Accurate temperature measurements in flames with high spatial resolution using Stokes Raman scattering from nitrogen in a multiple-pass cell

Abstract

A multiple-pass cell is aligned to focus light at two regions at the center of the cell. The two "points" are separated by 2.0 mm. Each probe region is 200 μm×300 μm. The cell is used to amplify spontaneous Raman scattering from a CH4-air laminar flame. The signal gain is 20, and the improvement in signal-to-noise ratio varies according to the number of laser pulses used for signal acquisition. The temperature is inferred by curve fitting high-resolution spectra of the Stokes signal from N2. The model accounts for details, such as the angular dependence of Raman scattering, the presence of a rare isotope of N2 in air, anharmonic oscillator terms in the vibrational polarizability matrix elements, and the dependence of Herman-Wallis factors on the vibrational level. The apparatus function is modeled using a new line shape function that is the convolution of a trapezoid function and a Lorentzian. The uncertainty in the value of temperature arising from noise, the uncertainty in the model input parameters, and various approximations in the theory have been characterized. We estimate that the uncertainty in our measurement of flame temperature in the least noisy data is ±9 K.

Spontaneous Raman Scattering Temperature Measurements and Large Eddy Simulations of Vibrational Non-equilibrium in High-Speed Jet Flames

“…Vibrational and rotational temperatures are calculated from the spectra by fitting to a simulated spectrum using a Levenberg-Marquardt algorithm to obtain the least-square error. 10 All distributions are assumed to be Boltzmann in order to infer a single temperature for each mode. The theoretical Raman spectrum at a given wavelength is given by 11…”

Vibrational Non-equilibrium Effects in Supersonic Jet Mixing

“…The functional form and the line shape parameters are described in detail in Ref. 20. The intensity calibration factors are obtained by taking the ratio of the CI Systems SR-20 cavity black body counts at 1000 K to the Planck's function at the same temperature 20 .…”

“…The Raman scattering from a non-equilibrium distribution is simulated by modifying the code that generates the Raman spectrum from an equilibrium distribution. The Raman code for equilibrium distribution was developed for the characterization of a multiple pass Raman instrument in a previous study 20 . The contribution to the total intensity at a given wavelength λ from a Stokes Raman transition is given by 21…”

Raman spectra of methane, ethylene, ethane, dimethyl ether, formaldehyde and propane for combustion applications

“…A similar version of the spectrograph has been previously used by KC et al [21] to acquire high resolution N 2 spectra in laminar flames. A pair of 150 mm diameter achromatic lenses (Linos Photonics f/2 and f/4), identical to the ones used for the "low resolution" system, focused the Raman signal at the entrance of a Holospec f/1.8i spectrograph (Kaiser Optical Instruments).…”