A Study of Quantitative Concentrations of Hydroxyl (OH) in Laminar Flat Flames Using Planar Laser Induced Fluorescence (PLIF)

A Thesis Presented

By

Adrienne Murphy Jalbert

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

August, 2011

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Abstract In these times of striving for cleaner, more efficient fuels, research is needed to provide understanding that will allow technologies to improve. Planar laser induced fluorescence (PLIF) is a popular, non-intrusive technique for studying flame structure. A radical species, hydroxyl (OH), is known to be found in the region between burned and unburned gases in a flame. By tuning the PLIF system to the proper wavelength, OH can be excited and its fluorescence can be imaged, resulting in a map of concentrations. Knowing where OH is located allows the flame front to be defined. Furthermore, if the concentrations of OH can be quantified then this numerical data can be used in theoretical models.

Methane is a well documented fuel and has data readily available for concentrations of OH at specific distances above the burner surface. This data was used to create a calibration curve which could then be applied to PLIF images to quantify the concentration of OH in fuels that do not have documented data. Hydrogen was selected as the fuel to be studied because of its promising future in alternatives to fossil fuels. Only by increasing our understanding of the fuel’s burning behavior will we be able to use it most efficiently. Data collected can be used to verify theoretical models that predict burning efficiency, emissions and work on how to improve them.

The works of past students were verified and improved upon. As each step of the establishment of the PLIF system was repeated to gain a full understanding of the system, it was well documented. All of these documents were collected into a thorough manual of procedures that will help the next student quickly and easily learn the system.

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Table of Contents List of Figures and Tables .............................................................................................................................. 4

Acknowledgements ....................................................................................................................................... 5

1. Background ........................................................................................................................................... 6

1.1. Purpose ......................................................................................................................................... 6

1.2. Literature Review .......................................................................................................................... 6

1.3. PLIF Basics ..................................................................................................................................... 7

1.3.1. Quantum Mechanics ............................................................................................................. 8

1.3.2. OH Specifics ........................................................................................................................... 9

2. Experiment .......................................................................................................................................... 10

2.1. Experimental Setup ..................................................................................................................... 10

2.2. Wavelength Selection ................................................................................................................. 11

2.3. Concentration Calibration ........................................................................................................... 14

2.3.1. Thermocouple Calibration Method .................................................................................... 15

2.3.2. LaVision Calibration Method ............................................................................................... 16

2.3.3. Calibration Method Comparison ............................................................................................. 16

2.3.4. Concentration Calibration Conclusion .................................................................................... 19

2.4. Transient System ......................................................................................................................... 20

2.4.1. System Establishment ......................................................................................................... 20

2.4.2. Current System Status ......................................................................................................... 21

3. Experimental Results .......................................................................................................................... 23

3.1. Concentration Calibration Validation ......................................................................................... 23

3.2. Fuel Flowrate Comparison .......................................................................................................... 27

3.3. Equivalence Ratio Comparison ................................................................................................... 30

4. Condensed Manual ............................................................................................................................. 35

5. Conclusions and Future Recommendations ....................................................................................... 36

References .................................................................................................................................................. 38

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List of Figures and Tables

Figure 1.1 - Basic concept of PLIF system ..................................................................................................... 8Figure 1.2 - Hypothetical energy level diagram, adapted from [12] ............................................................. 9Figure 1.3- OH fluorescence spectra showing Q1(8) transition .................................................................. 10Figure 2.1 - PLIF system .............................................................................................................................. 11Figure 2.2 - OH spectra of (1-0) band from 282nm to 284nm .................................................................... 12Figure 2.3 - Incorrect OH spectra without FCU enabled ............................................................................. 13Figure 2.4 - Calibration image with horizontal bands of constant concentration ...................................... 14Figure 2.5 - Concentration calibration sample plot .................................................................................... 15Figure 2.6 - LaVision calibration data (methane/air, P=1 bar, Φ=1) ........................................................... 16Figure 2.7 - Temperatures reported by a radiation corrected thermocouple at varying heights above a burner [19] .................................................................................................................................................. 17Figure 2.8- Thermocouple correction comparison of Fredette [4] to Cattolica [19] .................................. 18Figure 2.9 - Comparison between LaVision and thermocouple calibrations .............................................. 19Figure 2.10 - Constant volume cylinder in place for experiments .............................................................. 20Figure 2.11 - Andrews' successful timing system final images [12] ............................................................ 21Figure 2.12 - Weak transient image, methane/air, P=1atm, Φ=1 .............................................................. 22Figure - - H2 air=13.8) ...... 24Figure 3.2 - Comparison of three stoichiometric hydrogen-air flames with same calibration data ........... 24Figure 3.3 - Comparison between two independent concentration calibrations ...................................... 25Figure 3.4 - Comparing average concentration to maximum concentration ............................................. 26Figure 3.5 - Inconsistent PLIF image taken with clogged burner ................................................................ 26Figure 3.6 - Stoichiometric methane calibration image ............................................................................. 27Figure 3.7 - Calibrated stoichiometric methane-air flames with varying flowrates on common scale ...... 28Figure 3.8 - Calibrated stoichiometric methane-air flames with varying flowrates on independent scales

.................................................................................................................................................................... 28Figure 3.9 - Image post processing inconsistency test, all images should match ....................................... 29Figure 3.10 - Comparison of [OH] varying with total flowrates of methane-air and hydrogen-air ............ 30Figure 3.11 - H2-air flames with varying equivalence ratios displayed on a common concentration scale

.................................................................................................................................................................... 31Figure 3.12 - Effect of equivalence ratio on value and location of OH concentration ............................... 32Figure 3.13 - Flowmeter calibration curve .................................................................................................. 33Figure 3.14 - Adjusted calibration curve ..................................................................................................... 34Figure 5.1 - (L to R) Fredette's flame, clogged burner flame, post HCl bath flame .................................... 37

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Acknowledgements First and foremost I would like to acknowledge my adviser, Dr. Hameed Metghalchi. As busy as you are as the chair of mechanical and industrial engineering at Northeastern University, you still always had time to offer me assistance and positive encouragement. I would also like to thank you for telling me about the BS/MS program and inviting me to join your laboratory. I would never have reached this accomplishment if not for you.

To my lab mates, Casey Bennett and Ali Moghaddas, thank you for always dropping what you were doing and helping me out. Whether it was with equipment set up or a theoretical concept that I did not quite understand, neither of you ever hesitated to assist in any way you could. And thank you for keeping me sane when things got stressful.

Dr. Mohammad Janbozorgi and Ghassan Nicolas, my theoretical combustion colleagues, thank you for your support. Without the crash courses in chemical kinetics, Fortran and mathematics I would not have the understanding behind my experimental research which I now do.

My BS/MS predecessors, RJ Andrews and Colin Fredette, thank you for paving the way for me by setting up this system. I can only hope that my work will be as helpful to future students as yours was to me.

Many of the Mechanical and Industrial Engineering staff have come to my aid when I have needed them most. Bridget Smyser, you were always able to help me find chemicals that I needed in a pinch and keep things moving with laser safety installations. Joyce Crain, your speed and efficiency more than once got me out of a stressful jam. John Doughty, you could always help me find a tool or fix a part when no one else could.

Finally I would like to thank my supportive family. You have kept me focused on the end goal and picked me up at the toughest times when I did not think that I would ever graduate!

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1. Background

1.1. Purpose The purpose of this research is to study the concentration of hydroxyl (OH) in laminar flames to get a better understanding of flame structure and behavior. OH is found in the region between burned and unburned gases and can therefore be used to mark the flame front. This thesis will combine the master’s thesis works of Raymond Andrews and Colin Fredette, alumni of the Northeastern University laminar combustion laboratory. Andrews’ thesis was to set up a planar laser induced fluorescence (PLIF) system and design a timing system for transient combustion events in 2008. Fredette then developed a calibration method for quantifying concentrations of radical species using a stationary flame in 2009.

Along with combining past efforts of this lab, this thesis will also serve as a user’s guide to the PLIF system. Previous students have never overlapped with each other. This has resulted in each user of the PLIF equipment needing to teach themselves the entire system from scratch. While this is a very thorough way to ensure that the entire system is understood, it is also very time consuming. Learning a system of this complexity can easily take six months when thesis work is only being done part time. A manual of eighteen step by step procedures has been put together to accompany this thesis to help future students learn the system quickly and then be able to focus the bulk of their time on new developments in their research.

