Model-Based Inference of Flame Transfer Matrices From Acoustic Measurements in an Aero-Engine Test Rig

Flame dynamics, represented as a flame transfer matrix (FTM), is not directly measurable in test rigs and must be deduced from transfer matrix measurements of the combustion system. The burner-flame transfer matrix (BFTM) approach for FTM estimation is based on local pressure signals from microphones located upstream and downstream of the combustor. It combines acoustic measurements in nonreacting and reacting conditions, with the latter implicitly including flame dynamics. A simple matrix operation yields the FTM. However, this approach assumes loss-free wave propagation at constant speed of sound with no change in cross-sectional area between the microphones and the burner/flame. The present work demonstrates the limitations of these assumptions when applied to a test rig with effusion cooling, bypass annulus, and end contraction. This work proposes a method to infer the FTM for complex combustors by combining reactive transfer matrix measurements of the entire combustor with an accurate low-order model (LOM) of the test rig. This generalized method reduces to the BFTM approach as a special case. The Rolls-Royce SCARLET test rig, operating under realistic engine conditions, is used to analyze the capabilities of the proposed model-based inference method and the limitations of the BFTM approach. First, a LOM based on SCARLET's geometry and operating point is formulated using a generic FTM. This model visualizes the limitations of the BFTM approach concerning various physical and geometrical parameters. Finally, experimental data is used to infer the FTM of SCARLET using the proposed approach.