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.     

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.     

CARS investigations of quenching and photochemical reactions in the Na+H2 collision system

Summary We have carried out parallel studies of the quenching process in Na(3p)+H₂ collisions and the possible reactive process in Na(3p)+H₂ (v ^″=1,2,3) collisions. Rich CARS spectra which were obtained at H₂ pressure of 100 mbar and oven temperature of 600 K indicate the presence of vibrationally excited H₂ and photochemically produced NaH molecules. Temporal resolution of NaH CARS lines was employed in order to rule out competing collisional processes. We make use of resonantly enhanced CARS methods which enabled us to achieve very high sensitivity for NaH detection. Paper presented at the ?XI European CARS Workshop?, Florence, Italy, 23–25 March, 1992.     

Dual-pump coherent anti-Stokes Raman scattering thermometry in a sooting turbulent pool fire

We present a dual-pump coherent anti-Stokes Raman scattering (CARS) instrument, which has been constructed for the probing of temperature fluctuations in turbulent pool fires of meter-scale. The measurements were performed at the Fire Laboratory for Accreditation of Models and Experiments (FLAME) facility at Sandia National Laboratories, which provides a canonical fire plume in quiescent wind conditions, with well-characterized boundary conditions and access for modern laser-diagnostic probes. The details of the dual-pump CARS experimental facility for the fire-science application are presented, and single-laser-shot CARS spectra containing information from in-fire N₂, O₂, H₂, and CO₂ are provided. Single-shot temperatures are obtained from spectral fitting of the Raman Q-branch signature of N₂, from which histograms that estimate the pdf of the enthalpy-averaged temperature fluctuations at the center of the fire plume are presented. Results from two different sooting fire experiments reveal excellent test-to-test repeatability of the fire plume provided by FLAME, as well as the CARS-measured temperatures. The accuracy and precision of the CARS temperatures is assessed from measurements in furnace-heated air, where the temperature can be accurately determined by a thermocouple. At temperatures in excess of 500 K, the furnace results show that the CARS measurements are accurate to within 2-3% and precise to within ±3-5% of the measured absolute temperature.     

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.     

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.     

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.     

Coherent anti-Stokes Raman spectroscopy (CARS) of thev 3 band of methane in supersonic molecular beams

Supersonic molecular beams of methane are investigated in the expansion region using coherent anti-Stokes Raman scattering (CARS). Raman spectra of thev ₃ vibration with resolved rotational structure at low temperatures are reported. Comparison with calculated CARS spectra shows that the rotational distribution in the beam may be well described by a Boltzmann distribution. Temperatures are the same for all three nuclear spin modifications within the experimental error.     

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.     

用CARS技术确定拉曼退偏比的一种数据处理新方法

Yuika等人利用偏振CARS技术可以准确地确定分子的拉曼退偏比.其方法是，首先对不同检偏角_d所对应CARS谱峰的频率分布进行数学模拟，然后由所得系数随检偏角φ_d的变化求得使CARS信号中共振项消失的偏振角φ^０_d，最后由消失条件ρ＝-1/(tanθtanφ⁰_d)求出退偏比ρ，θ为产生CARS光的Pump光与Stokes光偏振方向的夹角.本文提出的数据处理方法，即交点法.同Yuika等人处理数据的方法相比，交点法毋需关于谱峰频率分布的知识，做法也更为简便.     

Collisional narrowing and spectral shift in coherent anti-stokes Raman spectra of molecular nitrogen up to 2500 bar and 700 K

CARS spectra of the N₂ Q-branch up to 2500 bar and 700 K have been measured. Calculated spectra based on theoretical models show significant disagreement with measured spectra above 300 bar so that CARS temperature measurements are in error by 130 K at 700 K and –150 K at 295 K. The spectral shift of the Q-branch reaches an asymptotic value corresponding to that measured in liquid nitrogen.     

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.     

