Measurements of fuel mixture fraction are made for a jet flame in an acoustic chamber. Acoustic forcing creates a

spatially-uniform, temporally-varying pressure field which results in oscillatory behavior in the flame . Forcing is at 22,27, 32, 37, and 55 Hz. To asses the oscillatory behavior, previous work included chemiluminescence, OH PUF, nitric oxide PUF imaging, and fuel mixture fraction measurements by infrared laser absorption. While these results illuminated what was happening to the flame chemistry, they did not provide a complete explanation as to why these things were happening. In this work, the fuel mixture fraction is measured through PUF of acetone, which is introduced into the fuel stream as a marker. This technique enables a high degree of spatial resolution of fuel/air mixture value. Both non-reacting and reacting cases were measured and comparisons are drawn with the results from the previous work. It is found that structure in the mixture fraction oscillations is a major contributor to the magnitude of the flame oscillations

Culick, F. E. C.

Kang, D. M.

Pinedo, C.

Ratner, A.

English

Caltech Authors - Main

Imaging of Fuel Mixture Fraction Oscillations in a Driven System using Acetone PLIF 
D. M. Kang, C. Pinedo, F. E. C. Culick 
Jet Propulsion CenterlDepartment of Mechanical Engineering 
California Institute of Technology 
Pasadena, California 
and 
A. Ratner 
Department of Mechanical Engineering 
University ofIowa 
Iowa City, Iowa 
Abstract 
Measurements of fuel mill.wre fraction are made for a jet flame in an acoustic chamber. Acoustic forcing creates a 
spatially-uniform, temporally-varying pressure field which results in oscillatory behavior in the flame . Forcing is at 22, 
27, 32, 37, and 55 Hz. To asses the oscillatory behavior, previous work included chemiluminescence, OH PUF, nitric 
oxide PUF imaging, and file! mixture fraction measurements by infrared laser absorption. While these results 
illuminated what was happening to the flame chemistry, they did not provide a complete explanation as to why these 
things were happening. In this work, the filel mixture fraction is measured through PUF of acetone, which is introduced 
into the fuel stream as a marker. This technique enables a high degree of spatial resolution of filellair mixture value. 
Both non-reacting and reacting cases were measured and comparisons are drawn with the results fi'om the previous work. 
It is found that structure in the mixture fraction oscillations is a major contributor to the magnitude of the flame 
oscillations. 
Introduction 
Combustion instabilities are caused by a coupling 
between thermo-acoustic and fluid-dynamic conditions 
present during the combustion processes. Combustion 
instability is defined here as 'the amplification of acoustic 
waves ' due to the thermo-acoustic coupling between 
energy release from the combustion process and acoustic 
waves inside the combustion chamber. The interaction 
between vortices (mixing), sound (acoustic, or pressure 
oscillations), and combustion heat release can lead to self-
excited oscillations that can cause structural damage. Any 
unsteadilless in the rate of combustion is a source of 
sound, generating pressure and velocity fluctuations [2]. 
Especially, as environmental issues becoming more 
important, lean premixed combustion schemes are being 
widely employed, which, while reducing the amount of 
NOx produced by lowering the flame temperature, have a 
greater tendency to cause instabilities since the 
combustion occurs near the lean blow-out limits [3]. 
A series of theoretical and experimental works [1, 4-
6] has been done on this subject in JPC, Caltech. The 
coupled effects of acoustic forcing with combustion heat 
release on species concentration was examined by Pun et 
al. [4,5] with OH-PUF under atmospheric pressure. In 
figure 1, the flame region is marked as region (1). Phase 
resolved imaging revealed phase-dependent response of 
the combustion process under low frequency (22 ~ 55Hz) 
acoustic excitations. Since this work is closely related to 
the current work in that both used the same combustion 
chamber, bumer, and acoustic excitations, it is very 
interesting and at the same time important to compare the 
effects of acoustic forcing on the energy release and 
filellair mixing. 
Femandez et al. [6] used an infrared laser technique 
to measure point-wise fuel/air mixing in terms of 
'Unmixedness factor' in the same experimental conditions 
as in the work by Pun et al. Unfortunately, a direct 
comparison of Pun ' s 311d Femandez's work is not possible 
because the technique employed by Femandez is time-, 
but not phase-, resolved. Femandez's did demonstrate 
that fuel /air mixing within the eductor block is heavily 
affected by the acoustic excitation . 
This effort aims to bridge these two studies by 
producing mixture fraction data that can be directly 
compared to both Femandez's unmixedness results and to 
Pun ' s OH PUF results. This work employs single 
frequency acoustic forcing at five different frequencies, 
just as the previous efforts, to examine how the local 
filellair mixing is affected by the imposed acoustic 
