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| Author | : Jonathan Teng-Chieh Liu |
| Publisher | : |
| Total Pages | : 121 |
| Release | : 2004 |
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| Author | : Gordon S. Humphries |
| Publisher | : |
| Total Pages | : 0 |
| Release | : 2017 |
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In-situ optical techniques offer one of the most attractive options for measuring species concentration and spatial distribution profiles in reacting environments, such as flames. The generally non-intrusive nature and spatial resolution of these techniques are now preferred over on extractive sampling, followed by analysis using techniques such as gas chromatography. In this thesis two laser absorption measurement techniques are applied to measure the soot distribution, and acetylene concentration profiles in a flat-flame burner. The in-situ measurement of the distribution of particulate matter in flames is a key step in understanding the mechanism of its formation. Most in-situ measurement systems for this purpose are based on laser induced incandescence where particles are heated using high power laser sources and the increased incandescence emission of the soot particles is detected. However as the soot cools by heat transfer to the surrounding gas, following laser heating, the pressure of the gas is increased creating an acoustic effect. Photoacoustic detection has been applied to quantify low concentrations of particulate matter in ambient air but there have been few applications of photoacoustic detection to the in-situ measurement of particulate matter formation in combustion processes. A novel simple approach using a modulated continuous wave diode laser is presented in this thesis. The measurements taken using this new technique are compared to measurements of the visible emission from the flame, and previous soot distribution measurements using laser induced incandescence. Absorption spectroscopy using near-infrared tunable diode lasers has been applied to measure species in several harsh environments such as aero-engine exhaust plumes, flames, and other industrial processes. Simple single pass absorption techniques are not always suitable for this purpose due to the low absorption of the target species, either due to low concentration or weak absorption line-strength at high temperatures. One method to increase the sensitivity of such techniques is by using cavity enhanced methods which increase the effective path length of the laser through the absorbing medium. One such cavity enhanced method is Cavity Ring-Down Spectroscopy (CRDS). CRDS uses a cavity constructed of highly reflecting mirrors, laser light is then coupled into this cavity and absorption measurements can be evaluated from the decay rate of light from the cavity. The design and, novel application of continuous wave CRDS to measure the concentration profile of acetylene in the flat-flame burner is presented. Difficulties in deriving an absolute acetylene concentration from the measured ringdown times were encountered due to the large number of interfering features. Serveral fitting and extraction techniques are applied and compared to attempt to overcome these difficulties.
| Author | : James R. P. Bain |
| Publisher | : |
| Total Pages | : 0 |
| Release | : 2012 |
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Tunable diode laser spectroscopy is a widely used technique for recovering quantitative gas information in a range of industrial applications. Established methods often use readily available, robust and low cost optoelectronic hardware in the near-infrared, with output wavelengths that coincide with the absorption spectra of several important gas species of interest, providing a versatile platform for gas analysis instrumentation. In this work the challenges associated with the recovery of gas information from harsh detection environments, particularly for aero engine diagnostics, are considered. For stand-alone instrumentation, calibration-free direct absorption measurements are highly advantageous yet calibrated techniques employing wavelength modulation spectroscopy are often favoured due to their significantly higher sensitivities. Recent developments have enabled calibration-free line shape recovery using lock-in amplifier detection of the residual amplitude modulation in wavelength modulated signals. These techniques have significant potential in harsh environments, but the overall sensitivity is limited by distortions to the recovered line shapes at high modulation amplitudes and by large background signals that saturate the detection electronics. In this thesis, solutions to these two problems are proposed, investigated and validated. A correction function is derived that is able to account for line shape distortions at arbitrarily high modulation indices. Application of the function depends upon knowledge of the experimental modulation index and two methods for extracting this information directly from the experimental signals are described. The full correction procedure has been experimentally validated. An investigation was made into the use of autobalanced photoreceivers, typically used for common mode noise cancellation, for direct absorption measurements and in a different configuration for nulling of the residual amplitude modulation (RAM) in wavelength modulation spectroscopy. Initial measurements suggest that removal of the background RAM can increase the lock-in detection sensitivity by over an order of magnitude. In addition an external amplitude modulator has been iv shown to be an effective method for producing sensitive absorption signals that are free of distortions, recoverable at frequencies that are outside the bandwidth of most environmental noise sources. A temperature sensor based on ratio thermometry of ambient water vapour absorption was designed and evaluated. The sensor is intended to provide accurate intake gas temperature information during aero engine ground testing when misting conditions prevent standard thermocouples from providing reliable data. Direct detection and second harmonic wavelength modulation spectroscopy experiments were undertaken in an environmental chamber, over the range 273-313K, to test the potential accuracy of the proposed system. Using a second harmonic peak height method, temperature information based on a calibration was able to recover temperature measurements with precision of ±0.4K however the overall accuracy suffered from a problematic calibration drift. Three engine test campaigns are described in which a range of recovery methods and potential optical system layouts are evaluated for the purposes of intake and exhaust mounted test bed sensor systems. The effects of extreme noise conditions were observed on a variety of measurements and favourable detection and modulation options were identified for the purpose of planning proposed future engine tests. Exhaust plume measurements of high temperature water vapour on the Rolls-Royce Environmentally Friendly Engine demonstrator established the viability of temperature and concentration measurements up to 850K.
