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Measurement of Hydroxyl (OH) Concentration of Transient Premixed Methane-Air
Flames by Planar Laser Induced Fluorescence (PLIF) Method
A Thesis Presented
by
Raymond James Andrews
to
The Department of Mechanical and Industrial Engineering
In partial fulfillment of the requirements for the degree of
Master of Science
in
Mechanical Engineering
in the field of
Thermofluids
Northeastern University Boston, Massachusetts
April, 2008
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Abstract
Northeastern University has established a new Planar Laser Induced Fluorescence system
for studying transient high temperature flames in constant volume combustion vessels.
This system includes a tunable laser system, charge-coupled device camera, combustion
vessel, and a computer used for experiment operation, data acquisition, and data analysis.
A timing system was developed which coordinates the pulsing laser beam, combustion
ignition, and image acquisition with high precision so that flames of a desired radii could
be imaged.
The timing system was proven using different fuel to air concentrations of methane-
mixtures with initial room temperature and atmospheric pressure. Snapshots of each
mixture’s OH concentration were taken at a similar flame radius. All images were
processed with calibration data which corrected for background offset, laser pulse energy
fluctuation, uneven energy distribution in the laser sheet, and image distortion. This
work has laid the groundwork for further experiments which will establish a database of
experimental data which can be used to validate chemical kinetics mechanisms.
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Table of Contents
LIST OF ILLUSTRATIONS AND TABLES................................................................................................ 4 ACKNOWLEDGEMENTS............................................................................................................................ 5 CHAPTER 1. BACKGROUND..................................................................................................................... 6 CHAPTER 2. PHYSICAL BACKGROUND ................................................................................................ 7
2.1 LIF BASICS ........................................................................................................................................... 7 2.2 THEORY................................................................................................................................................ 8 2.3 EXPERIMENTAL APPLICATIONS ........................................................................................................... 10 2.4 OH AND COMBUSTION ........................................................................................................................ 11
CHAP 3. EXPERIMENT DESIGN.............................................................................................................. 13 3.1 SAFETY............................................................................................................................................... 13
3.1.1 Laser safety ................................................................................................................................ 13 3.1.2 Camera safety............................................................................................................................. 13
3.2 FACILITIES .......................................................................................................................................... 14 3.2.1 Control and data acquisition computer....................................................................................... 15 3.2.2 Laser system............................................................................................................................... 15
Pump laser and peripherals ............................................................................................................................. 15 Dye laser ......................................................................................................................................................... 17
3.2.3 Experimental beam .................................................................................................................... 18 Energy monitor ............................................................................................................................................... 18 Sheet optics ..................................................................................................................................................... 19 Beam termination............................................................................................................................................ 20
3.2.4 Combustion system.................................................................................................................... 20 3.2.5 Detector...................................................................................................................................... 22
3.3 EXPERIMENTAL PROCEDURE............................................................................................................... 23 3.3.1 System tuning............................................................................................................................. 24
DaVis 7.1.1 software....................................................................................................................................... 24 Beam tuning.................................................................................................................................................... 25 Imaging a stationary flame.............................................................................................................................. 27
3.3.2 Peak finding ............................................................................................................................... 29 3.3.3 Pre-processing calibration.......................................................................................................... 31
Image correction ............................................................................................................................................. 31 Energy correction............................................................................................................................................ 32 Background subtraction .................................................................................................................................. 32 Sheet correction .............................................................................................................................................. 33
3.3.4 Data acquisition ......................................................................................................................... 34 Explanation of experiment timing................................................................................................................... 34
3.4 IMAGE POST PROCESSING.................................................................................................................... 37 3.5 IMAGE PRESENTATION ........................................................................................................................ 40
CHAPTER 4. PRELIMINARY RESULTS ................................................................................................. 41 CHAPTER 5. CONCLUSIONS AND RECOMMENDATIONS ................................................................ 42 REFERENCES............................................................................................................................................. 43 APPENDIX: ELEMENT WIRING DIAGRAMS........................................................................................ 46
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List of Illustrations and Tables Figure 1: Einstein radiative processes for 2 level atom, adapted from [15] ................................................... 7 Figure 2: Planar imaging ................................................................................................................................ 8 Figure 3: Einstein radiative processes for fluorescent 2 level atom ............................................................... 9 Figure 4: Beam-camera angle....................................................................................................................... 10 Figure 5: OH energy level diagram .............................................................................................................. 12 Figure 6: System overview........................................................................................................................... 14 Figure 7: Laser system.................................................................................................................................. 15 Figure 8: Nd:YAG diagram (top view) ........................................................................................................ 16 Figure 9: Dye Laser diagram (top view), adapted from [27] and [32].......................................................... 17 Figure 10: Energy monitor, adapted from [33]............................................................................................. 19 Figure 11: Sheet optics [34].......................................................................................................................... 19 Figure 12: Sheet optics Rayleigh Scattering................................................................................................. 20 Figure 13: Iris diaphragm and beam dump................................................................................................... 20 Figure 14: Four window cylindrical vessel with spark electordes................................................................ 21 Figure 15: Gas manifold layout .................................................................................................................... 22 Figure 16: Spark electrode............................................................................................................................ 22 Figure 17: Experiment workflow, adapted from [20]................................................................................... 23 Figure 18: DaVis: LIF project excerpt.......................................................................................................... 24 Figure 19: LIF project window highlights.................................................................................................... 25 Figure 20: Nd:YAG Remote Diagram [28] .................................................................................................. 26 Figure 21: Good beam burn pattern [28] ...................................................................................................... 26 Figure 22: Dye laser detail with beamtool positions .................................................................................... 27 Figure 23: Image Intensifier Control Window ............................................................................................. 28 Figure 24: Image intensifier-pulse timing .................................................................................................... 29 Figure 25: Peak finding acquisition settings................................................................................................. 30 Figure 26: Systematic errors, adapted from [31] .......................................................................................... 31 Figure 27: Image correction ......................................................................................................................... 32 Figure 28: Sample background subtraction .................................................................................................. 33 Figure 29: PTU control window................................................................................................................... 35 Figure 30: Timing summary ......................................................................................................................... 36 Figure 31: Ignition timing electronics .......................................................................................................... 37 Figure 32: Post processing project window.................................................................................................. 37 Figure 33: Batch processing program........................................................................................................... 38 Figure 34: Expanded post processing project window ................................................................................. 39 Figure 35: Example image............................................................................................................................ 39 Figure 36: Variations of an experimental image .......................................................................................... 40 Figure 37: Methane-air mixtures with initial room temperature and atm pressure....................................... 41 Figure 38: Wiring diagram of CPU .............................................................................................................. 46 Figure 39: Wiring diagram of CPU clusters ................................................................................................. 47 Figure 40: Wiring diagram of pump laser power supply.............................................................................. 48 Figure 41: Wiring diagram of energy monitor ............................................................................................. 48 Figure 42: Wiring diagram of NanoStar camera .......................................................................................... 49 Figure 43: Wiring diagram of shutter box .................................................................................................... 49 Figure 44: Wiring diagram of pulse generator ............................................................................................. 50 Figure 45: Wiring schematic of NAND chip [36] ........................................................................................ 50 Figure 46: Wiring schematic of ignition box [2] .......................................................................................... 51 Figure 47: Wiring schematic of transformer [2]........................................................................................... 51 Table 1: Lab 190 pulse energy [28].............................................................................................................. 16 Table 2: Batch operation reference list ......................................................................................................... 38
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Acknowledgements
My advisor and mentor, Professor Hameed Metghalchi, thank you for always
supporting me with positive enthusiasm. Thank you for giving me the opportunity to
study in the BS/MS program and work in the combustion lab. Your guidance has been
invaluable.
