A fiber-coupled, compact, remotely operated laser absorption instrument is developed for CO, CO2, and H2O measurements in reactive flows at the elevated temperatures and pressures expected in gas turbine combustor test rigs with target pressures from 1–25 bar and temperatures of up to 2000 K. The optical engineering for solutions of the significant challenges from the ambient acoustic noise (~120 dB) and ambient test rig temperatures (60 °C) are discussed in detail. The sensor delivers wavelength-multiplexed light in a single optical fiber from a set of solid-state lasers ranging from diodes in the near-infrared (~1300 nm) to quantum cascade lasers in the mid-infrared (~4900 nm). Wavelength-multiplexing systems using a single optical fiber have not previously spanned such a wide range of laser wavelengths. Gas temperature is inferred from the ratio of two water vapor transitions. Here, the design of the sensor, the optical engineering required for simultaneous fiber delivery of a wide range of laser wavelengths on a single optical line-of-sight, the engineering required for sensor survival in the harsh ambient environment, and laboratory testing of sensor performance in the exhaust gas of a flat flame burner are presented.

This paper presents a laser-based absorption technique for measuring temperature and CO concentration in high-pressure shock tubes. Two fundamental vibrations of CO (v" = 0, P8, 4.73 µm and v" = 1, R21, 4.56 µm) were selected for high-temperature sensitivity with a reduced influence from pressure broadening compared to previous work. Single-pass absorption (80 mm path length) was measured with two quantum-cascade lasers. The technique was demonstrated by measuring time-resolved temperature for non-reactive mixtures at 1100–1960 K and 1.2–9.7 bar. During partial oxidation of n-heptane, temperature and CO concentrations were measured with 4 µs time resolution at 1360–1670 K and 5.8–8.2 bar. Interference from broadband CO2 absorption was quantified and subtracted. Measured data in the burnout state are in excellent agreement with predictions from kinetics mechanisms (Mehl et al. Proc Combust Inst 33:193, 2011; Zhang et al. Combust Flame 172:116, 2016) over the entire range of operating conditions, which validates the performance of the current laser-absorption technique in reactive-mixture measurements. Additionally, time-resolved temperature and CO-concentration measurements agree well with predictions based on the Mehl et al. mechanism.

A novel single-ended mid-infrared laser-absorption sensor for time-resolved measurements of water mole fraction and temperature was developed and deployed within the annulus of a hydrogen/air-fed rotating detonation engine (RDE). The sensor transmitted two laser beams targeting mid-infrared water transitions through a single optical port on the outer wall of the cylindrical RDE annulus and measured the backscattered radiation from the RDE inner surface using a photodetector for a round-trip path of 1.52 cm. Optimizing the sensor's optical arrangement using numerical ray tracing to minimize interference from optical emission, beam steering, and scattered laser light from window surfaces was essential to sensor performance. Scanned-wavelength-modulation spectroscopy with second-harmonic detection and first-harmonic normalization was implemented to allow for frequency-domain multiplexing of the two lasers and to suppress non-absorbing interference sources such as beam-steering and emission. Tunable diode lasers near 2551 and 2482 nm were modulated at 100 and 122 kHz, respectively, and sinusoidally scanned across the peaks of their respective water transitions at 10 kHz to provide a measurement rate of 20 kHz and detection limit of 0.5% water by mole. Experimentally derived spectroscopic parameters enabled water and temperature sensing with respective uncertainties of 7.3% and 5.3% relative to the measured values. Time-resolved and time-averaged sensor measurements of gas temperature and water vapor mole fraction allow quantitative evaluation of the combustion progress at the measurement location and thus provide a design tool for RDE optimization. Broadly, this single-ended laser sensor should find applications in other combustion systems where optical access is limited. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

A laser-based sensor was developed and deployed for in situ monitoring of the gas temperature in the plenum after the arc heater and before the expansion nozzle in the 60 MW interaction heating facility at NASA Ames Research Center. Early measurements suggested the cold air added to the arc-heated gas to control enthalpy was not well mixed, and modifications were made to add a mixing volume in the plenum between adding air and the expansion. Measurements of the gas temperature conducted before and after this modification are presented. An external cavity diode laser was wavelength scanned to monitor the population of electronically excited (3s) oxygen atoms via absorption by the P-5(3)-S-5(2 degrees) transition near 777.2 nm. Assuming thermochemical equilibrium, the population of electronically excited 3s atoms provided a sensitive measurement of the gas temperature in the 4000-8000 K range expected in the arc-heater plenum. Time-resolved measurements were obtained on multiple lines of sight, and differences between the inferred temperatures for the different lines of sight revealed important details regarding the degree of flowfield uniformity in the arcjet plenum.

