Abstract
Reducing NOx emissions from gas turbines requires lean premixed combustion. However, the interactions between the combustor acoustic waves and the flame unsteadiness then become significant. This can lead to combustion instability (which can cause irreparable structural damage), and to enhanced levels of aero-engine exhaust noise. Unsteady combustion generates both acoustic waves and temperature fluctuations. The latter are often termed “entropy waves”; they advect with the flow and are silent in a non-accelerating flow. On accelerating entropy waves, as occurs at the interface between combustor exit and turbine inlet in a gas turbine, “entropy noise” is generated, as first described in the Marble & Candel paper of 1977. The reflected component of entropy noise will propagate upstream within the combustor to arrive back at the flame, and may influence the potential for combustion instability, while the transmitted component will affect aero-engine exhaust noise. In this presentation, it will be shown that accounting for entropy noise can affect combustion instability via mode destabilisation, mode stabilisation, mode switching and a purely acoustic-entropy instability. An improved model to account for the effect of entropy wave advection on entropy noise will be presented, along with models for predicting entropy noise generation in annular geometries exhibiting circumferential modes.