm
( X.N ) : R (mm) D = (29.4, 1.5) 0 o = (37.3, 2.1) 10 = (53.9, 3.8) 20
3"0~
( X , N ) : Z(mm) o = (62.4, 1.5) 80 o = (81.7, 1.4) 100 = (92.9, 1.4) 150 + = (94.9, 1.4) 200 x = (98.5, 1.3) 250
0,0
E :k -~.
T1= 348K m I = .50 g/s
4.0
1.2-
o,6-
0
561
RADIAL DISTRIBUTIONS FUEL OIL NO. 2
FUEL OIL NO. 6 P Q = 6 0 kPa, T a = 2 9 8
SPRAY
x ~ .
;/
x=(94930)
40
?- i
300
F[e,. 3. As in Fig. 2, but for No. 6 oil at 323K (top) and 348K (bottom). Other parameters as in Fig. 2. properties, since all other conditions are the same. For No. 6 oil, the liquid viscosity is 33 times greater than for No. 2 oil. The main difference between the two sprays is the relatively greater number of large droplets on the axis of the spray of the more viscous oil, with X greater by about a factor of 2. Figures 4 and 5 show the variation of the DSD with R at increasing downstream distances. The data in these two figures were obtained under the same conditions as those in the lower parts of Figs. 2 and 3. T w o features of these data are evident. One is the close similarity of the DSD in the outer region of the spray for both fuels. There is a high concentration of relatively large droplets at the periphery of each spray. The other is again the difference between the DSD in the cores of the two sprays. The spray of the lighter oil has a core of small droplets; the spray o f the heavy oil has many more large droplets in the core, including some of the largest in the spray. The data of Figs. 2 to 5 could be condensed by showing only the variation of the RosinRammler parameters X and N. However, Figs.
0.0 50
100 DIAMETER ,
150
200
/~rn
Fie,. 4. Radial variation of the DSD for No. 2 oil at 348K. Other parameters as in Fig. 2. 2 to 5 also show what would be involved in any attempt to correlate these X and N with the atomization parameters and fuel properties. For each set of parameters and properties, X and N also depend on location in the spray. Correlations of X and N would, therefore, have to be very specific not only to the atomizer used but also to the aerodynamics of the environment in which it operates. Because of their lack of generality, no attempt to develop such correlations has been made in this study. The measurement of local DSD has revealed the existence of large droplets at the edges of the flame. These droplets are very important in determining the burning characteristics of the spray. Flame photographs and schlieren flow visualization have shown that the flame of No. 2 oil under the conditions of Fig. 4 looks very much like a gaseous diffusion flame surrounded by some burning droplets quite close to the nozzle. These droplets either burn up very quickly or are entrained into the luminous flame and the surrounding layer of hot gas. No
562
PRACTICAL COMBUSTION DEVICES RADIAL DISTRIBUTIONS FUEL OIL NO. 6 Pa = 60 kPo, T ~ = 348 K a / f = 2.0, in, = . 5 0 g / s
3o
"]
,.o4 I I 0
/%<+%
/ i~\
z=50 mm
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:
50
100
150
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200
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1 4
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50
200
150
1.5 Z = 150
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I00
DIAMETER
150 ,
200
,u.m
Fro. 5. As in Fig. 4, but tot No. 6 oil.