| Author | : |
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| Total Pages | : 13 |
| Release | : 2005 |
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Tunable diode laser absorption measurements of gas temperature and water concentration were made at the exit of a model scramjet combustor fueled on JP-7. Multiplexed, fiber-coupled, near-infrared distributed feedback lasers were used to probe three water vapor absorption features in the 1.34 to 1.47 mum spectral region (2v1 and v1 + v3 overtone bands). Ratio thermometry was performed using direct absorption wavelength scans of isolated features at a 4-kHz repetition rate, as well as 2f wavelength modulation scans at a 2-kHz scan rate. Large signal-to-noise ratios demonstrate the ability of the optimally engineered optical hardware to reject beam steering and vibration noise. Successful measurements were made at full combustion conditions for a variety of fuel/air equivalence ratios and at eight vertical positions in the duct to investigate spatial uniformity. The use of three water vapor absorption features allowed for preliminary estimates of temperature distributions along the line of sight. The improved signal quality afforded by 2f measurements, in the case of weak absorption, demonstrates the utility of a scanned wavelength modulation strategy.
| Author | : Aamir Farooq |
| Publisher | : |
| Total Pages | : |
| Release | : 2010 |
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The development of diagnostics based on laser-absorption spectroscopy for combustion applications has been an important and active field of research over the past two decades due to the advantages of this non-intrusive optical sensing technique compared to traditional sampling-based sensing methods. Tunable diode laser (TDL) sensors, in particular, have shown the ability to provide in situ, time-resolved, line-of-sight measurements of temperature, gas species concentration, velocity, density, mass flux, and pressure in a variety of combustion environments. This thesis explores three new areas of TDL research: (a) extended near-infrared (NIR) diagnostics, (b) sensing under high-pressures, and (c) applications to chemical kinetics. Water vapor (H2O) and carbon dioxide (CO2) are attractive sensing targets for hydrocarbon-fueled systems as they are primary combustion products and their concentrations can be interpretrated to indicate combustion progress and efficiency. Both these gases have absorption spectra in the infrared (IR) region. Most previous TDL absorption sensors were designed to exploit robust telecommunications diode lasers and optical fiber technology in the 1.3-1.6 [mu]m (NIR) wavelength region. Recent developments in semiconductor diode-laser technology have extended the range of continuous wave (CW) room-temperature single-mode diode lasers to 2.9 [mu]m, allowing access to stronger vibrational bands of H2O and CO2 in the extended-NIR region. The first combustion diagnostics in the extended-NIR wavelength were demonstrated as part of this thesis work. The sensors were designed by selecting optimal transitions and then measuring the pertinent spectroscopic parameters in controlled laboratory environements. These sensors were then tested in the combustion environments of a flat flame and shock tube to validate their performance. These new sensors provide enhanced sensitivity and improved accuracy compared to previous TDL diagnostics. As part of this work, a novel diagnostic based on wavelength modulation spectroscopy (WMS) of CO2 was developed to make precise measurements of temperature behind reflected shock waves. This temperature diagnostic achieved an unprecedented uncertainty of
| Author | : Alan C. Eckbreth |
| Publisher | : CRC Press |
| Total Pages | : 630 |
| Release | : 2022-01-27 |
| Genre | : Technology & Engineering |
| ISBN | : 1000124622 |
This book examines the variety of potential laser diagnostic techniques and presents a considerable theoretical foundation elucidating physics relevant to the laser diagnostics. It explains the Raman-based approaches for major species and temperature measurements.
| Author | : Yufei Ma |
| Publisher | : MDPI |
| Total Pages | : 278 |
| Release | : 2020-03-05 |
| Genre | : Technology & Engineering |
| ISBN | : 3039283987 |
Trace gas sensing technologies are widely used in many applications, such as environmental monitoring, life science, medical diagnostics, and planetary exploration. On the one hand, laser sources have developed greatly due to the rapid development of laser media and laser techniques in recent years. Some novel lasers such as solid-state, diode, and quantum cascade lasers have experienced significant progress. At present, laser wavelengths can cover the range from ultraviolet to terahertz, which could promote the development of laser gas sensing technologies significantly. On the other hand, some new gas sensing methods have appeared, such as photothermal spectroscopy and photoacoustic spectroscopy. Laser spectroscopy-based gas sensing techniques have the advantages of high sensitivity, non-invasiveness, and allowing in situ, real-time observation. Due to the rapid and recent developments in laser source as well as the great merits of laser spectroscopy-based gas sensing techniques, this book aims to provide an updated overview of the state-of-the-art laser gas sensing technologies.