1.2. Literature Review Much work has been done studying the concentration of OH in flames using PLIF. Labs around the world have used laser diagnostics to study OH in both methane/air flat flames and transient flames. A thorough literature search was conducted using the Engineering Village literature database Compendex and the online search engine Google Scholar. The review first targeted quantitative flat flame PLIF then moved on to the more complex transient investigations.

A. Arnold et al. of Paul Scherrer Institut (PSI) [1] in Switzerland used a combination of 2-D PLIF and 1-D UV absorption spectroscopy to quantify the concentration of hydroxyl in a flat flame. The energy profile of the laser sheet was imaged using 1-D UV absorption spectroscopy before and after passing through the flat flame. Looking at the difference in energies at a certain height, the energy absorption of the flame at each height above the burner could be determined. Using the energy absorption and the known flame conditions, a 1-D algorithm could be used to calculate the concentration of the species of interest at a specific height [2]. This quantity could then be applied to the 2-D PLIF image of the flat flame for each height, quantifying the concentration. The data from Arnold’s work at PSI is actually reported in the user’s manual of the calibration burner [3] used in this thesis. According to LaVision (the company that developed much of the PLIF system equipment), Arnold’s data is to be used as calibration data for quantifying OH concentrations.

C. Fredette of Northeastern University in Boston developed another quantification method for the concentration of OH in flat flames for his master’s thesis [4].Using a thermocouple, Fredette measured the temperature of the flame at a specific height above the burner. This temperature, along with the known environmental conditions and flame species, could be entered into STANJAN [5], a chemical

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equilibrium computing program, to calculate the concentration of OH at the point of the thermocouple bead. See the Thermocouple Calibration Method section for more detail.

This first part of the review confirms that quantification of OH in methane/air flat flames has been sufficiently studied and documented already. Studying flat flames is a stepping stone to being able to analyze a transient combustion system. Data found by past flat flame works should be able to be applied to the more involved transient systems.

To decide what this thesis could do to add to knowledge in the combustion field it first had to be determined what has already been done and where the field currently stands in transient flame studies. Mansour of Cairo University studied flame kernel structure using OH laser induced predissociation fluorescence (LIPF) in 2007 [6]. The results of this study report qualitative OH concentrations only. In 2010, Müller of the Institute of Reactive Flows and Diagnostics in Germany used PLIF to study OH in flame kernels within an optically accessible internal combustion engine [7]. Again only qualitative data was collected. This literature review concluded that there is a void in current research in quantitative concentration profiles of laminar spherically propagating flames.

1.3. PLIF Basics Planar laser induced fluorescence (PLIF) is a technique commonly used to optically study the structure of flames. It can be used to visualize concentration and temperature fields. Typically, a tunable laser system is used that can be set to a precise wavelength (often accurate to within 0.001 nm) [8]. Once tuned, the beam is converted into a planar sheet which passes through the combustion event and excites a species of interest through photon absorption. The photon will only be absorbed if its energy is equal to the change in energy needed for a molecule to change energy states [9]. By knowing the energy required by the molecule of interest to change energy states, the laser can be tuned to the best wavelength that will allow excitation of only that species. Absorption will be followed by spontaneous emission of the photon. When this spontaneous emission occurs, photons are released at random in all directions. Any photons that happen to travel into the camera lens will then be captured by the camera’s intensifier. This means that the images taken do not collect all of the fluorescence but only a portion of it. Since spontaneous emission is completely random in direction, the probability of a photon going in any direction is equal to any other direction. Therefore, the image taken represents a relative distribution of molecules, not the total fluorescence of an event. See Figure 1.1 for a general overview of the PLIF system:

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Figure 1.1 - Basic concept of PLIF system

PLIF is a popular option for researchers because of its selectivity, sensitivity and non-intrusiveness. The selectivity comes from the tunable laser system. Being able to tune the wavelength to the thousandth of a nanometer (picometer) allows scientists to target exactly the species they are interested in, even if it transitions near a different species. This method, according to the LaVision Tunable LIF manual [8], can detect molecules in the sub-ppm range, making it a very sensitive data acquisition technique. Finally the fact that the procedure does not interfere with the combustion event is very beneficial. It is not necessary to add tracers to the air/fuel mixture or to disturb the flame in any way. This way the data collected will be comparable to real world situations without added experimental inaccuracies.

1.3.1. Quantum Mechanics In order to understand the data collected by a PLIF system, one must first understand the basic principles of quantum mechanics. According to quantum mechanics a molecule cannot exist with an arbitrary amount of energy. There are discrete levels of energy specific to each type of molecule known as stationary states that they must exist within. When energy is absorbed or emitted by a molecule it can jump to a different stationary state. Absorption occurs when a molecule gains energy equal to the amount of energy required to make it to jump from a lower energy state to a higher energy state. Emission occurs when the molecule, for some reason, releases energy and causes it to drop from a higher energy state down to a lower energy state. Causes of emission may include collision with another molecule or simply that the excited state is unstable and the molecule returns to a more stable lower energy state. When absorption and emission occur back to back it is the phenomenon known as fluorescence [10].

Diatomic molecules such as OH have three different types of energy: electronic, vibrational and rotational. According to the Born-Oppenheimer approximation, these three types of energy are all independent of each other. There are two different spectra that can be observed. The excitation spectrum is studied by sweeping the laser through a range of wavelengths for varying absorption and observing the amount of emission. This spectrum is covered in more detail in the Wavelength Selection section where peak finding is explained. Then there is the emission spectrum, which is studied by tuning the laser to one specific absorption wavelength and observing the fluorescence of different emission

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lines [8]. The one specific wavelength is selected by studying the excitation spectrum and finding at what wavelength the maximum excitation of the desired molecule occurs.

Quantum numbers are used to state the energy level of a molecule. While there are many different forms for reporting quantum numbers, they are often given in pairs as (v’, v’’) to give the (excited, ground) vibrational states. Vibrational separation energies are evenly spaced and much larger than rotational separation energies, which are not evenly spaced. This results in clusters of rotational energy levels separated by large vibrational energy gaps approximately 1000 times larger than rotational separations [11] (see Figure 1.2, not to scale)

Figure 1.2 - Hypothetical energy level diagram, adapted from [12]

1.3.2. OH Specifics Hydroxyl was selected as the radical to be studied in this work because it is known to be found in the region between burned and unburned gases in flames. This allows it to be used as a flame front marker. Northeastern’s combustion lab has a background in studying this species. The current PLIF system has been optimized by previous students to work with OH, [4] and [12].

The Q1(8) line within the OH A2Σ+←X2Π (1-0) transition band targeted in this study occurs when a wavelength of approximately 283.55 nm is absorbed. This absorption raises the molecule from its X2Π ground state to the A2Σ+

Figure 1.3

excited state from which it spontaneously emits photons with a wavelength around 308 nm. The Q1(8) line was recommended by LaVision because of its low temperature sensitivity [13]. The Q1(8) transition is expected to give off a strong fluorescence signal relative to neighboring transitions. below shows the Q1(8) emission level relative to other peaks at nearby wavelengths measured in arbitrary units.

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Figure 1.3- OH fluorescence spectra showing Q1(8) transition

It can be seen that Q1(8) is one of the highest peaks in the spectra. This fact coupled with its temperature insensitivity makes it an ideal wavelength to work at. The stronger the emission of OH is in experimental images, the higher the resolution of the concentration can be. For this work, the Q1(6) line actually ended up being used, as will be discussed in further detail in following sections on Wavelength Selection.