individual droplet combustion is seen outside the bulk of the luminous flame. The behaviour is very different tbr the heavier oil. The direct flame photographs (exposure of 1/4000 s) shown as Figs. fi(a) and (b) illustrate the difference. Both photographs are to the same scale, and correspond to the conditions of Figs. 4 and 5, respectively. T h e flame of No. 6 oil is much less uniformly luminous. Many small flames, associated with large single droplets or clusters of droplets, are clearly visible over its entire length. In fact, the whole flame is nmch less uniformly luminous than the flame of the lighter oil and appears to have a small-scale structure. This interpretation is reinforced by schlieren and schlieren/direct photographs which show, in addition, that many droplets remain outside the hot gas layer s u r r o u n d i n g the flame. It would appear, therefore, that the lighter oil is atomized into a finer spray, evaporates more quickly, and b u r n s predominantly as a single diffusion flame of the vapour formed in the core of the spray. T h e heavier oil atomized
u n d e r exactly the same conditions forms a coarser spray, evaporates more slowly, and burns in what seems to be a large n u m b e r of small diffusion flames, some of which are far removed from the bulk of the luminous region. The appearance of this flame strongly suggests that many of the large droplets are b u r n i n g individually. The measurements of flame radiation produced unexpected results, but these can be explained in terms of the different flame structures of the two fuels. Radiation from flames of No. 2 oil behaved similarly to that from diffusion flames of hydrocarbon gases. For the whole set of fuel and air temperatures, 't r ranged from about 34% for the lowest a/f ratio to a value about 10% at the highest all ratio. These results were quite independent of the air and fuel temperatures. The results for No. 6 oil were very different. They were very sensitive to both the air and the fuel temperatures. At the conditions of Figs. 4 and 5, xI* was about half of that for the lighter oil. The reason was incomplete combustion. Many large droplets of the heavier oil were quenched when only partly burned, and some at the outer edges of the spray did not b u r n at all. However, the calculation of 9 from Eq.2 was on the assumption that combustion was complete. At lower fuel and air temperatures, No. 6 oil b u r n e d even more poorly and xI* was only a few percent, and at 298K it would not burn at all. The flame temperatures of the two fuels were also very different. Figs. 7 and 8 show the mean temperatures measured for the two fuels u n d e r similar conditions. In this case, 9 was about 10% for both fuels. The temperature profiles for the lighter oil look like those of a horizontal gaseous diffusion flame showing the effects of buoyancy, with a peak value of 2000K at Z = 200 mm. (The thermocouple had melted at smaller values of Z.) For the heavier oil, the highest temperature measured at these conditions was 1600K, and the profiles close to the nozzle showed that there was little b u r n i n g in the core. The highest temperature measured in the flame of No. 6 oil u n d e r any conditions was only 1800K. A final result of the measurements was a very simple characterization of the flow field of the spray flame of the lighter oil. Profiles of the axial (horizontal) c o m p o n e n t of velocity were calculated from the measured profiles of mom e n t u m flux and temperature for the b u r n i n g spray whose temperature profiles are shown in Fig. 7, and compared with the corresponding far-field velocity profiles of the jet of atomization air alone (Re=5 x 104). It turned out that the two sets of profiles were similar in both
FIG. 6. exposure FIG. 6. exposure
(a) P h o t o g r a p h o f the b u r n i n g spray o f No. 2 oil p r o d u c e d under the same conditions as in Fig. 4; 0.25 ms; to same scale as in Fig. 6(b). (b) Photograph o f the b u r n i n g spray o f No. 6 oil produced under the same conditions as in Fig. 5; 0.25 ms; to same scale as in Fig. 6(a). 563
564
PRACTICAL COMBUSTION DEVICES TEMPERATURE PRORLES
T E M P E R A T U R E PROFILES
FUEL OIL NO. 2 P==96kPa, o/f
2000
FUEL OIL NO. 6 T==373K
= 2.0,
P==96kPa,
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Measurements o f the spatial variation of the droplet-size distributions in the cold sprays of liquid fuels provide new information which can be used together with more traditional techniques to explain the structure and behaviour of their flames. Using such measurements, it
. . . .
50 r
Conclusions
o
o I
500 -100
shape and width, but the peak velocities in the flame were greater by a factor between 2 and 3 than those in the air j e t at the same value of Z. (These velocities c o r r e s p o n d e d to pressures of the o r d e r of several P a below ambient measured in the flame.) Similar observations have been reported in similar circumstances 27'2s with slightly higher velocities as a result of buoyancy in a vertical flame. T h e effect o f buoyancy in the horizontal flame in this study was to deflect the velocity profiles upward. Figure 6(b) clearly shows that the same description does not apply to the heavier oil. The large droplets at the periphery of the spray of No. 6 oil evaporate slowly and have sufficient m o m e n t u m to penetrate through the shear layer of the atomizing air jet.
"1
Z = 100 mm
1500 j
FIG. 7. Temperature profiles in the flame of a spray of No. 2 oil. Tt- = 348K, To = 373K, r~ = 0.5 g/s, a/f= 2.0.
i ....
i
100 ,
. . . .
~
150
. . . .
t
200
. . . .
i
250
i'TIm
FIG. 8. Temperature profiles in the flame of a spray of No. 6 oil; conditions the same as in Fig. 7. was possible to explain the differences between the flames of two fuels with very different viscosity and volatility when they were sprayed from the same atomizer and with the same atomization parameters. It turns out that large differences in viscosity produce not only different mean droplet sizes, but also different spatial distributions of d r o p size and velocity. These were the main reasons for the differences in the flames. Comparisons o f the b u r n i n g of unconfined sprays undoubtedly exaggerate the difference between two fuels o f different volatility. With a long enough residence time in the presence o f hot walls, the combustion of the heavy oil would be more complete, with higher flame temperatures, and less quenching than was observed in this work. However, there is no reason to believe that the initial spray would be any different since it d e p e n d s mainly on the viscosity of the liquid, which depends only on its temperature. In a complementary study 18 to be r e p o r t e d elsewhere, a simulation showed that if their
COMBUSTION OF A PRACTICAL OIL SPRAY sprays have exactly the same initial DSD, a n d if o n the average they are exposed to the same e n v i r o n m e n t , No. 2 a n d No. 6 fuel oils b u r n essentially the same way, with very similar average droplet lifetimes.
