LES of longitudinal and transverse self-excited combustion instabilities in a bluff-body stabilized turbulent premixed flame

Combustion dynamics of a V-flame in an afterburner-type configuration are investigated using high-order compressible large eddy simulations (LES) and compared to experimental results. Both self-excited longitudinal (100 Hz) and transverse (1400 Hz) modes observed in the experiments are captured by LES and instability mechanisms are discussed. LES results for all modes are compared to a Helmholtz solver output, showing that the transverse mode appearing in the LES is the 1Lx-2Ty-OTz eigenmode of the chamber, affecting the velocity field symmetrically. The 1Lx fluctuation causes a symmetric flame roll-up which increases heat release rate fluctuations, closing the feedback loop. The 2Ty component of the mode is active along the flame holder axis and causes not only transverse fluctuations but also a reorganization of the mean flame along two main zones located on both sides of the zero acoustic velocity plane, a feature that has not been reported before. Dynamic mode decomposition (DMD) is used to extract the structure of the transverse mode from LES snapshots which is found to match the Helmholtz solver prediction. This study confirms the capacity of high-order LES to capture not only low-frequency oscillations but also high-order frequency transverse modes in combustion chambers. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.