| Author | : Wei Ren |
| Publisher | : |
| Total Pages | : |
| Release | : 2013 |
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Biofuels are classified as renewable because the carbon present in the vegetable oil or animal fat feedstocks originates from carbon dioxide already present in the atmosphere. One of the current focuses on biofuel-based combustion research is the design of advanced energy conversion devices using complex reaction mechanisms. The development of these mechanisms requires a large experimental database to ensure accuracy of computational predictions. Infrared laser-absorption diagnostics are widely used in combustion research for fast, sensitive, and non-intrusive measurements of species concentration, temperature, and pressure. This thesis explores three new areas of laser diagnostic research: (a) mid-infrared diagnostics, (b) sensing in multiphase flows, and (c) applications to shock tube chemical kinetics. A novel distributed-feedback quantum-cascade laser (DFB-QCL) near 4.7 um was investigated to develop a new mid-infrared absorption sensor for in situ measurements of carbon monoxide (CO) and temperature in combustion gases. The laser provides convenient access to the stronger vibrational bands of CO than was possible previously, enabling ppm-level detectivity with an optical path length of 10 cm at high temperatures between 1000-2000 K. Wavelength modulation spectroscopy with 1f-normalized 2f detection (WMS-2f/1f) of CO2 was developed for accurate temperature sensing in multiphase combustion flows. In this method, two tunable diode lasers with wavelengths near 2.7 um were used to measure time-varying gas temperature during the evaporation of shock-heated fuel aerosols. These recently developed mid-IR laser absorption diagnostics were then applied in studying the thermal decomposition of oxygenates (biofuel surrogates) by measuring species concentration time-histories behind reflected shock waves. In a particular study of methyl formate (the simplest biodiesel surrogate), the reaction rate constants of methyl formate unimolecular decomposition were measured using the mid-IR CO absorption behind reflected shock waves. Detailed comparisons of the measured methanol and CO time-histories with the model predictions were made. Sensitivity and reaction pathway analyses for these oxygenated fuel components were performed, leading to rate recommendations with improved model performance.
| Author | : |
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| Total Pages | : 72 |
| Release | : 2007 |
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Advanced optical diagnostic techniques relevant to propulsion were investigated. The techniques studied were based on laser spectroscopy, with emphasis on spectrally-resolved absorption and laser-induced fluorescence (LIF). Laser sources included tunable cw near-infrared diode lasers and tunable (or fixed-wavelength) pulsed lasers operated at ultraviolet (UV) or infrared (IR) wavelengths. The cw lasers were spectrally narrow, allowing study of innovative diagnostics based on spectral lineshapes, while the pulsed lasers provided intense bursts of photons needed for techniques based on LIF. Accomplishments of note included: (1) development of a new imaging diagnostic based on infrared planar laser-induced fluorescence (IR PLIF), (2) investigations of quantitative ultraviolet (UV) PLIF of NO and CO2 in high-pressure combustion environments, (3) the development of a new temperature diagnostic using UV absorption of CO2 for high-temperature combustion environments, (4) development of advanced wavelength-multiplexed diode laser absorption sensing of non-uniform temperature distributions, gas temperature in scramjet flows, and tunable mid-IR-based fuel sensing, and (5) further development of quantitative tracers to image fuel distribution using ketones and the aromatic toluene. The full spectrum of results was published in thirty-eight papers in the AIAA and peer reviewed literature, seven PhD theses, and forty-three presentations and invited lectures.
| Author | : Zachary Emanuel Loparo |
| Publisher | : |
| Total Pages | : 122 |
| Release | : 2020 |
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| ISBN | : |
High-temperature reacting systems are central to many fields including propulsion, power generation, and transportation. Studying such systems requires the use of experimental facilities such as shock tubes to obtain the relevant high temperature conditions, and non-intrusive diagnostic tools for monitoring parameters of interest in the reaction zone, including temperature and species concentration time-histories. Laser absorption spectroscopy offers high-speed, in-situ measurements of the reacting flow field and provides direct measurements of species concentrations and temperature. Applying absorption spectroscopy via different diagnostic strategies enables tailored measurements of these parameters across a variety of combustion systems for chemical kinetic model refinement and enhancing the fundamental understanding of combustion over a broader range of conditions, ultimately aiding in the development of more efficient and lower-emissions fuels and engines.