Nomenclature
a/f ~ C D D32 DSD
L M m
N
Q q"r R r
S SMD T V
vz X Z 2:
ratio of atomizing air mass flow to fuel mass flow empirical m e a s u r e of specific gravity [141,5/(pf/PH20)60oF] -- 131.5 specific heat of the liquid, kJ/kgK d r o p l e t diameter, txm Sauter m e a n diameter, ixm =- dV/dD fraction o f the liquid volume per u n i t d r o p l e t d i a m e t e r at the d i a m e t e r D, Ixm -1 latent heat o f evaporation, kJ/kg molecular weight, kg/kmol mass flow, kg/s or g/s e x p o n e n t in R o s i n - R a m m l e r distribution, dimensionless s t a n d a r d heat o f combustion, MJ/kg incident r a d i a n t heat flux, W/m s radial position of c e n t r e of laser beam, mm radial coordinate, m m distance f r o m the c e n t r e of the flame to the thermopile, m Sauter m e a n diameter, Ixm temperature, K fraction of total liquid droplet volume c o m p o n e n t of velocity in the axial direction, m/s characteristic d i a m e t e r in Rosin-Rammler distribution, Ixm axial position of c e n t r e of laser beam, mm axial coordinate, m m fraction of heat of c o m b u s t i o n radiated angle density, kg/m 3 surface tension, N / m kinematic viscosity, m2/s
Subscripts a b f
atomizing air boiling point fuel Acknowledgements
The authors wish to thank Dr. J.L. Rapanotti for taking part in useful discussions of this work. The research was supported by an Individual Operating Grant from the Natural Sciences and Engineering Research Council of Canada to T.A. Brzustowski.
565
REFERENCES 1. GIFFEN, E. AND MURASZEW,A.: The Atomization of
Liquid Fuels, p.218, Wiley, 1953. 2. LEFEBVRZ,A.H.: Prog. Energy & Comb. Sci., 6, 233, (1980). 3. LEFEBVRE,A.H.: Gas Turbine Combustion, p. 433, McGraw-Hill, 1983. 4. DICKINSON, D.R. ANn MARSHALL, W.R. Jr.: A.I.Ch.E. Journal 14,541 (1968). 5. WITTIG, S., A1GNER, M.,
SAKBANI, KH. AND
SATTELMAYER,TH. : Optical Measurements of Droplet Size Distributions: Special Considerations in the Parameter Definition for Fuel Atomizers, paper 13, AGARD CP-353, Oct. 1983. 6. Technical Data Book, American Petroleum Inst., Vols. I and II, 1970. 7. SWITHENBANK,J., et al.: Progress in Astronautics and Aeronautics, Vol. 53 (B.T. Zinn, Ed.), A.I.A.A., p. 421 (1977). 8. HODKINSON, J.P. AND GREENLEAVES, I.: J. Opt. Soc. Amer., 53, 577 (1963). 9. STEVENSON,W.H.: Spray and Particulate Diagnostics in Combustion Systems: A Review of Optical Methods, Paper presented at the Central States Section, The Combustion Inst., West Lafayette, Ind., April 1978. 10. FZLTON,P.G.: Measurement of Particle~Droplet Size Distributions by a Loser Diffraction Technique, Paper presented at the 2nd European Symp. on Particle Characterization, Niirnberg, W. Germany, Sept. 1979. 11. RL'SCELLO,L.V. AND HIRLEMAN, E.D.: Determining Droplet Size Distributions of Sprays with a Photodiode Array, Paper WWS/C1-81-49, Fall Meeting, Western States Section, The Combustion, Inst., Arizona State U., Oct. 1981. 12. VAN DE HULST, H.C.: Light Scattering by Small Particles, Dover Publications, 1981. 13. BORN, M. AND WOLF, E.: Principles of Optics, Pergamon Press, Sixth ed., 1980. 14. FOWLES,G.R.: Introduction to Modern Optics, Holt, Rinehart, and Winston, Inc., 2nd ed., 1975. 15. NEGUS, C. AND AZZOPARDI, B.J.: The Malvern Particle Size Distribution Analyzer: Its Accuracy and Limitations, U.K. Atomic Energy Authority, Harwell, Report AERE-R 9075, Dec. 1978. 16. FELTO~,', P.G., HAMIDI, A.A., AXt) ALGAL, A.K.: Multiple Scattering Effects on Particle Sizing by Laser Diffraction, Report No. 431 HIC, Dept. of Chem. Eng. and Fuel Tech., U. of Sheffield, Aug. 1984. 17. FELTON, P.G., HAMIDI, A.A., AND AIGAL, A.K.: Measurement of Drop Size Distribution in Dense Sprays by Laser Diffraction, Paper presented at ICLASS-85 and pub, in the proc. Vol.2, p. IVA/4/1, Imperial College, London, July 1985. 18. BREI~'A DE LA ROSA, A.: The Influence of Drop Size Distribution on the Structure of Unconfined Oil Sprays, Ph.D. Thesis, Dept. of Mech. Eng., U. of Waterloo, 1986.
