Modal Decomposition Analysis of Bluff-Body Stabilized Lean Premixed CH4/H2/air Flames Based on LES Data

Abstract

Unsteady flow and flame dynamics of bluff body stabilized CH4/H2/air premixed flames are investigated numerically with LES and modal decomposition techniques. Energy ranked and frequency dependent modes are identified with proper orthogonal (POD), spectral proper orthogonal (SPOD) and dynamic mode decompositions (DMD). Flow and flame coherent structures are extracted based on the axial velocity and heat release rate contours. Two cases, namely 100% CH4/air (V-flame) and 43.4% CH4 + 56.6% H2/air (M-flame), are selected for the analysis. Each case is acoustically excited in large eddy simulation (LES) with harmonic excitation signals at distinct frequencies and with a broadband excitation signal, to produce snapshots for modal decomposition analysis. In POD, the frequencies of the relevant modes are extracted with the discrete Fourier transform (DFT) of time coefficients, while in SPOD and DMD the extracted modes are frequency dependent by nature. Results are compared and commented.