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     

Hydrogen CARS thermometry in H2–N2 mixtures at high pressure and medium temperatures: influence of linewidths models

In order to improve the accuracy of H₂ CARS thermometry, H₂ Q-branch CARS spectra have been recorded for various H₂-N₂ mixtures in a high-pressure cell at different pressures and temperatures (up to 40 bar and 875 K). Due to the low spectral resolution of broadband CARS experiments, the relevant spectral lineshape factor is the linewidth ratio &(Q(3))/&(Q(1)), since Q(1) and Q(3) are the most intense lines of the Q-branch spectrum in this temperature range. For the first time, the speed-inhomogeneous effects are accounted for in the simulation of the CARS profiles. The evaluated temperatures are in good agreement with reference values obtained by thermocouples. The specific role on the accuracy of H₂ CARS thermometry of the speed inhomogeneity is carefully analyzed, in connection with the influence of the nitrogen concentration.     

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 thermometry revisited in lightof the intramolecular perturbation

The rigid rotor approximation (RRA) is commonly assumed in the Raman cross section used in thermometric analysis based on coherent anti‐Stokes Raman scattering (CARS). In this paper, we discuss instead the role of the coupling between molecular vibrations and rotations in view of the alterations found in the amplitude of CARS signals of basic molecules and, in the end, we demonstrate that the deviation of a few percent from the RRA results in corrections to the measured temperature that are comparable to the thermometric accuracy of very well‐known Q‐branch CARS measurements on nitrogen, which is unanimously regarded as the fundamental molecule in CARS thermometry. Copyright © 2010 John Wiley & Sons, Ltd.     

High resolution pulsed cars spectroscopy of the Q-branches of some simple gases

Coherent anti-Stokes Raman spectra of the Q-branches of N₂, O₂ and of v₁ of C₂H₂ have been measured with fairly high resolution (≈ 0.30 cm^-1) by means of a pulsed dye laser system. Calculated CARS spectra show very good agreement with the observed rotational Q-branch structure.     

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.     

Broadening of vibrational spectra of carbon dioxide upon absorption and condensation in nanopores

Coherent anti-Stokes Raman Spectroscopy (CARS) has been used to study the vibrational Q-branch with the frequency of 1388 cm^?1 of the ν₁ mode of carbon dioxide molecules filling a sample made of nanopore glass at room temperature (20.5°C). The measurements were carried out in a gas cell at pressures approaching saturation P _sat. When pressure was increased above 0.8 P _sat, in addition to the spectral component due to the gaseous phase molecules, the CARS spectra featured a component due to the molecules adsorbed on the pore walls. Simulation of spectra taking the interference of these two contributions into account enabled the estimation of the broadening of the vibrational molecular spectra in the adsorbed layer. The spectral width of the component due to the adsorbed molecules was nearly a factor of two times larger than that of molecules in the bulk liquid phase. At pressures above 0.94 P _sat, the spectral width of the component due to the adsorbed molecules decreased to values close to those measured in the bulk liquid phase, which corresponds to the condensation of molecules in nanopores.     

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.     

H2 vibrational spectral signatures in binary and ternary mixtures: theoretical model,simulation and application to CARS thermometry in high pressure flames

A summary of the main results obtained by the two groups in the field of H₂ vibrational spectral line signatures for various mixtures, in connection with CARS diagnostics of H₂–O₂ combustion systems, is presented. H₂–X systems may have specific large inhomogeneous spectral features, due to the dependence of the line broadening and line shifting on the (H₂) radiator speed, particularly at high temperature. Thus, careful attention has to be paid to rigorously analyze such features, both from the experimental point of view (Dijon) and from the theoretical one (Besançon). Applications of the present results to high-pressure H₂/air flame thermometry are also briefly described. They present an approach aiming to include the more recent basic results on coherent Raman line shape in CARS diagnostics, in order to improve the accuracy of temperature measurements.     

Multiplex CARS measurements in supersonic H2/air combustion

2 and O₂ multiplex coherent anti-stokes Raman spectroscopy (CARS) employing a single dye laser has been explored to simultaneously determine the temperature and concentrations of H₂ and O₂ in a hydrogen-fueled supersonic combustor. Systematic calibrations were performed through a well-characterized H₂/air premixed flat-flame burner. In particular, temperature measurement was accomplished using the intensity ratio of the H₂ S(5) and S(6) rotational lines, whereas extraction of the H₂ and O₂ concentrations was obtained from the H₂ S(6) and O₂ Q-branch, respectively. Details of the calibration procedure and data reduction are discussed. Quantification of the supersonic mixing and combustion characteristics applying the present technique has been demonstrated to be feasible. The associated detection limits as well as possible improvements are also identified. Received: 1 July 1997/Revised version: 29 September 1998     

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.     

Herman–Wallis correction in vibrational CARS of oxygen

Light molecules are subject to vibration–rotation (VR) interaction, which implies corrections to the rigid rotor approximation and, in particular, corrections to spectral line intensities are related to the so‐called Herman–Wallis (HW) factor. This problem is outlined here for the spectral response of some medium‐weight diatomics in the gas phase and probed by means of vibrational coherent anti‐Stokes Raman scattering (CARS) used for diagnostic reasons in combustion science. However, different from other works on this subject, we specialized our analysis to oxygen and, since the peculiarity of its anti‐bonding molecular orbital, we find that the VR coupling is responsible for deviations that compete with the effect of Raman line widths typical of collisional environments of hot gases at room pressure. The HW correction is ultimately demonstrated to affect O₂ CARS thermometry in such a manner that the accuracy for measurements at high temperatures can be improved. Copyright © 2011 John Wiley & Sons, Ltd.     