2. Experiment

2.1. Experimental Setup The laminar flame lab’s PLIF system consists of a pump laser, tunable dye laser, energy monitor, sheet optics, combustion device and camera. The pump laser is a Nd:YAG Spectra-Physics Quanta-Ray Lab 190. By adjusting the position of the Nd:YAG crystals, the solid lasing medium within the laser’s harmonic generator, the wavelength can be set to 1064nm, 532nm, 355nm or 266nm [14]. It is set for the current work to generate a 532nm wavelength beam. This beam leaves the pump laser and is directed into the dye laser by two right angle mirrors. The Sirah PrecisionScan SL dye laser uses a Rhodamine 6G liquid lasing medium dye to finely tune the pumped beam to a fundamental wavelength of 566nm. Before leaving the dye laser, the 566nm beam passes through a frequency doubling crystal which halves the wavelength to a final output of 283nm [15]. This ultraviolet beam passes through an energy monitor, where about 5% of each pulse’s energy is diverted, measured and recorded for future corrections [16]. The remaining 95% of the beam then passes through a set of sheet optics where a cylindrical lens converts the beam into a flat laser plane [17]. This planar laser then passes through the combustion event, inducing fluorescence. Depending on what type of experiment is being run either a flat flame burner (calibration and stationary flames) or a spark ignition constant volume cylindrical bomb

283.4 283.45 283.5 283.55 283.6 283.65 283.7 283.75

Rela

tive

Inte

nsit

y [a

rbit

rary

uni

ts]

Wavelength [nm]

Q1(8) Q2(7)

R2(16) Q21(8) Q12(7)

P1(4)

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(transient experiments) is used as a combustion device. Finally, a camera positioned perpendicular to the laser sheet captures an image of the fluorescence. A simple mock up of the PLIF system can be seen below in Figure 2.1:

Figure 2.1 - PLIF system

The camera has a filter on it that only allows wavelengths near 308nm to pass through to be imaged. The OH transition being excited by the 283nm beam emits photons around this wavelength so the light particles released by fluorescence enter the camera and are captured on the intensifier. The more photons there are, the more OH must be present and the brighter the image is. This allows us to locate OH throughout the plane of the flame that the laser excited. Raw images are recorded in arbitrary units of intensity counts where higher count areas mean higher OH concentrations.

The entire system was covered in great detail by Andrews and Fredette in their master’s theses [4] and [12]. For an in depth description of each piece of equipment, please refer to their theses.

2.2. Wavelength Selection At different wavelengths, different transitions of OH can be excited. Each of these transitions will emit different intensities of fluorescence. By finding the wavelength that induces the highest level of emission within a range of interest, experiments can be run using the optimum wavelength to capture images with the strongest fluorescence [8].

Experiments performed by previous students on this PLIF system were run using a UV wavelength of 282.672nm. After speaking with experts at LaVision, it was learned that there is normally a higher peak found around 283.55nm that is called the Q1(8) line. As mentioned in the section on OH Specifics, this transition line is recommended because of its low temperature dependence and fact that it is typically the highest peak within the (1-0) transition band. Exact wavelength locations of OH transitions can vary depending on the room temperature and laser tuning. Knowing that the (1-0) band occurs around 283nm, a scan was performed over a range of wavelengths to locate the highest transition line within the band. The current dye laser set up is optimized to study the (1-0) band. The Rhodamine 6G dye has its peak fluorescence efficiency of 28% at a fundamental wavelength of 566nm which, upon passing

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through the doubling crystal, is cut in half to 283nm. The doubling crystal in the dye laser, which doubles the frequency while halving the wavelength and keeping velocity constant, is compatible with wavelengths of 210 to 290 nm. This eliminates the option of studying the other common option of the (0-0) band, which occurs around 308nm.

Between recommendations from laser professionals, hardware optimizations that have already been performed and limitations of current equipment, it was decided to stay with the (1-0) band. Knowing that this band is around 283nm, a scan was performed from 284nm to 282nm to find the maximum fluorescence intensity in that range. A scan is performed by lighting the flat flame burner and taking a fluorescence image of the flame at each wavelength. The computer program averages the intensities in each image and plots this average against the wavelength that the picture was taken at, creating a plot similar to that shown below in Figure 2.2. Scans are performed starting at a higher wavelength and stepping down by 0.001nm at a time. They are run in decreasing wavelength because the stepper motor that controls the resonator of the dye laser is more accurate when stepping in this direction, allowing a step of only 1pm to be performed with confidence. A global maximum of the scan range is expected to be found at the suggested Q1(8) line around 283.55nm. The spectra obtained from scanning a stoichiometric methane-air flat flame is shown below in Figure 2.2:

Figure 2.2 - OH spectra of (1-0) band from 282nm to 284nm

The vertical axis of Figure 2.2 above is in arbitrary units (a.u.). Since only the wavelength of the highest peak is desired, the heights of the peaks are all relative and the only important information is which is the tallest. Therefore, the dependent units can be reported as arbitrary.

A few key peaks have been labeled in the OH spectra above. One of the first things to notice is that the Q1(8) line is not the global maximum of the scanned range. The maximum actually occurs at the Q1(6) line, which is around 282.92nm. Another point of interest is that the peak occurring at 282.67nm, where previous students conducted experiments, is shorter than the Q1(6) peak. The Q1(8) peak is approximately 60% of the height of the Q1(6) peak. The peak used by Andrews and Fredette at 282.67nm is about 93% of the maximum peak.

282 282.2 282.4 282.6 282.8 283 283.2 283.4 283.6 283.8 284

Rela

tive

Inte

nsit

y [a

.u.]

Wavelength [nm]

Q1(6)

Q1(8)

Peak used by predecessors

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Despite the fact that the Q1(8) line was recommended, it was decided to continue with the Q1(6) line for this work. The maximum intensity consistently occurred at this transition, giving the strongest concentration images possible with the current setup. All of the following work will be conducted at a wavelength of approximately 283.9nm. Peakfinding scans were run daily to ensure that the optimum wavelength was used for the exact room and laser conditions that day. The maximums found by these scans only varied by a few picometers from day to day.

The frequency conversion unit (FCU) is very sensitive to temperature and can de-tune overnight just because of fluctuations in room temperature [18]. To get the best results, an FCU table should be created in the Sirah 2.6 software every day that a peakfinding scan is being run. If the FCU table is not up to date, then the FCU should not be enabled in the peakfinding scan. When the FCU is enabled, that means that the doubling crystal and compensator will rotate along with the resonator as wavelengths are scanned. If the doubling crystal does not rotate during the scan, you may receive a peakfinding curve similar to that shown below in Figure 2.3:

Figure 2.3 - Incorrect OH spectra without FCU enabled

Two key points to notice about this bad peakfinding scan is that the X axis displays the fundamental wavelength (around 566nm). This should be a giveaway that the FCU is disabled. The second point is that instead of looking like the correct spectra shown previously in Figure 2.2, this spectra looks like a bell curve. While this may seem like it would be correct to have a maximum at the ideal wavelength surrounded by decreasing peaks, this is wrong. The overall bell distribution of peaks tells us that the FCU’s doubling crystal was not rotating with the resonator. This lead to the only tall peak being at the wavelength that the doubling crystal was tuned to. Instead of tuning the FCU to only one wavelength, an

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FCU table must be created that generates a relationship between wavelengths and peak crystal rotational location. The peakfinding scan can then access the data in this table and rotate the doubling crystal as needed during the scan to get the maximum intensity at each wavelength so that a true peak across the scanned range can be found.

2.3. Concentration Calibration The concentration calibration was performed using a McKenna flat flame burner made by Holthius & Associates (see Fredette’s thesis [4] for burner details). With a stable flat flame, horizontal bands of constant concentration can be seen, as shown in Figure 2.4:

Figure 2.4 - Calibration image with horizontal bands of constant concentration

Knowing the concentration at certain heights above the burner will allow the user to change the color scale from arbitrary counts to absolute concentration of the species being studied. To do this one would place a mask around a location within the flame with a known concentration. Then the known quantitative concentration at that location is entered. The DaVis software will then average the intensity counts of all of the points inside of the drawn mask and assign the given concentration to the calculated average intensity, adding a single point to a calibration curve. This is repeated at different locations with other known concentrations. The result will be a concentration curve similar to that shown in Figure 2.5 that the program can then apply to experimental images, converting the relative concentration counts to absolute concentrations.

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Figure 2.5 - Concentration calibration sample plot

Once the program has calculated a conversion from arbitrary intensity counts to quantitative concentration, the calibration data can be applied to any image in the project. This allows the user to conduct a calibration using the stationary flat flame with known concentrations and then map the data onto a transient flame with unknown concentrations. As long as the same environmental conditions and laser settings are used for the experimental images as are used for the calibration image, the concentration conversion will be valid. As long as it is being excited under the same conditions, a OH molecule should emit the same amount of energy regardless of what type of flame it is produced in, be it rich or lean, transient or stationary.

