Tomographic laser absorption spectroscopy using Tikhonov regularization

The application of tunable diode laser absorption spectroscopy (TDLAS) to flames with non-homogeneous temperature and concentration fields is an area where only few studies exist. Experimental work explores the performance of tomographic reconstructions of concentration and temperature profiles from wavelength-modulated TDLAS measurements within the plume of an axisymmetric McKenna burner. Water vapor transitions at 1391.67 nm and 1442.67 nm are probed using calibration free wavelength modulation spectroscopy with second harmonic detection (WMS-2f). A single collimated laser beam is swept parallel to the burner surface, where scans yield pairs of line-of-sight (LOS) data at multiple radial locations. Radial profiles of absorption data are reconstructed using Tikhonov regularized Abel inversion, which suppresses the amplification of experimental noise that is typically observed for reconstructions with high spatial resolution. Based on spectral data, temperatures and concentrations are calculated point-by-point. Here, a least-squares approach addresses difficulties due to modulation depths that cannot be universally optimized due to a non-uniform domain. Experimental results show successful reconstructions of temperature and concentration profiles based on two-transition, non-optimally modulated WMS-2f and Tikhonov regularized Abel inversion, and thus validate the technique as a viable diagnostic tool for flame measurements.Comment: This paper was published in Applied Optics and is made available as an electronic reprint with the permission of OSA. The paper can be found at the following URL on the OSA website: http://dx.doi.org/10.1364/AO.53.008095. Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under la

Industrial Applications of Tunable Diode Laser Absorption Spectroscopy

Tunable diode laser absorption spectroscopy (TDLAS) utilizes the absorption phenomena to measure the temperature and species concentration. The main features of the TDLAS technique are its fast response and high sensitivity. Extensive research has been performed on the utilization of diode laser absorption spectroscopy for the system monitoring and its control. The TDLAS technique gives self-calibrations to reduce the noise such as particles and dusts because the laser wavelength is rapidly modulated at kHz rates. In addition, two dimensional (2D) temperature and concentration distributions can be obtained by combining computed tomography (CT) with TDLAS. The TDLAS applications have been extensively studied with great progress. This chapter largely focuses on the engineering fields, especially the practical industrial applications

Tomographic imaging of combustion zones using tunable diode laser absorption spectroscopy (TDLAS)

This work concentrates on enabling the usage of a specific variant of tunable diode laser absorption spectroscopy (abbr. TDLAS) for tomogaphically reconstructing spatially varying temperature and concentrations of gases with as few reconstruction artifacts as possible. The specific variant of TDLAS used here is known as wavelength modulation with second harmonic detection (abbr. WMS-2f) which uses the wavelength dependent absorbance information of two different spectroscopic transitions to determine temperature and concentration values. Traditionally, WMS-2f has generally been applied to domains where temperature although unknown, was spatially largely invariant while concentration was constant and known to a reasonable approximation (_x0006_+/- 10% ). In case of unknown temperatures and concentrations with large variations in space such techniques do not hold good since TDLAS is a “line-of-sight” (LOS) technique. To alleviate this problem, computer tomographic methods were developed and used to convert LOS projection data measured using WMS-2f TDLAS into spatially resolved local measurements. These locally reconstructed measurements have been used to determine temperature and concentration of points inside the flame following a new temperature and concentration determination strategy for WMS-2f that was also developed for this work. Specifically, the vibrational transitions (in the 1.39 microns to 1.44 microns range) of water vapor (H2O) in an axi-symmetric laminar flame issuing from a standard flat flame burner (McKenna burner) was probed using telecom grade diode lasers. The temperature and concentration of water vapor inside this flame was reconstructed using axi-symmetric Abel de-convolution method. The two different sources of errors in Abel’s deconvolution - regularization errors and perturbation errors, were analyzed and strategies for their mitigation were discussed. Numerical studies also revealed the existence of a third kind of error - tomographic TDLAS artifact. For 2D tomography, studies showing the required number of views, number of rays per view, orientation of the view and the best possible algorithm were conducted. Finally, data from 1D tomography was extrapolated to 2D and reconstructions were benchmarked with the results of 1D tomography

Cost-Effective Quasi-Parallel Sensing Instrumentation for Industrial Chemical Species Tomography

