Experimental Investigation of Bleed Air Extraction through a Circumferential Gap in a Low Speed Axial Compressor Stage

For the supply of secondary air systems such as turbine cooling, oil sump and cabin ventilation, bleed air is taken from the compressor of aircraft engines. The design objective is to extract the required fluid with as little losses as possible. Bleed air is usually extracted at the casing downstream of the stator row. By shifting the extraction position by half a stage upstream, an improvement of the compressor efficiency could be achieved. In this measuring campaign the effect of a bleed air geometry is experimentially investigated downstream of the stator blade row as well as downstream of the rotor blade row of a 1.5-stage axial compressor. In the studies presented here, the differences of the configurations with respect to stage aerodynamics and aerodynamics of the bleed air geometry are shown. The aim is to investigate the potential of bleed air extraction downstream of the rotor blade row. Since the investigation of bleed air extraction in the rotating test rig has a higher complexity compared to the pure axial compressor, the following will not only deal with the results of the measurement campaign. In addition, some general considerations and lessons learned are discussed in relation to the bleed air removal research. INTRODUCTION Bleed air extraction plays an important role when it comes to the efficiency and stability of the axial compressor of aircraft engines and stationary gas turbines. Therefore an efficient bleed air system should meet different criteria. When extracting fluid for seconday air systems, the smallest possible influence on core flow of the compressor is desirable. Also the extracted fluid itself should experience minimum losses. The higher the static pressure recovery of the extraction port, the higher the pressure available for secondary air systems in the plenum. An aerodynamic improvement of static pressure recovery of the extraction ports would allow a shifting of the the extraction position to the upstream stages, this would have a positive impact on the specific fuel consumption. In the past, several studies have been presented on this topic, which dealt with the geometry and positioning of the bleed air removal port. Poenick [1] and Leishman [2],[3] investigated bleed air removal through circular holes. Zimmermann [4], Leishman [2], Peltier [5], [6], Gomes [7], [8], Grimshaw [9], [10] and Aufderheide [11] examined circumferential bleed ports. So far most research was done in non-rotating systems or in cascades. In the test section presented in the following, the aerodynamic behaviour of the bleed port geometry and the interaction with the 1.5-stage axial compressor is examined in a rotating system. In addition, previous investigations have mainly dealt with the bleed air extraction downstream of a row of stator blades. This is all too understandable, since downstream of the rotor blade row there are comparatively high speeds due to the swirl in the flow. At high extraction mass flows, there is therefore a justified concern that the bleed air duct will be blocked. Nevertheless, first fundamental investigations of the influence of the extraction position downstream of the rotor blade row are carried out here, which should lead to a better understanding of the effects on the stage aerodynamics. If, in future research projects, it were possible to convert much of the twist behind the rotor into static pressure, the extraction position downstream of the rotor blade row would have great potential.