2.3.1. Thermocouple Calibration Method The calibration method used for previous work in the laminar flame speed laboratory involved using a thermocouple to measure the temperature of a flat flame at different heights above the burner. With the measured temperature and knowing that the flame is in a standard environment (T=298K, P=1atm) the reactants and products of the flame could be entered along with this information into a program called STANJAN [5]. This program, developed by a professor at Stanford, uses the given temperature, pressure and species information to calculate the concentration of a desired species. Since the temperature measurement from a thermocouple is used, the calculated concentration is only valid at the thermocouple bead. In the DaVis software a circle can be drawn around the thermocouple bead in an image to tell the program where it is located. The program then averages the intensity counts within that circle and can apply the concentration calculated at that point. The thermocouple bead is then translated to a different height above the burner and the process is repeated. A similar concentration curve to the one above is formed and then applied to experimental images. See Colin F Fredette’s master’s thesis for a more detailed explanation of this calibration method [4].

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2.3.2. LaVision Calibration Method Another method of quantification would be to use the data presented in the LaVision calibration burner product manual [3]. Data used in the LaVision manual was experimentally collected by Arnold [1] and Cattolica [19]. The two references agree closely with each other as seen in figure Figure 2.6.

Figure 2.6 - LaVision calibration data (methane/air, P=1 bar, Φ=1)

The calibration data given in the manual defines known concentrations at specific heights above the burner surface in a stoichiometric methane/air flat flame (p=1bar, Φ=1).

Arnold collected data using a single laser pulse method as described in the literature review. Considering error from shot noise, temperature, transmission measurement and spectroscopic data inaccuracies Arnold et al estimated a total accuracy of 20% for their single shot data.

In 1982, Cattolica used two different methods, laser-absorption spectroscopy and molecular-beam mass spectrometry (MBMS), to find the quantitative concentration of OH in stoichiometric methane-air flat flames at atmospheric pressure. For his laser-absorption spectroscopy method, Cattolica used radiation corrected thermocouple measurements and assumed a Boltzmann distribution of the concentration of a measured rotational OH level. Unfortunately, the thermocouple correction used could not be found to compare with Fredette’s. The MBMS approach measured the mass to charge ratio of molecules in the flame and could then use the detected masses to determine concentration. Cattolica concluded that the peak OH concentrations of the two methods agreed within their estimated experimental error.

2.3.3. Calibration Method Comparison In order to compare the two techniques described previously, the thermocouple approach was performed using Fredette’s correction on a flame experimentally similar to those used by Arnold and Cattolica (matching stoichiometry and mixture flowrate). Then the concentrations calculated from the thermocouple method could be directly compared to those found in the manual.

One of Fredette’s recommendations for future users of the PLIF system at the end of his thesis was to verify the validity of the assumptions made for his thermocouple correction. This calibration comparison

1.0E+02

1.0E+03

1.0E+04

0 2 4 6 8 10

OH

mol

e fr

acti

on [p

pm]

height above burner [mm]

Arnold

Cattolica

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seemed like a good opportunity to check the thermocouple correction as well. The original correction was designed and used on a hydrogen-air flat flame. Since this work has dealt mostly with methane, the correction was tested on a methane-air stoichiometric flame. In the same Cattolica paper that LaVision specified for concentration calibration there is also data reporting temperatures versus the distance above the burner surface. Cattolica et al report using “radiation corrected thermocouple measurements” but never go into any further detail regarding what these corrections actually were. They simply present a plot of their temperatures in their paper, shown in Figure 2.7 below:

Figure 2.7 - Temperatures reported by a radiation corrected thermocouple at varying heights above a burner [19]

This plot was imported into Data Thief [20], a program that can take plots and extract numerical data. By simply marking three reference points with known coordinates then defining the beginning, end and color of the curve of interest, the program prints out data that can be imported into Excel. The solid curve labeled thermocouple, shown in Figure 2.7, was “stolen” for comparison with experimental data.

Cattolica reports having used a mixture flow rate of 0.316 liters/second with a stoichiometric methane-air flame (Φ=1). This flame was replicated experimentally to compare temperatures, being sure to take into account the correction factors for the Omega flow meters being used. Temperature measurements were taken every 0.5mm starting on the burner surface and working up to Cattolica’s reported maximum of 7mm. The experimental points are plotted along with Cattolica’s curve from Figure 2.7 above in Figure 2.8 below:

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Figure 2.8- Thermocouple correction comparison of Fredette [4] to Cattolica [19]

The experimental data set using Fredette’s correction reported temperatures on average 50 K lower than Cattolica. This tells us that either Cattolica was more conservative with corrections, or Fredette missed a contributor to energy loss from the thermocouple. Even still, the percent difference between experimental and expected measurements was at most 3.2% over the range of heights studied. While this seems like an acceptable difference for the temperatures, it creates an unacceptable difference between concentrations calculated by STANJAN (see Table 1), which was Fredette’s technique for calibrating. A worst case example was checked at the experimental peak temperature 1mm off the surface of the burner. The elements C, H, O, and N were entered into STANJAN along with species CH4 (1 mole), O2 (2 moles) and N2 (7.52 moles) as the reactant mixture components for a stoichiometric methane-air reaction. Additional species that were included were H2, O, N, NO, NO2, H2O, H2O2, OH, HO2, CO and CO2

Table 1 - Comparison of [OH] calculated using temperatures from experiment and literature

and the calculation constraints used were constant pressure and temperature. The results are shown below:

Cattolica Current Work Difference % Difference

Temperature [K] 1818 1760 58 3.3% STANJAN OH Mole Fraction 2.45E-04 1.55E-04 9.00E-05 58.1% STANJAN OH Mass Fraction 1.51E-04 9.54E-05 5.56E-05 58.3%

As seen in Table 1, the seemingly slight difference in temperature causes an unacceptably large discrepancy in OH mole fractions as calculated by STANJAN.

Then, the concentrations calculated using STANJAN were compared to the concentrations reported in literature. It was found that the estimated concentrations at specific heights above the burner from the thermocouple method were all about 2% of the concentrations from the LaVision method. To try to get

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a better idea of how the data compared the LaVision data was plotted against the thermocouple data resulting in Figure 2.9 below:

Figure 2.9 - Comparison between LaVision and thermocouple calibrations

The fact that there is a strong linear trend between the results of the thermocouple correction method and the LaVision method suggests that some kind of scaling factor was missed in the thermocouple method. It is not likely that an important contribution to energy loss was neglected when assumptions were made and is now causing this error. This may be a contributor but the fact that such a tiny percentage of the expected concentration was calculated means that there is something more significant happening here.

2.3.4. Concentration Calibration Conclusion Looking at the fact that Arnold and Cattolica used two different experimental methods and yet their data for methane/air flames still agreed closely, it was decided that they were likely more correct. This, along with the fact that this data was provided by the manufacturer, resulted in the conclusion that their data should be used for calibration. This project uses Arnold’s data because the experimental set up that Arnold et al used was closer to that of the current work. Being able to closely replicate the experiments led to greater confidence in the accuracy and applicability of the calibration data to the current work.

Fredette’s calibration method was not used in the current work because of the uncertainty introduced by the thermocouple. In order to get an accurate temperature reading of the thermocouple, corrections must be made for the radiation loss from the bead and the conduction loss down the lead wires [4]. The results from the thermocouple correction could not be confirmed by other experimental data found in literature. The added complexity of thermocouple corrections was avoided in the current work by simply using calibration data supplied by LaVision in the McKenna burner manual.

R² = 0.9933

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2.4. Transient System A constant volume cylindrical vessel with a central spark ignition system was used to study spherically expanding flames. Using partial pressures the cylinder would be filled with the desired ratio of fuel and air to bring the total system pressure to a pre-determined value. The ignition system would then create a spark in the center of the cylinder, initiating a flame which would then propagate outwards spherically until it extinguished on the internal wall of the vessel.

Figure 2.10 - Constant volume cylinder in place for experiments

Windows at the cylinders ends and on either side allowed images to be taken of the transient flame while a laser was pulsed through, inducing fluorescence of OH just like was done with the flat flame burner. Theoretically, if the same laser settings (including wavelength and power), pressure and initial temperature are used inside the cylinder as were used in performing the concentration calibration with the flat flame burner, the calibration data should be able to be applied to transient images, quantifying the concentration of OH.