Professor James Keck, thank you for providing our lab with constant challenges
and insightful perspective.
My colleague, Kian Eisazadeh Far, thank you for giving me daily encouragement,
breaking me into experimental research, and being a great friend. You have been a
wonderful teacher, I have learned so much from you passion for combustion.
To my other combustion lab colleagues: Mohammad Janbozorgi, thank you for
patiently explaining the complex science behind our work. You have always met my
requests with a smile. Matthew Gautreau, your enthusiasm has been wonderful as I have
trained you to replace me and take over the PLIF system. I am confident you will be very
successful and produce many great pictures. I would also like to acknowledge my
predecessor, Ed Shirk, for helping develop the PLIF system.
The Mechanical and Industrial Engineering department has been very supportive
of my work. Jeff Doughty, thank you for always sharing your insight and humor. Kevin
McCue, thank you for your resilience in helping me develop my timing system. I would
also like to thank John Doughty, Bridgette Smyser, and Joyce Crane for quickly coming
to my assistance whenever I needed help.
Finally I would like to thank my sister Elizabeth and my mother and father, Ray
and Mary Ellen. You have loved me unconditionally and devoted yourselves to my
success. I am so happy to make you proud.
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Chapter 1. Background
Validation of developed chemical kinetics mechanisms requires reliable experimental data. Experimental
data is used to establish databases of single ignition combustion events. These databases can be compared
with and used to determine the range of applicability and degree of detail of a mathematical computer
model.
The Northeastern University combustion lab in the basement of the Forsyth building is rooted in work
performed by Metghalchi and Keck studying laminar burning velocities of flames at high temperatures and
pressures at Massachusetts Institute of Technology [1]. The combustion lab has evolved and its capabilities
include burning speed measurement in a spherical combustion vessel and transient flame structure imaging
in a cylindrical vessel [2, 3]. The lab has recently been expanded to include a new laser spectroscopy
system for the purpose of studying flame structure by measuring species concentration. The objective of
this thesis is to establish this new planar laser induced fluorescence system and present the results from
tests which measured OH (hydroxyl) concentration of transient premixed methane-air flames. This thesis
also serves as a useful reference manual to any operator of this system.
Laser-induced fluorescence (LIF) is a spectroscopic method used to study parameters of a flame species by
exciting it with laser radiation, which causes a measurable spontaneous emission. Wood, the father of
infrared and ultraviolet photography, first photographed ultraviolet fluorescence in 1905 [4]. LIF has been
used to study diverse parameters including species concentration, temperature, velocity, and pressure [5-7].
If a set of sheet optics is used to transform the collimated beam into a planar laser sheet the LIF method is
called planar laser induced fluorescence (PLIF), dubbed by Hanson in 1986 [8]. PLIF was simultaneously
first reported as a method to measure OH in 1982 by Kychakoff and coworkers and Dyer and Crosley [9,
10]. A detailed history of LIF can be found in [11].
Measuring a constant volume combustion event complicates the PLIF experimental scheme as timing the
imaging with the transient flame and pulsing laser can be difficult. Cattolica and Vosen measured OH from
methane-air flames in a constant volume combustion chamber in 1984 [12]. More recently, Dreizler and
coworkers studied methane-air flames in a constant volume combustion chamber with a high speed system
[13]. Bradley and coworkers used qualitative PLIF from OH to study transient flame structure [14].
The purpose of this thesis is to present preliminary results obtained with a new PLIF system and detail its
operation for future users. First, physical background will be established. Next, experiment design will be
discussed and will include a description of experimental timing. Finally, results, conclusions, and
recommendations will be presented.
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Chapter 2. Physical Background
The following chapter summarizes the basic science behind the LIF spectroscopy method so that the reader
can have a working understanding of the theory driving the experimental research presented later. As PLIF
is a derivative of LIF and works by the same principles, they are used interchangeably as examples
throughout. John Daily details LIF background in molecular structure, spectroscopic concepts,
thermodynamics, and combustion chemistry [15]. Several books also present a collected overview of laser
spectroscopy [16-19]. This chapter first defines LIF and introduces its usefulness. Next underlying theory
is summarized. Finally, experimental considerations and the importance of the OH species are addressed.
2.1 LIF basics
The LIF method can be explained with a simple model: a species molecule is excited to a higher electronic
energy level by absorbing the energy of a laser beam photon which is tuned to a species electronic
transition. This state is unstable and the molecule will decay spontaneously and emit another photon of the
same energy as the original absorbed photon (see Figure 1). This emitted photon can be detected and used
to measure the population of the species.
Figure 1: Einstein radiative processes for 2 level atom, adapted from [15]
Figure 2 diagrams basic PLIF events. The tuned laser beam is transformed into a sheet with a set of optics.
The laser sheet excites the flame, causing it to fluoresce. The fluorescent signals are filtered and focused
by a set of collection optics to a camera, which finally transmits the data to a computer for processing.
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Figure 2: Planar imaging
LIF is an attractive spectroscopic technique because it is non-intrusive, capable of probing unstable flame
species, and sensitive enough to detect species in the ppm range [20]. LIF has several useful applications.
It can measure the population of a species’ excited state. It can be used to measure temperature, pressure,
or velocity. Beyond engineering applications, LIF is used in the study of energy transfer and chemical
processes by observing transient and steady state responses. Experimental results presented later use PLIF
to measure species concentration.
2.2 Theory Diatomic molecules consist of four independent internal energy storage modes: translation, rotation,
vibration, and electronic. Quantum mechanics predicts that molecules do not have an arbitrary amount of
energy, but instead exist in certain discrete energy states called stationary states, which can be characterized
by their energy modes. A photon is the elementary particle that carries electromagnetic radiation. The
energy of a photon is hv, where h is Planck’s constant and v is the optical frequency [15]. This optical
frequency is equal to the speed of light, c, divided by the photon’s wavelength, λ:
λcv =
LIF can be thought of as a classical collisional process. When an atom is resonantly stimulated by a laser
source it absorbs a photon with energy hv equal to the energy difference between electronic states, which
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produces a transition from a ground state to an excited state. The atom may return to its ground state
through the absorption of another laser photon (emitting two photons), collision with another molecule, or
through the spontaneous release of energy [11]. Spontaneous emission of a photon occurs because this
excited state is naturally unstable, and the atom decays to the ground state with lower energy. The radiative
lifetime of the excited state is very short (a few picoseconds) [15]. If a molecule absorbs a photon and then
emits another photon of a longer wavelength, it is considered fluorescent (see Figure 3).
Figure 3: Einstein radiative processes for fluorescent 2 level atom
The main disadvantage of PLIF is quenching, or extinction, of fluorescence at higher pressures by
numerous collisions of molecules [21]. Quenching and reabsorption issues can be mitigated by considering
the position of the fluorescence detector. The reemission of photons is coherent, or direction independent.
There is an equal chance that the emitted photon will depart its molecule in any direction because
reemission is isotropic. If only one molecule is considered, it does not matter at what angle to the laser
sheet the camera detector is positioned, it will absorb the same percentage of photons as any other position.
However, if a group of molecules is considered then the position of the camera is important in order to
eliminate system uncertainty due to reabsorption by neighboring molecules.
For example, consider Figure 4: the path of the emitted photon, or fluorescence, does not matter in a one
molecule system. But in a multiple molecule system the probability of collision between an emitted photon
and a neighboring molecule, and subsequent reabsoprtion, increases as the detection angle strays from 90
degrees because there is an increase in the photon path length, L, through the area where other molecules
are present. Reabsorption has the least negative impact when the camera is positioned at a right angle to
the laser beam.