Research to demonstrate in situ laser-absorption-based sensing of H2O, CH4, CO2, and CO mole fraction is reported for the product gas line of a biomass gasifier. Spectral simulations were used to select candidate sensor wavelengths that optimize sensitive monitoring of the target species while minimizing interference from other species in the gas stream. A prototype sensor was constructed and measurements performed in the laboratory at Stanford to validate performance. Field measurements then were demonstrated in a pilot scale biomass gasifier at West Biofuels in Woodland, CA. The performance of a prototype sensor was compared for two sensor strategies: wavelength-scanned direct absorption (DA) and wavelength- scanned wavelength modulation spectroscopy (WMS). The lasers used had markedly different wavelength tuning response to injection current, and modern distributed feedback lasers (DFB) with nearly linear tuning response to injection current were shown to be superior, leading to guidelines for laser selection for sensor fabrication. Non- absorption loss in the transmitted laser intensity from particulate scattering and window fouling encouraged the use of normalized WMS measurement schemes. The complications of using normalized WMS for relatively large values of absorbance and its mitigation are discussed. A method for reducing adverse sensor performance effects of atime-varying WMS background signal is also presented. The laser absorption sensor provided measurements with the sub-second time resolution needed for gasifier control and more importantly provided precise measurements of H2O in the gasification products, which can be problematic for the typical gas chromatography sensors used by industry.

Infrared laser-absorption spectroscopy (IR-LAS) sensors play an important role in diagnosing and characterizing a wide range of combustion systems. Of all the laser-diagnostic techniques, LAS is arguably the most versatile and quantitative, as it has been used extensively to provide quantitative, species-specific measurements of gas temperature, pressure, composition and velocity in both laboratory- and industrial-scale systems. Historically, most IR-LAS work has been conducted using tunable diode lasers; however, todays researchers have access to a wide range of light sources that provide unique sensing capabilities and convenient access to nearly the entire IR spectrum (approximate to 0.8 to 16 mu m). In particular, the advent of room-temperature wavelength-tunable mid-infrared semiconductor lasers (e.g., interband- and quantum-cascade lasers) and hyperspectral light sources (e.g., MEMS VCSELs, Fourier-domain mode-locked lasers, dispersed supercontinuum, and frequency combs) has provided a number of unique capabilities that combustion researchers have exploited. The primary goals of this review paper are: (1) to document the recent development, application, and current capabilities of IR-LAS sensors for laboratory- and industrial-scale combustors and propulsion systems, (2) to elucidate the design and use of IR-LAS sensors for combustion gases through a discussion of the modern sensor-design process and state-of-the-art techniques, and (3) to highlight some of the remaining measurement opportunities, challenges, and needs. A thorough review and description of the fundamental spectroscopy governing the accuracy of such sensors, and recent findings and databases that enable improved modeling of molecular absorption spectra will also be provided.

In this study we report a novel UV laser absorption diagnostic for sensitive and time-resolved measurement of methyl radicals (CH3) in shock tube kinetics studies, utilizing a frequency-quadrupled, high-repetition-rate (78 MHz) ps-pulsed Ti-sapphire laser targeted at the strong rovibronic transition of CH3 at 216.62 nm. This diagnostic employs a cavity-enhanced absorption spectroscopy (CEAS) strategy that amplifies the absorbance signal by a gain factor of about 50, which enables sub-ppm detection limit of CH3 to be achieved under typical shock conditions of 1200 K < T < 2500 K and P ∼ 1 atm, in a shock tube of ∼15 cm inner diameter. The use of a pulsed laser also enables an on-axis CEAS configuration, which yields a time resolution of 2–4 µs. For example, CH3 measurements were performed in shock tube decomposition experiments of dilute ethane and methane mixtures, demonstrating the substantial improvement in detection sensitivity of the current diagnostic compared to conventional single-pass absorption measurements. This new diagnostic promises to be a powerful tool for future shock tube chemical kinetics studies, and can be potentially extended to the detection of other UV-absorbing species.

A mid-infrared quantum cascade laser absorption sensor was developed for in-situ detection of NO2 in high-temperature gas environments. A cluster of spin-split transitions near 1599.9cm−1 from the ν3 absorption band of NO2 was selected due to the strength of these transitions and the low spectral interference from water vapor within this region. Temperature- and species-dependent collisional broadening parameters of ten neighboring NO2 transitions with Ar, O2, N2, CO2 and H2O were measured and reported. The spectral model was validated through comparisons with direct absorption spectroscopy measurements of NO2 seeded in various bath gases. The performance of the scanned wavelength modulation spectroscopy (WMS)-based sensor was demonstrated in a combustion exhaust stream seeded with varying flow rates of NO2, achieving reliable detection of 1.45 and 1.6ppm NO2 by mole at 600K and 800K, respectively, with a measurement uncertainty of ±11%. 2σ noise levels of 360ppb and 760ppb were observed at 600K and 800K, respectively, in an absorption path length of 1.79m.

