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Inelastic collisions in natural H2 are studied from the experimental and theoretical points of view between 10 and 140 K. Rotational populations and number densities measured by Raman spectroscopy along supersonic expansions of H2 provide the link between experimental and theoretical rotranslational state-to-state rate coefficients of H2 in the vibrational ground state. These rates are calculated in the close- scattering approach with the MOLSCAT code employing a recent ab initio H2–H2 potential. The calculated rates are assessed by means of a master equation describing the time evolution of the experimental rotational populations. The feasibility for obtaining the rates on the sole basis of the experiment is discussed. The dominant processes j1j2→j1′j2′ in the investigated thermal range are found to be 21→01>30→12>31→11, proving the importance of double processes such as 30→12. Good agreement is found between theory and experiment, as well as with earlier ultrasonic measurements of relaxation times. A spectral representation is proposed in order to visualize quantitatively the collisional contributions in any nonequilibrium time evolving process. [ABSTRACT FROM AUTHOR]

We report an experimental determination of the k00→02 rate coefficient for inelastic H2:H2 collisions in the temperature range from 2 to 110 K based on Raman spectroscopy data in supersonic expansions of para-H2. For this purpose a more accurate method for inverting the master equation of rotational populations is presented. The procedure permits us to reduce the measured k00→02 rate coefficient to the corresponding σ00→02 cross section in the range of precollisional energy from 360 to 600 cm-1. Numerical calculations of σ00→02 carried out in the frame of the coupled channel method are also reported for different intermolecular potentials of H2. A good agreement is found between the experimental cross section and the numerical one derived from Diep and Johnson’s potential [J. Chem. Phys. 112, 4465 (2000)]. [ABSTRACT FROM AUTHOR]

Self-broadened and N2, Ar, He-broadened halfwidth and pressure shift coefficients of the rotational transitions in the 0003–0000 band of 12C16O2 have been measured from laboratory absorption recorded at room temperature with a Fourier transform spectrometer. Comparison is made with the results of a theoretical calculation based on a semiclassical model. Good agreement is obtained for all the IR vibrational bands for which data are available. It has been shown that the shifts mainly originate from vibrational dephasing. Infrared line shifts have been compared with results obtained from stimulated Raman spectroscopy and we show that they are not consistent. A number of possible explanations have been discussed to account for this discrepancy. [ABSTRACT FROM AUTHOR]

Using a high resolution Raman spectrometer, we have measured Ar-broadening coefficients in the ν2 Q branch of C2H2 for 22 lines at 295 K, 20 lines at 174 K, and 16 lines at 134 K. These lines with J values ranging from 1 to 23 are located in the spectral range 1970.9–1974.3 cm−1. The collisional widths are obtained by fitting each spectral line with a Rautian profile. The resulting broadening coefficients are compared with theoretical values arising from close coupling and coupled states calculations. A satisfactory agreement is obtained at room as well as at low temperatures, especially for odd J lines. By comparing broadening coefficients at 295, 174, and 134 K from a simple power law, the temperature dependence of these broadenings has been determined both experimentally, and theoretically. [Copyright &y& Elsevier]

Theory and experiment are combined in a novel approach aimed at establishing a set of two-body state-to-state rates for elementary processes ij→ℓm in low temperature N2:N2 collisions involving the rotational states i,j,ℓ,m. First, a set of 148 collision cross sections is calculated as a function of the collision energy at the converged close-coupled level via the MOLSCAT code, using a recent potential energy surface for N2–N2. Then, the corresponding rates for the range of 2≤=T≤=50 K are derived from the cross sections. The link between theory and experiment, aimed at assessing the calculated rates, is a master equation which accounts for the time evolution of rotational populations in a reference volume of gas in terms of the collision rates. In the experiment, the evolution of rotational populations is measured by Raman spectroscopy in a tiny reference volume (≈2×10-3 mm3) of N2 traveling along the axis of a supersonic jet. The calculated collisional rates are assessed experimentally in the range of 4≤=T≤=35 K by means of the master equation, and then are scaled by averaging over a large set of experimental data. The scaled rates account accurately for the evolution of the rotational populations measured in a wide range of conditions. Accuracy of 10% is estimated for the main scaled rates. [ABSTRACT FROM AUTHOR]

This work presents the first quantitative Raman measurements of temperature, mass fractions, and mixture fractions in two turbulent ammonia/hydrogen/nitrogen-air diffusion jet flames simulating 14% (CAJF14) and 28% (CAJF28) partial ammonia cracking ratio with Reynolds numbers of 11,200. The precision and accuracy of the system were validated in laminar counterflow and laminar jet flames to ensure data quality. The scalar structure in turbulent flames is examined using conditional mean and rms radial profiles, scatterplots in mixture fraction space, as well as statistics conditioned on mixture fraction and physical spaces. These results are compared with opposed diffusion flow 1D simulation with multi-component transport model and Le=1 transport model to visualize the effects of differential diffusion on the scalar structure at various axial positions. Finally, localized extinction was found in CAJF14 but not for the more stable CAJF28, and the probability of localized extinction was less than 2.5%. The dataset is provided as supplemental material and can be used for the validation of numerical models. [ABSTRACT FROM AUTHOR]

Ammonia has shown great potential as a carbon-free fuel, in particular for marine transportation and energy production. Its low laminar flame speed, and the tradeoff between ammonia slip and NOx emission, pose challenges for industrial applications, and a more in-depth understanding of the combustion of ammonia is therefore needed. Raman spectroscopy is a powerful diagnostic often employed to investigate turbulence chemistry interactions and resolve the thermo-chemical structure of hydrogen and hydrocarbon-air flames. This work extends Raman spectroscopy to the instantaneous and spatially resolved measurement of major species concentrations and temperature in ammonia flames. The lack of detailed ammonia spectra at high temperatures, the strong flame luminosity, and fluorescence interference are the major obstacles to the implementation of Raman spectroscopy to ammonia flames. This paper introduces a novel approach to estimate the temperature dependence of the Raman signal and of fluorescence interference contributions from a series of counterflow diffusion flames. This approach was first validated in high-pressure hydrogen flame, and then extended this approach to high-pressure ammonia flames to obtain the Raman response of ammonia and its crosstalk. A high-contrast optical shutter is sufficient to suppress flame luminosity. Finally, species concentrations and temperature profiles from measurements are shown, and their accuracy and precision are discussed. [ABSTRACT FROM AUTHOR]