Browsing by Author "Yalin, Azer, advisor"
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Item Open Access Development and testing of a multiplexing system for laser ignition of large bore natural gas engines(Colorado State University. Libraries, 2011) Reynolds, Adam Robert, author; Yalin, Azer, advisor; Willson, Bryan, committee member; Roberts, Jacob, committee memberConventional electric spark plugs present a hindrance to the continuing goals of higher efficiency and reduced emissions for large-bore natural gas engines. In order to achieve these goals, higher compression ratios and higher air-to-fuel ratios must be achieved relative to those currently allowed by conventional spark plugs. Laser ignition has been shown to work farther into the lean limit, and contrary to conventional electric spark plugs, laser sparks are easier to produce at higher pressures. Laser ignition has also been shown to reduce NOx emissions. This work presents efforts to design, build, and test a single-laser-to-multiple-cylinders multiplexed laser ignition system for use with a large bore natural gas engine. A fiber based laser delivery system was found to work for laser ignition on the bench-top. Results of bench top tests are presented.Item Open Access Development and testing of a solid core fiber optic delivery system and ultraviolet preionization for laser ignition(Colorado State University. Libraries, 2012) Wilvert, Hurley Nicholas, author; Yalin, Azer, advisor; Marchese, Anthony, committee member; Rocca, Jorge, committee memberLaser ignition of natural gas engines has shown potential to improve many facets of engine performance including brake thermal efficiency, exhaust emissions, and durability as compared with traditional spark ignition. Laser ignition technology has yet to transition to industry primarily because no system for reliably and safely delivering the laser pulse to the combustion chamber exists. This thesis presents a novel fiber optic delivery approach using solid core multimode step index silica fibers with large cladding diameters (400 μm core, 720 μm cladding). Testing was done on the fibers to determine their response to bending, vibration, high power input, and long duration beam transmission. It was found that in configurations representative of what is required on a real engine, and in the presence of vibration, reliable spark formation could be achieved in pressures as low as 3.4 bar using a specially designed optical spark plug. Comparative tests between the fiber delivered laser ignition system and a traditional J-gap spark plug were performed on a single cylinder Waukesha Cooperative Fuel Research (CFR) engine running on bottled methane. Tests were run at three different Net Mean Effective Pressures (NMEP) of 6, 8, and 12 bar at various air-fuel ratios. Results indicate reliable performance of the fiber and improved engine performance at high NMEP and lean conditions. Thesis research also includes initial studies into the use of dual laser pulses for plasma formation and ignition. In this approach, a first ultraviolet pulse preionizes a volume of air while a second overlapped pulse adds additional energy. Electron density measurements reveal the ultraviolet beam generates substantial preionization even with no visual breakdown, and Schlieren images are used to study the interaction between the two beams at atmospheric and lower pressures.Item Open Access Effect of additives on laser ignition and compression ignition of methane and hydrocarbons in a rapid compression machine(Colorado State University. Libraries, 2016) Boissiere, Andrew, author; Marchese, Anthony, advisor; Yalin, Azer, advisor; Van Orden, Alan, committee memberDespite recent efforts to develop new energy systems that do not rely on combustion of fossil fuels, internal combustion (IC) engines powered on fossil fuels (i.e. gasoline, diesel or natural gas) will remain as an integral component of the global energy portfolio for years to come and increasing the efficiency of IC engines will be a necessary means to reduce fossil fuel consumption and greenhouse gas emissions. In this study, the effect of fuel additives on natural gas and gasoline spark ignited engines were investigated using laser ignition and compression ignition experiments performed in a rapid compression machine (RCM). The goal of the laser ignition study was to examine the effect of additives to extend the lean limit of natural gas engines, while the goal of the compression ignition experiments were to examine the ability of fuel additives to decrease knock propensity of gasoline fuels. For the laser ignition study, methane/air mixtures containing various fuel additives at temperatures and pressures representative of the compressed conditions inside an internal combustion engine were ignited in the RCM. An Nd:YAG laser operating at a wavelength of 1064 nm was used to