Numerical analysis of the thermoacoustics of hydrogen-air flames

• Applicant: Professor Dr.-Ing. Wolfgang Schröder
• Subject Area: Fluid Mechanics
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 497570045

In this project, the thermoacoustics of lean premixed hydrogen flames is analyzed numerically. The analysis focuses on the thermoacoustic source mechanisms, i.e., besides the heat release the terms due to sound velocity fluctuations and entropy gradients are determined, which have to be known to suppress thermoacoustic instabilities, e.g., in gas turbine combustion chambers, and to achieve a carbon free and low NOx power generation in the future. Unlike for methane combustion, the high diffusivity of hydrogen in the hydrogen-air mixture yields thermodiffusive instabilities in hydrogen combustion, which generate sound through the intensified unsteady flame movement. This sound generation due to thermodiffusive instabilities does not appear for methane flames, which is why it has not been thoroughly investigated yet. Therefore, it is the overall objective of the current project to determine the effect of thermodiffusive instabilities on the thermoacoustic source mechanisms. The computation of the thermoacoustic sources requires a realistic prediction of the local flame speed and hence, thermodiffusive instabilities. In the current project, this is achieved by direct numerical simulation (DNS) with detailed chemistry. A hybrid approach is used for the thermoacoustic source analysis. That is, the thermoacoustic sources are computed by the DNS solution and their acoustic emission is determined by solving the acoustic perturbation equations in a computational aeroacoustics simulation. The amplitudes and the phases of the signals of the thermoacoustic source terms are of special interest to develop thermoacoustic stability criteria for hydrogen combustion. In the first three-year period of the proposal, two-dimensional hydrogen flames are computed to cover a meaningful range of parameters. Since the equivalence ratio has a significant impact on the thermodiffusive instability, the thermoacoustic sources are first determined for a range of equivalence ratios. Then, hydrogen flames excited by time-harmonic modulations of the inflow velocity are simulated and the impact of the thermodiffusive instability on the thermoacoustic flame response to velocity fluctuations is analyzed. The results of this project yield fundamental findings to avoid or control thermoacoustic instabilities.

• DFG Programme: Research Grants
• Co-Investigator: Professor Dr.-Ing. Heinz Pitsch