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Figure 4: Beam-camera angle
2.3 Experimental applications The most common type of experimental configuration is pulsed excitation with integrated detection [15].
Pulsed lasers give advantageous higher power beams and are described later. The LIF signal is collected
over a gated period of time. The disadvantage of this type of system is that pulse repetition rates are
relatively slow, and so time dependent flows are difficult to image. The LIF signal may be detected by
several different types of devices. A charge-transfer device was used in the experiments and is described
later. As mentioned earlier, LIF can be used to measure population, velocity, temperature, and pressure.
Any LIF system is quite sensitive. Work is done with wavelengths tuned to the picometer and timing
controlled to the nanosecond. It is important that precision, accuracy, and uncertainty be considered.
Calibration is the process that defines precision, helps removes systematic error, and allows an LIF signal
to be transformed from a relative intensity value to an absolute measurement. This is done by relating
experimental results to a controlled, well-defined calibration source whose absolute values are known. A
flat-flame burner is typically used as a calibration source for flame research [22].
Several sources of systematic uncertainty exist. They include shot to shot variations in laser power,
fluorescence reabsorption, laser beam attenuation, interference, and scattering effects. Many things can
happen to both the laser beam and the target fluorescent species. Shot to shot energy variation occurs when
the beam pulse used for calibration has a different energy than the beam pulse used in a later experiment.
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The uneven distribution of energy in each pulse will become magnified when it becomes a laser sheet. The
laser pulse will lose photons and attenuate as it moves through the target medium. A photon emitted from a
target species molecule may be reabsorbed by another similar molecule. Interference and scattering from
any reflective surface, including all windows, must be considered. These sources of error can be corrected
using several techniques described in section 3.3.3 Pre processing calibration.
2.4 OH and combustion OH, or hydroxyl, is a molecule consisting of an oxygen atom and a hydrogen atom connected by a covalent
bond. OH is studied by combustion researchers because it is formed in the flame front, between burned and
unburned gas [23]. OH is used as a flame front marker because it is formed with combustion and quickly
disappears as the water product is formed in the burned gas region. The study of OH has also produced a
variety of data which is used to validate complex chemical reaction models, making an OH study a
standard part of many flame investigations [24, 25]. Preliminary results presented later measure the
concentration of the OH species in a transient premixed methane-air flame.
An electronic spectrum is composed of bands which arise from transitions between electronic states.
Numerous vibrational bands are allowed and are labeled by their vibration quantum number in the excited
and ground electronic state: A-X (vexcited, vground), or (v´, v´´). OH excitation is strongest in the (0, 0)
band (zero excited vibrational level to zero ground vibrational level) and occurs at a wavelength near 310
nm. Detection of the (0, 1) band is also popular because the comparably weaker signal is still detectable,
but better suppresses scattered light [23]. Excitation in the (1,0) is popular for systems using tunable dye
lasers because this band suffers less from attenuation than the nearby (0,0) band. Experiments were
conducted in the (1, 0) band at a wavelength near 285 nm. Rotational transitions can also be characterized
and are labeled according to rotational sublevels: R, Q, and P branches [15]. See Figure 5 for a simple
energy level diagram of OH that includes vibration numbers and popular excitation transitions. Similar OH
energy level diagrams can be found in [11, 15, 21].
Considering Figure 5 and Figure 3 demonstrates how complex real molecular behavior is compared to the
simple two-level atom model. The response of a simple atomic system to laser excitation can be described
by solving the simultaneous rate equations for each quantum state. Collisional rates that dominate the
response of OH to excitation are developed by Daily by considering the two electronic level diatomic
molecule under thermal and laser excitation [15].
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Figure 5: OH energy level diagram
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Chap 3. Experiment Design
3.1 Safety
The laser system presents serious danger to both operator health and sensitive system elements. While this
section presents an overview of necessary caution, it should not be treated as a complete review of safety.
Before operating the system users must familiarize themselves with safety guidelines in [26-28] and take
necessary training courses from the Northeastern University Office of Environmental Health and Safety.
These courses include chemical hygiene training, hazardous waste training, and laser safety awareness
training. The following sections present basic laser safety and instructions about taking care of the
sensitive Nanostar camera.
3.1.1 Laser safety
The PLIF laser system is a class-IV high power laser. The beam is a safety and fire hazard. Accidental
exposure to a beam directly or by reflection could cause severe and instantaneous damage to an operator’s
skin or eye. Protective eyewear should be worn at all times. A controlled area that experiments are
conducted in must be secured while the laser is operating and prominent warning signs should be posted.
Unnecessary exposure to radiation should be avoided by avoiding looking at the output beam, avoiding
bodily intersection with the beam, and avoiding the wearing of reflective jewelry such as a watch. The
beam should travel through enclosures whenever possible. A high level of environment ambient light
should be maintained to prevent operator pupil dilation. A discussion of current laser system safety
measures is included in the system maintenance section.
The dye used as a lasing medium in the dye laser presents additional safety considerations. Dyes are often
made of toxic substances and the operator should wear protective glasses, respirator, and cover all skin
when working with dye. Coumarin 153 was used to make the dye used in the experiments. Coumarin 153
can cause skin irritation, should never be ingested, and must be carefully disposed of. If a spill or leak of
Coumarin 153 occurs it should be dry-mopped clean. A liquid, such as water, should never be used to
clean up a spill.
3.1.2 Camera safety
The Nanostar camera will be permanently damaged if a laser beam is focused, directly or by reflection, on
the camera’s chip or intensifier. The protection cap should remain on the camera whenever it is not being
used for imaging. The proper filter should always be used when imaging a flame. The camera can become
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saturated and damaged if the image intensity is greater than 4096 counts. The intensity value must be
monitored by the user as images are taken. This can be done by displaying a min-max resolution by right
clicking on the image and selecting Data and display properties... >> Add Ons.
3.2 Facilities
A general experimental system diagram is given in Figure 6. The system was designed and installed by
LaVision. An experimental recording of a species concentration is acquired by triggering a charge-coupled
device (CCD) camera to take a picture at the same time that a laser beam pulse is passing through a
transient flame in the constant volume cylindrical vessel. The experimental laser pulse is emitted from a
dye laser, which is pumped by a Nd:YAG laser. The pump beam leaves the Nd:YAG laser, passes through
an open shutter box and is reflected by two mirrors into the dye laser. The dye laser generates the
experimental pulse. This pulse passes through an energy monitor, which is used to monitor pulse to pulse
energy fluctuations. The experimental pulse is next turned into a sheet with set of sheet optics. The size of
the sheet is controlled by truncating the beam with an iris diaphragm. The beam enters the cylindrical
vessel, passes through the combustion event, and terminates in the beam dump. The CCD camera is
triggered by using the Q-switch signal from the control computer and a set of timing electronics.
Corresponding wiring connection diagrams can be found in Appendix: Wiring.
Figure 6: System overview
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3.2.1 Control and data acquisition computer
The computer, which runs the LaVision software package, DaVis 7.1.1, is used for all aspects of the
experiment including adjusting the system, acquiring data, and post-processing results. The LaVision built
central processing unit includes a programmable timing unit (PTU). Data communication lines from the
lasers, energy monitor, and camera are input to the computer. A unique USB dongle is also plugged into
the back of the computer and prevents the use of software on more than one machine at a time. See
LaVision software manuals for more information about this software package [29, 30].
