Theory and application of reverberated direct and indirect noise

Combustion noise can be classified as direct or indirect, the former related to the flame’s unsteady heat release, and the latter to the acceleration/deceleration of convected flow disturbances. An analytical and numerical analysis of the direct and indirect noise produced by the generation and acceleration of entropy waves in a reflective environment is presented. A model for the direct noise associated to the generation of entropy waves is produced, building on previous work to account for the injection of mass and/or momentum. The effect of repeated reflections on low-frequency perturbations (characteristic of Entropy Wave Generators) is determined analytically. These results are then implemented in a set of limit cases, showing the limit behaviours of such systems. Low-order simulations are carried out using the Cambridge Entropy Wave Generator experiment as a test case, in which entropy waves are generated by an electric heater and accelerated through a subsonic or sonic (choked) orifice plate. Due to the clear separation of direct and indirect noise in the experimental results, direct and indirect noise transfer functions can be extracted from the experimental data for the first time, and compared directly with existing theoretical models. The backwardpropagating indirect noise generated at an orifice plate is shown to be significantly higher than predicted by existing theoretical models, both in the subsonic and sonic conditions.