ignite methane/air mixtures ranging in equivalence ratio from stoichiometric down to 0.4 using a rapid compression machine (RCM). Experiments were conducted to determine the lean limit, minimum spark energy (MSE), and minimum ignition energy (MIE). Three different fuel additives at varying concentrations were tested. The results show that laser ignition exhibits a stochastic behavior which must be interpreted statistically. A 90% probability of occurrence is used to evaluate the MSE and MIE which resulted in MSE90=2.3 mJ and MIE90=7.2 mJ for methane/air mixtures of equivalence ratio equal to 0.4. The lean limit, defined as greater than 90% of the theoretically possible heat release, was found as equivalence ratio of 0.47 for methane/air mixtures. All three fuel additives resulted in a reduction of the baseline methane/air MIE, while only DTBP and NM resulted in a reduction of the lean limit. For the compression ignition study, the effects of various fuel additives on the auto-ignition characteristics of gasoline reference fuels were studied in the RCM. Fuel additives were added to stoichiometric fuel/air mixtures of liquid gasoline surrogate fuels and were auto-ignited in a RCM. Experiments were conducted to determine the ignition delay, heat release rate, and net heat release of the gasoline surrogate/air mixtures with and without fuel additives. Five different gasoline fuel additives were tested in an Iso-Octane and Toluene Reference base fuel. The results show that the majority of the additives increased the reactivity and decreased the ignition delays of the base fuels. However, a select few of the tested additives decreased the reactivity and increased the ignition delays of the base fuel at select conditions, which could be beneficial to increasing the efficiency of internal combustion engines.Item Open Access Evaluation of a non-thermal plasma generator for plasma-assisted combustion in an oil burner(Colorado State University. Libraries, 2013) Doyle, Jake Downin, author; Yalin, Azer, advisor; Joshi, Sachin, advisor; Marchese, Anthony, committee member; Collins, George, committee memberThe addition of plasma to a combustion system has the potential to increase the combustion efficiency and reduce harmful emissions by reforming hydrocarbon fuels. The ability for plasma to reform fuel to create hydrogen-rich synthesis gas has been shown by other researchers. The work presented in this thesis includes the characterization of a plasma generator patented by Clean Diesel, LLC and testing an oil burner that was modified to use the plasma generator for combustion enhancement. The plasma was generated by six electrodes with a circulating high voltage pulse created by a signal generator and high voltage transformers. The plasma is characterized through optical emission spectroscopy and with electrical measurements, where it was shown to be a non-thermal plasma operating in the glow-to-arc transition region. The plasma generator was then implemented into an oil burner where its thermal efficiency and emissions were compared to that of a stock Riello F10 burner. Testing showed similar efficiencies for the modified and stock burners (contrary to previous testing that showed improvements due to plasma assistance). Carbon monoxide and nitrogen oxides were considered as the key pollutants, and it was shown that NOx emissions exceeded that of the stock burner, although CO levels were reduced. Further testing was performed with additional modifications such as fuel spray type, electrode insulation, and plasma frequency, although none showed significant improvements in its operation. The results have led to the realization that a more volumetric plasma that can provide longer residence time for fuel interaction is likely needed for effective fuel reforming.Item Open Access Fiber delivery and diagnostics of laser spark ignition for natural gas engines(Colorado State University. Libraries, 2008) Joshi, Sachin, author; Yalin, Azer, advisor; Willson, Bryan, advisorLaser ignition via fiber optic delivery is challenging because of the need to deliver pulsed laser beam with relatively high energy and sufficient beam quality to refocus the light to the intensity required for creating spark. This dissertation presents work undertaken towards the development of a multiplexed fiber delivered laser ignition system for advanced lean-burn natural gas engines. It also describes the use of laser ignition system to perform in-cylinder optical diagnostics in gas engines. Key elements of the dissertation includes: (i) time resolved emission spectroscopy (TRES) of laser sparks in air to investigate the dependence of spark temperatures and electron number densities on ambient gas pressures, (ii) optical characterization of hollow core fibers, step-index silica fibers, photonic crystal fibers (PCFs) and fiber lasers, (iii) development and on-engine