3.2.2 Laser system The laser system generates the experimental beam and is defined as the pump laser, including its laser head
and power supply, peripherals which pass the pump laser beam to the dye laser, and the dye laser (see
Figure 7). The laser system is configured using software to produce a desired wavelength. It is also
required to manually fine tune the laser in order to get a good beam, which is characterized by both shape
and intensity. The process of aligning the laser system is described in section 3.3.1 Experimental
procedure, System Tuning.
Figure 7: Laser system
Pump laser and peripherals The pump laser is a Spectra Physics Quanta-Ray Lab-190 Pulsed Nd:YAG Laser. The Lab-190 is optically
pumped by a flashlamp. The flashlamp has a lifetime of 30 million pulses, which are tracked with a
counter on the side of the power supply. Neodymium-doped yttrium aluminum garnet (Nd3+:Y3Al5O12, abbreviated Nd:YAG) crystals are used as a lasing medium. The Lab-190 uses Q-switching, or giant pulse
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formation, to produce a pulse output beam. This allows the production of light pulses with high peak
power, much higher than would be produced by the same laser if it were operating in a continuous wave
mode. The following table gives pulse energy data for the Lab-190 [28]. A 355 nm wavelength and 10 Hz
beam pulse repetition rate, yielding 250 mJ per pulse, was used for presented experiments.
Table 1: Lab 190 pulse energy [28]
Rep Rate (Hz) 10 30 50 100
Wavelength (nm) Energy (mJ/pulse)
1064 1000 800 600 325
532 500 400 250 120
355 250 200 100 50
266 110 60 25 20
The laser head is powered by a power supply which incorporates two different cooling water systems.
Chilled building water must be supplied to it at 3.8 liters/ minute and 40-60 psi. De-ionized water is used
to cool the laser head and can be replaced by removing the cover of the power supply. The de-ionized
water level can be seen at the side of the power supply. The Nd:YAG laser is operated manually by
remote control or externally by the DaVis software. The pump laser’s optics are simply diagrammed below
in Figure 8.
Figure 8: Nd:YAG diagram (top view)
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The pump beam’s path to the dye laser is diagramed in Figure 7. The pump beam leaves the Nd:YAG laser
head and passes through the laser shutter, which is controlled by a manual switch or remotely by the
computer [31]. The beam continues through a safety tube which limits escaping radiation and deters
operators from breaking the invisible beam path. The beam is reflected by a mirror box, which can be
adjusted using three knobs, and passes through another set of safety tubes and mirror box to be directed into
the dye laser.
Dye laser The dye laser is a Sirah Precision Scan SL Dye Laser. The dye laser is pumped by the pump beam and uses
a liquid dye as a lasing medium. Dyes can be used for a wider range of wavelengths than a solid lasing
medium (such as the crystals used in the Nd:YAG pump laser), and are thus well suited for tunable lasers.
The laser’s wavelength must be tunable, or finely controllable, in order to best excite and cause the desired
species to fluoresce. Dye beam emission is inherently broad and is made narrower by using a system of
gratings, mirrors, prisms, and other lenses. The Sirah Precision Scan SL Dye Laser configuration can be
seen in Figure 9.
Figure 9: Dye Laser diagram (top view), adapted from [27] and [32]
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The dye must be circulated through the dye laser system because dyes quickly degrade as they are exposed
to intense light. Two pumps, the Sirah PC 500 and 1000, circulate dyes of different concentrations through
the system. Dye quality will decay over time, yielding poor intensity images. The dye should be changed
after several months of continuous use of the laser system. Always wear proper safety clothing when
working with dye. Changing dye solution is described in detail in section M.3 of [27]. In short, new dye
solutions must be prepared, the dye circulators must be emptied, cleaned, and filled with the new dye
solution, and the old dye solution must be properly disposed.
A Coumarin-153 and ethanol dye solution was used in experiments to image OH. 1.60 grams of Coumarin-
153 is dissolved in 1.00 liter of ethanol solvent with the help of a magnetic stirrer and stir bar. 700
milliliters of this solution is used in the smaller PC-500 circulator. An additional 700 milliliters of solvent
is added to the remaining 300 milliliter solution, creating a more diluted 1000 milliliter solution. This
diluted solution is put into the large PC-1000 circulator. Coumarin-153, also known as Coumarin 540A,
should be purchased in 1.6 gram quantities to avoid measuring it out. Coumarin should always be handled
over a disposable surface, such as an absorbent BenchGuard paper, so that any spill can be easily cleaned.
3.2.3 Experimental beam
The dye laser pulse, or experimental pulse, is emitted from the Sirah Dye Laser. The beam passes through
an energy monitor, turns into a sheet by passing through a set of sheet optics, is truncated by an iris
diaphragm, enters the cylindrical vessel, passes through the combustion event, and terminates in the beam
dump (see Figure 6).
Energy monitor A LaVision online Energy Monitor measures dye laser pulse energy. Images are corrected for pulse to
pulse energy fluctuation by simultaneously acquiring camera images and laser energy signals using the
energy monitor. The signal is directly proportional to the energy value, and can be normalized with respect
to a reference value.
A beamsplitter cube, which contains a quartz plate, is located in the path of the laser beam and is used to
couple out a small amount (less than 5%) of the laser energy. This small amount of laser energy then
passes through a series of attenuators, diffusers, and filter. These control the amount of light detected by a
photodiode and are adjusted for the current laser power and wavelength selection. The light that passes
through this series is then detected by a broadband photo sensor. The analog data is stored onboard until
the PC software requests the data. Figure 10 diagrams the general energy monitor configuration.
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Figure 10: Energy monitor, adapted from [33]
Sheet optics After leaving the energy monitor the experimental beam must be turned into a flat sheet. This is
done by using two cylindrical divergent lenses that change the laser beam into a light sheet. These lenses
are encased as a set of sheet optics. The aperture angle, α, of the light sheet is determined by the focal
length, f, of the cylindrical lens and the beam diameter, d, at the lens position. Figure 11, adapted from
[34], illustrates the relation tan(α/2) = d/(2f). Currently a sheet optics system with focal length of 50 mm is
used. Figure 12 is an image of beam Rayleigh scattering before and after the sheet optics and displays the
transformation of the collimated beam into a diverging sheet.
Figure 11: Sheet optics [34]
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Figure 12: Sheet optics Rayleigh Scattering
Beam termination The experiment exits the sheet optics as a laser sheet and passes through a Newport Iris Diaphragm with 25
and 1.5 mm maximum and minimum apertures. The aperture can be made smaller to cut out unwanted top
and bottom portions of the laser sheet and is particularly useful for calibration when only a thin part of the
laser sheet is passed over the surface of a flat flame burner. The laser beam passes in and out of the
experiment in the combustion vessel via two small windows and terminates in a CVI Laser Series 48 Beam
Dump, where it is absorbed (see Figure 13).
Figure 13: Iris diaphragm and beam dump
3.2.4 Combustion system
The experimental apparatus used to generate combustion events are described. The combustion system
includes the combustion vessel, where combustion occurs, the gas manifold system which is used to fill the
combustion vessel with the appropriate fuel-oxidizer mixture, and the ignition system, which initiates
combustion.
The combustion events occurred in a four window constant volume cylindrical vessel diagramed in Figure
14. The four window cylindrical vessel is based on a two window cylindrical vessel used for imaging
flame structure in a shadowgraph system [2]. It is constructed of 316 SS and measures 13.5 cm in diameter
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and 13 cm long. Two large Pyrex windows provide a line of sight through the vessel and allow the
combustion event to be imaged by the Nanostar camera. Two smaller windows allow the experimental
laser sheet to pass into the vessel, through the flame, and out of the vessel to terminate in the beam dump.
