Cold and hot air injection upstream of the first rotor tip of a multistage compressor was tested experimentally. The compressor operating range was extended toward lower mass flow by more than 60% indicating a better throttling capability when air injection was activated. A strong dependency of the stability enhancement on the injected mass flow and injection velocity was found. Both increasing injection mass flow rate and increasing injection velocity led to a considerable extension of the throttling line. Comparable enhancements were achieved when reducing the number of nozzles and hence the injection mass flow. It was also found that injection of hot air, at temperatures comparable to air that bled off at a following stage, had no penalty on the stability enhancement. Investigation of the influence of air injection on radial work distribution showed that only small amounts of injected air were sufficient to lead to a significant radial work redistribution. This in turn changed the operating point of the first stage, leading to axial rematching and thus changed the whole operational behavior of the compressor.

This paper majorly aims to identify and understand the driving flow phenomena when the blading aspect ratio of a 1.5-stage axial compressor is increased so that its overall axial length is reduced. The blading is representative for a state-of-the-art high-pressure compressor (HPC) front-stage design. As part of the investigation steady-state RANS simulations are performed to evaluate the impact on its performance and operability. Moreover, an optimized high aspect ratio (HAR) design is introduced to recover performance penalties. In order to achieve the desired reduction in axial stage length at constant blade row spacing and blade height, numerous possible combinations of increased rotor and stator aspect ratios exist. The impact on compressor efficiency and surge margin will be more or less severe, depending on the chord length reduction in rotor and stator. One intermediate combination of both changes in rotor and corresponding stator aspect ratio is analyzed in detail. The results show that by reducing rotor chord length, the compressor’s stability is predominantly compromised, whereas a shorter stator chord has a bigger impact on efficiency than the rotor. For each HAR configuration, profile loss is increased through a reduced blade chord Reynolds number and a higher profile edge thickness-to-chord ratio. Secondary loss is significantly reduced. However, this effect is extenuated by an increased endwall boundary layer thickness-to-chord ratio. Ultimately, this yields a diminished overall stage efficiency. In general, current HPC blade designs exhibit a lower initial rotor aspect ratio compared to the stator vanes. Consequently, an equivalent stage length reduction has a less crucial impact on Reynolds number — hence profile loss — for rotor blades than for stator vanes. Thus, regarding efficiency, there is an optimum of balancing rotor and stator chord length reduction yielding the least efficiency drop. On the contrary, the stability margin for the compressor stage analyzed is primarily driven by the rotor’s clearance-to-chord ratio. Hence, at constant tip clearance an increase in the rotor’s aspect ratio is proportional to the resulting lack of stability. However, specific compressor design modifications are introduced in order to recover the stability margin without adversely affecting design point efficiency, such that the optimized HAR compressor stage exhibits at least the same performance specifications of the baseline design. This study’s findings also encourage that increasing the blading aspect ratio is a feasible measure for reducing the compressor’s overall axial length aiming a compact design. An optimized HAR compressor allows additional design flexibility, which provides potential for performance improvements.

Steady tip injection with re-circulated air as a reliable method for high pressure compressor stabilization has the disadvantage to reduce the efficiency due to the usage of already compressed air. Therefore, the minimization of recirculation air and/or a large extension of the stable operation range is mandatory. The paper describes an alternative approach by using an active compressor stability control system with modulating valves. The system was tested at a high pressure compressor. It recognized evolving stall cells reliable. The achieved available stable throttling range of the compressor (in terms of mass flow) was more than doubled by active controlled injection compared to steady tip injection at 90% aerodynamic speed. The other way round, depending on the tip flow structure and the aerodynamic speed injection mass flow saving up to 50% at a similar surge margin compared to steady injection was achieved.

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Within the European research project NEWAC (New Aero Engine Core Concepts), a multi-stage highpressure compressor equipped with a tip injection system upstream of the first rotor was tested in three different configurations at MTU Aero Engines. One aim of the test campaign was to investigate the effects of tip injection on the compressor performance. This paper gives an overview of the influences of tip injection on the characteristics of the first three stages. Following an outline of the motivation for tip injection, it is assessed to what extent the surge behavior of the tested compressor is affected by tip injection. The assessment is made by evaluating the surge line extension due to tip injection. If the injection system is applied in an engine, re-matching of its turbo components will occur. Due to this fact, the focus is placed on evaluating the benefits afforded by the use of the injection system in an aircraft engine. The test results are integrated in an existing engine model for a next generation geared turbofan engine and off-design simulations are performed. In this way, changes in surge margin due to tip injection are evaluated. In addition to mere tip injection tests, measurement data of test cases is analyzed, in which mass flow recirculation was simulated. The evaluation of these tests is discussed analogously to the test cases of tip injection. It is found that tip injection prevents the generation of stall cells almost completely. The analyses also show that the stage matching of the multi-stage compressor is changed by tip injection at the front stage. According to the synthesis calculations carried out, recirculation increased the surge margin at part speed by up to about 35% relative to the reference compressor without tip injection.