Determination of temperature and concentration of molecular nitrogen,oxygen and methane with coherent anti-stokes raman scattering

Temperature and concentration measurements by Coherent Anti-Stokes Raman Scattering (CARS) of molecular nitrogen, oxygen and methane were carried out. A comparison of corrected thermocouple and CARS temperature measurements in a high-temperature furnace up to 2000 K is presented. The temperature dependent CARS spectra of N₂ and O₂ are evaluated by a simulation program. Agreement between CARS and thermocouple temperatures is obtained within 40 K for N₂ and 80 K for O₂. Good agreement is found between measurements and calculations of the decrease of CARS intensity with temperature. Various quick-fit methods for N₂-and O₂-temperature measurements from temperature sensitive spectral parameters were tested. Temperature dependent CARS spectra of thev ₁-fundamental of methane are recorded and the methane CARS intensity as function of temperature is measured.     

Rotational CARS N2 thermometry: validation experiments for the influence of nitrogen spectral line broadening by hydrogen

Rotational coherent anti‐Stokes Raman spectroscopy (CARS) in fuel‐rich hydrocarbon flames, with a large content of hydrogen in the product gases (∼20%), has in previous work shown that evaluated temperatures are raised several tens of Kelvin by taking newly derived N₂ H₂ Raman line widths into account. To validate these results, in this work calibrated temperature measurements at around 300, 500 and 700 K were performed in a cell with binary gas mixtures of nitrogen and hydrogen. The temperature evaluation was made with respect to Raman line widths either from self‐broadened nitrogen only, N₂ N₂ [energy‐corrected‐sudden (ECS)], or by also taking nitrogen broadened by hydrogen, N₂ H₂ [Robert–Bonamy (RB)], Raman line widths into account. With increased amount of hydrogen in the cell at constant temperature, the evaluated CARS temperatures were clearly lowered with the use of Raman line widths from self‐broadened nitrogen only, and the case with inclusion of N₂ H₂ Raman line widths was more successful. The difference in evaluated temperatures between the two different sets increases approximately linearly, reaching 20 K (at T ∼ 300 K), 43 K (at T = 500 K) and 61 K (at T = 700 K) at the highest hydrogen concentration (90%). The results from this work further emphasize the importance of using adequate Raman line widths for accurate rotational CARS thermometry. Copyright © 2010 John Wiley & Sons, Ltd.     

Rotational CARS for simultaneous measurements of temperature and concentrations of N2, O2, CO, and CO2 demonstrated in a CO/air diffusion flame

Rotational coherent anti-Stokes Raman spectroscopy (CARS) has over the years demonstrated its strong potential to measure temperature and relative concentrations of major species in combustion. A recent work is the development and experimental validation of a CO₂ model for thermometry, in addition to our previous rotational CARS models for other molecules. In the present work, additional calibration measurements for relative CO₂/N₂ concentrations have been made in the temperature range 294-1246 K in standardized CO₂/N₂ mixtures. Following these calibration measurements, rotational CARS measurements were performed in a laminar CO/air diffusion flame stabilized on a Wolfhard-Parker burner. High-quality spectra were recorded from the fuel-rich region to the surrounding hot air in a lateral cross section of the flame. The spectra were evaluated to obtain simultaneous profiles of temperature and concentrations of all major species; N₂, O₂, CO, and CO₂. The potential for rotational CARS as a multi-species detection technique is discussed in relation to corresponding strategies for vibrational CARS.     

Narrow-band BoxCARS applied to CO2 laser discharges

Summary CARS (Coherent Anti-Stokes Raman Scattering) has developed into a powerful tool for studying molecular systems. One of its possibilities is to derive vibrational and rotational temperatures as well as concentrations of molecules from measurements of the energy level population differences. A very good spatial resolution of CARS technique is one of its important advantages. This feature has been utilized for making spatially resolved measurements of the vibrational and rotational temperatures of N₂ in a d.c.-excited transverse-flow CO₂ laser discharge. Apart from that also spectra of CO₂, CO and O₂ in the discharge have been taken, which allowed us to evaluate the spatial distributions of those components in the discharge. Additionally first investigations of a microwave-excited CO₂ laser module have been performed for comparison. Paper presented at the ?XI European CARS Workshop?, Florence, Italy, 23–25 March, 1992.     