excitation and how this relates to the observed flame 
behavior. By comparison with earlier work [6], it also 
becomes possible to understand the evolution of fuel /air 
mixing. 
The phase dependence of the mixing process is 
evaluated by collecting images at random phase values for 
each frequency . These images are then sorted by phase, 
with phase defined as a best-fit sinusoid of the acoustic 
oscillation (as measured by the pressure transducer). 
Imaging of Fuel Mixture Fraction Oscillations in a Driven System using Acetone PLIF 
D. M. Kang, C. Pinedo, F. E. C. Culick 
Jet Propulsion Center/Department of Mechanical Engineering 
California Institute of Teclmology 
Pasadena, California 
and 
A. Ratner 
npn~rttl1pnt ()fMpr.h~nir.~l Pnuinpprinu 
°1 
Ignition 
- - - Steady 
Combustion 
3 
TIME (sec) 
4 
Fuel/air mixture behavior is assumed to vaty only with 
phase and spatial location , which enables averaging of 
images in each portion of phase (10° degree increments 
for this shldy). The phase-averaged images can then be 
compared to assess phase-dependent behavior. This is 
similar to the technique employed by Pun in the shldies of 
OH PLIF. 
The imaging performed here was planar laser induced 
fluorescence of acetone. The acetone was employed as a 
fuel marker to visualize the fuel/air mixing region directly 
beneath the flame, as illustrated in figure 1. Acetone is 
seeded into the fuel stream and then imaged to show the 
distribution of fuel in the flow. This technique enables us 
to see the 2-D distribution of fuel , and consequently, 
compute the degree of fuel /air mixing. 
Acetone PLIF has been previously shldied for 
various purposes [7-13]. Thurber et al. [7, 8] used this 
teclmique to determine temperahlre. Many of the other 
studies [8, 9, 11-13] focused on measurement of fuel 
concentration using the acetone-PLIF where the acetone 
had been similarly introduced as a fhel marker.. For most 
of these studies, as in the work by Demayo et al. [12] , a 
single image was taken as a representative of the system, 
and then the unmixedness was calculated based on this. 
In effect, the two dimensional image was collapsed to a 
single unmixedness value. In this paper, 2-D maps of 
unmixedness factor at each point of the image are 
calculated, as well as the phase and frequency dependence. 
hlCf(t'\ir/A~~Jj)nc mlxtllre 
Figure 1. Regions of interest; regions (1) flame region 
[4, 51 (OH-PLIF), (2) at the neck of the eductOl' block 
[61 (lnfrared lasel'), (3) in the mixing region (acetone-
PLIF) 
Experimental Configuration 
Figure 2 shows a schematic of the acoustic chamber 
and figure 3 illustrates how acetone is seeded into the fuel 
stream. The chamber exhaust is open to the atmosphere 
at the top, providing an acoustically open exit condition. 
A pair of acoustic drivers are sealed to a pair of air jet 
2 
film cooling rings (to prevent heat failure of the drivers), 
which are in him sealed to opposite sides of the steel 
structure. These acoustic drivers are cooled by air flow. 
The acoustic drivers are 12 inch diameter sub woofers 
that can continuously handle 400 W of power. A 1000W 
power amplifier along.with a function generator provides 
the input signal and power to the acoustic drivers. The 
acoustic chamber is made of aluminum, with a ring at the 
bottom, which creates a closed-end acoustic condition, 
The ring has two sets of inlet louvers cut on opposing 
sides to allow air to flow into the hlbe, while maintaining 
acoustically closed end condition. 
The burner (fig 1,2) is a traditional jet-mixed burner 
with flame anchoring about in the middle of the quartz 
tube depending on the fuel /air mixhlre flow rate. The thel 
is 50% methane and 50% nitrogen. The tube is 5.72cm 
wide on each side and 11.43cm tall. PLIF imaging of 
acetone, with excitation at 280 nm, is performed at the 
bottom portion of the quartz tube where no flame is 
present. 
Exhaust 
c 
d 
t 
e ..... _ .. 
.... ,t-
a 
b 
Figure 2. The combustion chamber. (a): loud 
speakers, (b) pressure transducer, (c) fused-silica 
burner tube, (d) eductor block(see fig. 1), (e) fuel spud, 
alTows at the bottom indicate the ail' inlet 
Fuel seeded W(th acatone 
Figure 3. Acetone scedcl' 
Figure 4 is a simplified schematic of the laser and 
imaging systems. An intensified CCD camera is used for 
the image acquisition, while a National Instnunent data 
acquisition board (NI PCI 6014) along with pressure 
transducer (PCB 106B50) is used to measure and record 
the pressure signal. All timing is lin ked to the ND: YAG 
pump laser. The ND:YAG outputs a high power laser 
beam at 532 nm which then drives a dye laser. The dye 
laser produces a 560 nm beam which. in turn, is frequency 
doubled to 280 nm for excitation of acetone. Lenses 
elongate the height and narrow the width (where the 
minimum sheet waist is at the image center). 
1=====i532 rwn S60nm 280nm 
tCGD Camera 
DAQ Computer 
Figure 4. Laser and imaging system configuration 
The PLIF signal is captured by an intensified CCD 
camera with a maximum resolution of 512 by 512 pixels. 
An area of 5.5 by 4.1 cm is imaged with 160 by 160 
microns corresponding to each camera pixel (344 by 256 