demonstration of a multiplexer to deliver the laser beam from a single laser source to two engine cylinders via optical fibers, and (iv) demonstration of simultaneous use of laser sparks for ignition and Laser Induced Breakdown Spectroscopy (LIBS) to measure in-cylinder equivalence ratios in a Cooperative Fuel Research (CFR) engine. For TRES of laser sparks, the ambient gas pressure is varied from 0.85 bar to 48.3 bar (high pressures to simulate elevated motored in-cylinder pressures at time of ignition in advanced gas engines). At later stages (~1μs) of spark evolution, spark temperatures become comparable at all pressures. Electron number densities increase initially with increasing ambient gas pressure but become comparable at pressures greater than ~20 bar. The effects of launch conditions and bending for 2-m long hollow core fibers are studied and an optimum launch f/# of ~55 is shown to form spark in atmospheric pressure air. Spark formation using the output of a pulsed fiber laser is shown and delivery of 0.55 mJ nanosecond pulses through PCFs is achieved. Successful multiplexed laser ignition of a CAT G3516C gas engine via hollow core fibers is shown. LIBS analysis conducted at equivalence ratios from 0.6 to 0.95 in the CFR engine show a linear variation and linear correlation (R2 > 0.99) of line intensity ratio (Hα/O777 and Hα/Ntot) with equivalence ratio.Item Open Access Investigation of dual-pulse laser plasmas for ignition of fuel-air mixtures(Colorado State University. Libraries, 2020) Butte, Carter Vincent, author; Yalin, Azer, advisor; Marchese, Anthony, committee member; Bailey, Ryan, committee memberProgress towards more complex combustion applications has demanded more advanced and versatile ignition techniques. One attempt to address some of the concerns associated with well-established techniques such as spark plugs and igniters is laser plasma ignition. Advantages of laser ignition include flexibility of spark location and timing, reduced NOx formation, leaner engine operation, increased combustion efficiency, and greater system longevity at elevated pressures. Additionally, the non-intrusive nature of laser plasmas results in more unperturbed kernel evolution, as mounting hardware is not required. This is an advantage when compared with spark plug or igniter electrodes which typically act as heat sinks quenching the flame. However, large input energies, complications with beam delivery, and undesirable kernel dynamics have impeded field implementation. Our approach to address these challenges uses a dual-pulse laser plasma where an ultraviolet (UV) beam preionizes a gas mixture and a second near infrared (NIR) beam increases the energy and ionization state of the gas. The use of this technique decouples the processes responsible for ionization, predominantly multiphoton ionization (MPI) and electron avalanche ionization (EAI) through inverse bremsstrahlung absorption, and allows for tailoring of plasma properties through adjustments to beam energies and delay time. Recent work has shown that dual-pulse laser plasmas not only reduce energy requirements but also enhance ignition characteristics such as combustion efficiency, particularly around the lean limit.6 The present thesis serves to fill voids in the existing literature with regards to plasma properties and ignition characteristics in various fuels, as well as present a new resonant preionization scheme targeting molecular oxygen at λ=287.5 nm. Four laser plasmas are investigated in this work: non-resonant single pulse (λ=1064 nm), non-resonant dual-pulse (preionization at λUV=266 nm with energy-addition at λNIR=1064 nm), resonant single-pulse (λREMPI=287.5 nm), and resonant dual-pulse plasma (preionization at λREMPI=287.5 nm and energy addition at λNIR=1064 nm). Each of these plasmas are analyzed for electron density and gas temperature using combined Rayleigh Thomson scattering, and are studied for ignition of propane-air, methane-air, and hydrogen-air mixtures. In the analysis, these experimental results are combined with past results to give a comprehensive picture of the ignition abilities of single pulse and dual-pulse plasmas in propane-air, methane-air, and hydrogen-air mixtures. Together, knowledge of plasma properties and ignition characteristics give us a more complete picture of the capabilities and limitations of each plasma for combustion applications.Item Open Access Laser ignition for internal combustion engines via fiber optic delivery(Colorado State University. Libraries, 2009) DeFoort, Morgan, author; Yalin, Azer, advisor; Willson, Bryan, advisorIn the effort to reduce emissions and improve the efficiency of Otto cycle engines, the ignition system is often a limiting factor. Many "high energy" ignition systems have been developed, but almost all of these are based on traditional electric arc spark plugs. Laser ignition