OH and CH2O were imaged in a premixed, cavity-anchored, ethylene-air turbulent flame using a high resolution planar laser-induced fluorescence (PLIF) system. The electrically-heated, continuous flow facility (UVa Supersonic Combustion Facility, Configuration E) consisted of a Mach 2 nozzle, an isolator with fuel injectors, a test section with a cavity flame holder and optical access, and an extender. Standard test conditions comprised total temperature 1200 K, total pressure 300 kPa, local equivalence ratio near 0.4, and local Mach number near 0.6. OH PLIF data was also collected for a case with reduced total temperature and another with reduced equivalence ratio. OH and CH2O were excited in separate experiments with light sheets at 283.55 nm and 352.48 nm, respectively. A light sheet of approximate thickness 25 ?m illuminated the stream-wise midplane. This plane was imaged for 120 mm downstream of the backward-facing step. The intensified camera system imaged OH with magnification 1.97, a square 6.67 mm field of view, and in-plane resolution of 39 ?m. The smallest observed OH structures observed were approximately 100 ?m wide. The CH2O PLIF image signal was much weaker; the smallest observed structures were approximately 200 ?m wide. Composite fluorescence images were computed for the observed area

Chelliah, Harsha K.

Cutler, Andrew D.

Danehy, Paul M.

Geipel, Clayton M.

Hashem, Zeid

Rockwell, Robert D.

Spelker, Christopher A.

NASA Technical Reports Server

Diagnostics Colloquium 
 
1 
10th U. S. National Combustion Meeting 
Organized by the Eastern States Section of the Combustion Institute 
April 23-26, 2017 
College Park, Maryland 
 
High-Resolution OH and CH2O Visualization in a Premixed 
Cavity-Anchored Ethylene-Air Flame in a M = 0.6 Flowfield 
 
Clayton M. Geipel1,*, Robert D. Rockwell1, Harsha K. Chelliah1,  
Andrew D. Cutler2, Christopher A. Spelker2, Zeid Hashem2, Paul M. Danehy3 
 
1Department of Mechanical and Aerospace Engineering, 
University of Virginia, Charlottesville, Virginia 22904, USA 
2Department of Mechanical and Aerospace Engineering, 
The George Washington University, Washington, District of Columbia 20052, USA 
3NASA Langley Research Center, Hampton, Virginia 23681, USA 
*geipel@virginia.edu 
 
Abstract: OH and CH2O were imaged in a premixed, cavity-anchored, ethylene-air turbulent flame 
using a high-resolution planar laser-induced fluorescence (PLIF) system. The electrically-heated, 
continuous flow facility (UVa Supersonic Combustion Facility, Configuration E) consisted of a 
Mach 2 nozzle, an isolator containing flush-wall fuel injectors, a combustor with a cavity 
flameholder and optical access, and an extender. Standard test conditions comprised total 
temperature 1200 K and total pressure 300 kPa, local equivalence ratios near 0.4, and local Mach 
number near 0.6 in the combustor. OH and CH2O were excited in separate experiments with planar 
laser sheets at 283.55 nm and 352.48 nm, respectively. A laser sheet of approximate thickness 25 
μm illuminated a streamwise plane that bisected the cavity. This plane was imaged between the 
backward-facing step and 120 mm downstream of the step. The intensified camera system imaged 
OH with magnification 1.91, a square 6.67 mm field of view, and in-plane resolution 39 μm. The 
smallest observed OH structures were approximately 110 μm wide. CH2O PLIF was captured with 
poorer resolution; structures as small as 200 μm were observed. Composite fluorescence images 
were computed for the observed area. 
Keywords: scramjet, cavity flameholder, PLIF, high-resolution visualization 
 