2.4.1. System Establishment The master’s thesis of RJ Andrews [12] was to establish a timing system for transient combustion events. Andrews successfully set up a PLIF system that could image a spherical flame at the radius desired. His final images proving his success are shown in Figure 2.11:

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Figure 2.11 - Andrews' successful timing system final images [12]

Unfortunately he did this with a now outdated version of the software, DaVis 7.1.1. Since then the software was updated to DaVis 7.2 and many of the menus and settings used were eliminated. Therefore, the present work includes re-learning the transient system and trying to optimize it with the current software.

2.4.2. Current System Status Much time was spent trying to exactly replicate Andrews’ settings in the new software. After speaking with the developers of the recording software it was learned that transient imaging was far simpler with the new DaVis 7.2. All of the same settings that were used for imaging the flat flame would again be used and just one delay would be entered that would allow the flame to expand to the desired radius before imaging. Once a satisfactory timing between laser and camera was established it could be applied to any flame, regardless of stationary or transient. This realization greatly simplified matters and allowed the bulk of time and energy to be spent on optimizing images.

Transient images taken were weak, to the point of barely being detectable within the background noise, as shown in the raw image below in Figure 2.12:

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Figure 2.12 - Weak transient image, methane/air, P=1atm, Φ=1

This weak image was thought to be the result of low power output of the dye laser. The pump laser power was consistently around 4W throughout this work. The dye laser was much more variable and ranged from a minimum of 160mW up to a maximum of 240mW. Past experiments by Fredette were run around 260mW. In an attempt to increase the power the Rhodamine 6G dye was changed and all components in the dye laser were tuned. Changing the dye did not solve the problem of weak signal.

After speaking with LaVision engineers, it was found that experiments were not being conducted at the ideal wavelength. Having performed peakfinding scans WITHOUT the FCU enabled for previous experiments, a wavelength of approximately 282.93nm was being used to start with. LaVision engineers suggested using the Q1(8) transition of OH which occurs around 283.55nm, as mentioned previously in the Wavelength Selection section. A peakfinding scan was performed with the flat flame burner and the FCU enabled and the Q1(8) peak was found to be at 283.545nm. Even though this was not the highest peak of the scan (recall Figure 2.2) the dye laser was set to this wavelength and the transient experiments were attempted again. This new wavelength was coupled with another suggestion from LaVision to use the laser on a different setting. Typically for transient images the laser lays dormant until the record button is pressed. The laser then flashes one single time according to the timing set in the software to induce fluorescence in time for imaging. It was recommended to try running the laser on adjust mode which would have the laser flash repeatedly at 10Hz. Then, when the record button is pressed, the image acquisition would be timed to fall on one of these laser flashes. The idea behind this was that sometimes the first shot of the laser after lying dormant is not as powerful as the repeated flashes just by nature of the laser. In an attempt to improve the transient images both of these suggestions were combined but the result was a weak image similar to that shown in Figure 2.12.

Unfortunately, time ran out before a good transient image could be taken with the system. The fact that there is clearly an image of the spherical flame, weak as it may be, in Figure 2.12 tells us that the timing set up is correct. Having been able to successfully image a flat flame proves that the camera and laser

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are working together properly. The partial pressure filling method of the cylindrical vessel was validated using a gas chromatograph as part of the master’s thesis of another student in this lab, Casey Bennett [21]. The cylinder and fill lines were checked for leaks to make sure that the mixture was getting to and staying in the cylinder. No leaks were found. The remaining possibilities could be that the laser power is not high enough or that there is simply not much OH to image in the transient flames being created.

3. Experimental Results

3.1. Concentration Calibration Validation To determine if the concentration calibration technique using the LaVision data was valid, it was first checked if the calibration could be repeated reliably. To do this, first a stoichiometric methane-air flame with flowrates matching the total mixture flowrate of Cattolica (0.316 liters/second) was imaged. The image was corrected by subtracting out the background noise, zeroing out laser energy fluctuations and correcting for laser sheet inconsistencies. The processed image was then sent to the concentration calibration module in DaVis 7.2 where the calibration data from LaVision could be applied to the image at specific heights above the burner. This created a calibration curve similar to that in Figure 2.5 that was stored in the project and could be applied to any image to convert color counts to concentration using the developed relationship, as discussed in the concentration calibration section.

Once a calibration curve was obtained, stoichiometric hydrogen-air flames with the same mixture flowrate were imaged. Three sets of hydrogen images were taken. Each set of 100 images was averaged together, corrected in a similar method as the methane flame and then calibrated for concentration using the calibration curve. A note should be made here: the calibration curve created with the methane flame should be able to be applied to any flame, flat or transient, regardless of fuel type, so long as the flame was imaged using the exact same dye laser wavelength and power as the calibration image and is at the same environmental conditions. OH will fluoresce with a consistent wavelength and emit a consistent strength signal so long as it is excited by a consistent wavelength and power. To ensure that you are not confusing energy transitions of OH and applying incorrect concentrations, be sure to create a new concentration calibration curve every day. Peak wavelength and power of the dye laser can change over night without even touching a thing! The FCU is highly temperature sensitive and can de-tune rapidly. This will drastically change the color scale counts on which the concentration calibration is based.

The concentration at each pixel along a vertical line, representing a constant radial location, was drawn through the maximum concentration point and data was exported for each of the three calibrated flame images (see one of these in Figure 3.1 below with vertical line along which data was taken):

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Figure 3.1 - - H2 air

This data from along the vertical line in

=13.8)

Figure 3.1 could then be used to plot concentrations against height above the burner at the same radial position in three different images. The three different images were all stoichiometric hydrogen-air flames with equivalent flowrates. If the calibration and post processing method is repeatable, all three data sets should be equal. The plot of these three tests is shown in Figure 3.2:

Figure 3.2 - Comparison of three stoichiometric hydrogen-air flames with same calibration data

As can be seen in Figure 3.2 all three tests are quite similar. The vertical offset in the areas of low concentration are likely due to inconsistencies in background imaging. So this test showed that hydrogen flames can be imaged repeatedly and come up with consistent data through post processing.

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However, these three images all shared one common set of calibration data. The real question is, can a concentration calibration be performed repeatedly and reliably?

The next step was to simply perform the entire process again to see how two sets of data taken on different days might compare to eachother. Another methane flame was imaged. A new concentration calibration curve was developed using the same data. Three more hydrogen-air flame images were taken and calibrated. At this point all six data sets (three from calibration 1 and three from calibration 2) were adjusted so that the lowest reported concentration was set to zero. The minimum concentration of each set was simply subtracted from each point of the data set, translating each curve without changing its actual shape. This added step was just to clean up the noise that the background subtractions had missed during image processing. At his point, the consistency of concentration calibration is being tested, not the background subtraction, so it was deemed acceptable to cancel out this noise. In some cases there were negative concentrations which of course do not make sense.

Having decided that the three test curves from calibration 1 were similar, they were averaged together. The three test curves from calibration 2 were also averaged and a plot was created that could only compare the two calibrations in the same manner that Figure 3.2 compared the three tests within calibration 1. This resulted in Figure 3.3:

Figure 3.3 - Comparison between two independent concentration calibrations

Looking at Figure 3.3, it is clear that the curves from two completely independent calibrations agree closely with each other. This has now shown that not only the imaging process is repeatable but the complete concentration calibration is repeatable as well.

One area of concern in this study was the assumption that concentration of OH is constant across the burner at each height from the surface. Looking back at the fluorescence image in Figure 3.1, this is clearly not true. In order to test just how inaccurate this assumption was, concentrations along horizontal lines were studied to get the average concentration across the image at each height. This was

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performed for both calibration data sets and plotted against distance from burner surface. These averaged results were then compared to the concentrations along only the vertical line containing the maximum concentration. As can be seen in Figure 3.4 below, averaging the concentrations at each height greatly decreased the maximum concentration from 2400 ppm to 1500 ppm.