The vessel is fitted with spark electrodes which are used to create center point ignition. It is filled through
an evacuated 3 mm tube which connects the bottom of the vessel to a gas manifold system
Figure 14: Four window cylindrical vessel with spark electordes
The gas manifold shown in Figure 15 connects the four window cylindrical vessel to a vacuum pump and
fuel, oxidant, and diluent tanks. The same gas manifold is used to service other combustion vessels. Three
gauges measure the pressure of the constituent being metered to the vessel and are used to monitor a vessel-
filling process that uses partial pressures to achieve the correct fuel-oxidant-diluent mixture.
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Figure 15: Gas manifold layout
The ignition system consists of an ignition box, transformer, and spark plugs and is based on the system
presented in [2]. The ignition box uses an AC power source to charge a 100 μF capacitor. It is triggered by
a set of timing electronics, described later. Once triggered, it releases a single DC pulse from the capacitor.
This pulse is sent to the transformer circuit, which has a turn ratio of 1:100, producing about 3000 V. This
high voltage pulse is sent to the spark electrodes. The spark electrodes are two automotive type spark plugs
with their ground prongs filed off (see Figure 16).
Figure 16: Spark electrode
3.2.5 Detector
LaVision NanoStar camera is used to image photons emitted by the excited species molecule. It is an
intensified charge-coupled device (ICCD) camera, made up of an intensifier and charge-coupled device
(CCD). The 25mm diameter image intensifier has three basic elements: a photocathode, a single stage
micro-channel plate (MCP), and a phosphor screen. The intensifier allows the detector to be gated to a
small amount of time. Photons emitted by the experiment pass through a wavelength selecting filter, are
received by the photocathode, and photoelectrons are generated. The photoelectrons are accelerated and
- 23 -
then multiplied inside the MCP and finally accelerated again toward the phosphor screen. The phosphor
screen converts multiplied electrons back to photons, which are transmitted to the 2/3” CCD sensor and
then finally out of the camera by a fiber optic line. The camera has a frame rate of 8 Hz and 1280 times
1024-pixel array image area [35].
3.3 Experimental procedure
A PLIF combustion experiment consists of warming the system up, calibrating for system inhomogeneities,
collecting experimental data, and post processing this data. While an experiment event may take only a
few moments, considerable time must be devoted to establishing a working, calibrated, and reliable
experimental set up. This section describes how to complete basic tasks, establish data used for calibration,
conduct an experiment, and finally combine calibration and experimental data into a package of useful
results. This section also includes a detailed explanation of experimental timing, which is critical to make
the experiment work. Figure 17 diagrams experimental steps. The LIF manual, [20], should be used as a
companion reference to this section.
Figure 17: Experiment workflow, adapted from [20]
- 24 -
3.3.1 System tuning
This section introduces system operation including how to navigate the software’s basic functions,
manually tune the system to get a good beam, and how to take a picture of a stationary flame. Before
practicing these functions the system must first be powered up. Turn on the computer and the camera. It
takes a few minutes for the camera to warm up; it will flash red and green until it is ready. Open the
cooling water valves that supply the Nd:YAG laser power supply. Check that the remote’s lamp energy
oscillator is set to zero and then turn on the power supply by the remote. Turn on both dye circulators and
then turn on the dye laser. Once the camera glows a steady green it is ready to be used and the DaVis
software can be opened.
DaVis 7.1.1 software The DaVis software, version 7.1.1, controls the PLIF system and is used to operate the experiments and
process results. It is opened from the computer desktop. The user should log in as an expert. A list of past
projects, which are named by date and time created, will appear automatically in a folder titled My
Projects. The user can open one of the past projects, or begin a new project by selecting the New project
icon. The new project should be designated as an LIF project. The DaVis: LIF project home screen will
open and includes many useful icons noted in Figure 18.
Figure 18: DaVis: LIF project excerpt
Clicking the New icon in the DaVis: LIF project window will open the recording dialog window. This
window, highlighted in Figure 19, can be used to tune the PLIF system, take sample images, and record
experimental data. The interactive mode is used for adjusting the system only and not used for recording
data. It can be used to Take a single image or continuously Grab a live movie. The grabbing is stopped by
selecting the stop sign icon. Recording data is accomplished in the Recording panel: select a recording
mode and select Start Recording. Settings of each component in the PLIF system can be viewed and
controlled in the settings display.
- 25 -
Figure 19: LIF project window highlights
Beam tuning
The laser system is tuned by hand in order to develop the best experimental beam. This is a dangerous
process because the operator has an increased chance of exposure to radiation or of breaking the laser beam
path. Safety precautions should be taken. First, the Nd:YAG laser harmonic generator is adjusted by finely
tuning crystal translation arms. Next, a beamtool and adjustable mirror boxes are used to position the pump
beam so that it enters the dye laser correctly. For these procedures operate the pump laser remotely. The
remote lamp energy OSC should begin at zero (START), the source should be FIXED, and the mode should
be Q-SW (see Figure 20).
- 26 -
Figure 20: Nd:YAG Remote Diagram [28]
Chapter seven of the dye laser manual details the pump laser harmonic generator tuning [28]. The
harmonic generator is accessed by a door in the top of the pump laser head. With the laser pumping, the
operator dials the first translational arm until a peak laser excitation is seen emitted from the harmonic
generator. Next, the second translation arm is dialed until the desired peak excitation is attained. Laser
alignment paper can be used to check the quality of the pump beam by recording the burn pattern. A good
beam will leave a burn pattern of even concentric circles (see Figure 21). Troubleshooting bad burn
patterns is addressed in chapter six of the dye laser manual [28].
Figure 21: Good beam burn pattern [28]
Once a good pump beam is attained it must be properly steered into the dye laser. This is done with an
iterative process that includes adjusting the beam path and checking its position. The beam path is adjusted
- 27 -
by changing how the mirror boxes are anchored to the frame that supports the laser system and how the
mirrors are positioned inside of their boxes (see Figure 7). The mirrors can be adjusted by twisting three
control knobs. The beam path position is checked by interrupting the beam and noting its position. The
beam should enter and leave each mirror box at the center of each box opening. A piece of thick paper can
be placed at a mirror box opening to detect the beam position. The beam must also enter the dye laser at
the correct location and angle. This is checked by using a supplied beamtool to interrupt the beam path in
two locations (see Figure 22, not to scale). The mirror positions must be methodically adjusted until the
beam is centered in each mirror box opening and in both of the beamtool bull’s-eyes, which are at a height
of 58 and 75 mm.
Figure 22: Dye laser detail with beamtool positions
Imaging a stationary flame A stationary flame, such as a Bunsen burner, can be used to synchronize the camera with the beam pulse.
The DaVis software is used to operate the laser system by triggering the Q-switch. This establishes a
software internal datum that correlates with the pumping of the dye laser. To operate the laser by the
software set the Source and Mode of the remote to external (EXT). The laser is switched ON in Laser
Control in the Device Settings panel. Once the laser is pulsing, its timing is not changed. The camera is
triggered from the computer (see Appendix: Wiring). This trigger closely synchronizes the camera with the
pulse, but must be fine tuned for the type of desired image.
- 28 -
The camera is synchronized with the beam pulse using the DaVis software. In a LIF Project: Recording
window (see Figure 23), the Image Intensifier control is opened in Device Settings. The camera-beam sync
is fine tuned using Delay and Gate. Delay represents a time delay from when the camera receives the
computer trigger signal to when data acquisition begins. Gate is the length of data acquisition. Note that
both values are in microseconds.
