Experimental Investigation of the Fundamental Entropy Noise Mechanism in Aero-Engines

Entropy noise caused by combustors increases rapidly with rising Mach number in the nozzle downstream of the combustion chamber. This is experimentally shown with a dedicated test facility, in which entropy waves are generated in a controlled way by unsteady electrical heating of fine platinum wires immersed in the flow. Downstream of the heating module called Entropy Wave Generator (EWG) the pipe flow is accelerated through a convergent-divergent nozzle with a maximum Mach number of 1.2 downstream of the nozzle throat. Parameters like mass flux of the flow, nozzle Mach number, amount of heating energy, excitation mode (periodic, pulsed or continuously), and propagation length between Entropy Wave Generator and nozzle have been varied for the analysis of the generated entropy noise. The results are compared with the results of a one-dimensional theory found in early literature.