566
PRACTICAL COMBUSTION DEVICES
19. HIRLEMAN,E.D.: On-Line Calibration Techniquefor
Laser Diffraction
Droplet
Sizing
Instruments,
A.S.M.E. Paper 83-GT-232, 1983. 20. Dot)ce, L.: Appl. Optics, 23, 2415 (1984). 21. HIRLEMAN, E.D., OECHSLE, V., AND CHIGIER, N.A.: Optical Eng. 23, 610 (1984). 22. HIRLEMAN, E.D.: Modeling of Multiple Scattering
Effects in Fraunhofer Diffraction Particle Size Analysis, Paper presented at the Canadian and Western States Sections, The Combustion Institute, 1986 Spring Technical Meeting, Banff, Alberta, April 1986. 23. GL'LDER, O.L., BILLINGHAM, R., AND CHELL1NGWORTH, F.W.: Intermittent Spray Characterization
and Spray Ignition at High Pressures and Temperature.s: Description of an Experimental Set-up, Paper
24. SMYTH, K.C., MILLER, J.H., DORFMANN, R.C., MALLARG, W.G., AND SANTORO, R.J.: Comb. and Flame 62, 157 (1985). 25. Arwva, A.M. AND WHITELAW, J.H.: Velocity,
Temperature, and Species Concentrations in Unconfined Kerosene Spray" Flames, A.S.M.E. Paper 81-WA/HT-47, 1981. 26. NOWACK, C.F.R., J. Phys. E: Scientific Instruments 3, 21 (1970). 27. CHmXER, N.A. AXD STYLES,A.C.: Sixteenth Symposium (International) on Combustion, p. 619, The Combustion Institute, 1977. 28. CHmlER, N.A. Axn STYLES, A.C.: EvaporationCombustion of Fuels, (J.T. Zung, Ed.), p. 111, Advances in Chemistry Series, #166, A.C.S., Washington, D.C., 1978.
presented at the Canadian and Western States Section, The Combustion Institute, 1986 Spring Technical Meeting, Banff, Alberta, April 1986.
COMMENTS C. R. Negus AERE Hatwell, UK. Our studies on a kerosene-fired furnace (similar in size, etc., to yours), using photography and phase-difference particle sizing, have all shown the droplets to lie predominantly within the lmninous flame front. However, your photographs show a considerable number of droplets outside the flame front. Do these droplets represent a significant fraction of the total fuel? What happens to these droplets? Is this situation common, or is your system poorly optimised? Author's Reply. The droplets lying outside the flame represented a small fraction of the fuel for No. 2 oil, and a larger fraction for No. 6 oil. In the case of the lighter oil, these droplets are eventually entrained into the flame and burned. In the case of the heavier oil, many of them fall out unburned. This situation is not common in furnaces. One
hopes that it is also rare in liquid incinerators, where just a few unburned large droplets evaporating downstream of the flame could mean the difference between acceptable and unacceptable performance. Our system was, of course, not optimized for complete combustion as in a furnace or combustor. On the contrary, it was designed to show the greatest possible effect of extreme variations of fuel properties on the atomization and combustion of a spray from a given nozzle operating under a given set of conditions. The lack of control over the air flow surrounding the spray and the absence of hot walls exaggerated the effects of initial spray momentum and fuel volatility. By showing the effects in an exaggerated form, our experiments have pointed the way to what has to be done to compensate for varying fuel properties in order to operate a real system in optimal fashion.