Hydrogen CARS thermometry in a high-pressure H2–air flame. Test of H2 temperature accuracy and influence of line width by comparison with N2 CARS as reference

This work describes a further step towards the determination of the temperature accuracy of H₂ Q-branch CARS (Coherent Anti-Stokes Raman Scattering) at high pressure with regard to the influence of the H₂ line widths. In laminar steady H₂/air flames in the pressure range 1–15 bar and at fuel-rich conditions with stoichiometries between two and four, quasi-simultaneous temperature measurements were performed with H₂ and N₂ CARS. The temperature values deduced from H₂ CARS are in good agreement with the reference temperature from N₂ CARS. The influence of different line-width contributions on the accuracy of H₂ Q-branch thermometry was investigated in detail. Received: 10 April 2001 / Revised version: 22 May 2001 / Published online: 18 July 2001     

The application of CARS for temperature measurements in high pressure combustion systems

The determination of accurate temperatures from CARS N₂ Q-branch spectra in premixed flames is discussed for pressures up to 40 bar. The influence of collisional line narrowing in the CARS spectra is modelled by a MEG fitting law. It takes into account collisions of N₂ with CO₂ and H₂O. The analysis of the CARS data showed that the non-resonant background has an increasing influence on temperature with increasing pressure. Little influence on the quality of the fit between theory and experiment was found. Since there is a danger of residual systematic temperature deviations, which cannot be identified from the quality of the fit, spontaneous rotational Raman scattering is employed as an independent measuring technique.     

Study of processes of translational,rotational, and vibrational relaxation based on CARS spectra line shapes

This paper reviews the various physico-chemical processes responsible for actual linewidths encountered in high-resolution coherent anti-Stokes Raman spectroscopy (CARS). Most of the experimental data are based on linewidth measurements using a pulseamplified CARS spectrometer with an emission bandwidth (FWHM) of 2×10^–3 cm^–1. Detailed rotational and vibrational relaxation constants have been obtained from the analysis of theQ-branch profiles of C₂H₂, N₂, CH₄, and SiH₄.     

Time-domain measurements of S-branch N2–N2 Raman linewidths using picosecond pure rotational coherent anti-Stokes Raman spectroscopy

Time-resolved dual-broadband picosecond pure rotational CARS has been applied to measure self-broadened S-branch N₂–N₂ Raman linewidths in the temperature range 294–1466 K. The coherence decays were detected directly in the time domain by following the J-dependent CARS signal decay as a function of probe delay. The rotational Raman N₂–N₂ linewidths were derived from these time-dependent decays and evaluated for thermometric accuracy. Comparisons were made to the energy-corrected sudden (ECS) and modified exponential gap (MEG) dynamical scaling laws, and the results were used to quantify the sensitivity of nanosecond rotational CARS thermometry to the linewidth model employed. The uncertainty based on the linewidth model used in pure N₂ was found to be 2 %. The merits and limitations of this rapid method for the determination of accurate Raman linewidths are discussed.     

CARS temperature measurement in a LOX/CH4 spray flame

Coherent anti‐Stokes Raman scattering (CARS) spectroscopy has been used to investigate cryogenic liquid oxygen/gaseous methane (LOX/CH₄) flames on a medium‐size test facility at a pressure of 0.24 MPa and mass flow of 0.025 kg/s. Single‐shot, broadband CARS spectra with simultaneous detection of the Q‐branches of hydrogen and water molecules were recorded with good signal‐to‐noise ratio. Temperature was deduced from the H₂ and H₂O CARS profiles. The spatial temperature distribution in a comparatively harsh environment has been measured successfully. The measurements took place in the windowed combustion chamber of the DLR M3 test facility, aiming to provide data for validation of rocket combustor modeling. Copyright © 2010 John Wiley & Sons, Ltd.     

