Effects of Flow Noise in Characterization of Acoustic Liners at High Mean Flow

Bias flow acoustic liners are used to suppress thermo-acoustic instability in jet engines. This paper aims to identify the problems associated with the flow noise and the inability of acoustic loads to characterize acoustic liners in the presence of high mean flow. At high mean flow, conventional loads deviate from the criteria of being an effective acoustic load, as flow noise dominates the sound source. Three different loads, an open-end termination, an expansion chamber, and a quasi-anechoic chamber, have been used in the present work. The acoustic parameters absorption coefficient and reflection coefficient are presented to quantify the ability of acoustic loads considered at different mean flow rates. The results suggest the need to design an acoustic load that could absorb flow noise and enables in-duct characterization of acoustic elements in the presence of high mean flow. At the same time, loads must not obstruct the path of flow.