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Rotor blade vibrations observed in the Darmstadt transonic compressor rig are investigated in this paper. The vibrations are nonsynchronous and occur in the near stall (NS) operating region. Rotor tip flow fluctuations traveling near the leading edge (LE) against the direction of rotation (in the rotor relative frame of reference) with about 50% blade tip speed are found to be the reason for the occurrence of the vibrations. The investigations show that the blockage at the rotor tip is an important factor for the aeroelastic stability of the compressor in the NS region. It is found that by application of a recirculating tip injection (TI) casing treatment, the aeroelastic stability increases as a result of reduced blockage in the rotor tip region.

The numerical and experimental investigation of non-axisymmetric casing treatments is complex compared to axisymmetric ones like circumferential grooves. Hence they are still rarely investigated, at least regarding combinations of numerical and experimental work on the same configuration. The casing treatment under investigation is capable of significantly broadening the operating range of a tip-critical transonic compressor. This is accomplished without an efficiency penalty at design conditions. It is demonstrated that the application of a casing treatment over the rotor might impose negative effects on the downstream stator that would have to be considered in future designs. Earlier work by the authors was focused on the experimental investigation, in particular of transient effects near stall. In this paper, RANS and URANS simulations are presented in detail. These are used to further study and explain the causes of the effects that were found experimentally. Additional experimental data are added where appropriate, continuing the validation of the numerical methods.

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.

Application of non-axisymmetric Casing Treatments (CTs) can extend the operating range of a transonic compressor significantly. Recent CT designs have proven successful at achieving operating range extension without efficiency loss under design conditions. Two different CT designs were investigated on a high-speed one and a half stage test rig using extensive instrumentation. The stage setup is representative of the front stage of a modern high-pressure compressor. Results of Particle Image Velocimetry (PIV) measurements taken in the blade tip region underneath the Casing Treatment show a significantly modified flow structure compared to the Smooth Casing reference case. Blockage zone, secondary flow and shock structures are affected by the CT, especially in highly throttled operating conditions. The stall inception process of the system with Axial Slots shows unexpected behavior, with modal activities that are not observed without CT. These activities are resolved using unsteady wall pressure and Hot Wire measurements.

A state-of-the-art transonic compressor rotor has a distinct potential for increased efficiency if modified for improved interaction with an axial-slot type casing treatment. Reducing the number of blades and thus the surface lowers friction losses but increases tip clearance effects and deteriorates the stall margin due to the higher aerodynamic blade loading. The latter two negative effects can be compensated for by the casing treatment, thus restoring the required stall margin and gaining an overall reduction of losses. For the specific compressor rotor under investigation, the potential in polytropic efficiency is as high as 0.7%. The present study was performed using time-accurate CFD (URANS) simulations. Both the reference rotor as well as the modified design are analyzed regarding their interaction with the casing treatment. The traceability of the conclusions is assured by interpreting the detailed flow phenomena. The newly designed rotor is found to be favorably influenced by the casing treatment at design operating conditions whilst the reference only benefits at throttled operating points. Casing treatments are commonly used to broaden the operating range of existing compressors without changing the design of the compressor rotor itself. This study aims to show the possible transformation of this potential in the stall margin into efficiency at design operating conditions by implementing an appropriate rotor design.

Variable inlet guide vanes enhance the efficiency and stability of modern transonic compressors. The current quest for compact and highly efficient aero-engines requires for higher stage-loading and small axial gaps between adjacent blade rows, increasing interaction between blade rows and introducing further unsteadiness into the flow. Modeling these interactions is relevant to jet engine implementation. Recent advancements in numerical simulation of unsteady flow in multiple blade rows create an additional way to investigate the flow patterns formed by the unsteady interaction. In the case of transonic compressors, the experimental and numerical database is small. The current article will contribute to this database by investigating the flow downstream of an inlet guide vane and the influence this flow has on the passage in a transonic compressor. Unsteady flow phenomena are resolved by piezoresistive wall pressure tappings. Numerical and experimental data show that the interaction of blade rows influences the formation of the tip leakage vortex. Analyzing the vortex structure within the transonic compressor stage it is possible to show how the blade rows interact and how geometric modifications in the VIGV tip gap model influence simulation results.

This paper presents the experimental investigation of the stall inception of a state-of-the-art transonic compressor that resembles a typical front stage of a commercial jet engine. The compressor was designed and manufactured in cooperation with Rolls-Royce Deutschland and assembled in the Darmstadt Transonic Compressor test facility at Technische Universität Darmstadt, Germany. Utilising the 1.5-stage design it is possible to investigate stall inception for different rotor inflow conditions. The aerodynamic features prior and during stall inception are investigated by means of unsteady pressure probes in the casing above the rotor. All stall inception events have a spike-type stall character. However, in some cases the spikes are preceded by modal-type activities. In addition to the unsteady wall pressure, speed lines are presented. The combination of the data shows a clear effect of measurement position and averaging process on the shapes of the speed lines but did not allow to predict the type of stall inception.