1. Introduction 
High-resolution OH and CH2O PLIF imaging was performed at the University of Virginia 
Supersonic Combustion Facility (UVaSCF) on a premixed ethylene-air flame that was anchored 
and stabilized by a cavity flameholder. The UVaSCF is a continuous-flow, electrically-heated, 
clean-air ground test facility with optical access that produces an enthalpy simulating a Mach 𝑀 
= 5 scramjet flight. This work corresponds to UVaSCF “modified Configuration E” [1]: a 𝑀 = 2 
nozzle followed by an isolator with fuel injectors, a combustor test section, and an outlet.  Tests 
were performed with total temperature 1200 K, total pressure 300 kPa, local equivalence ratios 
near 0.4, and 𝑀 = 0.6 at the combustor.  
Nonintrusive optical diagnostic techniques such as planar laser-induced fluorescence (PLIF) are 
often used to characterize turbulent flame structures [2]. PLIF is a technique in which a 
cylindrical lens expands a circular laser beam in a given direction and then a spherical lens 
collimates the beam in the given direction and focuses it in the orthogonal direction, creating a 
https://ntrs.nasa.gov/search.jsp?R=20170004913 2019-08-29T22:26:22+00:00Z
Diagnostics Colloquium 
 
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thin sheet of laser light.  The wavelength of this sheet is tuned to match an absorption band of a 
chemical species of interest, usually an intermediate product of combustion.  The species emits 
broadband fluorescence, which is captured by an intensified camera.  This provides an 
instantaneous, planar map of qualitative or quantitative concentration measurements.  PLIF 
images can be compared to computational species concentration results in order to refine 
computational and theoretical models. 
Previous studies have imaged OH [3] and CH2O [4] at UVaSCF; the current work presents data 
at a higher spatial resolution, resolving smaller flame structures. The resolution of PLIF images 
depends on both the resolution of the imaging and camera system and on the laser sheet 
thickness.  Turbulent flows are highly three-dimensional, and PLIF signal is integrated over the 
sheet thickness.  Obtaining a sharp image of small structures is therefore dependent on creating a 
thin laser sheet. 
The smallest possible perturbations in a turbulent flow are on the order of the Kolmogorov 
length scale 𝜂.  This is the length over which turbulent kinetic energy can be dissipated by 
viscosity. Previous particle image velocimetry experiments and computational investigations of 
the current flowpath [5] have yielded estimates for the integral length scale 𝐿 = 5 mm, root-mean 
square velocity 𝑢 = 50 m/s, and kinematic viscosity 𝜐 = 4 to 5 m2/s. The expression [6] 𝜂 =
(𝜐/𝑢)3/4𝐿1/4 yields 𝜂 = 7 to 8 µm.  The smallest flame structures are expected to be around ten 
times larger than the Kolmogorov length scale [7]. Therefore, a PLIF system that has both planar 
resolution and laser sheet thickness smaller than 70 µm should be able to observe the smallest 
flame structures in the flow. 
2. Set-up 
A long-duration visible light image of the cavity-stabilized flame is shown in Figure 1; the 
coordinate system shown originates in the plane of the backward-facing step at the downstream 
projection of the center of the nozzle exit.  One wall had a copper insert with a cavity of step 
height 𝐻 = 9 mm. Two side walls of the test section had quartz windows for camera access.  The 
wall opposite the cavity had a quartz window for laser sheet insertion.   
 
 
1. 500 mm focal length cylindrical lens 
2. Spatial filter 
3. 500 mm focal length cylindrical lens 
4. Relay mirrors 
5. 50 mm (200 mm) focal length cylindrical lens 
6. 300 mm focal length spherical lens 
7. planar mirror periscope 
8. flame 
Figure 1: Cavity-anchored flame. Figure 2. OH (CH2O) PLIF optics. Not to scale. 
The PLIF beam path is described in Figure 2. A Q-switched Spectra-Physics Nd:YAG laser 
created a 1064 nm beam at 20 Hz.  After frequency-doubling, the 532 nm beam pumped a Sirah 
Cobra-Stretch dye laser.  For OH PLIF, the frequency-doubled dye laser output was tuned to 
 