A dual-wavelength diode laser-based absorption sensor for standoff point measurements of water film thickness on an opaque surface is presented. The sensor consists of a diode laser source, a foil as backscattering target, and off-axis paraboloids for collecting the fraction of the laser radiation transmitted through the liquid layer via retro-reflection. Laser wavelengths in the near infrared at 1412 and 1353 nm are used where the temperature dependence of the liquid water absorption cross section is known. The lasers are fiber coupled and the detection of the retro-reflected light was accomplished through a multimode fiber and a single photodiode using time-division multiplexing. The water film thickness at a given temperature was determined from measured transmittance ratios at the two laser wavelengths. The sensor concept was first validated with measurement using a temperature-controlled calibration cell providing liquid layers of variable and known thickness between 100 and 1000 µm. Subsequently, the sensor was demonstrated successfully during recording the time-varying thickness of evaporating water films at fixed temperatures. The film thickness was recorded as a function of time at three temperatures down to 50 µm.

We report the first application of cavity-enhanced absorption spectroscopy (CEAS) with a ps-pulsed UV laser for sensitive and rapid gaseous species time-history measurements in a transient environment (in this study, a shock tube). The broadband nature of the ps pulses enabled instantaneous coupling of the laser beam into roughly a thousand cavity modes, which grants excellent immunity to laser-cavity coupling noise in environments with heavy vibrations, even with an on-axis alignment. In this proof-of-concept experiment, we demonstrated an absorption gain of 49, which improved the minimum detectable absorbance by ~20 compared to the conventional single-pass strategy at similar experimental conditions. For absorption measurements behind reflected shock waves, an effective time-resolution of ~2 μs was achieved, which enabled time-resolved observations of transient phenomena, such as the vibrational relaxation of O(2) demonstrated here. The substantial improvement in detection sensitivity, together with microsecond measurement resolution implies excellent potential for studies of transient physical and chemical processes in nonequilibrium situations, particularly via measurements of weak absorptions of trace species in dilute reactive systems.

We report measured line intensities and temperature-dependent broadening coefficients of NH3 with Ar, N2, O2, CO2, H2O, and NH3 for nine sQ(J,K) transitions in the ν2 fundamental band in the frequency range 961.5-967.5 cm-1. This spectral region was chosen due to the strong NH3 absorption strength and lack of spectral interference from H2O and CO2 for laser-based sensing applications. Spectroscopic parameters were determined by multi-line fitting using Voigt lineshapes of absorption spectra measured with two quantum cascade lasers in thermodynamically-controlled optical cells. The temperature dependence of broadening was measured over a range of temperatures between 300 and 600 K. These measurements aid the development of mid-infrared NH3 sensors for a broad range of gas mixtures and at elevated temperatures.

A multi-wavelength diode laser absorption sensor was investigated for real-time monitoring of the thickness, solute concentration and temperature of thin films of liquid solutions, and the concept was demonstrated with aqueous NaCl solutions. The sensor monitors the transmittance of laser intensity of four near-infrared diode lasers through a thin liquid film deposited on a quartz plate. The variation of the absorption spectrum of the solution with temperature and solute concentration was used to select wavelengths for determining film thickness, NaCl concentration and liquid temperature from ratios of the transmission measurements. The spectral database needed to select laser wavelengths was measured by a Fourier transform infrared spectrometer in the near infrared (5500–8000 cm−1) for NaCl solutions between 5 and 26 wt% and temperatures between 298 and 338 K. A prototype sensor was constructed, and the sensor concept was validated with measurements using a calibration cell consisting of two parallel quartz plates providing liquid layers of variable thickness (200–1500 μm), NaCl concentration (5–15 wt%) and temperature (293–318 K). The sensor was demonstrated for real-time constant temperature evaporation of an evaporating liquid film deposited on a heated quartz window.

