5 kHz thermometry in a swirl-stabilized gas turbine model combustor using chirped probe pulse femtosecond CARS. Part 2. Analysis of swirl flame dynamics

Abstract

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.

Helical vortex core dynamics and flame interaction in turbulent premixed swirl combustion: A combined experimental and large eddy simulation investigation

“…While the recent effort to study flow and flame dynamics and structures in turbulent combustion remained strong, both experimentally [32][33][34][35][36] and numerically, [37][38][39][40][41][42] very few studies joined the strengths of experimental and numerical tools to gain insight into the vortical structures that we observe in such reacting flows. The goal of this work is to combine experiments and simulations in order to identify the origin and dynamics of flame and flow structures in a model swirl burner.…”

Impact of moderate pump–Stokes chirp on femtosecond coherent anti‐Stokes Raman scattering spectra

“…Femtosecond (fs) coherent anti‐Stokes Raman scattering (CARS) spectroscopy has emerged as a powerful tool for temperature and concentration measurements over the last decade . Previous studies have shown that Fourier transform‐limited (FTL) pump and Stokes pulses with pulse duration on the order of 50–100 fs can generate giant Raman coherences in which all the Q‐branch transitions for a molecule such as N 2 are excited with nearly the same efficiency and phase .…”