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 spectroscopy of molecules and clusters in supersonic jets

Supersonic molecular beams of D₂, CH₄, NH₃, and C₂H₄ are investigated in the expansion region employing collinear coherent anti-Stokes Raman spectroscopy (CARS). The analysis of rotationally resolved CARS spectra allows the determination of temperatures in the beam. The rotational relaxation as a function of stagnation pressure and separation from the nozzle is studied by recording theQ branch for D₂ and the ₃ R andS branches for CH₄. Rotational temperatures for NH₃ are determined by investigating the complete ₃ band. At strong stagnation conditions broad structures arise which can be attributed to the formation of NH₃ clusters. For C₂H₄ the ₅ band with resolved rotational structure is reported. Again, at larger distances from the nozzle, broad structures are observed. They are assigned to the ₁ and ₅ vibrations in the C₂H₄ cluster.     

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     

Vibrational Analysis of Coherent Anti-Stokes Resonance Raman Spectra from Bacteriorhodopsin Containing Isotopically Substituted Retinal Chromophores

Vibrational spectra recorded by coherent anti-Stokes resonance Raman scattering (CARS) from bacteriorhodopsin (BR) samples containing isotopically substituted (²H and ¹³C) retinal chromophores were measured using high repetition rate, low-power, picosecond pulsed excitation (λ₁=580 nm and λ_s=640±3 nm). These picosecond resonance CARS (PR/CARS) data were analyzed via third-order susceptibility relationships [χ ^{( 3 )} ] to obtain band origins, bandwidths, relative intensities, and electronic phase factors assignable to all significant vibrational Raman features in the 1490–1700 cm⁻¹ wavenumber region (the ethylenic stretching and C = N–H rocking or Schiff base modes). Isotopic substitution selectively places ²H at C₁₅, ¹³C singly at the C₁₀ position and at the C₁₄ position, and ¹³C simultaneously in positions of C₁₄ and C₁₅. Each isotopic BR sample was examined not only in H₂O, but also in D₂O, which places a ²H at the Schiff base nitrogen of the retinal. In addition, PR/CARS data were recorded from each isotopic BR sample following either light adaptation [i.e. the BR sample contained a single retinal isomer (all- trans , 15- anti or BR-570)] or dark adaptation [i.e. the BR sample contained a mixture of comparable amounts of retinal isomers (BR-570 and 13- cis , 15- syn or BR-548)]. Excellent agreement was found between the vibrational features observed by PR/CARS and those obtained from spontaneous resonance Raman measurements from the same isotopically substituted BR pigments. Several new vibrational features were also found from the PR/CARS data. Vibrational Raman data from three of the isotopic BR samples in D₂O are reported for the first time.     

CARS spectra of thermally excited SF6 molecules

CARS spectra of thev ₁ mode of thermally excited SF₆ were calculated numerically. The influence of the vibrational quasicontinuum on the CARS spectra has been considered by introducing different types of the homogeneous broadening at different vibrational levels. The appearance of additional lines in the CARS spectrum due to mixing of high-lying vibrational levels by Fermi coupling was considered numerically in the frame of a simple model. A comparison of calculated and experimental spectra has been made.     

CARS diagnostics of a CO2-laser pyrolytic experiment for the production of nanosized ceramic powders

Spatial distributions of rotational temperatures and molecular number densities of C₂H₂ and H₂ were measured with CARS during the production of ultrafine SiC powders in a laser pyrolytic process flame. By means of a CO₂ laser, the reaction gases SiH₄ and C₂H₂ (or alternatively C₂H₄) are converted into SiC and H₂. From the CARS measurements temperature gradients are determined between 8.8 × 10⁵ K/m and 1.6 × 10⁶ K/m with corresponding heating rates of 1.8 × 106 K/s and 1.3 × 10⁶ K/s. The CARS data also allow an estimation of the gas expansion behaviour in the reaction zone. Moreover, they show that diffusive velocity components of the hydrogen in the hot reaction zone do not exceed 0.4 m/s.     

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.