Experimental investigation of the hydrodynamic effects upon convecting entropy waves in nozzle flows

Abstract Entropy noise induced by hot spots (entropy waves) is the least explored mechanism of combustion generated sound. Emission of entropy noise is subject to convection of entropy wave throughout the combustor exit nozzle. Nonetheless, the highly diffusive flows in combustors can dissipate entropy waves partially and even totally and hence, suppress the noise generation. Yet, the annihilation of entropy waves in this process is still poorly understood. In particular, no investigation exists on the evolution of entropy waves in nozzle flows. To address this issue, two low-speed, nozzle configurations supplied with fully developed flows at the inlet are examined experimentally. Entropy waves are generated by a controllable electric heater embedded inside the flow. A set of fast-response thermocouples are arranged at the entrance and exit sections of the nozzle to record the spatio-temporal evolution of the entropy wave during its passage thought the nozzle. The acoustic wave, generated by the conversion of entropy waves to sound, is further measured by differential pressure sensors. The results show that hydrodynamic characteristics of the flow such as Reynolds number and turbulence intensity as well as nozzle geometry dominate the survival of entropy wave. Analysis of the spatio-temporal coherence of the wave reveals that there is a predictable frequency threshold above that the wave is spatially distorted. This frequency limit determines the validity range of the commonly used, one-dimensional theoretical models of entropy wave that are currently applied to the whole spectral domain.