Figure 3.4 - Comparing average concentration to maximum concentration

This discrepancy is due to inconsistencies of the flat flame. These inconsistencies could be caused by build up within the bronze sintered plug of the burner that is highly calibrated to evenly distribute the air-fuel mixture across the surface of the burner. When the flat flame burner was first being used for this thesis, it had been sitting in a dusty lab untouched for over a year. The first images taken showed signs of extreme clogging. Figure 3.5 was taken with a clogged burner:

Figure 3.5 - Inconsistent PLIF image taken with clogged burner

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Clearly this flame is nowhere near flat. The manufacturer was contacted and a hydrochloric acid bath was performed per their recommendation. After completing the HCl bath, images were greatly improved to look more like that shown in Figure 3.1 above. However, the flame is still not perfectly flat. Had time permitted, the burner would have been sent back to California for a complete recalibration. Images taken after the HCl bath were sent to the manufacturer were approved as acceptable for continuing study. A complete recalibration would have reduced the difference between average and maximum concentrations shown in Figure 3.4, but has been left as a recommendation for the next student who uses the PLIF set up.

3.2. Fuel Flowrate Comparison A study was performed to see how the flowrate of the air-fuel mixture affects the concentration and location of OH in stoichiometric flat flames. It is expected that as the flowrate is increased, the maximum concentration will increase. Since the fuels being studied are hydrocarbons, a higher flowrate means more hydrogen which allows for more OH formation. This study will serve as the basis for understanding some of the differences between methane-air and hydrogen-air stoichiometric flames needed to continue this work.

First, a concentration calibration was performed using the same total flowrate used by Cattolica in [19]. This resulted in the calibration image shown in Figure 3.6below:

Figure 3.6 - Stoichiometric methane calibration image

In an attempt to correct for the inconsistency of the burner which creates disturbances in the smooth bottom of the flame, only the portion of the flame outlined in the box in Figure 3.6 above was used for the concentration calibration. This was done because this portion of the flame had the smoothest bottom and contained the maximum intensity in the image. This rectangle contains heights above the burner from approximately zero to ten millimeters.

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Once a concentration calibration curve was created using the image above, it was applied to four different stoichiometric methane-air flames. Methane flowrates of between 0.75 and 4 SLPM, stepping 0.25 SLPM at a time, were selected and used with calculated air flowrates to maintain an equivalence ratio of one. 100 images were taken at each flowrate. For each flowrate the images were averaged, energy corrected, background subtracted, sheet processed and then calibrated for concentration using the curve created from Figure 3.6 and Cattolica’s data. The final images from four of these flowrate tests can be seen below, all set to the same concentration scale in Figure 3.7:

Figure 3.7 - Calibrated stoichiometric methane-air flames with varying flowrates on common scale

It is immediately apparent that the concentration increases with flowrate. Also, the thickness of the flame front increases with flowrate. This detail is easier to see when each image is scaled to show its own maximum concentration on their respective color scales, as was done for Figure 3.8 below:

Figure 3.8 - Calibrated stoichiometric methane-air flames with varying flowrates on independent scales

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The minimum of each concentration color scale was set to zero and the maximum concentration in the image was found by the software and set to the maximum of the color scale. It can be seen clearly now in Figure 3.8 that the red region of high OH concentration, which marks the flame front, becomes thicker as the flowrate of methane is increased while maintaining stoichiometry. This lead to the understanding of how important it is to match not only the equivalence ratio but the volumetric flowrate as well for calibration data to be applicable to a flame image.

A detail worth noting in Figure 3.8 is the smoothness of the bottoms of the flames. As the flowrate increases, the bottoms of the flames become considerably less smooth. Another detail that is perhaps the most noticeable thing when comparing the four images is that the background of the 4 SLPM image is blue, predicting an OH concentration of around 2000 ppm throughout, while the rest of the backgrounds are black with concentrations around zero. This would mean that there are 2000 ppm of OH on the stainless steel of the burner and in the ambient air when the flowrate is 4 SLPM, which of course is not true! This discrepancy could mean that the background subtraction did not work properly and 2000 ppm have been added to each pixel, giving a falsely high maximum concentration. Or perhaps there was a fluctuation in ambient light captured during the 4 SLPM test that made the common background image incompatible with the experimental image. Looking at Figure 3.10 below, it can be seen that the data point for 4 SLPM for methane does not fall as close to the trendline as the other points, further indicating that this may be an error. This issue was further investigated and it is believed to be due to inconsistencies in batch processing within the DaVis software. To test the repeatability of the software, a stationary flame was lit and three sets of 100 images were taken right in a row, within less than a minute of each other. These three sets were then processed using the exact same background image, sheet image and concentration calibration. Theoretically, all three final images would be the same, but as can be seen in Figure 3.9 below, they clearly are not:

Figure 3.9 - Image post processing inconsistency test, all images should match

In the images above, all color scales were set to have a minimum of zero and a maximum of the highest concentration in each image. The left most image has a maximum concentration of about 3200 ppm. The solid light blue background of that same image is predicted to be around 400 ppm. The right most picture has a maximum concentration of about 2600 ppm and a background of almost zero. This strongly supports the suspicion that the entire image has simply been shifted in intensity. After conducting this post processing consistency test, it was decided that final images would be rejected if their backgrounds were not close to zero. Many processed images would end up having negative OH concentrations, which of course does not make sense. Or, as seen in Figure 3.8, the entire image would

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be scaled too high. When dealing with numerical data, these data sets could be corrected by subtracting the lowest concentration from each point to zero out the background.

A similar flowrate comparison was repeated for stoichiometric hydrogen-air flames, still using the methane-air flame for concentration calibration. It was again found that concentration of OH increased with the flowrate.

Figure 3.10 - Comparison of [OH] varying with total flowrates of methane-air and hydrogen-air

When plotted on the same graph, it becomes obvious that methane has much higher concentrations of OH at comparable flowrates to hydrogen. The maximum concentrations of OH increase by almost 280 ppm per SLPM for stoichiometric methane-air flat flames but by only about 40 ppm per SLPM for stoichiometric hydrogen-air flat flames. This study has shown that hydrogen flames give off a much weaker LIF signal than methane flames. This knowledge can be used in future experiments. Knowing what to expect of experimental images relative to the calibration image can help confirm that the system is working properly.

3.3. Equivalence Ratio Comparison Once the calibration procedure was validated and the effect of flowrate on [OH] in stoichiometric flames was studied, the next topic of interest for this work was to study the effect of equivalence ratio on quantitative [OH] in a hydrogen-air flame. A key question that arose when starting this equivalence ratio study was, in order to have comparable flames, should the fuel flowrate be kept constant and the oxidizer flowrate changed to achieve the desired equivalence ratio? Or should the total mixture flowrate be kept constant and both fuel and oxidizer flowrates adjusted to sum up to the same total each time? For all of the studies perfomed in this work, flowrates and equivalence ratios were determined using the ideal gas law. Knowing that flat flames would occur at atmospheric pressure, room temperature and using the universal gas constant, a direct ratio of volumetric flowrates and moles could be used. Looking at this fact, it was thought that in order to make flames that are truly comparable, the number of moles

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of the premixed air-fuel mixture per minute should be kept constant. This would mean, assuming ideal gas behavior, that the total mixture flowrate must remain the same. After this consideration, it was decided that tests would be run using Cattolica’s total mixture flowrate of 0.316 liters per second to have flames be comparable to each other and to literature.

Just as was done in the flowrate study, a methane flame was first imaged to establish a calibration curve for the post processing of experimental images to be taken. Then, 100 images were taken of hydrogen-air flames at each equivalence ratio of interest. Images at each ratio were averaged, post processed and calibrated for quantitative concentration. Five equivalence ratios were studied and their final images can be seen, all displayed on a common scale, in Figure 3.11 below:

Figure 3.11 - H2-air flames with varying equivalence ratios displayed on a common concentration scale

Thanks to the common concentration scale used for the five images in Figure 3.11 above, it can be clearly seen that the concentration of OH decreases as the equivalence ratio increases. This is not at all the expected result. The peak concentration was expected to be at stoichiometric conditions, with the concentration decreasing as the mixture moves away from stoichiometric in either direction. Average concentrations at each height above the burner along the full height of each image was extracted and plotted to get a more quantitative comparative view of these results, as shown in Figure 3.12:

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Figure 3.12 - Effect of equivalence ratio on value and location of OH concentration

There are a few interesting points to note about Figure 3.12 above. First off, the visually drawn conclusion that the maximum concentration increases as the equivalence ratio decreases is quantitatively confirmed, which is unexpected and will be discussed in greater detail. Also, notice that the peak concentration of each equivalence ratio occurs around the same height off the burner surface (approximately 1mm). One detail that was not easily noticed in the flame images is that the higher peak concentrations seem to more rapidly decrease in concentration as height above the burner increases, with the exception of Φ=1.2. Compare the shallow slope from 1 to 5 mm of the Φ=1.1 curve to the nearly vertical Φ=0.8 curve across the same range. This shows that the flame front of the lean mixtures is much thinner than that of the rich mixtures. This would be expected because in a lean flame, all of the fuel gets consumed whereas in a rich flame, excess fuel will be able to travel further away from the burner surface before being burned.