pixels). The PLIF signal goes through a UV high-pass 
filter which filters out the scatters from the 300 nm 
excitation beam. Fluorescence occurs between 350 and 
550 nm in wavelength. 
Results 
For experiments with acoustic forcing, the images are 
post-processed in a phase-resolved manner-binning all the 
images in each phase range (10 degrees each). Each 
image is tagged with its own phase information, then 
afterwards moved to the bin corresponding to the tag. 
Images in each bin are read in and convelied to 
mathematical variables to calculate averages and standard 
deviations. Some selected data are presented here. 
A sample acetone - PLIF image is shm;vn in figure 5 
(one of 900 similar images in this data set). It is assumed 
that the intensity or the brightness of each pixel is linearly 
proportional to the local fhel concentration ([14] showed 
that this induces up to 5% error, mainly due to CCD non-
linearity). As is visible in the image in figure 5, the bright 
3 
core part is denser with fuel, surrounded by less dense 
mixing regions, with pure air on the outside. 
To compare these images to previous data, 
unmixedness needs to be defined for image data. Here, 
unmixedness is defined as: 
(}2 
U=----------
(xmax - < X»· « X > -xmin ) 
(I) 
where (J is the standard deviation of fuel concentration at 
one pixel, x. fhel concentration, and <>, the average at the 
pixel. However, this measure of unmixedness can either 
be an indication of how unstable the fhel/air mixing is at a 
point or that the fuel concentration values are high. 
Figure 5. Acetone PLIF image (16-bit Tiff), reduced to 
225 x 300 pixels to highlight the laser - sheet region. 
Some of the high unmixedness values (figure 6, 
equation (1» are, as expected, located in the center fuel 
core region. To either side of the core, relatively low 
unmixedness is seen, implying that fuel and air in this 
region experience stable mixing. Outside of that region, 
there occur high unmixedness values, likely indicating 
unstable mixing rather than high fuel concentration . The 
boundary area has low unmixedness values since it is 
mostly pure air. 
Figure 6. 2-D map of Unmixedness factor (37Hz, 
60-70 degree phase, by equation (1» 
Another way of defining unmixed ness is as follows. 
a 2 U=-----(l-<x»'<x> 
(2) 
where cr is the standard deviation and <x> average of fuel 
concentration over the eutire 2-D image, instead of oue 
poiut with many measurements. A 2-D image collapses to 
a point value by tlus definition. and this is very usefitl 
when the phase resolved behavior of mixing rate over a 
period of time. 
It has been shown by Fernaudez et al. [6] that high 
values of unmixedness occur in the shear mixing layer of 
the flow (30-60% from the center, see figure 7). There is 
not much variation between cases with different 
excitation frequencies , but they show clear position 
dependency. 
0.15 - 01"« 
- 22hz 
= ;~ I 
- 37hz: 
- 55hz 
~ 30 40 50 GO 70 00 90 100 
NORMALIZED RADIAL LOCATION (rI R) 
Figure 7. Temporal Unmixedness facto!' [6] at the neck 
of the eductor block by equation (1) 
FllIther downstream from the eductor block, it seems 
the shear mixing zone seems to have moved and widened 
to 30 - 80 % region. The basic structure of the 
unmixedness distribution seems only to have been 
smeared (as is expected for a mixing process). The 
seemingly higher values of unmixedness factor 
downstream (fig 8) compared to those upstream values 
(fig. 7) does not necessarily mean more uusteady behavior 
of li.!ellair mixtme downstream, but, rather, the difference 
of the measurement techniques aud their resolutions. In 
the core region close to the center, the variation is very 
small as expected fi'om previous results. This shows the 
core region is still filel dominant and needs fUither mixing. 
Qualitatively speaking, it seems that while the filellair 
mixing keeps the same stmcture downstream, the curves 
are less stiff; meaning the degree of instability in fuel /air 
mixing is decreasing downstream. Currently it is not 
possible to make any quantitative comparison since there 
is as yet no numerical correlation between the infrared 
absorption and the acetone-PLIF measurements. 
4 
G' 
- "'" 
- 27'" 
--'"'' - "",, 
Figure 8. Temporal Unmixedness factor inside the 
mixing I'egion with elevations (a) 1 em, (b) 2 cm, and 
(c) 3 em above the eductor block by equation (1). 
Figure 9 shows the comparison of unmixedness 
factors at three different distances from the eductor block 
(1,2 , 3 cm each) at 37 Hz. As the flow goes down stream, 
it is shown that better and more stable mixing - smaller 
unmixedness-occurs in the outer region of 60-80% 
locations. 
(J.4S,..~_~ _ _ ~"""" __ f"'_~_"~1_-_"_mr-:''':''~'"',,:;)_-c==,..-, 
1 = ~g: 1 
- 30. 0.1 
O,3S 
O.10!--;10:c---:O"'C----cJ<l~--;I;;.-. - -;;.,C---, .. ;;;----;.,, --; .. :;--= ..-~,00 
'Ito dlClfKffrom Il1o eorttr 
Figure 9. Temporal Unmixedness factors at each 
location with excitation fl'equency at 37 Hz 
-0.25 ·0,2 -0,15 .0 1 ..0,05 0 . O,OS G 1 0,15 0.2 0':5 
Figul'e 10. Distdbution of fuel concentration by phase 
at 37 Hz. Contours show the difference from the 