Chemical Species Tomography (CST) has been widely applied for imaging of critical gas-phase parameters in industrial processes. To acquire high-fidelity images, CST is typically implemented by line-of-sight Wavelength Modulation Spectroscopy (WMS) measurements from multiple laser beams. The modulated transmission signal on each laser beam needs to be a) digitised by a high-speed analogue-to-digital converter (ADC); b) demodulated by a digital lock-in (DLI) module; and c) transferred to high-level processor for image reconstruction. Although a fully parallel data acquisition (DAQ) and signal processing system can achieve these functionalities with maximised temporal response, it leads to a highly complex, expensive and power-consuming instrumentation system with high potential for inconsistency between the sampled beams due to the electronics alone. In addition, the huge amount of spectral data sampled in parallel significantly burdens the communication process in industrial applications where in situ signal digitisation is distanced from the high-level data processing. To address these issues, a quasi-parallel sensing technique and electronic circuits were developed for industrial CST, in which the digitisation and demodulation of the multi-beam transmission signals are multiplexed over the high-frequency modulation within a wavelength scan. Our development not only maintains the temporal response of the fully parallel sensing scheme, but also facilitates the cost-effective implementation of industrial CST with very low complexity and reduced load on data transfer. The proposed technique is analytically proven, numerically examined by noise-contaminated CST simulations, and experimentally validated using a lab-scale CST system with 32 laser beams.Comment: Submitted to IEEE Transactions on Industrial Electronic

Development of a Single-Ended Laser-Absorption Spectroscopy Sensor for High-Temperature Gases

Tunable diode-laser-absorption spectroscopy (TDLAS) sensors have been one widely-used laser-diagnostic technique that offers great potential for non-intrusive, time-resolved and multi-parameter sensing in combustion systems. These sensors have been used for performance testing, model validation and feedback control of combustors [1–9]. During operation, monochromatic laser light with a specific wavelength is transmitted through the test gas and collected on a photodetector. Gas conditions such as temperature and composition are then inferred by comparing the measured amount of light that is absorbed with that predicted by spectroscopic models. In this thesis, the design and demonstration of a compact single-ended laser-absorption spectroscopy (SE-LAS) sensor for measuring temperature and H2O in high-temperature combustion gases is presented. The primary novelty of this work lies in the design, demonstration, and evaluation of a sensor architecture which uses a single lens to provide single-ended, alignment-free measurements of gas properties in a combustor without windows. The sensor is demonstrated to be capable of sustained operation at temperatures up to at least 625 K and is capable of withstanding direct exposure to high-temperature (≈1000 K) flame gases for long durations (at least 30 min) without compromising measurement quality. The sensor employs a fiber bundle and a 6-mm diameter AR-coated lens mounted in a 1/8” NPT-threaded stainless-steel body to collect laser light that is backscattered off native surfaces (e.g., a combustor wall). Distributed-feedback (DFB) tunable diode lasers (TDLs) with a wavelength near 1392 nm and 1343 nm were used to interrogate well characterized H2O absorption transitions using wavelength-modulation spectroscopy (WMS) techniques. The sensor is demonstrated with measurements of gas temperature and H2O mole fraction in a propane-air burner with a measurement bandwidth up to 25 kHz. In addition, this work presents an improved wavelength-modulation-spectroscopy spectral-fitting technique which reduces computational time by a factor of 100 compared to previously developed techniques

Environmental Application of High Sensitive Gas Sensors with Tunable Diode Laser Absorption Spectroscopy

Due to the fact of global warming, air quality deterioration and health concern over the past few decades, great demands and tremendous efforts for new technology to detect hazard gases such as CH4, CO2, CO, H2S, and HONO have been performed. Tunable diode laser absorption spectroscopy (TDLAS) is a kind of technology with advantages of high sensitivity, high selectivity, and fast responsivity. It has been widely used in the applications of greenhouse gas measurements, industrial process control, combustion gas measurements, medicine, and so on. In this chapter, we will briefly summarize the most recent progress on TDLAS technology and present several kinds of gas sensors developed mainly by our group for various field applications. These could expand from energy, environment, and public safety to medical science

Tunable diode-laser absorption-based sensors for the detection of water vapor concentration, film thickness and temperature

