Compositional inhomogeneities as a source of indirect combustion noise: Physical mechanisms and fuel effects

Indirect combustion noise has been recognized as a main mechanism of engine-core noise in aviation gas-turbines. This noise mechanism is commonly associated with the distortion of temperature inhomogeneities and vortical structures that generate excess noise as they interact with mean-flow gradients. Recently, a new indirect noise-source mechanism was identified that arises from the interaction of mixture inhomogeneities and associated variations in the Gibbs free energy with the mean-flow gradients in the nozzle. By employing different levels of model fidelity in representing combustion noise, this work examines relative contributions of different noise source mechanisms, and the model fidelity that is necessary to accurately describe their relevant physical processes. For this, idealized one-dimensional compact and non-compact nozzle-analysis is performed, which is augmented by multidimensional transient flow-field calculations considering the linearized Euler equations. It is shown that the non-compact nozzle theory is necessary to accurately capture phase-cancellation effects between entropy and composition noise, and the linearized Euler formulation is required to account for complex mode shapes of the perturbations exiting the combustor and entering the downstream nozzle.