Indirect combustion noise (entropy and vortex noise), which arises from inhomogeneous temperature and velocity fields at the exit of the combustion chamber and the inlet of the turbine, will be verified experimentally and quantified with respect to the overall noise generation due to its increasing relevance in future engine designs.The generation of entropy waves was verified by numerical simulations (CAA/CFD) at Hermann-Föttinger-Institut of the Technische Universität Berlin. It could be verified experimentally by a DLR team with a designated generic experiment (EWG). Currently, a similar approach is pursued to verify the existence of vortex noise.A model combustion chamber operated with methane gas with its exit coupled to a nozzle will be used to investigate the indirect combustion experimentally and quantify the amount of indirect noise compared to the total noise generation.
The combustion chamber can be operated in a regime which exhibits self-excited resonance. Furthermore, the fuel supply can be modulated which in turn generates entropy waves. The acoustic field within the combustion chamber and the exhaust duct can be measured with designated probe microphones. Time resolved measurements of the temperature by twin-thermocouple probes, of the density by filtered Rayleigh scattering (FRS) or an laser-induced grating technique (TGS) which was already applied successfully in a test setup will be used to quantify the inhomogeneous fields of temperature and density which generates the indirect combustion noise.
For an extensive study of these phenomena, the thermal output, the ratio of fuel and air, the length and the outlet geometry of the combustion chamber can be varied as well as the frequency and amplitude of artificial excitation.