Diagnostics Colloquium 
 
3 
283.5525 nm, to excite the temperature-independent [3] [8] Q1(8) transition of OH.  The width of 
the laser sheet was decreased by a spatial filter (which was created using the entrance slit for a 
spectrometer) in which the gap between the blunt sides of two steel blades was set by an 
adjustable knob. By narrowing the gap around the beam focus, the broad wings of the laser sheet 
were removed. The thickness of the OH PLIF laser sheet waist was measured using a 2448 x 
2048 pixel, 16-bit CCD Point Grey beam-profiling camera (with the ultraviolet filter removed.) 
The laser sheet had full width at half-maximum of approximately 25 μm.  Figure 3 shows 
profiles of laser sheet intensity at different spatial filter settings.  The 125 μm setting was used 
for this data; smaller values caused slow ablation of the spatial filter. 
PLIF images were taken using a PI-Max 4 intensified CCD camera with a square 13.1 mm 
sensor.  OH PLIF images were taken with a 100 mm focal length, f/2.8 Cerco camera lens, with 
203.2 mm of extension tubes and a Semrock FF02-320/40-30-D filter (bandpass from 310 to 340 
nm) to block laser reflection and other interferences. The camera setup for OH PLIF was 
characterized using a USAF-1951 resolution target.  The system had magnification 1.91, 
corresponding to a field-of-view 6.86 x 6.86 mm across 512 x 512 pixels (or 13.4 μm/pixel) after 
2 x 2 binning. The in-plane resolution was about 39 μm, based on a contrast transfer function of 
50%. Three different motorized translation stages moved the sheet optics and the camera 
together to image different areas of the flow.  Relay mirrors were used such that the laser sheet 
focus did not change during translation. A fourth motorized translation stage was used to move 
the camera towards or away from the laser sheet, adjusting the focus. 
For CH2O PLIF, laser output was tuned to 352.484 nm, using the ?̃?1𝐴2 − ?̃?
1𝐴140
1 transition 
system commonly used to excite CH2O [9].  Compared to the OH PLIF experiment, changes 
were made to the optical setup to promote a higher signal-to-noise ratio.  The cylindrical lens 
focal length (Element 5 in Figure 2) was increased to 200 mm, increasing laser sheet intensity by 
a factor of four.  The camera was fitted with a visible light f/2 lens.  The system had 
magnification 2.14, corresponding to a field-of-view 6.11 x 6.11 mm across 256 x 256 pixels (or 
23.9 μm/pixel) after 4 x 4 binning.  The camera used a GG-395 Schott filter to block laser 
reflection. 
3. Methods and data processing 
To investigate a large area, images were taken while the translation system’s x-axis motor was in 
motion. Camera focus was periodically adjusted. Background image sets were acquired with the 
laser blocked.  Mean background images have been subtracted from all presented results. 
 
Figure 3. Effect of spatial filter setting on laser sheet intensity profile. Settings are not 
absolute aperture widths; they are offset by an unknown constant value. 
 
0
0.5
1
-100 -50 0 50 100
In
te
n
si
ty
 (
A
U
)
Distance (μm)
open
200 μm
150 μm
125 μm
100 μm
Diagnostics Colloquium 
 
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For OH PLIF, the camera travelled along the x-axis at 0.25 mm/s while acquiring images at 10 
Hz, resulting in 40 images for every millimeter. Six x-direction sweeps were acquired from x/H = 
0 to 14.10, and from y/H = -1.75 to 1.61.  
CH2O PLIF images were captured at 20 Hz while the camera travelled at 0.5 mm/s.  Five x-
direction sweeps were captured from x/H = 0 to 14.01, and from y/H = -1.75 to 0.99. Imaging 
was attempted closer to the cavity wall, but the ablation of the wall by the laser created bright 
interference that obscured flame structure in many images. 
4. Results and discussion 
 
Figure 5. Compilation of OH PLIF images across the test section. False color. 
    