The development and demonstration of a four-color single-ended mid-infrared tunable laser-absorption sensor for simultaneous measurements of H2O, CO2, CO, and temperature in combustion flows is described. This sensor operates by transmitting laser light through a single optical port and measuring the backscattered radiation from within the combustion device. Scanned-wavelength-modulation spectroscopy with second-harmonic detection and first-harmonic normalization (scanned-WMS-2f/1f) was used to account for variable signal collection and nonabsorption losses in the harsh environment. Two tunable diode lasers operating near 2551 and 2482 nm were utilized to measure H2O concentration and temperature, while an interband cascade laser near 4176 nm and a quantum cascade laser near 4865 nm were used for measuring CO2 and CO, respectively. The lasers were modulated at either 90 or 112 kHz and scanned across the peaks of their respective absorption features at 1 kHz, leading to a measurement rate of 2 kHz. A hybrid demultiplexing strategy involving both spectral filtering and frequency-domain demodulation was used to decouple the backscattered radiation into its constituent signals. Demonstration measurements were made in the exhaust of a laboratory-scale laminar methane-air flat-flame burner at atmospheric pressure and equivalence ratios ranging from 0.7 to 1.2. A stainless steel reflective plate was placed 0.78 cm away from the sensor head within the combustion exhaust, leading to a total absorption path length of 1.56 cm. Detection limits of 1.4% H2O, 0.6% CO2, and 0.4% CO by mole were reported. To the best of the authors' knowledge, this work represents the first demonstration of a mid-infrared laser-absorption sensor using a single-ended architecture in combustion flows.

A dual-wavelength diode laser absorption spectroscopy (DLAS)-based sensor makes use of a retro-reflective foil as scattering target and time-division multiplexing (TDM) to enable remote measurements of the water film thickness in the 50-1000 μm range.

A novel quantum cascade laser (QCL) absorption sensor is presented for high-sensitivity in situ measurements of ammonia (\(\hbox {NH}_3\)) in high-temperature environments, using scanned wavelength modulation spectroscopy (WMS) with first-harmonic-normalized second-harmonic detection (scanned WMS-2f/1f) to neutralize the effect of non-absorption losses in the harsh environment. The sensor utilized the sQ(9,9) transition of the fundamental symmetric stretch band of \(\hbox {NH}_3\) at \(10.39\,{\upmu }\hbox {m}\) and was sinusoidally modulated at 10 kHz and scanned across the peak of the absorption feature at 50 Hz, leading to a detection bandwidth of 100 Hz. A novel technique was used to select an optimal WMS modulation depth parameter that reduced the sensor’s sensitivity to spectral interference from \(\hbox {H}_2\hbox {O}\) and \(\hbox {CO}_2\) without significantly sacrificing signal-to-noise ratio. The sensor performance was validated by measuring known concentrations of \(\hbox {NH}_3\) in a flowing gas cell. The sensor was then demonstrated in a laboratory-scale methane-air burner seeded with \(\hbox {NH}_3\), achieving a demonstrated detection limit of 2.8 ± 0.26 ppm \(\hbox {NH}_3\) by mole at a path length of 179 cm, equivalence ratio of 0.6, pressure of 1 atm, and temperatures of up to 600 K.

Detonation-based engines represent a challenging application for diagnostics due to the wide range of thermodynamic conditions involved (T 500-3000 K, P 2-60 atm) and the short time scales of change ( 10− 6 to 10− 4 sec) associated with such systems. Non-intrusive laser absorption diagnostics can provide high time-resolution and have been employed extensively in shock tube kinetics experiments (P 1-20 atm), offering high potential for application in detonation environments with modest utilization to date [1-4]. Limiting factors in designing effective tunable laser absorption sensors for detonation engines can be divided into two sets of challenges: high-pressure, high-temperature absorption spectroscopy and harsh thermo-mechanical environments. The present work, conducted in a high-pressure shock tube and operating detonation combustor, addresses both sets of difficulties, with the objective of developing time-resolved, in-situ temperature and concentration sensors for detonation studies.

A high-bandwidth absorption sensing technique for iso-butene (iC4H8) was developed to measure transient species concentration behind reflected shock waves for combustion kinetics studies. Direct measurements of iC4H8 were enabled by monitoring absorption in the infrared near 11.3μm using a novel pulsed external-cavity quantum cascade laser (ECQCL) with a repetition rate of 600kHz. Optimal wavelength selection for high-temperature combustion gases was first determined by a spectral survey at 1000K near the peak of the absorption band (878cm−1 to 892cm−1) using the ECQCL. Absorption cross section measurements of iC4H8 at 881.4cm−1, the selected high-temperature wavelength, were then conducted from 800K to 1800K behind reflected shocks to characterize temperature dependence at modest pressures (4–8atm). The species-specific technique was subsequently demonstrated by time-resolved (100kHz) measurements of iso-butene decay during thermal decomposition (1280–1480K). First-ever shock tube measurements of iC4H8 yields from iso-octane pyrolysis (1070–1300K) were also produced, with a detection limit of ∼100ppm. Experimental results were compared to recent kinetic models to illustrate the potential of this diagnostic for analyzing combustion chemistry.