Investigation of the onset of acoustic instability in turbulent combustion systems
Final Report Abstract
Two copies of a non-premixed combustor based on the backward-facing step flame stabilization geometry were developed for work al IIT Madras and TU Darmstadt. Earlier, a map of onset of combustion instability was obtained by testing the combustor over a wide range of flow and geometric condilions. In the present project period, phase-locked ensemble-averaged PIV under combustion conditions revealed the fluctuation ofthe recirculation zone and the flame during the onset of combustion instability relative to before the onset. High-speed chemiluminescence imaging revealed that combustion occurred in vortical structures, and the spectra of the chemiluminescent intensity exhibited dominant peaks at the same frequency as that observed in the acoustic spectra. Fuel injection fluctuations were in phase with the chemiluminescent fluctuations, showing the link between the two, and the potential for an additional mechanism to contribute to instability near blow-off conditions. High-speed PIV of cold flow past the backward-facing step clearly revealed the presence of two dominant mechanisms of fluid dynamic fluctuations, one the step-mode, and another, the K-H instability in the stretched shear layer of the step-mode, and the interaction between the two modes. High-speed imaging of Rayleigh scattering from flames has been attempted in the present study to map densily fluctuations as a source of sound. As a validation, helium mixing in ambient air has been tested to begin with, and signals from helium, nitrogen, and mixtures of the two, have been mapped for calibration. Large eddy simulation of the DLR-A flame has been carried out and the material derivative of the density fluctuations is used as a source term in acoustic simulations to predict combustion noise with good agreement. A formal decomposition of the fluid dynamic governing equations taking into account both lime-scale and length-scale disparities between the flow and acoustics in the low mean Mach number limit shows the source term from the acoustics lo the flow is in the form of the acoustic Reynolds number and the source term from the flow to the acoustics is in the form of the pressure work due to dilatation from combustion heat release, which identically equals the heat release rate when the time-scales of the two processes match. Further work is to be carried out in some respects of processing the high-speed PIV images, the high-speed Rayleigh scattering imaging, on the experimental side, and in attempting confined flow simulations to consider acoustic feedback with LES, and to perform turbulent flow computations with the formal flow decomposition equafions, on the theoretical/numerical side.
Publications
- "Numerical Simulation of Acoustic Excitation of Laminar Flow Past of a Backward-Fac ing Step," 12th Asian Congress on Fluid Mechanics, Daejeon, South Korea, Aug. 18-21, 2008
Balaji, C. and Chakravarthy, S. R.
- "Onset of Thermo-Acoustic Instability in a Non-Premixed Backward-Step Combustor," 44th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Hartford, Connecticut, USA, 20-23 July 2008
Shreenivasan, O. J. and Chakravarthy, S. R.
- "Oscillatory Response ofa Ducted Non-Premixed Flame: Variable Density and Shear Layer Effects," 44th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Hartford, Connecticut, USA, 20-23 July 2008
Balaji, C. and Chakravarthy, S. R.
- "PIV of Mode Shifts During Combustion-Acoustic Lock-on in a Non-Premixed Backward-Fac ing Step Combustor," 12th Asian Congress on Fluid Mechanics, Daejeon, South Korea, Aug. 18-21,2008
Shreenivasan, O. J. and Chakravarthy, S. R.
