5 kHz thermometry in a swirl-stabilized gas turbine model combustor using chirped probe pulse femtosecond CARS. Part 1: Temporally resolved swirl-flame thermometry

Abstract

a b s t r a c tSingle-laser-shot temperature measurements at 5 kHz were performed in a gas turbine model combustor using femtosecond (fs) coherent anti-Stokes Raman scattering (CARS). The combustor was operated at two conditions; one exhibiting a low level of thermoacoustic instability and the other a high level of instability. Measurements were performed at 73 locations within each flame in order to resolve the spatial flame structure and compare to previously published studies. The measurement procedures, including the procedure for calibrating the laser system parameters, are discussed in detail. Despite the high turbulence levels in the combustor, signals were obtained on virtually every laser shot, and these signals were strong enough for spectral fitting analysis for determination of flames temperatures. The spatial resolution of the single-laser shot temperature measurements was approximately 600 μm, the precision was approximately ±2%, and the estimated accuracy was approximately ±3%. The dynamic range was sufficient for temperature measurements ranging from 300 K to 2200 K, although some detector saturation was observed for low temperature spectra. These results demonstrate the usefulness of fs-CARS for the investigation of highly turbulent combustion phenomena. In a companion paper, the time-resolved fs CARS data are analyzed to provide insight into the temporal dynamics of the gas turbine model combustor flow field.

Simple model of coherent anti‐Stokes Raman scattering signals generated by means of linearly chirped ultrashort Gaussian laser pulses

“…One of the most known applications of CARS with ultrashort pulses is for gas sensing . Within this context, the theoretical tool developed in the previous sections can now be employed to reproduce known experimental results and, by doing so, the ability to extract physical and chemical information will be put to test.…”

“…A further comparison between the CARS model with chirped laser pulses and gas‐phase measurements can be made along the same lines as the above‐mentioned applicative examples of rotational CARS thermometry. This time we turn our attention to chirped vibrational CARS that is characterized by spectra made of one broad vibrational band with many replica resulting from interferences with the nonresonant CARS background . In this experimental approach, some chirp is introduced in the probe pulse only, and a simulation that reproduces an experimental signal measured at 298 K by Dennis et al is provided in Figure .…”

“…This time we turn our attention to chirped vibrational CARS that is characterized by spectra made of one broad vibrational band with many replica resulting from interferences with the nonresonant CARS background . In this experimental approach, some chirp is introduced in the probe pulse only, and a simulation that reproduces an experimental signal measured at 298 K by Dennis et al is provided in Figure . The theoretical model is based on the laser parameters available in Dennis et al and is represented by the red line of Figure .…”

“…In this experimental approach, some chirp is introduced in the probe pulse only, and a simulation that reproduces an experimental signal measured at 298 K by Dennis et al is provided in Figure . The theoretical model is based on the laser parameters available in Dennis et al and is represented by the red line of Figure . The black line, instead, shows the experimental measurement corresponding to the result digitally extracted from figure 3 of Dennis et al The current model follows satisfactorily the experiment except for some details that deserve some explaining.…”

“…In recent years, CARS with chirped femtosecond (fs) laser pulses is increasingly attracting the interest of many groups at the experimental level and, following the progress of the experimental effort, many researchers have tried to formulate theoretical models that incorporate chirp effects . The nonlinear optical problem is rather intricate and, owing to the difficulty of treating chirped laser pulses, the current models are based on numerical methods, which are often limited to chirp in one single laser pulse and rarely consider chirp in more than one pulse .…”