High stability and efficiency are the main two objectives in the design of an axial-flow compressor. Stability usually reduces at higher stage loading, and the stability margin critically drops in transient operation and through the life cycle of an engine. A major reason for this to happen is the growing tip gap. A recirculating tip blowing casing treatment has shown the ability to enhance stability. To be able to use it as a stability control system at varying tip clearances in aircraft engines, the behavior of this casing treatment at different tip clearances was considered important and investigated in this paper. The present study investigates in depth the ability of a tip blowing casing treatment to postpone stall at three different tip clearances. The results prove a substantial beneficial effect for design and increased tip gaps and show some negative impact of the casing treatment for a small tip gap. The study is carried out on a 1.5 stage research compressor. The investigated rotor was already investigated with an axial-slot casing treatment for different tip gap heights at the Institute for Flight Propulsion. The design of a recirculating tip blowing casing treatment is simulated with an equivalent numerical setup. A tip blowing casing treatment consists of a bleed port connected to a tip blowing upstream of the rotor. The streamwise pressure gradient drives the tip blowing with a high injection velocity. A design speed line is simulated for three tip clearance values with and without the tip blowing casing treatment. The impact of the interaction between the tip blowing and the tip gap vortex is analyzed. A detailed analysis of the passage flow is conducted. A comparison of the stall margin is made. The study is carried out using URANS simulations.

Stability issues are often restricting the design space of axial-flow compressors. Casing treatments have shown the ability to enhance stability on existing designs in a late design phase. Prior studies have identified the positioning of the casing treatment as an important parameter for both stability and overall efficiency. Particularly for axial casing treatments, the downstream position of the fluid removal was found to be relevant. Within the present study the focus is put on a tip blowing casing treatment consisting of an axially and circumferentially discrete bleed port connected to an upstream injection port. For this kind of casing treatment, an accurate positioning of the fluid removal has high importance. Therefore, in the present study a parametric variation of the bleed port's axial position is carried out at design speed. Minimizing losses at sufficient stall margin improvement requires recirculation mass flows dependent on operating point. In support of that goal of mass flow self-regulation criteria for the fluid removal port position are derived. The analysis of the flow field, of the pressure distribution and of the shock-vortex-interaction help to identify an ideal position. The study is carried out on the isolated rotor of a 1.5 stage research compressor arrangement. NomenclatureCT = casing treatment TBCT = recirculating tip blowing CT CSCT = circumferential-slot CT ASCT = axial-slot CT CTDI = CT duct inlet CTDO = CT duct outlet SC = smooth casing DP = design point operating condition PE = peak efficiency operating condition NS = near stall operating condition SM = stall margin (V)IGV = (variable) inlet guide vane (U)RANS = (unsteady) reynolds averaged navier-stokes y + [-] = non-dimensional wall distance [Pa] = static pressure [Pa] = total pressure ̇ [ / ] = recirculation mass flow rate Π [-] = total pressure ratio [K] = total temperature Ma [-] = Mach number [-] = heat capacity ratio [m²] = nozzle cross section R [J/kg/K] = gas constant Downloaded by UNIVERSITY OF QUEENSLAND on July 30, 2015 | http://arc.aiaa.org |

Tip blowing and axial slot casing treatments have shown their ability to enhance the stability of a transonic axial compressor with different effects on efficiency. For an effective application of these casing treatments, a good knowledge of the influence of the casing treatment on the rotor flow field is important. There is still a need for more detailed investigations, in order to understand the interaction between the treatment and the near casing 3D flow field. For transonic compressor rotors this interaction is more complex, as super- and subsonic flow regions alternate while interacting with the casing treatment. In the present study, an axial slot and a tip blowing casing treatment, which have been developed and optimized for the same tip critical transonic axial compressor rotor (reference rotor) by Streit et al. [1] and Guinet et al. [2], are subject of the investigation. Both casing treatment types showed their capabilities to enhance the compressor stability without losing by means of CFD simulations. Since the higher compressor stability allows a higher blade loading, Streit et al. reduced the blade number of the rotor. Thus, the efficiency was increased due to the reduction of friction losses. However, applying the tip blowing casing treatment to the reduced rotor shows a negative effect on the efficiency. Both casing treatment types recirculate flow from a downstream to an upstream location of the rotor and reinject it to enhance the near casing flow field. Although the working principle of the two casing treatment types are similar, the transfer of the casing treatments from the reference to the reduced rotor show different trends in efficiency. Therefore, the effect of recirculation cannot explain the difference in efficiency. Hence, applying axial slots must include additional flow features, compared to recirculation channels. Compensating effects as in circumferential groove casing treatments and other flow interactions between the near casing flow field and the slot flow are considered. These additional mechanisms of the axial slot casing treatment will be identified and isolated by comparing the two different casing treatment types. The numerical simulations are carried out on a 1.5 stage transonic axial compressor using URANS simulations.

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The design space of axial-flow compressors is restricted by stability issues. Different axial-type casing treatments have shown their ability to enhance compressor stability and to influence efficiency. Casing treatments have proven to be effective, but there still is need for more detailed investigations and gain of understanding for the underlying flow mechanism. Casing treatments are known to have a multitude of effects on the near-casing 3D flow field. For transonic compressor rotors these are more complex, as super- and subsonic flow regions alternate while interacting with the casing treatment. To derive design rules it is important to quantify the influence of the casing treatment on the different tip flow phenomena. Designing a casing treatment in a way that it antagonizes only the deteriorating secondary flow effects can be seen as a method to enhance stability while increasing efficiency. The numerical studies are carried out on a tip-critical rotor of a 1.5 stage transonic axial compressor. The examined recirculating tip blowing casing treatment, which consists of a recirculating channel with an air off-take above the rotor and an injection nozzle in front of the rotor. The design and functioning of the casing treatment is influenced by various parameters. A variation of the geometry of the tip blowing, more specifically the nozzle aspect ratio, the axial position or the tangential orientation of the injection port, was carried out to identify key levers. The tip blowing casing treatment is defined as a parameterized geometric model and is automatically meshed. A sensitivity analysis of the respective design parameters of the tip blowing is carried out on a single rotor row. Their impact on overall efficiency and their ability to improve stall margin is evaluated. The study is carried out using URANS simulations.