Investigation of Plenum Inlet Distortion and its Impact on Compressor Total Pressure and Swirl Distortion
Authors: Lavan Kumar Gundeti , Vunnam Kiran , Bouldin Bruce , Sheoran Yogi 
Published: November 2013

Aircraft auxiliary power unit (APU) compressors experience a wide range of total pressure and swirl distortion levels as a result of the APU inlet geometry and different flight conditions and wind directions. Compressors are designed to operate with certain levels of distortion while avoiding surge and stall and maintaining acceptable stress levels. For a new APU development program, the engine compression systems are subjected to a series of tests to evaluate their structural integrity and operability response to different levels of distortion.
Most large APUs incorporate two compressor systems. The engine compressor (E/C) makes up the compression system of the gas turbine engine. The load compressor (L/C) supplies air to the airplane for use by its environmental control system (ECS) and for main engine starting. The E/C and the L/C are situated opposite each other within a plenum and jointly pull air from this common plenum. Because both compression systems are positioned deep within the plenum, it is not possible to apply a distortion inducing blockage or measure distortion levels (similar to ones defined in SAE ARP1420) directly at the compressor faces. Instead, it is customary to apply the blockage using metal plates at the inlet to the plenum. This station is typically called the Aerodynamic Interface Plane (AIP) which is defined as the interface between the customer inlet system and the engine inlet plenum. An AIP defined at the plenum entry plane combined with a unique distortion descriptor defined at this plane are useful for many shaft engine applications. This distortion descriptor is directly related to the angular momentum entering the plenum which is converted inside of the plenum into compressor inlet swirl at the compressor eyes. The resulting distortion levels are measured using an array of static pressure, total pressure and total temperature probes placed just downstream of the blockage plates.
Because of the physical distance between where the distortion is generated and the APU compressor faces, the amount of distortion seen by the compressors for a given blockage plate configuration is unknown. Consequently, a series of steady state CFD analyses were conducted to evaluate the airflow pattern within the plenum and the resulting compressor face distortion for a given level of distortion induced at the plenum inlet. Both total pressure and swirl distortion levels are evaluated. This CFD analysis evaluated several different plate configurations which result in 15% to 50% blockage of the plenum inlet. Additionally, four different APU plenum configurations were analyzed, some from existing APU installations and another from an APU installation currently in development.
This paper discusses the theories behind plenum distortion and its impact on compressor performance. Honeywell’s methodology for the calculation of plenum distortion is described and its benefit in understanding the influence of distortion on compressor operability is explained. This paper describes details of the CFD analysis and presents predicted airflow patterns within the plenums and distortion levels at the compressor faces. The aerodynamics of the different plenum shapes are contrasted and compared with suggestions for future APU plenum designs. Flow visualization techniques are used on an existing plenum to validate the CFD analysis.