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.     

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.     

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.     

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.     

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.     

The application of single-pulse CARS for temperature measurements in a turbulent stagnation flame

N₂ Q-branch CARS spectra have been recorded and evaluated for temperature determination in a turbulent, premixed CH₄/air stagnation flame with a burner of 40 mm diameter and 22 kW thermal load. Temperature histograms on the flame axis at different distances from the stagnation plate have been measured. Problems of practical applicability are addressed, including those arising from the limited spatial resolution of the BOXCARS geometry, from an insufficient dynamic range of the diode array detector, and from a memory effect of the detector in the case of measurements in highly turbulent flame areas with strong intermittency. Some information is given on the computerized acquisition and on the evaluation of the large amounts of data that are necessary for extensive investigations in large combustion systems.     

H2 vibrational CARS thermometry

TheQ-branch spectra of molecular hydrogen have been used to determine rotational temperatures bycoherentanti-StokesRamanscattering (CARS). A supersonic jet expansion through a constant flow nozzle of adjustable temperature served as hydrogen source in the temperature range 300 to 2500 K. The analysis of the high-temperature data reveals details of the internal energy transfer in jet expansion.     

Effects of laser linewidth on coherent antiStokes Raman spectroscopy

Expressions for the generated antiStokes spectral density in coherent antiStokes Raman spectroscopy (CARS) are obtained, which take into account the finite linewidths of laser sources and which may be used to analyse observed spectra. Lorentzian and gaussian laser lineshapes are taken as special cases, which enable further analytic results for single and multiline CARS spectra to be derived. Emphasis is placed on scanning and broadband (multiplex) CARS techniques, and the choice of laser sources discussed from the spectral point of view. As examples of multiline spectra an analytical account of a periodic spectrum is presented and temperature measurement from Q-branch spectra is treated.     

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%.     

Parameter studies in practical nitrogen CARS thermometry using standard and advanced fitting codes

A systematic study of the influence of the collisional narrowing and the cross coherence effect on the temperature analysis of N₂-Q branch-CARS spectra at atmospheric pressure is presented. A comparison of calculated spectra over a temperature range 300–2000 K reveals that the standard theory neglecting these effects leads to temperature errors of +1.7% under flame conditions, when the nonresonant background is suppressed. This result is supported by the analysis of experimental CARS temperature measurements on a standard laminar diffusion flame. Furthermore, the temperature misreadings originating from erroneous slit function parameters and laser linewidth were investigated.     

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.     

The impact of laser field statistics in determination of temperature and concentration by multiplex USED CARS

Experiments have been performed which demonstrate that the ratio of resonant to nonresonant third-order susceptibilities measured in multiplex USED CARS are affected by the time correlation of the pump fields. Comparing the resonant to nonresonant signal ratio obtained with decorrelated fields to the ratio obtained with correlated fields, a relative increase of 2.5 was measured for the nitrogen Q-branch, whereas a corresponding increase of 1.9 was observed for the hydrogen Q(1)-line. To compare our experimental results with theoretical calculations of the spectral shapes, the nonresonant third-order susceptibilities of a number of gases were re-evaluated by calibrating to the nonresonant susceptibility of argon.     

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.     

Collisional line widths of autoperturbed N2: Measurements and quantum calculations

In this work we present new experimental and theoretical values for the line broadening coefficients of the Q-branch Raman lines of autoperturbed N₂. For the experimental determination of the coefficients, high resolution stimulated Raman spectra of the Q-branch of N₂ at different pressures were obtained at 77, 194 and 298 K. Simultaneously, quantum dynamical calculations, performed on two potential energy surfaces, were carried out for the system between 77 and 298 K, rendering a set of theoretical line broadening coefficients that could be directly compared to those obtained from the present measurements and the previous ones. Within the limit of considering the colliding molecules distinguishable we discuss the ortho and para contributions to the pressure broadening cross-sections. Because such calculations are time consuming we indicate routes to circumvent this difficulty. We observe a reasonable agreement between theoretical and experimental values of the collisional line broadening coefficients at all the studied temperatures.     

The methane ν1(a1) vibrational state rotational structure obtained from high-resolution CARS-spectra of the Q-branch

The gaseous methane ν₁(a₁), Q-branch coherent anti-Stokes Raman scattering (CARS) spectra have been investigated at a resolution of 0.002 cm^?1. A complex rotational structure of the resolved Q-branch has been experimentally observed. This structure can be ascribed to strong tetrahedral splitting of the rotational levels of the upper vibrational state, which possibly occurs due to Fermi resonance between the ν₁(a₁) and 2ν₂(a₁) vibrational energy levels which are close to each other. An assignment of the observed spectral lines has been made, yielding the rotational constants B, D, and D_t for the ν₁(a₁) vibrational state of the methane molecule. The absolute Raman frequency ν₁ of the purely vibrational transition has been found.     

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.     

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.     

Temperature measurements in CO2-laser-induced pyrolysis flames for SiC and ternary SiC/C/B powder synthesis by means of CARS

The hot luminescent reaction zones of CO₂-laser-induced pyrolysis flames using SiH₄/C₂H₂ gas mixtures with different silane to acetylene ratios and with and without diborane additives were investigated by means of H₂ Q-branch CARS spectroscopy, leading to spatial temperature profiles in gas flow direction. In the case of B₂H₆ additive to the stoichiometric SiH₄/C₂H₂ mixture a high temperature plateau ( 800–1000 K) of the reactant gas volume develops already several millimetres before reaching the CO₂-laser focus line. This precursor preheating zone could be explained by the catalytic effect of boron atoms or boron-containing intermediate species in the flame. A similar behaviour for acetylene-rich flames operating at half laser power was not observed.     

Absorption strength measurement of the ν1 band of methyl chloride

The absorption strengths of the Q-branch manifolds of the ν₁ band of methyl chloride have been measured. The results have been used to deduce the band strength which is 32.1 ± 2.9 cm^?2 atm^?1 at 297 K. The P-branch absorptions have been investigated to determine the possibility of determining a vibration-rotation factor for the band. This factor is 1 + (2αβ + γ) ～ 1.026.     

Intensity measurements in the 4·54μ fundamental of CH3D at low temperatures

The integrated intensities of the J-multiples R(4) through P(12), including the Q-branch, of the 4·54μ fundamental of CH₃D have been measured at 100°K, 150°K, 200°K, 250°K, and 298°K. Comparison of the measured line strengths with values calculated using symmetric-top formulae suggests strong intensity anamolies.