Identification of entropy waves in a partially premixed combustor

Unsteady combustion generates not only acoustic waves, but also fluctuations of the burnt gas temperature — referred to as entropy waves. These waves are convected by the mean flow through the combustor and result in conversion to acoustic energy when accelerated in an exit nozzle. The upstream traveling acoustic wave can then couple with the unsteady heat release of the flame and cause self-excited thermoacoustic instability, particularly at low frequencies (“rumble”). In this work, large eddy simulation (LES) is combined with system identification (SI) to determine the entropy transfer function (ETF) of a partially premixed, swirl-stabilized flame with hydrogen enrichment. We compare the single-input single-output (SISO) entropy transfer function identified from a broadband-forced LES with air mass flow modulation to the one obtained experimentally through tunable diode laser absorption spectroscopy with wavelength modulation spectroscopy (TDLAS-WMS) to measure temperature fluctuations. Then, multiple-input single-output (MISO) identification is applied to time series data obtained from simultaneous modulation of air and fuel mass flow to estimate the individual contributions of perturbations in velocity and equivalence ratio to entropy response. Equivalence ratio fluctuations are found to be the dominant generation mechanism of entropy waves. Finally, the entropy transfer function is identified at various positions in the combustion chamber to analyze the decay of entropy waves governed by convective dispersion.