Finally, notice that the concentration is not zero at a height of zero, as one might expect. Looking back up at any of the fluorescence images you will see sort of a blue glow below the flames. This blue glow is the concentration that is plotted below the surface of the burner. The cause of this is most likely fluorescence reflecting off of the burner surface. Fluorescence occurs in all directions and the camera only collects the photons that enter its intensifier. This means that there are photons which, when emitted, travel down towards the burner surface where they are reflected into the camera. Since the event of fluorescence lasts only 20ns and the gate of the camera is 200ns, some reflections will certainly be collected.

It is clear that quantitatively, the concentration of OH is higher at the lean equivalence ratios than at stoichiometric conditions, which should not be the case. This raised concerns that perhaps the flowmeters are off calibration and that the desired stoichiometries were not actually being experimentally created. To test this, a rotameter was connected to the Omega digital flowmeters that are used to monitor oxidizer and fuel flowrates to the flat flame burner. Calibration curves were created

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by plotting the flowrate measured by the Omega meter against the value measured by the Dwyer Instruments rotameter. Data was collected for air, methane and hydrogen. Since the Omega meters are calibrated for N2

Figure 3.13

and the Dwyer meter is calibrated for air, correction factors from the two manufacturers were used to convert the displayed flowrates on the meters to the actual flowrates of the specific gases being studied. This test resulted in the plot shown in below:

Figure 3.13 - Flowmeter calibration curve

Since both meters are reporting standard liters per minute (SLPM), the resulting curves are expected to be linear with a slope of one. As can be seen by the equations of each line displayed on the plot in Figure 3.13, this is not the case for all three gases. The slope of the air line is almost one, so the measurement of air flowrate taken by the Omega meter is deemed accurate. The methane and hydrogen curves however are far from having a slope of one. The two fuels do have r-squared values close to one though, which means that their data has a strong linear correlation. This leads to the belief that simply the manufacturer’s scaling factor is off. The dwyer scaling factor is a physical conversion using the specific gravity of the fuel so it is trusted. The Omega conversion factors however are simply numbers taken from a table with no physical background. The fact that the Omega meters are digital also leads to the thought that over time they more easily fall out of calibration than a rotameter, which simply depends on the properties of the fluid being measured and the size, shape and density of the channel and float, which are not likely to change much over time.

This lead to the conclusion that the Omega conversion factors for methane and hydrogen should be adjusted to make the calibration curves have a slope of one. By adjusting the conversion factor for methane from 0.72 to 0.59 and the conversion factor for hydrogen from 1.01 to 0.95, Figure 3.13 was adjusted to Figure 3.14:

y = 0.9667x + 0.2056R² = 0.9982

y = 0.7436x + 0.1818R² = 0.9927

y = 0.2521x + 0.4391R² = 0.9985

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Figure 3.14 - Adjusted calibration curve

The vertical shift of the hydrogen curve giving it a large y-intercept is of concern because this means that the hydrogen readings on the Omega meter experience a shove error reporting approximately 1.7 SLPM too low for each measurement. After accounting for the new conversion factors and this shove error for hydrogen, it was found that the stoichiometries had all been shifted. Correcting for this shift and looking back at the data displayed in Figure 3.12, it was found that the peak concentration actually did occur around stoichiometric conditions. A table showing the original and converted equivalence ratios is shown below:

Table 2- Corrected equivalence ratios using calibration curves

original Φ corrected Φ 0.5 0.73 0.6 0.82 0.7 0.9 0.8 0.97 0.9 1.06

1 1.13 1.1 1.21 1.2 1.3 1.3 1.4 1.4 1.49 1.5 1.58

These corrected equivalence ratios were calculated in a spreadsheet that was created to calculate equivalence ratio by entering the flowrates displayed on the Omega meters. A cell for the Omega conversion factor was adjusted to the new factors found from the calibration curves and the shove error

y = 0.9993x + 0.2443R² = 0.9927

y = 1.0058x + 1.6644R² = 0.9985

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was added to the flowrate after conversion. As seen in Table 2 above, the peak concentration that was originally believed to have occurred at the lean equivalence ratio of 0.8 actually occurs at approximately stoichiometric conditions of Φ=0.97.

Fredette had performed a flowmeter calibration curve procedure of his own, using the stoichiometries and peak temperatures to adjust the conversion factor given by Omega. When his corrections were used, data did not match theory even though his results had worked in 2009. This leads to a suspicion that the Omega flowmeters change overtime, slowly falling out of calibration. This means that as each student starts to work with the system they should check the meters in the way that they see most fit to confirm that they are truly creating the flames with the equivalence ratio desired.

4. Condensed Manual One of the most significant contributions of this thesis to the laminar flame lab was the creation of a condensed manual outlining all of the key procedures needed to run a PLIF experiment. Before this new condensed manual existed, the user had to rely on a separate manual for each piece of equipment, for both software packages and for the overall system making a total of over ten manuals to flip through. When a question arose, for example, about the camera, it was never known if the answer would be found in the camera manual, the software manual or the system manual. Just figuring out which manual to go to became a time consuming process. Even when the proper manual and section was found, detail would often be given for a specific example that was not similar to the Northeastern PLIF set up, making the information difficult to translate to the question at hand. Over the past two years as these manuals were deciphered and new tips and tricks were suggested from professionals at LaVision or Spectra Physics, the essential procedures specific to the laminar flame lab’s PLIF system were documented so that future students would have one go to source for experimental procedures.

In the past, no two students working on the PLIF system have overlapped, greatly slowing down the progress that could be made on the system by new students needing to learn the incredibly complex system from scratch every couple of years. The aim of this manual is to allow a new student to get the system up and running quickly so that the majority of their time can be spent collecting data and furthering the understanding of flame structure in laminar flames. This manual certainly does not replace all of the other manuals, but acts as more of a quick start guide. When more detail is needed than is given in the condensed manual, the original manuals from the manufacturers should be consulted.

One example of a procedure that has been made more clear by detailing it specifically to this lab is the peakfinding scan. Selection of wavelength is essential in PLIF experimentation. Someone first starting out with this system probably does not know the best range to scan nor what step size to use. While the manufacturer’s manual [8] gives a very thorough description of how to go through the peakfinding process in the DaVis software, they are doing it for a different OH transition with a different range. They also go into quite a bit of detail on steps that are not necessary to complete a successful scan for our specific system and can greatly complicate and confuse the process for a new student. The biggest problem with this manufacturer’s manual procedure for peakfinding is that it does not once mention

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that this scan must be coupled with data taken in a different program in order for it to be completed correctly! A relationship between FCU position and resonator must be established in the Sirah Control software before a peakfinding scan in the LaVision DaVis software can be properly conducted. Since the peakfinding procedure is in a LaVision manual, it does not once mention the Sirah software. This lead to peakfinding being performed incorrectly in this lab for over a year! Finally, a LaVision engineer visited and walked through the peakfinding process, showing this essential step of establishing the relationship between FCU and resonator. Since not all DaVis software is coupled with Sirah software, this procedure specific to our lab, was left out of the manufacturers manual. Without having spoken the the engineer at LaVision, this step would never have been known. See the Wavelength Selection section for sample peakfinding excitation spectra that were taken with and without the coupled Sirah step to learn more about what this key step really does.