Figure 23: Image Intensifier Control Window
It is advised that for recording species concentration a gate of 0.1 ms be used. 0.1 ms is 100 ns, ten times
larger than the beam pulse length of 10 ns. It is necessary to try to center the beam pulse in the middle of
the gate (see Figure 24). Finding the beam pulse and then positioning it in the middle of a small gate time
takes a little trial and error. First begin with a larger gate value, perhaps 0.5 ms (allowing you to find the
beam pulse quicker than if your gate value were 0.1). Once proper safety measures for operators and
camera are ensured, excite a stationary flame with the laser and take an image. Begin with a delay time of
zero and increase it until the flame is imaged. Next reduce the gate value until it is 0.1 ms and the flame is
still imaged. Finely adjust the delay value until an image with consistent maximum intensity is imaged.
- 29 -
Figure 24: Image intensifier-pulse timing
3.3.2 Peak finding
As discussed in Chapter 1, fluorescence is created by exciting a species at a radiation transition level with a
laser beam. While it is generally known where these bands exist for a given species, the tunable dye laser
allows the wavelength of the excitation beam to be specified to the .002 nm. Using the peak finding feature
of the DaVis software, it is possible to incrementally scan a range of wavelengths where peak excitation is
expected. The found peak can then be used in experiments to deliver images with strongest intensity.
It is necessary to incrementally scan a range of laser beam wavelengths where excitation is expected in
order to determine where maximum excitation will occur. The scan should be of an excited stationary
flame, such as a Bunsen burner. After setting up the scan, the computer will incrementally scan a range of
wavelengths and form a chart that compares intensity value to wavelength. Before the scan can be
conducted, the proper acquisition settings, mask definition, scan range, and increment value must be input
into the peak finding program. The peak finding program is accessed in the DaVis LIF project window
(see Figure 18).
- 30 -
Figure 25: Peak finding acquisition settings
The acquisition settings are set by selecting the Acquisit. icon in the Settings panel. New acquisition items
are inserted and deleted by using the icons at the top of the panel. Any existing recording scheme should
be deleted and a new one built that is identical to the one in Figure 25. First load the Scan Sirah Laser 1
from the Scanning tab. As a sub item, insert the Image Acquisition camera item from the Image
Acquisition tab. Finally insert the Lambda Scan Processing item from the Miscellaneous tab. The start and
end wavelengths and scan increment can be entered by selecting the Scan Sirah Laser 1 item. The start
wavelength should be greater than the end wavelength. The increment should be to -.001 to achieve a fine,
decreasing wavelength scan. The operator can specify the number of images taken at each wavelength in
the Image Acquisition item in the Acquisit. panel. Five images per wavelength yield confident results
without lengthening the scan process too much.
A picture of an excited stationary flame can be taken using the same controls as described before. The
excited flame fills a small area of the total image window. It is necessary to apply a mask to this image so
that only the flame portion of the image contributes to the peak finding process. The flameless masked
region is a dead space where excitation never occurs. Including this flameless region in the peak finding
process would only make it difficult to notice changes in flame excitation intensity. The mask is applied by
right-clicking the taken flame image and selecting Send to >> Mask definition. A new window opens
which is used to define the mask. After mask definition, this window is closed and the mask can be named.
If several masks are defined then the desired mask for peak finding can be specified under the Setup item in
the Acquisit. panel. The scan can be started by selecting Start acquisition. A continuously updating chart
comparing intensity to wavelength will build itself as the scan progresses. At the end of the scan process
the operator can choose to switch the experimental wavelength to the new found peak wavelength.
- 31 -
3.3.3 Pre-processing calibration
A raw experimental image displays an intensity distribution across a field of pixels. The raw image does
not take into account that detected fluorescence is a function of many things in addition to species
population. It is necessary to collect calibration data in pre processing and use this data to correct the raw
image in post processing. It is important that all calibration data be collected in the same conditions as the
experimental data and that post processing steps occur in the correct order. Correction removes systematic
uncertainty including environment interference, shot to shot laser power variation, uneven energy across
the laser sheet, and perspective distortion (see Figure 26). Calibration can also include relating a relative
intensity distribution to a known physical unit. The following section details calibration procedures before
an experimental image is taken and the reasons for these procedures. Later, in Image post processing, the
procedures for incorporating calibration with experimental images will be described.
Figure 26: Systematic errors, adapted from [31]
Image correction Image correction calibrates the image field from a unit of pixels to a unit of length (e.g. mm) and corrects
for distortion between the camera’s perspective and the actual experimental plane. This is done by using a
plate with an array of crosses with known dimensions and spacing. The plate is placed in the experimental
plane (for the 4 window combustion vessel it is convenient to tape it in front of the spark electrodes). The
image calibration wizard is accessed through the DaVis: LIF project home screen (see Figure 18). As an
- 32 -
exaggerated example, Figure 27 displays a summary of the image calibration of an experimental plane at an
acute angle to the camera. The calibration plate is imaged and data about the cross dimensions is input.
The operator selects three adjacent crosses and the computer finds the remaining crosses, boxing them in
green. Using the input distance between each cross, the computer is able to flatten the image and assign a
real unit of length, removing any distortion the camera angle created.
Figure 27: Image correction
Energy correction Fluctuations of total pulse energy of the light source used for excitation are corrected for using energy
correction. A signal value which is directly proportional to the energy value is recorded by the energy
monitor and imbedded in each recorded image’s attributes. Each recorded image can then be normalized to
a reference energy value. The energy monitor is activated in the Device Settings panel. The USB Energy
Monitor 1 should be on the ON mode. The energy monitor is triggered by the Q-switch. If the energy
monitor is used without a lasing pump laser an error will be issued.
Background subtraction
Each experimental image if offset by surrounding scattered light and the camera’s dark current. Dark
current is a low amount of current that flows through the camera even though no photons are hitting the
photocathode. The background image is recorded separately from the experimental image and simply
subtracted from it in post processing (see Figure 28). Background images are recorded by selecting the
Background image recording mode in the Recording panel of the LIF project: Recording window. A set of
ten images should be recorded at once. The number of images recorded can be accessed through Acquisit.
>> Image acquisition. It is assumed the dark current distortion is insignificant compared to the offset
created by the reflected laser beam. The set of background images should be recorded with a pumping
laser beam, filter, combustion vessel, and identical gain and gate values in order to mimic an experimental
shot. There should be no flame.
- 33 -
Figure 28: Sample background subtraction
Sheet correction Laser beams have a cross-sectional energy distribution which is transformed in a beam sheet to an energy
profile perpendicular to the beam axis. One part of this profile will likely have a consistently different
amount of energy than another. Results would be inaccurate if it was assumed the laser sheet has a uniform
intensity distribution. It is necessary to avoid this error by acquiring knowledge about the energy
distribution in the laser sheet.
Rayleigh scattering, the scattering of radiation by particles much smaller than the wavelength of the
radiation, is used to develop a profile of the sheet energy distribution. Rayleigh scattering must be detected
first by using the Grab recording mode before any calibration data is recorded. The same experimental
setup should be used, less the band pass filter. Without the filter there is risk of damaging the intensifier
and the peak intensity should be closely monitored so that the maximum 4096 counts is not exceeded.
Direct reflections of the laser beam into the camera must be avoided. The combustion vessel must be
removed from the field of view in order to avoid camera saturation and record clean sheet images.
The laser power should be gradually increased until the scattering is imaged. The gate time may also need
to be increased. The scattering will appear weak compared to dust particles in the air (see sheet in Figure
12). The low scattering signal is accounted for by sampling many images. Once a satisfactory scattering
profile is imaged, record it in the same manner as the background image. Select the Sheet images recording
mode and an image set of 100 images.