High temperature O2-CARS thermometry

O₂ temperature measurements at T=1910K have been performed by coherent anti-Stokes Raman scattering (CARS) inside a homogeneously heated gas volume of a tube furnace. The oxygen CARS spectrum can now be modeled accurately within the higher vibrational levels of the Q-branch manifold populated at flame temperatures using recently available spectroscopic data and collisional broadening coefficients.     

Laser-induced fluorescence determination of flame temperatures in comparison with CARS measurements

Temperature profiles in several premixed low pressure H₂/O₂/N₂ flames and in an atmospheric pressure CH₄/air flame were determined by laser-induced fluorescence (LIF) and by CARS experiments. In the LIF study, temperatures were derived from OH excitation spectra, CARS temperatures were deduced from N₂ Q-branch spectra. The present study is the first quantitative comparison of these two methods for temperature determination in flames burning at pressures up to 1 bar. The resulting temperatures showed good agreement.     

CARS thermometry in a transversely heated graphite-tube atomizer used in atomic absorption spectrometry

An experimental setup for gas-temperature diagnostics using Coherent Anti-Stokes Raman Scattering (CARS) in very small sample volumes with great spatial and temporal temperature variations is presented. Studies have been started of a newly designed transversely heated graphite-tube atomizer for atomic absorption spectrometry (Perkin-Elmer, 4100 ZL). For the investigations, high spatial resolution and precise time synchronization of the measurements and also automatic control of the time-dependent intensity of the anti-Stokes signal were realized. The graphite-tube atomizer also offers the possibility of high-temperature-gas spectroscopy. A high-temperature CARS spectrum of CO₂, recorded in the graphite tube at 2300 K, is shown for the first time. A number of hot-band transitions in the spectral region of the Fermi doublet at a Raman shift from 1230 to 1450cm^–1 was observed and assigned, making possible the use of CO₂ as spectroscopic thermometer gas, similar to N₂.     

High-temperature N2-CARS-thermometry

N₂-temperature measurements have been performed by coherent anti-stokes Raman scattering (CARS) up to more than 3200 K using a commercial heated graphite tube furnace. Surface radiation pyrometry served as the reference technique yielding good agreement between pyrometric and CARS-based temperature determination, resulting in an overall temperature uncertainty of ±80 K for a spectrum obtained at 1 bar and 3230 K.     

H2 CARS thermometry in a fuel-rich,premixed, laminar CH4/air flame in the pressure range between 5 and 40 bar

To establish H₂ CARS thermometry at high pressure, accumulated H₂ Q-branch CARS spectra were recorded in the exhaust of a fuel-rich CH₄/air flame at pressures between 5 and 40 bar. Temperatures were deduced by fitting theoretical spectra to experimental data points. The Energy-Corrected Sudden (ECS) scaling law was employed to set up an empirical model for the calculation of H₂ linewidths in high-pressure hydrocarbon flames with H₂ as a minority species. Experimental H₂ CARS spectra could be simulated very accurately with this model. The evaluated temperatures agreed well with reference temperatures obtained by spontaneous rotational Raman scattering of N₂.     

Single‐beam coherent anti‐Stokes Raman scattering (CARS) spectroscopy of gas‐phase CO2 via phase and polarization shaping of a broadband continuum

Coherent anti‐Stokes Raman scattering (CARS) spectroscopy of gas‐phase CO₂ is demonstrated using a single femtosecond (fs) laser beam. A shaped ultrashort laser pulse with a transform‐limited temporal width of ∼7 fs and spectral bandwidth of ∼225 nm (∼3500 cm⁻¹) is employed for simultaneous excitation of the CO₂ Fermi dyads at ∼1285 and ∼1388 cm⁻¹. CARS signal intensities for the two Raman transitions and their ratio as a function of pressure are presented. The signal‐to‐noise ratio of the single beam–generated CO₂ CARS signal is sufficient to perform concentration measurements at a rate of 1 kHz. The implications of these experiments for measuring CO₂ concentrations and rapid pressure fluctuations in hypersonic and detonation‐based chemically reacting flows are also discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