Figure 6. OH PLIF intermittency boundaries. Based on threshold signal intensity 6,000 counts. 
For OH PLIF, Figure 4 shows selected single shot images, while Figure 5 shows a compilation of 
single shots across the test section.  In these images, OH signal marks the location of combustion 
products in the flow. The edge of the signal is the interface between reactants and products. This 
 (a)        (b)        (c)  
Figure 4. Selected OH PLIF images. False color. Downstream of cavity.  
Image center coordinates: x/H = (a) 5.00, (b) 5.05, (c) 5.66, y/H = 0.51. 
x 
y 
x 
y 
x 
y 
Diagnostics Colloquium 
 
5 
marks the general location of the flame front [10]. The images show evidence of flamelets [11], 
where reactants and products are separated by wrinkled flames on the order of 100 μm wide. 
Intermittency was calculated across the test section. For a given pixel location, intermittency [3] 
is defined as the fraction of images in which significant OH signal is detected. Figure 6 shows 
flame boundaries with 10% and 90% intermittency. This gives a visualization of the region in 
which the flame front oscillates. 
The images were analyzed for the smallest flame structures in the flow.  Figure 7 shows an OH 
PLIF intensity profile (at the yellow line) across a small flame structure (in the lower right 
quadrant of the first image in Figure 4.)  It has a full width at half-maximum of approximately 
110 μm, and is representative of the smallest structures observed.  This is slightly larger than the 
predicted size for the smallest flame structures (70 – 80 μm.)  No structures near the predicted 
resolution of the system (39 μm) were observed.  This suggests that this OH PLIF system is 
capable of resolving all flame structure length scales for this turbulent compressible flowfield. 
 
For CH2O PLIF, selected single shots are presented in Figure 8. CH2O signal marks the preheat 
zone of the flame [10], which in some locations is significantly wider than the flame thickness. 
The smallest observed CH2O were larger than the smallest OH features, approximately 200 μm 
wide.  This may be due to significantly decreased signal intensity as well as decreased spatial 
resolution caused by increased pixel binning.  CH2O PLIF images suffered from a much smaller 
signal-to-noise ratio than OH PLIF.  CH2O PLIF images were also frequently marred by bright 
interference created by the laser sheet ablating the copper wall. 
 
5. Conclusions 
This work has demonstrated an OH PLIF system that can resolve premixed turbulent flame 
structures in a high-speed compressible reacting flow as small as 25 x 40 x 40 μm3. However, the 
         
Figure 7. OH PLIF small structure profile. 
0
2000
4000
6000
0 200 400 600 800 1000
In
te
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si
ty
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co
u
n
ts
)
Distance (μm)
Data Gaussian fit
                             
Figure 8. Selected CH2O PLIF images. False color. In the mixing region adjacent to the 
cavity. Image center coordinates: x/H = (a) 4.28, (b) 4.39, (c) 4.44, y/H = 0.62. 
,  
x
y 
Diagnostics Colloquium 
 
6 
observed CH2O PLIF signal was much weaker than the OH PLIF signal, resulting in decreased 
resolution. Work is ongoing to improve these issues with high-resolution CH2O PLIF. 
6. Acknowledgements 
The authors would like to thank Damien Lieber, Justin Kirik, Dr. Chris Goyne, and Dr. Ross 
Burns for their help with this project. This research is funded by the National Science Foundation 
award #1511520 (program officer Dr. Song-Charng Kong) and the Air Force Office of Scientific 
Research award #FA9550-15-0440 (technical monitor Dr. Chiping Li.) 
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Capability for Dual-Mode Scramjet Experiments, 53rd AIAA Aerospace Sciences Meeting, 2015. 
[2] K. N. Gabet, R. A. Patton, N. Jiang, W. R. Lempert, J. A. Sutton, High-speed CH2O PLIF 
imaging in turbulent flames using a pulse-burst laser system, Applied Physics B 106 (2012) 569-
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[3] L. M. L. Cantu, E. C. A. Gallo, A. D. Cutler, P. M. Danehy, R. D. Rockwell, C. T. Johansen, 
C. P. Goyne, J. C. McDaniel, OH PLIF Visualization of a Premixed Ethylene-fueled Dual-Mode 
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[8] S. O’Byrne, I. Stotz, A. Neely, R. Boyce, N. Mudford, F. Houwing, OH PLIF Imaging of 
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7.   References 


High-Resolution OH and CH2O Visualization in a Premixed Cavity-Anchored Ethylene-Air Flame in a M = 0.6 Flowfield

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