- 34 -
3.3.4 Data acquisition
Necessary pre processing steps must be taken before running an experiment. It is possible to proceed with
an experiment only after a stationary flame has been imaged, background and sheet data sets are taken, the
energy monitor is functioning correctly, and image distortion is corrected for. The experimental
combustion event can then be prepared.
The combustion vessel is filled with the desired fuel-oxidant-diluent mixture using the gas manifold. The
radius of flame to be imaged is selected by inputting a time delay between ignition and image capture. This
time delay is entered as an Initial delay in the PTU Delays panel. The next section describes experiment
timing in detail. Experiments are conducted with 100% laser power and a gain value of 80. Once the
combustion vessel is filled and the correct delay is selected, insure that a good laser beam is pumping and
conduct the experiment by selecting Start recording with the Experiment images recording mode. Only
one image should be taken.
Explanation of experiment timing A successful experimental image incorporates taking an image of a laser beam pulse passing through a
combustion event. The NanoStar Camera can take an image every 125 milliseconds (a frame rate of 8 Hz).
Each beam pulse lasts approximately ten nanoseconds (or 10-5 ms). Neither the camera nor the beam
pulsing is fast enough to take more than one image during a short 50 ms combustion event. A sophisticated
timing system is needed so that not only can the camera capture a beam pulse moving through the transient
flame, but also so that a specific point in the combustion event (e.g., 15 ms after ignition) can be imaged.
With the laser pulse and image intensifier synchronized it is possible to develop a timing scheme that
incorporates the combustion event. In summation, a trigger signal is input into an ignition box, which
generates a pulse that enters a transformer, which causes a set of electrodes to generate a spark, beginning
the combustion event. This ignition sequence happens so fast that it can be thought of as instantaneous
with respect to other timing events.
The trigger for the ignition sequence must come from the computer, which is already synchronized with the
laser pulsing. The computer uses a SYNC line to trigger the camera. This line is external and connects the
TTL I/O PORT to the Camera PTU PORT A CAMERA (see Appendix Wiring Figure 39). The time
between the SYNC signal and the image acquisition can be input by the user as Initial delay in the PTU
Delays window (see Figure 29): LIF Project: Recording >> Device Settings >> PTU Delays. The same
SYNC line is split and used to also trigger the ignition sequence. Thus changing Initial delay also changes
- 35 -
the time between combustion ignition and data acquisition. The initial delay is input in ms, and has been
found to provide a repeatable delay between SYNC and camera trigger signals. The timing scheme is
summarized in Figure 30, note that PTU start, Initial Delay, Camera shutter, Camera readout, and
Acquisition signal correlate to the same items in Figure 29.
Figure 29: PTU control window
- 36 -
Figure 30: Timing summary
The SYNC signal is a clock signal and consists of several square waves. It cannot be directly input as an
ignition box trigger because the ignition box should receive only one trigger signal. Multiple trigger
signals may damage the ignition box or create multiple sparks, which will corrupt the experiment. An
electronics scheme has been developed that truncates the SYNC signal, removing every wave after the first
and so delivering only one trigger signal to the ignition box.
The SYNC signal is input into a Tektronix tds5104 Oscilloscope and used to trigger a negative polarity
pulse out of the AUX OUT BNC connector. The oscilloscope must be set to use the channel that the
SYNC line is plugged into as its trigger source. It must also be on NORM mode, set for SINGLE, and DC
coupling for a positive slope. The negative polarity pulse from the AUX OUT is inverted and turned into a
positive polarity pulse using a chip with a Quad 2-input NAND gate chip. This positive polarity pulse is
used to trigger a PHILIPS 5715 Pulse Generator, which emits a single square wave upon reception of the
positive polarity pulse. This single square wave is input as the ignition box trigger signal. Figure 31
diagrams this sequence. Specific wiring diagrams of the pulse generator and NAND gate are found in
Appendix: Wiring Figure 44 and Figure 45.
- 37 -
Figure 31: Ignition timing electronics
3.4 Image post processing Image post processing combines pre-processing calibration data with experimental images and yields a
calibrated final set of data suitable for analysis. Calibration data and experimental images are combined
using the Batch processing program in the DaVis software (see Figure 32).
Figure 32: Post processing project window
- 38 -
The batch processing program is run for the background, sheet, and experimental data images as shown in
Figure 17. Each processing operation is loaded from an operation panel and should be ordered according to
Figure 17. For example, Figure 33 depicts the operation list for an experimental image. Note that you can
save any operation list for future use. Table 2 lists where to find relevant operations and includes notes
about settings for each operation. Once the operation list is established, the images are processed by
selecting Start Processing.
Figure 33: Batch processing program
Table 2: Batch operation reference list
Operation Group Comments energy correction intensity correction user value should be 100 Backgr. subtraction Project Parameter: Sheet/front lighting, Background File: select desired file Sheet correction Project Sheet File: Reference Set Sheet processing Project requires user discretion average statistics Parameter: select average from options
Once post processing is complete the post processing image tree can be expanded to reveal each processing
step (see Figure 34). A post processed experimental image, which lies at the bottom of the tree, can be
selected for analysis. An example image of a methane-air stochiometric mixture with initial conditions of
room temperature and atmospheric pressure is shown about 15 ms into combustion in Figure 35. Note that
the combustion event is an expanding sphere of flame. Through the windows the camera sees this flame as
a circle. The laser sheet illuminates only a portion of this circle, which is what is seen in the Figure 35.
- 39 -
Figure 34: Expanded post processing project window
Figure 35: Example image
- 40 -
3.5 Image presentation
Calibrated experimental images must be analyzed with care to best understand the data within the images.
It is possible to manipulate the images by changing the color palette, spectrum resolution, and gamma
weight in addition to other factors. These parameters can be accessed by right clicking on the image >>
Data and display properties... >> Image.
Figure 36 displays poor ways of presenting the same image as in Figure 35. In the left image a resolution
that is very high dampens the data. The center image falsely weights lower intensity signals, giving the
illusion of high intensity where it is not. The right image uses only a monochromatic spectrum, which does
not best differentiate between intensity levels. A better image analysis is presented in the next chapter.
Figure 36: Variations of an experimental image
- 41 -
Chapter 4. Preliminary Results
A preliminary set of experiments was conducted with methane-air mixtures at initial conditions of room
temperature and atmospheric pressure. Mixtures of different fuel to air equivalence ratios (0.8, 1.0, and
1.2) were imaged. While the analysis of these results is interesting, their demonstration of the success of
the timing system is most important. Post processed experimental images, not calibrated for concentration,
are displayed in Figure 37. Each image has been cropped so that only the OH excitation remains.
Figure 37: Methane-air mixtures with initial room temperature and atm pressure
OH was excited by an experimental beam with a 282.672 nm wavelength. Each image displays a flame
with a 34 mm outer radius of fluorescence. This was achieved by changing the initial delays as described
previously in the experimental timing section. It was possible to pick good initial delays based on a
database of movies of flame propagation taken with a high speed camera in a shadowgraph system. Figure
37 proves the ability to coordinate the timing of the laser pulse, ignition, and picture taking. While the
images are not calibrated for concentration in a ppm unit, they can be compared relative to one another.
The stochiometric mixture displays a much higher concentration of OH, as expected.
- 42 -
Chapter 5. Conclusions and Recommendations
A PLIF system for imaging transient flames has been established. A tunable laser system, CCD camera,
combustion vessel, timing system, and calibration methods have all been integrated to produce snapshots of
flame species concentrations. It has been shown that timing can be controlled well enough to record
images of flames with different propagating rates at desired radii. Now that the system is established,
future work is needed to produce a database of results useful to chemical kinetics modelers.
