Transfer functions of lean fully- and technically-premixed jet-stabilized turbulent hydrogen flames

Abstract

Understanding the response of multi-jet turbulent hydrogen-air flames to acoustic forcing is key for the development of future carbon-neutral gas turbine combustors. In this paper, we present flame transfer functions (FTFs) deduced from burner and flame transfer matrices obtained with acoustic measurements for fully-premixed (FP) and technically-premixed (TP) conditions, in conjunction with their analytical models. The matrix burner used in this study produces an array of sixteen turbulent lean hydrogen-air jet flames. Its acoustic transfer matrix is analytically modeled, with experimental validation. It exhibits a significant frequency dependence due to the non-compactness of the burner with respect to the acoustic wavelength considered. Our results show that the conical jet-stabilized flames have a typical low-pass filter behavior in the FP case, while in the TP case, they exhibit a frequency dependent gain modulation originating from the combination of mass flow and equivalence ratio oscillations. Using distributed time delay (DTD) models, we identify the dominant disturbances controlling the FTF data measured at different equivalence ratios and bulk velocities, and show that they can be well collapsed by using the associated Strouhal numbers. To unravel the smooth transition of FTF between the FP and TP cases, staging of the fuel is employed in the present study. We demonstrate that the features of the FTFs for staging conditions ranging from FP to TP are strongly correlated with the fuel staging ratio and can be well reproduced by a linear superposition of the FTFs of the pure FP and TP cases.