Boundary conditions to represent the wave impedance characteristics of axial compressors

Abstract

Thermo-acoustic oscillations can damage gas turbines. Numerical predictions guide the combustor design to prevent the occurrence of this hazardous resonance phenomenon. Uncertainties, such as incomplete knowledge of boundary conditions, challenge this strategy. For the appropriate estimation of combustor inlet boundary conditions, we develop a model mimicking the reflection characteristics of an axial compressor. The non-compact formulation relies on individual blade rows’ performance data and allows thereby estimating the phase delay information. The evolution of the reflection coefficients is analysed for different operating conditions and related to the slope of the compressor characteristics. We find that the impedance at the compressor discharge varies significantly with frequency, resulting in wave interactions with all blade rows. Only towards compressor choke, high flow velocities at the compressor discharge restrict the penetration depth of incident waves resulting in reflection coefficients with minor frequency dependence. That even small phase components of the reflection coefficient are crucial for thermo-acoustic stability prediction is demonstrated in an example.