Experimental and numerical investigation into the propagation of entropy waves in a small-scale rig

© 2016, American Institute of Aeronautics and Astronautics Inc, AIAA. All Rights Reserved. Entropy waves are an important source of indirect combustion noise and potentially contribute to the generation of thermoacoustic instabilities in gas-turbine combustors. Entropy fluctuations generated by unsteady combustion are known to disperse and diffuse as they convect towards the combustor exit. However, very few studies on the decay of entropy waves can be found in the literature, and therefore the main aim of this paper is to better characterise the propagation phenomenon of these waves using both experiments and numerical simulations. Entropy waves were experimentally investigated using a smallscale rig, based on a choked straight duct. Entropy fluctuations were generated by igniting fuel from a pulsed supply, enabling accurate control of the excitation. The fluctuating temperature was measured at several locations along the duct as well as the acoustic pressure, allowing us to characterise the propagation of entropy waves. Numerical simulations, based on a compressible code and LES approach, were also carried out in order to better understand the phenomena occurring during the propagation of such waves. In the LES entropy waves were generated by introducing a fluctuating temperature at the inlet of a straight duct. Several computations were performed varying different parameters including the frequency of excitation and bulk velocity of the flow. Both experimental and numerical results demonstrate that the amplitude of entropy wave fluctuations decays along the duct as a function of wave parameters and propagation distance. Results obtained in the present investigation can be directly exploited to improve the modelling of entropy wave dispersion in low-order acoustic network codes.