Single-laser-shot temperature measurements at 5 kHz were performed in a model gas turbine combustor using femtosecond (fs) coherent anti-Stokes Raman scattering (CARS). The combustor was operated at a global equivalence ratio of 0.65 and 10 kW thermal power. Measurements were performed at various locations within the flame in order to resolve the spatial flame structure and compare to previously published studies. Power spectral density analysis of the temperature measurements yielded the characteristic thermo-acoustic pulsation frequency previously reported at 308 Hz. These results demonstrate the usefulness of fs-CARS for the investigation of highly turbulent combustion phenomena. 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 2400 K, although some detector saturation was observed for low temperature spectra.
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.
Chirped probe pulse femtosecond coherent anti-Stokes Raman scattering (CPP fs-CARS) thermometry was performed at 5 kHz in a hydrogen jet diffusion flame with an air co-flow. Measurements were performed at different heights and radial locations within the jet diffusion flame, up to 16 nozzle exit diameters downstream (x/d = 16). The near-nozzle measurements were characterized by large, organized, buoyancy-driven instabilities that become more chaotic at the downstream locations x/d ≥ 4. The diffusion flame results highlight temperature fluctuations characteristic of the buoyancy-driven Kelvin-Helmholtz-type instability and provide new insights into the transient structure of these flames. At some measurement locations, the time-varying temperatures ranged from 300 K to nearly 2400 K. The CPP fs-CARS signal intensity is a factor of approximately 1000 times lower at 2400 K compared with 300 K. A dual-channel detection system was used to increase the dynamic range of the CARS measurements. The determination of temperature from the single shot spectra is discussed in detail. Laser and detection system parameters were determined from CPP fs-CARS spectra obtained from a near-adiabatic laminar calibration flame apparatus. The temperature precision of the system was determined from these calibration measurements and was found to be better than 2.0% at 2200 K. The influence of an instrument response function on spectral fitting parameters is systematically assessed. Published 2015.
We have performed a detailed analysis of the temperature field in a turbulent swirl flame operating with a self-excited thermo-acoustic instability. The temperature field was measured using 5 kHz chirpedprobe-pulse (CPP) femtosecond (fs) coherent anti-Stokes Raman scattering (CARS). The measurements are described in detail in the part 1 companion article. In this paper, part 2, a detailed analysis of the time-resolved temperature measurements and simultaneous pressure measurements is performed to provide insight into the dynamics and structure of the swirl-stabilized flame. This work is the first to capture the dynamics of the flame, flow, and coupled flow-flame processes using high-fidelity, spatially-and temporallyresolved thermometry in a flame of practical relevance. The time-averaged contour plot of the temperature field indicates that the flame is very flat and stabilizes approximately 10 mm downstream of the burner face. In this region, there are very significant temperature fluctuations indicating a very high level of unsteadiness. The temperature probability distribution functions (PDFs) are clearly bimodal in this region near the injector face. A Fourier analysis of the temperature time series revealed multiple coherent oscillatory modes. The strongest oscillation was found to be coherent and in-phase with an acoustic resonance at 314 Hz, as expected from the Rayleigh criteria for the unstable flame. An analysis of the phase-conditioned average temperature fields show clearly an axial pumping of low-temperature reactants, which are consumed after a convective delay and result in a spike in the global heat-release rate. Continued analysis also revealed a 438 Hz oscillation that was found to correspond with the dynamics of convective transport by a helical precessing vortex core (PVC). The structure of the PVC, and its interaction with the flame, were studied based on the presence of this characteristic frequency in the power spectral densities computed throughout the flow. The precision and time-resolution of the CPP fsCARS measurements was also sufficient to enable computation of the integral timescales as well as the PDFs of the temporal temperature gradients. A sample of state space trajectories were used to provide insight into the nature of coupling between the narrowband acoustic resonance and the broadband spectrum of turbulent flame processes.
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