Sound propagation in a lined annular duct with mean swirling flow was investigated via the normal mode analysis of three-dimensional(3-D) Linearized Euler equations

and was calculated as a generalized eigenvalue problem by a spectral collocation technique.Axial eigenmodes spectrum which are mainly composed of nearly/purely acoustic modes and nearly/purely convected modes with mean swirl and relative unsteady disturbances can then be determined.Mean swirl may even change the number of the propagating acoustic modes

and consequently affect the cut-on/cut-off characteristics of sound propagation.After validation of the approach

the effects of flow parameters and liner on the acoustic modes and convected modes are discussed.