A calibration system is being established which uses a simple burner. The burner is made by Tridelta
Siperm and includes a fluidizing stainless steel sintered pad. A burner like this can be used as a simple flat
flame burner. It is connected to the gas manifold and a stationary flame is maintained across its pad. It is
possible to select a thin part of the laser sheet with the iris diaphragm and pass this through the flame. The
flame OH excitation can be imaged and OH concentration can be attached to the image with confidence
based on computer models of the flame generated knowing data including mass flow rate, fuel type, and
pressure. Concentration images are then used to calibrate the concentration of transient flame images.
This work has laid the groundwork for further experiments which will be able to measure species
concentrations of numerous fuels and establish a database of experimental data which can confidently be
used to validate chemical kinetics mechanisms.
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References
[1] M. Metghalchi, J. C. Keck, "Laminar burning velocity of iso-octane-air, methane-air and methanol-air at high temperature and pressure," Proceedings of Eastern Section of the Combustion Institute, Hartford, CT, 1977.
[2] F. Parsinejad, “Experimental and theoretical studies on flame propagation and
burning speeds of JP-8, JP-10 and reformed fuels at high temperatures and pressures,” Ph.D. dissertation, Northeastern University, Boston, MA, USA, 2005.
[3] K. Eisazadeh Far K., R. Andrews, et. al., “Experimental study of fuel/O2/diluent
(argon, helium, nitrogen) premixed flames at high pressures and temperatures,” presented at Eastern States Section Meeting of Combustion Institute, Charlottesville, VA, 2007.
[4] R.W. Wood, “The fluorescence of sodium vapour and the resonance radiation of
electrons”, Philosophical Magazine, Vol. 10, p. 513, 1905. [5] K. Kohse-Höinghaus, “Laser techniques for the quantitative detection of reactive
intermediates in combustion systems,” Prog. Energy Combust. Sci., Volume 20, pp. 203-279.
[6] N.M. Laurendeau, “Temperature measurements by light-scattering methods,” Prog.
Energy Combust. Sci., Volume 14, pp. 147-170. [7] S.S. Penner, C.P. Wang, and M.Y. Bahadori, “Laser diagnostics applied to
combustion systems,” Twentieth Symposium (International) on Combustion, The Combustion Institute, 1984, pp. 1149-1176.
[8] R.K. Hanson, “Combustion Diagnostics: Planar Imaging Techniques,” Twenty-First
Symposium (International) on Combustion, The Combustion Institute, 1986, pp. 372-380.
[9] G. Kychakoff, et. al., “Quantitative visualization of combustion species in a plane,”
Applied Optics, Volume 21, pp. 3225-3227, 1982. [10] M.J. Dyer and D.R. Crosley, “Two dimensional imaging of OH laser-induced
fluorescence in a flame,” Optics Letters, Volume 7, pp. 382-384, 1982. [11] Y. Hicks, “Multi-dimensional measurements of combustion species in flame tube
and sector gas turbine combustors,” Ph.D. dissertation, Michigan State University, East Lansing, MI, USA, 1996.
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[12] R.J. Cattolica and S.R. Vosen, “Two-dimensional measurements of the [OH] in a constant volume combustion chamber,” Twentieth Symposium (International) on Combustion, The Combustion Institute, 1984, pp. 1273-1282.
[13] A. Dreizler, S. Lindenmaier, U. Maas, et. al., “Characterization of a spark ignition system by planar laser-induced fluorescence of OH at high repetition rates and comparison with chemical kinetic calculations,” Applied Physics B, vol. 70, pp. 287- 294, 2000. [14] D. Bradley, C.G. Sheppard, R. Woolley, et. al., “The development and structure of flame instabilities and cellularity at low Markstein numbers in explosions,” Combustion and Flame, vol. 122, pp. 195-209, 2000 [15] J.W. Daily, “Laser induced fluorescence spectroscopy in flames,” Progress in Energy and Combustion Science, vol. 23, pp. 133-199, 1997 [16] W. Demotröder, Laser Spectroscopy, Basic Concepts and Instrumentation, Third Ed. Berlin: Springer, 2003 [17] C.N. Barnwell, Fundamentals of Molecular Spectroscopy. Berkshire, England: McGraw-Hill. [18] D. Andrews and A. Demidov, Ed., An Introduction to Laser Spectroscopy. New York: Plenum Press, 1995. [19] A.C. Eckbreth, Laser Diagnostics for Combustion Temperature and Species. Cambridge, MA: Abacus Press, 1988. [20] LaVision Technical Staff, Tunable LIF Product-Manual, LaVision GmbH, 2005. [21] T. Yokomae, “Planar laser induced fluorescence (PLIF) of H2-O2 combustion,” M.S. thesis, The University of Texas at Arlington, Arlington, TX, 2003. [22] S. Prucker, W. Meier, and W. Stricker, “A flat flame burner as calibration source for combustion research: Temperatures and species concentrations of premixed H2/air flames,” Rev. Sci. Instrum., Vol. 65, pp. 2908-2911, 1994. [23] V. Sick and N. Wermuth, “Single-shot imaging of OH radicals and simultaneous OH radical/ acetone imaging with a tunable Nd: YAG laser,” Applied Physics B, vol. 79, pp. 139-143, 2004 [24] M. Versluis, L. Georgiev, M. Martinsson, et. al., “2-D absolute OH concentration profiles in atmospheric flames using planar LIF in a bi-directional laser beam configuration,” Applied Physics B, vol. 65, pp. 411-417, 1997
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[25] E.W. Kaiser, K. Marko, D. Klick, et. al., “Measurements of OH Density Profiles in Atmospheric-Pressure Propane-Air Flames,” Combustion Science and Technology, vol. 50, pp. 163-183, 1986 [26] LaVision Technical Staff, Laser Induced Fluorescence for DaVis 7.1 System Manual, LaVision GmbH, 2006. [27] LaVision Technical Staff, Sirah Dye Laser Product-Manual, LaVision GmbH, 2004. [28] Spectra-Physics Technical Staff, Pulsed Nd: YAG Lasers User’s Manual, Spectra- Physics, 2002. [29] LaVision Technical Staff, DaVis 7.1 Software Manual, LaVision GmbH, 2006. [30] LaVision Technical Staff, Imaging Tools Software Manual, LaVision GmbH, 2006. [31] LaVision Technical Staff, Laser Shutter Product-Manual, LaVision GmbH, 2004. [32] Sirah Technical Staff, Sirah Pulsed Dye Laser Service-Manual, LaVision GmbH, 2007. [33] LaVision Technical Staff, Energy Monitor Device-Manual, LaVision GmbH, 2005. [34] LaVision Technical Staff, Sheet Optics (divergent) Device-Manual, LaVision GmbH, 2007. [35] LaVision Technical Staff, NanoStar Datasheet, LaVision GmbH, 2006. [36] Texas Instruments Technical Staff, Quadruple 2-Input Positive Nand Gates, Texas Instruments, 2003.
- 46 -
Appendix: Element Wiring Diagrams
Figure 38: Wiring diagram of CPU
- 47 -
Figure 39: Wiring diagram of CPU clusters
- 48 -
Figure 40: Wiring diagram of pump laser power supply
Figure 41: Wiring diagram of energy monitor
- 49 -
Figure 42: Wiring diagram of NanoStar camera
Figure 43: Wiring diagram of shutter box
- 50 -
Figure 44: Wiring diagram of pulse generator
Figure 45: Wiring schematic of NAND chip [36]
- 51 -
Figure 46: Wiring schematic of ignition box [2]
Figure 47: Wiring schematic of transformer [2]