represents a fundamentally different approach to igniting gas mixtures and opens the door to improvements in fuel-lean engine operation and high-pressure combustion environments. Yet the promise of laser ignition remains unexploited, as practical systems have not been developed. In this contribution, we work towards the goal of developing a practical laser ignition system for stationary natural gas engines. Specifically, we focus on fiber optic delivery of the laser beam to the engine, thereby making a significant advance relative to past open-air (free-space) configurations. A combination of modeling and experimentation has been used to develop the needed fiber optic delivery systems, culminating in the first demonstration of fiber-optically delivered laser ignition on an engine.Item Open Access Nanometer-scale machining with extreme ultraviolet lasers(Colorado State University. Libraries, 2013) Bravo, Herman, author; Yalin, Azer, advisor; Rocca, Jorge J., advisor; Marconi, Mario, committee memberThis thesis demonstrates the feasibility of direct machining in the nanometer scale using Extreme Ultraviolet (EUV) laser radiation. Laser machining of materials has been widely used for the development of micromechanical components and devices. Advances in technology further motivate the extension of laser machining of microstructures to smaller dimensions. The advent of high repetition rate table top EUV lasers has opened the possibility of extending laser machining to the nanometer-scale. It has been previously demonstrated that single laser shots from a 46.9 nm wavelength capillary discharge laser can ablate very clean holes with a diameter as small as 82 nm on polymethyl methacrylate (PMMA) photoresist. This thesis extends previous work by demonstrating nanometer-scale machining of polymers with a focused EUV laser beam. Sub-200 nm wide trenches several micrometers in length were machined on PMMA. These are,to our knowledge, the smallest ablated trenches machined with a focused laser beam. This work also discusses the study of warm plasmas created by EUV laser irradiation of solid targets in which single-photon photoionization is the dominant energy absorption mechanisms. Low-absorption (silicon, Z=14) and high-absorption (chromium, Z=24, and silver, Z=47) targets were heated by ~ 1 ns duration pulses from a 46.9 nm wavelength EUV laser. The spectra obtained agree with 1 1/2 dimension simulations in showing that the Si plasmas are significantly colder and less ionized, confirming that in contrast to plasmas created by optical lasers the plasma properties are largely determined by the absorption coefficient of the target material.Item Open Access NOx formation in methyl ester, alcohol, and alkane droplet autoignition and combustion: PLIF measurements and detailed kinetic modeling(Colorado State University. Libraries, 2014) Grumstrup, Torben, author; Marchese, Anthony J., advisor; Yalin, Azer, advisor; Kreidenweis, Sonia, committee member; Olsen, Daniel B., committee memberNumerous studies have shown that diesel engines fueled by fatty-acid methyl ester biodiesel often exhibit slightly increased production of oxides of nitrogen (NOx) in comparison to petroleum diesel. A number of explanations for this increase have been proposed. One theory, which has been supported by optical engine test results, suggests that the presence of oxygen atoms in the methyl ester fuel molecule results in a leaner premixed autoignition zone, thereby increasing in-cylinder temperatures and promoting Zel'dovich NOx production. Other experiments have suggested that the unsaturated methyl esters in biodiesel cause an increase in CH radical production (and/or other potential precursors such as C2O) which in turn increases Fenimore NOx formation. In this work, these hypotheses are explored experimentally and computationally by considering autoignition and combustion of single, isolated methyl ester, alcohol and alkane droplets. Experiments were conducted in which the planar laser-induced fluorescence (PLIF) spectroscopy technique was applied to burning liquid fuel droplets in free-fall. A monodisperse stream of droplets was generated by a piezoelectric device and passed through a resistively heated ignition coil. A pulsed laser beam from a Nd:YAG-pumped dye laser (10 Hz, 10 ns width) was formed into a sheet and passed through the droplet flame. The dye laser was tuned to excite hydroxyl (OH) at 282.9 nm and nitric oxide (NO) at 226.0 nm. The resulting fluorescence was imaged by a Cooke Corporation DiCam Pro ICCD digital camera. Band pass filters were utilized to reject laser light scattering while admitting fluorescence wavelengths. Due to the small fluorescence signal, many fluorescence images were averaged together to create a useful average image; approximately 250 and 1000 images were averaged for OH and NO spectroscopy, respectively. Finally, pixel