Peakfinding is just one of almost twenty different procedures that have been tailored specifically to our PLIF system here at Northeastern. Some of the procedures included in this condensed manual do not exist anywhere else. They are simply tips and tricks that were picked up from visits from LaVision and Spectra Physics engineers. One of these new procedures is a step by step walkthrough of how to clean the flashlamp assemblies. When a Spectra Physics engineer came to replace an electrical board in the Sirah dye laser in the spring of 2011, he also peaked the power of the pump and dye lasers and careful notes were taken to pick up some professional tips that could be passed on to future students. One of his tricks was to clean the two flashlamp assemblies inside the pump laser with a 10% hydrochloric acid solution. The engineer said that one of the main reasons that he has found for decrease in pump power is a buildup of some kind of condensation inside these assemblies, making their reflective surfaces cloudy. If the internal surfaces of the flashlamps are cloudy, they cannot as efficiently reflect photons to generate a strong beam. By disassembling the flashlamps and wiping down each component, this condensation can be removed and the reflectivity restored, greatly increasing the pumps power which will directly increase the dye laser power.

Other procedures covered in the manual include regular maintenance to keep the system running smoothly, experimental image processing, how to set up and fill the constant volume combustion chamber and more.

5. Conclusions and Future Recommendations A concentration calibration method developed by the company that designed and set up the PLIF system has been tested. It greatly simplifies the calibration process, removing the need for a thermocouple with radiation corrections and chemical equilibrium calculations. By simply applying the given concentrations to an experimentally similar flame, a calibration curve can be created and applied to different fuels and flame types so long as environmental conditions and laser settings are kept constant.

Finally, in the past two master’s theses to come out of this lab issues with the Omega flowmeters have been noticed. In this thesis the conclusion that the calibration changes with time was drawn. An

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adjustment method using a rotameter was developed to draw a calibration curve and correct the flowrates displayed on the digital Omega meters.

The creation of a condensed PLIF system manual will speed up the learning curve of future students. The theoretical and technical knowledge placed in the manual took about two years to gather. Hopefully, having all of this information in one place will allow a new student to quickly get a handle on using the system and be able to spend their time conducting experiments and analyzing data instead of troubleshooting and learning the system.

A recommendation that will greatly improve the calibration system would be to send the burner back to Holthius & Associates and have it rebuilt. Over the years of sitting in the lab unused, dust and other foreign particles may have built up inside the porous plug of the burner. As a result, the “flat flame” is not as flat as it could be. When Fredette was using the new burner, flames were as flat as they should be with a freshly calibrated burner. After lying dormant for almost two years, the flame images appeared rough with voids in the flame where air/fuel mixture could not travel through the bronze plug of the burner. Upon recommendation from Holthius & Associates, a hydrochloric acid soak was performed in an attempt to dissolve and dislodge any foreign particles inside the burner. The progression of flat flame photographs is shown in Figure 5.1 below:

Figure 5.1 - (L to R) Fredette's flame, clogged burner flame, post HCl bath flame

While the hydrochloric acid bath clearly made a big difference, the flame is still not back to its original quality and should be professionally re-calibrated.

A second recommendation to improve the performance of the PLIF system would be to have a laser technician come to campus and tune the dye laser. A professional tuning should only be scheduled once the laser system is understood fairly well and the user can take both flat flame and transient images comfortably. This way you can get the most out of a visit from someone by knowing what questions to ask. Also, the laser power decreases over time so you want to be able to run experiments the day of or the day after a professional tuning. One thing that would be worth trying while the laser engineer is in the lab would be to measure power both with their power meter and this labs power meter to compare values and find what the peak on our Newport meter is. This was not done during the last professional tuning and it is now left unknown if the pump actually lost a watt of power overnight or if there was just a difference between the two power meters.

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References [1] A. Arnold, R. Bomback, B. Käppeli, A. Schlegel, “Quantitative measurements of OH concentration fields by two-dimensional laser-induced fluorescence,” Applied Physics B, vol. 64, pp.579-583, 1997.

[2] H.M. Hertz and M. Aldén, “Calibration of Imaging Laser-Induced Fluorescence Measurements in Highly Absorbing Flames,” Applied Physics B, vol. 42, pp. 97-102, 1987.

[3] LaVision GmbH, “Calibration Burner Product-Manual,” Anna-Vandenhoeck-Ring 19, D-37081 Göttingen, Germany, 2009.

[4] C. Fredette, “Quantitative Hydroxyl (OH) Concentration Calibration by Use of a Flat Flame Burner, Thermocouple and Planar Laser Induced Fluorescence (PLIF) System,” MS thesis, Northeastern University, Boston, MA, 2009.

[5] W.C. Reynolds, STANJAN, Chemical Equilibrium Calculation, Colorado State University, http://navier.engr.colostate.edu/tools/equil.html, August 2011.

[6] M. Mansour, N. Peters and L. Schrader, “Experimental study of turbulent flame kernel propagation,” Experimental Thermal and Fluid Science, vol. 32, issue 7, pp. 1396-1404, 2008.

[7] S.H.R. Müller, B. Böhm, M. Gleißner, S. Arndt and A. Dreizler, “Analysis of the temporal flame kernel development in an optically accessible IC engine using high-speed OH-PLIF,” Applied Physics B, vol. 100, pp. 447-452, 2010.

[8] LaVision GmbH, “Tunable LIF Product-Manual,” Anna-Vandenhoeck-Ring 19, D-37081 Göttingen, Germany, 2007.

[9] R.E. Stevens, H. Ma, C.R. Stone et al., “On planar laser-induced fluorescence with multi-component fuel and tracer design for quantitative determination of fuel concentration in internal combustion engines,” Proc. IMechE, vol. 221 Part D: J. Automobile Engineering, 2007.

[10] S.M. Blinder, “Introduction to Quantum Mechanics in Chemistry, Materials Science and Biology.” Complementary Science Series, Elsevier Academic Press, 2004.

[11] P. Signell, “Diatomic Molecules: Properties From Rotation-Vibration Spectra,” Project PHYSNET, Physics Bldg., Michigan State University.

[12] R. Andrews, “Measurement of Hydroxyl (OH) Concentration of Transient Premixed Methane-Air Flames by Planar Laser Induced Fluorescence (PLIF) Method,” MS thesis, Northeastern University, Boston, MA, 2008.

[13] LaVision GmbH, “Flamemaster Training: Introductory Presentation,” Anna-Vandenhoeck-Ring 19, D-37081 Göttingen, Germany, 2011.

[14] Spectra-Physics, a division of Newport Corporation, “Quanta-Ray PRO-Series Lasers: Pulsed Nd:YAG Lasers, User’s Manual,” Mountain View, CA. November 2002.

http://navier.engr.colostate.edu/tools/equil.html�

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[15] Sirah Laser-und Plasmatechnik GmbH, “Pulsed Dye Laser: Service Manual,” Germany, 2006.

[16] LaVision GmbH, “Energy Monitor: Device-Manual,” Anna-Vandenhoeck-Ring 19, D-37081 Göttingen, Germany, 2005.

[17] LaVision GmbH, “Sheet Optics (divergent): Device-Manual,” Anna-Vandenhoeck-Ring 19, D-37081 Göttingen, Germany, 2004.

[18] T. Horstmann, C. Gray, LaVision technical staff, www.lavision.com, 2011.

[19] R.J. Cattolica, S. Yoon, E.L. Knuth, “OH Concentration in an Atmospheric-Pressure Methane-Air Flame from Molecular-Beam Mass Spectrometry and Laser-Absorption Spectroscopy,” Combustion Science and Technology, Vol. 28, pp. 225-239, 1982.

[20] B. Tummers, DataThief III. 2006 http://datathief.org/

[21] C. Bennett, “Laminar Burning Speed and Flame Structure of 1,1-Difluoroethane (HFC-152A)/Air and Difluoromethane (HFC-32)/air Mixtures,”MS thesis, Northeastern University, Boston, MA, 2008.

http://www.lavision.com/�http://datathief.org/�

Northeastern UniversityAugust 01, 2011A study of quantitative concentrations of hydroxyl (OH) in laminar flat flames using planar laser induced fluorescence (PLIF)Adrienne Murphy JalbertRecommended Citation

List of Figures and TablesAcknowledgements1. Background1.1. Purpose1.2. Literature Review1.3. PLIF Basics1.3.1. Quantum Mechanics1.3.2. OH Specifics

2. Experiment2.1. Experimental Setup2.2. Wavelength Selection2.3. Concentration Calibration