average fuel distribution, normalized by the average 
value at that point. 
5 
Phase dependence of the mixing behavior is shown in 
figure 10. Here higher valued contours indicate higher 
than average fuel concentration for that region (reds and 
yellows), while the low value contours indicate lower 
than the average filel concentration (dark and light blue). 
Each axis is presented in pixel, and the domain size is 55 
x 42 mm. All the contours are scaled to values from -0.3 
to 0.3. The phase is that of excitation pressure wave. For 
37 Hz, the highest overall filel concentration occurs at the 
pressure node (0, 180 degree), then decreases 
subsequently through 30(210) and 60(240) degrees until 
the pressure anti-Hodes (90, 270 degree) , and so on. The 
phase lead is approximately 90 degrees in this case. In 
the latter part of the phase, the fluctuation seems a little 
bit smeared by a possible secondalY mode which is hard 
to see clearly at present phase resolution (10 degrees). 
The phase lead or lag is different for each frequency. The 
magnitude of the variation will be determined by 
evaluating the global unmixedness defined by equation 
(2). 
Conclusion 
As expected, the acetone-PLIF produced result, with 
a very low signal-to-noise ratio where only minimal 
adjustment was required for post-processing. So far, only 
qualitati ve comparisons have been performed. 
The uHmixedness defined by equation (1), a measure 
of variation in fuel /air mixing, showed the same structural 
shape downstream of eductor block as in the upstream 
region, and it was observed that as the flow progresses 
downstream, more mixing reduces the variation in the 
level of mixing. The same tendency was found at all 
frequencies investigated, showing that the impact on the 
mixing is not a frequency resonant effect. 
From the behavior of fllel concentration, it can be 
concluded that the oscillations occurring inside the 
mixing region are closely related to the previousl y 
reported flame oscillation behavior [4, 5]. The plwse lead 
and lag are very important in determining the response of 
fuel /air mixing due to the eftects of acoustic forcing, and 
analysis will continue in an effort to e)..1:ract a measured 
response (transfer) fimction of the mixing region. 
Acknowledgements 
This work was supported in part by the Califomia 
Institute of Technology and partly by the Air Force Office 
of Scientific Research (AFOSR) under Grant No. F49620-
03-1-0384 (Dr. Mitat Birkan , Program Manager). 
References 
1. Culick F. E. C., "A Note on Rayleigh ' s Criterion", Co 
mbustion Science and Technology, Vol. 56, pp. 159-1 
66, 1987. 
2. Dowling, A.P., "VOItices, Sound and Flames - a dama 
ging combination ", The Aeronautical Joumal , pp 105-1 
16, 2000. 
3. Venkataraman. K.K. , Preston, L.H., Simons, D.W., Le 
e, B.J., Lee J.O. , Santavicca, D.A. , "Mechanism of co 
mbustion instability in a lean premixed dump combust 
or", AIAA JoullIal of Propulsion and Power, 15(6):90 
9-918, 1999. 
4. Pun, W. , Ratner. A., and Culick, F.E.C., " Phase-Resol 
ved Chemiluminescence of an Acoustically Forced Jet 
Flame at Frequencies < 60 Hz", 40U1 AlAA Aerospace 
Sciences Meeting & Exhibit, AIAA-2002-0194, 2002. 
5. Pun , W., Palm S.L. , and Culick, F.E.C., "PLIF Measur 
ements of Combustion Dynamics in a BUlller under Fo 
rced OscillatOIY Conditions", 36th AlAAIASME/SAE/ 
ASEE Joint Propulsion Conference and Exhibit", AlA 
A-2000-3123 , 2000 
6. Femandez, V. , Ratner, and A.. Culick, F.E.C., "Measu 
red Influence of Oscillations in Fuel Mixture Fraction 
on Flame Behavior", Proceedings of the 3rd Joint Meet 
ing of the Combustion Inst. , 2003 
7. Thurber, M.e. , Orisch, F., and Hanson, R.K. , "Temper 
attire imaging with single- and dual-wavelength aceto 
ne planar laser-induced fluorescence" , Optical letters, 
Optics Society of America, Vo!' 22, No.4, pp. 251-25 
3, 1997 
8. Thurber, M.C., and Hanson, R.K. , "Simultaneous ima 
ging oftemperattlre and mole fraction using acetone pi 
anar laser-induced fluorescence", Exp. of Fluids, Vo!' 
30, pp. 93-101 , 2001 
9. Thurber, M.C. , and Hanson. R.K. , "Pressure and comp 
osition dependences of acetone laser-induced fluoresc 
ence with excitations at 248, 266, and 308nm", App!. 
Phys. B, Vol. 69, pp. 229-240, 1999 
10. Blyant, R.A. , Donbar, J.M. , and Driscoll, J.F ., "Aceto 
ne laser induced fluorescence for low pressure/low te 
mperattlre flow visualization", Exp. in Fluids, Vol. 28, 
pp. 471-476, 2000 
II. Meyer, T. R. , King, O. F., "Accuracy and resolution is 
sues in NO/acetone PLIF measurements of gas-phase 
molecular mixing." Exp. in Fluids, 32(6): 603-61 
1, 2002 
12. Demayo, T. N. , Leong, M.Y. , Samuelsen, G. S., 
Holdeman, J. D., "Assessing Jet-Induced Spatial 
Mixing in a Rich, Reacting Crossflow", J. of Prop. and 
Power, 19(1), 14-21 , 2003 
13. Yip, B., Miller, M. F. , "A Combined OHiAcetone 
PLIF Imaging Tecimique for Visualizing Combusting 
Flows." Exp. in Fluids, 17(5): 330-336, 1994 
14. Ratner, A. , Driscoll, J. F. , Huh, H., and Bryant, R. A., 
"Combustion efficiencies of supersonic flames," 
AIAA Journal of Propulsion and Power, 17 (2): 301-
307 MAR-APR 2001. 
6 