intensity of the averaged fluorescence image was integrated about the droplet center to create qualitative radial profiles of OH and NO concentration. Profiles were generated for a number of oxygenated fuels and one pure hydrocarbon: methanol, ethanol, 1-propanol, methyl butanoate, methyl decanoate, and n-heptane. To quantitatively interpret the contribution of Zel'dovich and Fenimore NOx mechanisms on NOx formation in the vicinity of igniting liquid droplets, detailed numerical droplet combustion simulations were conducted. The transient, spherically symmetric droplet combustion modeling featured detailed gas-phase kinetics, spectrally resolved radiant heat transfer, and multicomponent gas transport. New chemical kinetic mechanisms were created by appending NOx chemical kinetics to existing detailed methanol, methyl butanoate, and n-heptane mechanisms. In the computations, non-oxygenated (heptane) and oxygenated (methyl butanoate, methanol) fuel droplets are introduced into a hot (1150 K) air ambient whereupon the liquid vaporizes, thus producing a stratified fuel/air mixture that thermally autoignites after an ignition delay period. The computational results suggest that NOx formation in stratified fuel/air mixture in the vicinity of a cold liquid droplet is influenced greatly by the detailed full NOx chemistry (Fenimore, Zel'dovich and N2O) and cannot be fully explained by considering only the Zel'dovich NOx route. The computations also suggest, however, that the stoichiometry of the premixed autoignition zone in the laminar gas phase surrounding a spherical droplet differs from that observed in turbulent diesel spray ignition. In single droplets, irrespective of the fuel used, autoignition always initiates in the relatively hot lean region far from the droplet. In diesel sprays, depending on the thermodynamic conditions and fuel reactivity, ignition can occur in lean or rich regions by virtue of turbulent transport of heat and mass. In large molecular weight fuels like n-heptane or petroleum diesel fuel, this is often in mixtures which are quite rich (Φ ≈ 3). To underscore the difference between turbulent spray ignition and ignition of a single droplet, the most reactive mixture fraction and critical scalar dissipation rate were derived for the case of turbulent ignition The results show that for a turbulent non-premixed flame to ignite, two requirements must be met: (1) the fuel/air mixture fraction must be equal or similar to the most reactive mixture fraction, (2) the local scalar dissipation rate must be less than the critical scalar dissipation rate. Due to the effect of scalar dissipation rate on transport and mixing in turbulent, non-premixed flames, it is concluded that, at least as far as autoignition is concerned, autoignition of spherically-symmetric isolated fuel droplets has limitations as physical model for ignition of diesel sprays in compression ignition engines. However, the computations clearly show that transient NOx formation in presence of thermal and concentration gradients cannot be adequately described by the Zel'dovich NOx mechanism, which has consequences with regards to capability of computational engine simulations to accurately predict NOx formation.Item Open Access Total and differential sputter yields of boron nitride(Colorado State University. Libraries, 2011) Topper, James Leo IV, author; Yalin, Azer, advisor; Williams, John, committee member; Menoni, Carmen, committee memberA quartz crystal microbalance (QCM) based system was used to measure total and differential sputter yields of Boron Nitride targets due to Xenon ion bombardment. The system used a four-grid ion source to generate a (nearly) mono-energetic beam of ions with a low singles-to-doubles ratio that remains well-collimated at low energies. Boron Nitride sputter yields were measured as a function of Xenon ion energy and incidence angle. Total yield measurements are found by integrating differential yield data taken by the QCM. The measurement system was validated by sputtering Molybdenum, for which the measured yields compared well with published data. For Boron Nitride, complementary weight loss sputter yield measurements were conducted, but were found to be less accurate due to moisture absorption effects when samples were exposed to atmosphere. The effects of neutralization of both the ion beam and target surface were investigated, and a plasma bridge neutralizer (PBN) was used to prevent surface charge buildup on the target. The measured total sputter yields were found to vary strongly due to neutralization conditions, and were found to be approximately a factor of two higher than those available in the literature when aggressive target and beam neutralization was applied. The angular shape of the differential sputter yield profiles was described with fit parameters from the Modified Zhang equation.