On the decay of entropic-compositional sources of indirect noise in combustors

For nearly half a century, indirect combustion noise in gas turbine combustors was entirely attributed to entropy waves, as the convecting hot spots. However, recent studies identified another source of indirect noise called compositional waves, which consists of convecting chemical blobs. Understanding the evolution of this new source during its journey throughout the combustor requires attention due to the unknown physics of the turbulent, heat transferring flow in which it moves. In the current study, a hot chemical blob including a mixture of combustion products is introduced at the channel inlet. During its convection along the channel, degeneration of various thermal and chemical components of the entropic-compositional wave is investigated in the frequency domain using large-eddy simulations. It is shown that the wave annihilation due to wall cooling, as found in real combustors, can exceed those imposed by the flow hydrodynamics. Through a coherence analysis, it is found that mixture fraction is mainly responsible for deteriorating the chemical sources and that the contribution of potential function is comparatively smaller. Overall, it is concluded that compared to entropy waves, compositional waves are up to 20% more prone to decay.