Imaging of fuel mixture fraction oscillations in a driven system using acetone PLIF

Imaging of Fuel Mixture Fraction Oscillations in a Driven System using Acetone PLIF 
D. M. Kang, C. Pinedo, F. E. C. Culick 
Jet Propulsion CenterlDepartment of Mechanical Engineering 
California Institute of Technology 
Pasadena, California 
and 
A. Ratner 
Department of Mechanical Engineering 
University ofIowa 
Iowa City, Iowa 
Abstract 
Measurements of fuel mill.wre fraction are made for a jet flame in an acoustic chamber. Acoustic forcing creates a 
spatially-uniform, temporally-varying pressure field which results in oscillatory behavior in the flame . Forcing is at 22, 
27, 32, 37, and 55 Hz. To asses the oscillatory behavior, previous work included chemiluminescence, OH PUF, nitric 
oxide PUF imaging, and file! mixture fraction measurements by infrared laser absorption. While these results 
illuminated what was happening to the flame chemistry, they did not provide a complete explanation as to why these 
things were happening. In this work, the filel mixture fraction is measured through PUF of acetone, which is introduced 
into the fuel stream as a marker. This technique enables a high degree of spatial resolution of filellair mixture value. 
Both non-reacting and reacting cases were measured and comparisons are drawn with the results fi'om the previous work. 
It is found that structure in the mixture fraction oscillations is a major contributor to the magnitude of the flame 
oscillations. 
Introduction 
Combustion instabilities are caused by a coupling 
between thermo-acoustic and fluid-dynamic conditions 
present during the combustion processes. Combustion 
instability is defined here as 'the amplification of acoustic 
waves ' due to the thermo-acoustic coupling between 
energy release from the combustion process and acoustic 
waves inside the combustion chamber. The interaction 
between vortices (mixing), sound (acoustic, or pressure 
oscillations), and combustion heat release can lead to self-
excited oscillations that can cause structural damage. Any 
unsteadilless in the rate of combustion is a source of 
sound, generating pressure and velocity fluctuations [2]. 
Especially, as environmental issues becoming more 
important, lean premixed combustion schemes are being 
widely employed, which, while reducing the amount of 
NOx produced by lowering the flame temperature, have a 
greater tendency to cause instabilities since the 
combustion occurs near the lean blow-out limits [3]. 
A series of theoretical and experimental works [1, 4-
6] has been done on this subject in JPC, Caltech. The 
coupled effects of acoustic forcing with combustion heat 
release on species concentration was examined by Pun et 
al. [4,5] with OH-PUF under atmospheric pressure. In 
figure 1, the flame region is marked as region (1). Phase 
resolved imaging revealed phase-dependent response of 
the combustion process under low frequency (22 ~ 55Hz) 
acoustic excitations. Since this work is closely related to 
the current work in that both used the same combustion 
chamber, bumer, and acoustic excitations, it is very 
interesting and at the same time important to compare the 
effects of acoustic forcing on the energy release and 
filellair mixing. 
Femandez et al. [6] used an infrared laser technique 
to measure point-wise fuel/air mixing in terms of 
'Unmixedness factor' in the same experimental conditions 
as in the work by Pun et al. Unfortunately, a direct 
comparison of Pun ' s 311d Femandez's work is not possible 
because the technique employed by Femandez is time-, 
but not phase-, resolved. Femandez's did demonstrate 
that fuel /air mixing within the eductor block is heavily 
affected by the acoustic excitation . 
This effort aims to bridge these two studies by 
producing mixture fraction data that can be directly 
compared to both Femandez's unmixedness results and to 
Pun ' s OH PUF results. This work employs single 
frequency acoustic forcing at five different frequencies, 
just as the previous efforts, to examine how the local 
filellair mixing is affected by the imposed acoustic 
excitation and how this relates to the observed flame 
behavior. By comparison with earlier work [6], it also 
becomes possible to understand the evolution of fuel /air 
mixing. 
The phase dependence of the mixing process is 
evaluated by collecting images at random phase values for 
each frequency . These images are then sorted by phase, 
with phase defined as a best-fit sinusoid of the acoustic 
oscillation (as measured by the pressure transducer). 
Imaging of Fuel Mixture Fraction Oscillations in a Driven System using Acetone PLIF 
D. M. Kang, C. Pinedo, F. E. C. Culick 
Jet Propulsion Center/Department of Mechanical Engineering 
California Institute of Teclmology 
Pasadena, California 
and 
A. Ratner 
npn~rttl1pnt ()fMpr.h~nir.~l Pnuinpprinu 
°1 
Ignition 
- - - Steady 
Combustion 
3 
TIME (sec) 
4 
Fuel/air mixture behavior is assumed to vaty only with 
phase and spatial location , which enables averaging of 
images in each portion of phase (10° degree increments 
for this shldy). The phase-averaged images can then be 
compared to assess phase-dependent behavior. This is 
similar to the technique employed by Pun in the shldies of 
OH PLIF. 
The imaging performed here was planar laser induced 
fluorescence of acetone. The acetone was employed as a 
fuel marker to visualize the fuel/air mixing region directly 
beneath the flame, as illustrated in figure 1. Acetone is 
seeded into the fuel stream and then imaged to show the 
distribution of fuel in the flow. This technique enables us 
to see the 2-D distribution of fuel , and consequently, 
