The paper describes the flow conditions found inside an optimized transonic, high pressure ratio centrifugal compressor rotor operated with a rotor tip speed of 586 m/s. The rotor was coupled with a vaned diffuser of rather small exit radius ratio. Measurements and calculations were carried out on this device. A newly developed 3-component hybrid Laser velocimeter was applied for the optical measurements inside the rotating system. For the first time the new measurement technique delivered three velocity components instead of only two for such a compact machine with very narrow flow channels. An available 3D calculation system with an embedded Navier-Stokes solver was used for the theoretical approach. The flow field development is described with the help of optical measurement results taken at measurement planes located at the rotor inlet and exit as well as at 40% and 60% flow path length. The measured 3D effects were highest in the middle of the flow channel, where the flow is turned from axial to radial. Here the curvature effects in the meridional plane and the enhancement of secondary flow are significant. The agreement between the theoretically and experimentally obtained third velocity component is quite good, especially in the area with strong secondary flows.
For the development of the latest generation of axial compressors, it is necessary to enlarge the design space by using advanced aerodynamic processes. This enables a further increase in efficiency and performance. The use of a tandem blade configuration in a transonic compressor row provides the possibility to enlarge the design space. It is necessary to address the design aspects a bit more in detail in order to efficiently apply this blading concept to turbomachinery. Therefore, in the current study, the known design aspects of tandem blading in compressors will be summed up under the consideration of the aerodynamic effects and construction characteristics of a transonic compressor tandem. Based on this knowledge, a new transonic compressor tandem cascade (DLR TTC) with an inflow Mach number of 0.9 is designed using modern numerical methods and a multi-objective optimization process. Three objective functions as well as three operating points are used in the optimization. Furthermore, both tandem blades and their arrangement are parameterized. From the resulting database of 1246 members, a final best member is chosen as the state-of-the-art design for further detailed investigation. The aim of the ensuing experimental and numerical investigation is to answer the question, whether the tandem cascade resulting from the modern design process fulfills the described design aspects and delivers the requested performance and efficiency criteria. The numerical simulations within the study are carried out with the DLR flow solver TRACE. The experiments are performed at the transonic cascade wind tunnel of DLR in Cologne. The inflow Mach number during the tests is 0.9, and the AVDR is adjusted to 1.3 (design value). Wake measurements with a three-hole probe are carried out in order to determine the cascade performance. The experimental results show an increase in losses and a reduction of the cascade deflection by about 2 deg compared to the design concept. Nevertheless, the experimental and numerical results allow a good understanding of the aerodynamic effects. In addition, planar PIV was applied in a single S1 plane located at midspan to capture the velocity field in the wake of blade 1 in order to analyze the wake flow in detail and describe its influence on the cascade deflection and loss behavior. Finally, an outlook will be given on what future tandem compressor research should be focused.
A variety of measurement techniques have been applied to investigate the flow phenomena occurring in advanced turbomachinery components. In an ongoing research project the performance and internal flow field of a high pressure ratio centrifugal compressor is being investigated. Based on the results obtained the compressor was redesigned and resulted in an improved impeller and diffuser. The stage was designed for a total pressure ratio of 6:1 and a mass flow rate of 2.55 kg/s, the corresponding rotor tip speed was 586 m/s, resulting in transonic flow conditions at the inlet of the diffuser vane passage at a static temperature of 260°C. The improvement of the impeller flow and the broadening of the stable operating range was predicted by CFD and successfully verified by means of laser-2-focus (L2F) measurements.Current activities in the research program involve the use of particle image velocimetry (PIV) to analyze and further improve the understanding of the complex flow phenomena inside the vaned diffuser. The study includes phase-resolved measurements of the flow inside a diffuser vane passage with respect to the impeller blade position. Both, instantaneous and phase-averaged velocity fields will be presented.Optical access is given via a quartz glass window, which was manufactured to match the 3-D contour of the diffuser casing in order to minimize disturbance of the near-wall flow. An adjustable periscope light sheet probe was specifically designed for this application and delivered the laser light sheet (LLS) to the desired vane span positions for the flow field investigation. The periscope probe was purged with compressed air during operation for cooling and to protect the open outlet of the probe from seeding particle deposits.The flow field results obtained with PIV will be used for validation of the CFD calculations. Based on this approach a better understanding of the complex flow characteristics is expected, which will finally result in a further improvement of the compressor performance.
The flow through a transonic compressor cascade shows a very complex structure due to the occurring shock waves. In addition, the interaction of these shock waves with the blade boundary layer inherently leads to a very unsteady flow behavior. The aim of the current investigation is to quantify this behavior and its influence on the cascade performance as well as to describe the occurring transonic flow phenomena in detail. Therefore, an extensive experimental investigation of the flow in a transonic compressor cascade has been conducted within the transonic cascade wind tunnel of DLR Institute of Propulsion Technology at Cologne. In this process, the flow phenomena were thoroughly examined for an inflow Mach number of 1.21. The experiments investigate both the laminar and the turbulent shock wave boundary layer interaction within the blade passage and the resulting unsteady behavior. The experiments show a fluctuation range of the passage shock wave of about 10% chord for both cases, which is directly linked with a change of the inflow angle and of the operating point of the cascade. Thereafter, Reynolds-averaged Navier–Stokes (RANS) simulations have been performed aiming at the verification of the reproducibility of the experimentally examined flow behavior. Here, it is observed that the dominant flow effects are not reproduced by a steady numerical simulation. Therefore, a further unsteady simulation has been carried out to capture the unsteady flow behavior. The results from this simulation show that the fluctuation of the passage shock wave can be reproduced but not in the correct magnitude. This leads to a remaining weak point within the design process of transonic compressor blades because the working range will be overpredicted. The resulting conclusion of this study is that the use of scale-resolving methods such as LES or the application of DNS is necessary to correctly predict unsteadiness of the transonic cascade flow and its impact on the cascade performance.
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