A Centrifugal Compressor Operability Correlation With Combined Total Pressure and Swirl Distortion

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Inlet swirl distortion has become a major area of concern in the gas turbine engine community. Gas turbine engines are increasingly installed with more complicated and tortuous inlet systems, like those found on embedded installations on Unmanned Aerial Vehicles (UAVs) and Auxiliary Power Units (APU). These inlet systems can produce complex swirl distortion in addition to total pressure distortion. The effect of swirl distortion on engine or compressor performance and operability must be evaluated. The gas turbine community is developing methodologies to measure and characterize swirl distortion. There is a strong need to develop a database containing the impact of a range of swirl distortion and total pressure distortion on compressor operability. In one aircraft installation of an APU, the compressor experienced a wide range of combined total pressure and swirl distortion levels as a result of the APU inlet geometry. In this unique installation, the inlet system includes a check valve at the entrance of the inlet plenum through which all the airflow to the APU compressor passes. During APU operation, the check valve flaps are pushed open by the force of the airflow entering the plenum. The orientation of the check valve with respect to the compressor centerline introduces a varying amount of combined total pressure and swirl distortion. Consequently, it was necessary to test and map the compressor for a range of check valve positions in order to find an orientation with the lowest surge margin loss. The check valve comprises of two “D” shaped flaps hinged at the diameter of a circular flow entrance to the plenum. The plenum forms a “U’ shaped volume which allows the flow to turn 90 degrees from the check valve into the compressor annulus. A series of compressor tests was conducted where the check valve was rotated on its base such that the check valve had various clocking orientations with respect to the compressor axis. The check valve orientation was varied by lining the flap hinge line parallel to the compressor centerline (defined as zero degrees) then rotating the check valve in 30 degree increments from this reference all the way up to 150 degrees. Compressor speed lines were mapped at each check valve orientation and the surge margin loss/gain from a nominal undistorted surge line was determined. Fully 3D CFD analyses of each of these check valve orientations were run. The CFD models included the full valve and plenum geometry up to the compressor annulus. From the CFD analysis, the ARP1420 (Reference 1) total pressure distortion descriptors and the AIR5686 (Reference 2) swirl distortion descriptors were calculated on the annular plane at the compressor eye. Using these total pressure and swirl distortion descriptors determined for each check valve orientation along with the surge margin change available from the test data, a correlation of operability behavior was successfully obtained. This paper discusses the lessons learned and a recommended methodology todetermine an operability correlation for combined total pressure and high swirl distortion inlet systems.