compute the degree of fuel /air mixing. 
Acetone PLIF has been previously shldied for 
various purposes [7-13]. Thurber et al. [7, 8] used this 
teclmique to determine temperahlre. Many of the other 
studies [8, 9, 11-13] focused on measurement of fuel 
concentration using the acetone-PLIF where the acetone 
had been similarly introduced as a fhel marker.. For most 
of these studies, as in the work by Demayo et al. [12] , a 
single image was taken as a representative of the system, 
and then the unmixedness was calculated based on this. 
In effect, the two dimensional image was collapsed to a 
single unmixedness value. In this paper, 2-D maps of 
unmixedness factor at each point of the image are 
calculated, as well as the phase and frequency dependence. 
hlCf(t'\ir/A~~Jj)nc mlxtllre 
Figure 1. Regions of interest; regions (1) flame region 
[4, 51 (OH-PLIF), (2) at the neck of the eductOl' block 
[61 (lnfrared lasel'), (3) in the mixing region (acetone-
PLIF) 
Experimental Configuration 
Figure 2 shows a schematic of the acoustic chamber 
and figure 3 illustrates how acetone is seeded into the fuel 
stream. The chamber exhaust is open to the atmosphere 
at the top, providing an acoustically open exit condition. 
A pair of acoustic drivers are sealed to a pair of air jet 
2 
film cooling rings (to prevent heat failure of the drivers), 
which are in him sealed to opposite sides of the steel 
structure. These acoustic drivers are cooled by air flow. 
The acoustic drivers are 12 inch diameter sub woofers 
that can continuously handle 400 W of power. A 1000W 
power amplifier along.with a function generator provides 
the input signal and power to the acoustic drivers. The 
acoustic chamber is made of aluminum, with a ring at the 
bottom, which creates a closed-end acoustic condition, 
The ring has two sets of inlet louvers cut on opposing 
sides to allow air to flow into the hlbe, while maintaining 
acoustically closed end condition. 
The burner (fig 1,2) is a traditional jet-mixed burner 
with flame anchoring about in the middle of the quartz 
tube depending on the fuel /air mixhlre flow rate. The thel 
is 50% methane and 50% nitrogen. The tube is 5.72cm 
wide on each side and 11.43cm tall. PLIF imaging of 
acetone, with excitation at 280 nm, is performed at the 
bottom portion of the quartz tube where no flame is 
present. 
Exhaust 
c 
d 
t 
e ..... _ .. 
.... ,t-
a 
b 
Figure 2. The combustion chamber. (a): loud 
speakers, (b) pressure transducer, (c) fused-silica 
burner tube, (d) eductor block(see fig. 1), (e) fuel spud, 
alTows at the bottom indicate the ail' inlet 
Fuel seeded W(th acatone 
Figure 3. Acetone scedcl' 
Figure 4 is a simplified schematic of the laser and 
imaging systems. An intensified CCD camera is used for 
the image acquisition, while a National Instnunent data 
acquisition board (NI PCI 6014) along with pressure 
transducer (PCB 106B50) is used to measure and record 
the pressure signal. All timing is lin ked to the ND: YAG 
pump laser. The ND:YAG outputs a high power laser 
beam at 532 nm which then drives a dye laser. The dye 
laser produces a 560 nm beam which. in turn, is frequency 
doubled to 280 nm for excitation of acetone. Lenses 
elongate the height and narrow the width (where the 
minimum sheet waist is at the image center). 
1=====i532 rwn S60nm 280nm 
tCGD Camera 
DAQ Computer 
Figure 4. Laser and imaging system configuration 
The PLIF signal is captured by an intensified CCD 
camera with a maximum resolution of 512 by 512 pixels. 
An area of 5.5 by 4.1 cm is imaged with 160 by 160 
microns corresponding to each camera pixel (344 by 256 
pixels). The PLIF signal goes through a UV high-pass 
filter which filters out the scatters from the 300 nm 
excitation beam. Fluorescence occurs between 350 and 
550 nm in wavelength. 
Results 
For experiments with acoustic forcing, the images are 
post-processed in a phase-resolved manner-binning all the 
images in each phase range (10 degrees each). Each 
image is tagged with its own phase information, then 
afterwards moved to the bin corresponding to the tag. 
Images in each bin are read in and convelied to 
mathematical variables to calculate averages and standard 
deviations. Some selected data are presented here. 
A sample acetone - PLIF image is shm;vn in figure 5 
(one of 900 similar images in this data set). It is assumed 
that the intensity or the brightness of each pixel is linearly 
proportional to the local fhel concentration ([14] showed 
that this induces up to 5% error, mainly due to CCD non-
linearity). As is visible in the image in figure 5, the bright 
3 
core part is denser with fuel, surrounded by less dense 
mixing regions, with pure air on the outside. 
To compare these images to previous data, 
unmixedness needs to be defined for image data. Here, 
unmixedness is defined as: 
(}2 
U=----------
(xmax - < X»· « X > -xmin ) 
(I) 
where (J is the standard deviation of fuel concentration at 
one pixel, x. fhel concentration, and <>, the average at the 
pixel. However, this measure of unmixedness can either 
be an indication of how unstable the fhel/air mixing is at a 
point or that the fuel concentration values are high. 
Figure 5. Acetone PLIF image (16-bit Tiff), reduced to 
225 x 300 pixels to highlight the laser - sheet region. 
Some of the high unmixedness values (figure 6, 
equation (1» are, as expected, located in the center fuel 
core region. To either side of the core, relatively low 
unmixedness is seen, implying that fuel and air in this 
region experience stable mixing. Outside of that region, 
there occur high unmixedness values, likely indicating 
unstable mixing rather than high fuel concentration . The 