CARS investigation of collisional shift of the hydrogen Q‐branch transitions by water at high temperatures

A high‐resolution (∼0.1 cm⁻¹) spectroscopic method based on the application of a Fabry–Pérot interferometer to the spectral analysis of the coherent anti‐Stokes Raman scattering (CARS) signal from an individual Raman transition was used to obtain single‐shot spectra of hydrogen Q‐branch transitions directly in the flame of a pulsed, high‐pressure H₂/O₂ combustion chamber. Simultaneously with the Fabry–Pérot pattern, a broadband CARS spectrum of the complete H₂Q ‐branch structure was recorded in order to measure the temperature of the probe volume. During every cycle of the combustion chamber, a pressure pulse together with single‐shot CARS spectra, providing information on individual line shapes and medium temperature, was recorded. On the basis of the experimental data, the temperature dependences of lineshift coefficients for several Q‐branch lines of hydrogen molecules under collisions with water molecules were determined in the temperature range 2100 < T < 3500 K, and an empirical ‘fitting law’ for H₂ H₂O lineshift coefficients is proposed. Copyright © 2010 John Wiley & Sons, Ltd.     

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.     

Dual-broadband CARS temperature measurements in hydrogen-oxygen atmospheric pressure flames

Dual-broadband coherent anti-Stokes Raman scattering (CARS) spectroscopy of the Q₀₁-branch of H₂ has been employed for thermometry in an atmospheric-pressure hydrogen-oxygen flame. The aim was to investigate the applicability of the technique for single-shot temperature evaluation and to analyse the precision of the measurements. The results are presented of temperature and relative H₂ density mapping of the flame in the temperature range of 700-2800 K. The achieved precision of single-shot measurements was 3-5%.     

Intensity convolutions of CARS spectra

A derivation of the convolution integral for finite bandwidth pump and Stokes sources in CARS is given. The resulting “partially coherent” convolution confirms previously published results, and is compared to the widely used incoherent treatment. The effect of intensity convolution choice on multiplex CARS thermometry is evaluated by reducing N₂ data taken in an ethylene-air diffusion flame. N₂ CARS thermometry using multimode pumps is ordinarily not very sensitive to convolution.     

Detection of acetylene by electronic resonance-enhanced coherent anti-Stokes Raman scattering

We report the detection of acetylene (C₂H₂) at low concentrations by electronic resonance-enhanced coherent anti-Stokes Raman scattering (ERE-CARS). Visible pump and Stokes beams are tuned into resonance with Q-branch transitions in the v₂ Raman band of acetylene. An ultraviolet probe beam is tuned into resonance with the – electronic transition of C₂H₂, resulting in significant electronic resonance enhancement of the CARS signal. The signal is found to increase significantly with rising pressure for the pressure range 0.1–8 bar at 300 K. Collisional narrowing of the spectra appears to be important at 2 bar and above. A detection limit of approximately 25 ppm at 300 K and 1 bar is achieved for our experimental conditions. The signal magnitudes and the shape of the C₂H₂ spectrum are essentially constant for UV probe wavelengths from 233.0 to 238.5 nm, thus indicating that significant resonant enhancement is achieved even without tuning the probe beam into resonance with a specific electronic resonance transition. PACS 42.65.Dr; 42.62.Fi; 42.65.-k     

Raman Spectra of Nitrogen,Carbon Dioxide,and Hydrogen in a Methane Environment

Changes in the Raman spectra of N₂, H₂, and CO₂ are studied in the range of 200–3800 cm^–1 depending on the concentration of surrounding CH₄ molecules at a fixed medium pressure of 25 atm and temperature of 300 K. It has been found that changes in the spectral characteristics of purely rotational H₂ lines in a CH₄ medium are negligible, while the Q-branches of the v₁/2v₂ Fermi dyad in СO₂ become narrower and wavenumbers of its high-frequency component and v₁ band of N₂ decrease. In addition, under these conditions, the ratio of intensities of the CO₂ Fermi dyad Q-branch varies in proportion to the concentration of surrounding molecules of CH₄. The obtained data will be used in diagnosing the composition of natural gas using Raman spectroscopy.     

On the sensitivity of rotational CARS N2 thermometry to the Herman–Wallis factor

Purely rotational spectral signals of coherent anti‐Stokes Raman scattering (CARS) from nitrogen molecules are studied as a function of the vibration–rotation interaction that weakens the rigid rotor approximation under which the dominant terms of the Raman cross section are calculated. The effect of the vibration–rotation interaction is quantified by means of the Herman–Wallis (HW) factor, and different approaches to its determination are evaluated in terms of their relative contribution to the CARS intensity and thermometric measurements made in a fuel‐rich hydrocarbon flame. Known HW factors are contrasted with more complete expressions of recent derivation, and it is found that relative line strength adjustments amount to about a few percent. Such differences result in temperature corrections of less than 1%. This value should be considered for the definition of the ideal thermometric accuracy of the technique but it is of minor importance in comparison with other sources of uncertainty (e.g. Raman line widths) that emerge from the complexity typical of reactive gas mixtures. Copyright © 2011 John Wiley & Sons, Ltd.     

Precise measurement of the depolarization ratio from photoacoustic Raman spectroscopy

A new method for the accurate determination of the Raman depolarization ratio is reported with an improved setup for photoacoustic Raman spectroscopy (PARS). The precise measurement is achieved by measuring the dependence of the acoustic signal intensity on the cross‐angle between the polarizations of two incident laser beams. We demonstrate this sensitive and simple method with several gaseous molecules, such as CH₄ and H₂. The measured results of depolarization ratios agree well with the theoretical values with an upper error limit of ± 0.005, which is comparable to that with polarization‐resolved CARS spectroscopy. Copyright © 2007 John Wiley & Sons, Ltd.