Temperature and species concentration are fundamental parameters in combustion-related systems. For optimizing the operation and minimizing the pollutant emissions of combustion devices and to provide validation data for simulations, quantitative measurement techniques of these parameters are required. Laser-based diagnostic techniques are an advantageous tool for in-situ non-intrusive measurement in combustion related systems, e.g. flame reactors, combustors, and shock tubes. Fiber-based multiplexed tunable diode laser absorption spectroscopy (TDLAS) is attractive and employed in this thesis because of compact, rugged packaging, low cost, reliability and relative ease of use. In the present work, water (H2O) is chosen as the target species for the technique, since it has a rich absorption spectrum in the vapor-phase and a broad-band absorption spectrum for the liquid-phase in the near infrared region (NIR). TDLAS two-line thermometry is used to determine the temperature in gas-phase systems with homogenous temperature distribution. However, in many practical environments, temperature varies along the beam path. For this case the temperature-binning technique is used for retrieving non-uniform temperature distributions from line-of-sight (LOS) absorption data with multiplexed five-color absorbance areas. In this thesis, TDLAS was applied to determine the spatially-resolved temperature information inside a low-pressure nanoparticle flame synthesis reactor. The temperature distribution was obtained by assuming the temperature to be constant in variable lengths along the LOS. The length fractions for the temperature values along the LOS are determined using postulated temperature bins. Quantitative knowledge of liquid film thickness is important in many industrial applications. One example is Diesel engine exhaust gas aftertreatment, where NOx reduction via selective catalytic reduction (SCR) is accomplished in the exhaust using sprays of water/urea solutions. In this thesis a novel TDLAS sensor was developed to simultaneously measure the water film thickness, film temperature and vapor-phase temperature above the film. For this sensor four individual NIR wavelengths were selected for optimized sensitivity of the technique. The sensor was first validated using a calibration tool providing known film thicknesses and temperature, and then applied to open liquid water films deposited on a transparent quartz plate. In a collaborative project the technique was also compared with imaging measurements based on laser-induced fluorescence and Raman scattering, respectively. Furthermore, the TDLAS sensor was applied to determine time series data of liquid water film thickness resulting from impinging water jets and subsequent film evaporation on the wall of a gas flow channel.Temperatur und Spezies-Konzentration sind elementare Kenngrößen in Verbrennungssystemen. Um den Betrieb von Verbrennungs- und Reaktionsprozessen zu optimieren, die Schadstoffemission zu minimieren und außerdem Validierungsdaten für Simulationen zu generieren, sind quantitative Messungen dieser Kenngrößen notwendig. Laserbasierte Diagnostik-Methoden sind nützliche Verfahren für die berührungslose in-situ Messung innerhalb von Verbrennungssystemen wie z.B. Brenner, Flammenreaktoren und Stoßwellenrohren. Absorptionsspektroskopie mit mehreren faserbasierten und abstimmbaren Laserdioden (tunable diode laser absorption spectroscopy, TDLAS) wurde in dieser Arbeit wegen des kompakten, robusten Aufbaus, der kostengünstigen Komponenten und der Zuverlässigkeit aufgrund der optischen Fasern verwendet. In der vorliegenden Arbeit wurde Wasser (H2O) als Untersuchungssubstanz für diese Methode ausgewählt, da es in zahlreichen technisch relevanten Prozessen, im nahen Infrarot-Bereich (NIR) in der Gasphase ein schmalbandiges und in der flüssigen Phase ein breitbandiges Absorptionsspektrum besitzt. Die TDLAS-zwei-Linien-Thermometrie wird zur Temperaturbestimmung in Verbrennungssystemen mit homogener Temperaturverteilung benutzt. In anwendungsnahen Systemen jedoch ändert sich die Temperatur entlang des Strahlweges. In diesem Fall ist ein Temperatur-binning-Verfahren nötig, um aus einer Absorptionsmessung entlang einer Sichtlinie auch auf ungleichförmige Temperaturverteilungen rückschließen zu können. In der vorliegenden Arbeit wurde TDLAS mit einer Kombination von fünf Wellenlängen eingesetzt, um räumlich aufgelöst Temperaturen innerhalb eines Niederdruck-Nanopartikel-Synthesereaktors zu bestimmen. Dabei wurden Temperaturen bestimmt, indem diese in variablen Längen entlang der Sichtlinie als konstant angesehen wurde. Die Längenanteile dieser Wegstrecken mit verschiedenen Temperaturen wurden für vordefinierte Temperaturbereiche bestimmt. Die quantitative Kenntnis der Filmdicke von flüssigen Filmen ist wichtig für zahlreiche industrielle Anwendungen, z.B. die NOx-Reduktion mittels einer Wasser/Harnstoff-Lösung in selektiv-katalytischer Reduktion (selective catalytic reduction, SCR) im Abgas von Dieselmotoren. In der vorliegenden Arbeit wurde ein neuartiger TDLAS-Sensor entwickelt, um gleichzeitig Filmdicke, Filmtemperatur und Wasserdampftemperatur oberhalb des Films zu messen. Die vier eingesetzten NIR-Wellenlängen wurden hierbei auf optimale Empfindlichkeit hin ausgewählt. Der Sensor wurde zuerst in einer Kalibrationszelle mit bekannter Filmdicke und Filmtemperatur validiert und dann an einem freien Film auf einer transparenten Quarzglas-Platte getestet. Zusätzlich wurde der TDLAS-Sensor verwendet, um zeitaufgelöst die Filmdicke während der Einspritzung- und Verdampfungsprozesse innerhalb eines Strömungskanals zu bestimmen