Comparison Between the Effects of Zero and Non-Zero Flow Acceleration on the Entropy Wave Decay: An Experimental Study

Entropy wave, as the convecting hot spot, is one of the sources of combustion instabilities, which is less explored through the literature. Convecting in a highly turbulent flow of a combustor, entropy waves may experience some levels of dissipation and deformation. In spite of some earlier investigations in the zero acceleration flow, the extent of the wave decay has not been clear yet. Further, there exist no results upon the wave decay in non-zero accelerated flows. This is of crucial importance, as the wave passes through the end nozzle of the combustor or gas turbine stages. The current experiment, therefore, compares the wave decay in both flow of constant and variable bulk velocity, meaning, respectively, a uniform pipe and a convergent nozzle. The comparison will aid the theoretical models to reduce complexity by simplifying the relations of non-zero acceleration flow to those of no acceleration, as followed by the earlier effective-length method. Reynolds number and inlet turbulence intensity are considered as the governing hydrodynamic parameters for both investigated flows. The entropy wave is generated by an electrical heater module and detected using fast-response thermocouples. The results show that the entropy wave variation is point-wise and frequency-dependent. The accelerated flow of the nozzle is generally found to be more dissipative in comparison with the zero acceleration flow.