Experimental Investigation on Aerodynamic Unsteadiness in a Full Scale Gas Turbine Midframe Sector

Aerodynamic unsteadiness generated upstream of the combustor basket via the complicated geometry of a modern gas turbine can lead to incomplete combustion, reduced efficiency, greater pressure drop, flashback, and reduced part life. The MidFrame section encompasses the main gas path from the compressor exit to the turbine inlet. Diffuser performance, support struts, transition pieces, and other flow obstructing geometries can lead to flow unsteadiness which can reduce performance. This study uses a combination of thermal anemometry, pressure microphone, and wall mounted accelerometer measurements to determine the primary unsteadiness frequencies and target their source. Diffuser performance is shown to have a significant impact on the downstream flow behavior. Inlet conditions are modified to provide a separated bottom wall and a fully attached compressor exit diffuser (CED) condition at an area average inlet Mach number of 0.26. Unsteadiness levels are seen to increase as a result of the separated inlet condition while the mean flow characteristics are slightly altered due to the varying exit trajectory of the main core from the CED, nevertheless the overall level of unsteadiness/turbulence is low for such a complex flow field (8 to 11 %). Results of this study can help diagnose and prevent the aforementioned issues for complicated geometries where simple flow experiments fall short.