Raman spectroscopic study of a laminar hydrogen diffusion flame in air

Spontaneous Raman spectroscopy has been employed for time-averaged, spatially-resolved measurements of temperature and species concentration in an axisymmetric, laminar hydrogen diffusion flame in quiescent air. Temperatures were obtained from vibrational Q-branch raman spectra of N₂, O₂, and H₂ and the rotational Raman spectra of N₂ and H₂, and concentrations of H₂, and N₂ were determined. The results are compared to existing numerical nonequilibrium calculations for the conditions of this experiment. Significant differences between experimental and predicted temperature and concentration profiles are observed. In particular, the flame is larger in both diameter and length and the flame zone is thicker than predicted. Some possible sources of the discrepancies are discussed.     

Rotational temperature behaviour in supersonic jet expansions of molecular nitrogen

The rotational temperature behaviour in adiabatic jet expansion of molecular nitrogen is investigated over a wide range of stagnation density and nozzle diameter values using coherent anti-Stokes Raman spectroscopy (CARS). Estimates of rotational collisional numbers are made for both (N₂+N₂) and (N₂+Ar) systems. The aspect of cluster formation for both systems is also discussed.     

CARS spectral fitting with multiple resonant species using sparse libraries

The coherent anti‐Stokes Raman spectroscopy (CARS) technique is often used in the study of turbulent flames. Fast and accurate algorithms are needed for fitting CARS spectra for temperature and multiple chemical species. This paper describes the development of such an algorithm. The algorithm employs sparse libraries whose size grows more slowly with number of species than a regular library. It was demonstrated by fitting synthetic ‘experimental’ dual‐pump CARS spectra containing four resonant species (N₂, O₂, H₂ and CO₂), both with added noise and without it, and by fitting experimental spectra from a H₂ air flat flame produced by a Hencken burner. In the four‐species example, the library was nearly an order of magnitude smaller than the equivalent regular library (fitting times are correspondingly faster), and the fitting errors in the absence of added noise were negligible compared to the random errors associated with fitting noisy spectra. When fitting noisy spectra, weighted least squares fitting to signal intensity, as opposed to least squares fitting or least squares fitting to square root of intensity, minimized random and bias errors in fit parameters. Copyright © 2011 John Wiley & Sons, Ltd.     

CARS studies of vibrationally excited nitrogen at low pressures

Coherent anti-Stokes Raman spectroscopy (CARS) was used to measure the vibrational temperature of microwave-excited nitrogen in a N₂–CO–He mixture. CARS spectra, originating from the N₂-vibrational levelsv=0 up tov=3, have been recorded by both narrowband scanning of the resonance region as well as by broadband OSA detection. For the microwave-excited N₂ molecules a vibrational temperatureT _v (N ₂ = (2130±110K) and a lower limit of detection forN ₂(v = 3) = 1.2 x 10¹⁵ cm^–3 was established. The CARS results were independently confirmed by simultaneously recorded and spectrally resolved CO infrared fluorescence studies.     

Multi-color CARS for hydrogen-fueled scramjet applications

An approach to CARS, with considerable potential as a diagnostic for advanced airbreathing propulsion devices, is described and demonstrated. The technique is capable of providing simultaneous measurements of the major constitutents in H₂/air combustion. The method employs both broadband and narrowband Stokes beams to generate CARS via two, two-color and two, three-color wave mixing processes. Temperature and concentration information are simultaneously available from N₂, H₂, and H₂O. This permits the disappearance of fuel and appearance of product to be monitored. An analysis of the technique's capabilities and future improvements are discussed.