boundary area has low unmixedness values since it is 
mostly pure air. 
Figure 6. 2-D map of Unmixedness factor (37Hz, 
60-70 degree phase, by equation (1» 
Another way of defining unmixed ness is as follows. 
a 2 
U=-----
(l-<x»'<x> 
(2) 
where cr is the standard deviation and <x> average of fuel 
concentration over the eutire 2-D image, instead of oue 
poiut with many measurements. A 2-D image collapses to 
a point value by tlus definition. and this is very usefitl 
when the phase resolved behavior of mixing rate over a 
period of time. 
It has been shown by Fernaudez et al. [6] that high 
values of unmixedness occur in the shear mixing layer of 
the flow (30-60% from the center, see figure 7). There is 
not much variation between cases with different 
excitation frequencies , but they show clear position 
dependency. 
0.15 
- 01"« 
- 22hz 
= ;~ I 
- 37hz: 
- 55hz 
~ 30 40 50 GO 70 00 90 100 
NORMALIZED RADIAL LOCATION (rI R) 
Figure 7. Temporal Unmixedness facto!' [6] at the neck 
of the eductor block by equation (1) 
FllIther downstream from the eductor block, it seems 
the shear mixing zone seems to have moved and widened 
to 30 - 80 % region. The basic structure of the 
unmixedness distribution seems only to have been 
smeared (as is expected for a mixing process). The 
seemingly higher values of unmixedness factor 
downstream (fig 8) compared to those upstream values 
(fig. 7) does not necessarily mean more uusteady behavior 
of li.!ellair mixtme downstream, but, rather, the difference 
of the measurement techniques aud their resolutions. In 
the core region close to the center, the variation is very 
small as expected fi'om previous results. This shows the 
core region is still filel dominant and needs fUither mixing. 
Qualitatively speaking, it seems that while the filellair 
mixing keeps the same stmcture downstream, the curves 
are less stiff; meaning the degree of instability in fuel /air 
mixing is decreasing downstream. Currently it is not 
possible to make any quantitative comparison since there 
is as yet no numerical correlation between the infrared 
absorption and the acetone-PLIF measurements. 
4 
G' 
- "'" - 27'" --'"'' - "",, 
Figure 8. Temporal Unmixedness factor inside the 
mixing I'egion with elevations (a) 1 em, (b) 2 cm, and 
(c) 3 em above the eductor block by equation (1). 
Figure 9 shows the comparison of unmixedness 
factors at three different distances from the eductor block 
(1,2 , 3 cm each) at 37 Hz. As the flow goes down stream, 
it is shown that better and more stable mixing - smaller 
unmixedness-occurs in the outer region of 60-80% 
locations. 
(J.4S,..~_~ _ _ ~"""" __ f"'_~_"~1_-_"_mr-:''':''~'"',,:;)_-c==,..-, 
1 = ~g: 1 
- 30. 
0.1 
O,3S 
O.10!--;10:c---:O"'C----cJ<l~--;I;;.-. - -;;.,C---, .. ;;;----;.,, --; .. :;--= ..-~,00 
'Ito dlClfKffrom Il1o eorttr 
Figure 9. Temporal Unmixedness factors at each 
location with excitation fl'equency at 37 Hz 
-0.25 ·0,2 -0,15 .0 1 ..0,05 0 . O,OS G 1 0,15 0.2 0':5 
Figul'e 10. Distdbution of fuel concentration by phase 
at 37 Hz. Contours show the difference from the 
average fuel distribution, normalized by the average 
value at that point. 
5 
Phase dependence of the mixing behavior is shown in 
figure 10. Here higher valued contours indicate higher 
than average fuel concentration for that region (reds and 
yellows), while the low value contours indicate lower 
than the average filel concentration (dark and light blue). 
Each axis is presented in pixel, and the domain size is 55 
x 42 mm. All the contours are scaled to values from -0.3 
to 0.3. The phase is that of excitation pressure wave. For 
37 Hz, the highest overall filel concentration occurs at the 
pressure node (0, 180 degree), then decreases 
subsequently through 30(210) and 60(240) degrees until 
the pressure anti-Hodes (90, 270 degree) , and so on. The 
phase lead is approximately 90 degrees in this case. In 
the latter part of the phase, the fluctuation seems a little 
bit smeared by a possible secondalY mode which is hard 
to see clearly at present phase resolution (10 degrees). 
The phase lead or lag is different for each frequency. The 
magnitude of the variation will be determined by 
evaluating the global unmixedness defined by equation 
(2). 
Conclusion 
As expected, the acetone-PLIF produced result, with 
a very low signal-to-noise ratio where only minimal 
adjustment was required for post-processing. So far, only 
qualitati ve comparisons have been performed. 
The uHmixedness defined by equation (1), a measure 
of variation in fuel /air mixing, showed the same structural 
shape downstream of eductor block as in the upstream 
region, and it was observed that as the flow progresses 
downstream, more mixing reduces the variation in the 
level of mixing. The same tendency was found at all 
frequencies investigated, showing that the impact on the 
mixing is not a frequency resonant effect. 
From the behavior of fllel concentration, it can be 
concluded that the oscillations occurring inside the 
mixing region are closely related to the previousl y 
reported flame oscillation behavior [4, 5]. The plwse lead 
and lag are very important in determining the response of 
fuel /air mixing due to the eftects of acoustic forcing, and 
analysis will continue in an effort to e)..1:ract a measured 
response (transfer) fimction of the mixing region. 
Acknowledgements 
This work was supported in part by the Califomia 
Institute of Technology and partly by the Air Force Office 
of Scientific Research (AFOSR) under Grant No. F49620-
03-1-0384 (Dr. Mitat Birkan , Program Manager). 
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6 


A Combined OHiAcetone PLIF Imaging Tecimique for Visualizing Combusting Flows.&quot; Exp.

A Note on Rayleigh ' s Criterion&quot;,

A Note on Rayleigh's Criterion&quot;,

Aceto ne laser induced fluorescence for low pressure/low te mperattlre flow visualization&quot;,

Assessing Jet-Induced Spatial Mixing in a Rich, Reacting Crossflow&quot;,

Combustion efficiencies of supersonic flames,&quot;

Measu red Influence of Oscillations in Fuel Mixture Fraction on Flame Behavior&quot;,

Mechanism of co mbustion instability in a lean premixed dump combust or&quot;,

Phase-Resol ved Chemiluminescence of an Acoustically Forced Jet Flame at

Phase-Resol ved Chemiluminescence of an Acoustically Forced Jet Flame at Frequencies < 60 Hz&quot;,

PLIF Measur ements of Combustion Dynamics in a BUlller under Fo rced OscillatOIY Conditions&quot;,

Pressure and comp osition dependences of acetone laser-induced fluoresc ence with excitations at 248, 266, and 308nm&quot;,

Simultaneous ima ging oftemperattlre and mole fraction using acetone pi anar laser-induced fluorescence&quot;,

Temper attire imaging with single- and dual-wavelength aceto ne planar laser-induced fluorescence&quot;

Temper attire imaging with single- and dual-wavelength aceto ne planar laser-induced fluorescence&quot;,

VOItices, Sound and Flames - a dama ging combination &quot;, The Aeronautical Joumal ,

VOItices, Sound and Flames - a dama ging combination &quot;, The Aeronautical Joumal,

http://authors.library.caltech.edu/22063/1/Kang_DM_Imaging_of_Fuel_Mixture.pdf

Imaging of fuel mixture fraction oscillations in a driven system using acetone PLIF

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