Anton Weber

Numerical Investigation of Casing Treatment Mechanisms With a Conservative Mixed-Cell Approach

A conservative mixed-cell approach of second-order accuracy is presented and applied to investigate the mechanisms of a self-recirculating casing treatment coupled with a transonic compressor rotor. The mixed cell is a computational cell that may show up at the zonal interface boundary, the face of which is partially solid and partially fluid, if the azimuthal open area of casing treatment does not fully contact with the whole annulus of blade passage. The mixed-cell approach is essentially an extension of the conservative zonal approach by incorporating special mixed-cell handling at the zonal interface and it allows a great flexibility to the grid generation for the patched zones with the best grid topology. The mixed-cell approach is extremely useful for solving the unsteady interaction problems within turbomachinery and its application for simulating the coupled flow through the rotor and the casing treatment is reported. The calculated results and analysis reveal an effective stall margin extension of the casing treatment herein by weakening or even destroying the tip leakage vortex, and expose the different tip flow topologies between the cases with the casing treatment and with the untreated smooth wall. It is found that the casing treatment only slightly decreases the overall efficiency at the design point, but it is beneficial to the overall efficiency at the off-design operating conditions and it can improve the inflow conditions to the downstream stator blade row as well.© 2003 ASME

3D Transonic Flow in a Compressor Cascade With Shock-Induced Corner Stall

An experimental and numerical study of the transonic flow through a linear compressor cascade with endwalls was conducted. The cascade with a low aspect ratio of 1.34 was tested at an inlet Mach number of 1.09 and a Reynolds number of 1.9×106. Detailed flow visualizations on the surfaces and 5-hole probe measurements inside the blading and in the wake region showed clearly a 3-dimensional boundary layer separation on the blade surface and the sidewall, and a severe corner stall induced by a strong 3D shock system at blade passage entrance.The experimental data has been used to validate and improve the 3D Navier-Stokes code TRACE. Results showed an excellent resolution of the complex flow field. Surface pressure distributions on the entire blade surface and the endwalls, flow angle and total pressure contours within the blade passage and the wake are compared with the experimental results. An analysis of the secondary flow of this highly staggered cascade did not show the classical corner vortex. Instead, a severe flow deviation and partly reverse flow near the walls is seen. The flow solver helped to identify a weak ring vortex that originates from the passage sidewall. Surface oil flow pictures on the blade contour and the sidewall are in qualitatively good agreement to numerical surface streaklines. A considerable improvement of the numerical results could be achieved by a gradual grid refinement especially in the corner region and by successive code development.Copyright

3-D Transonic Flow in a Compressor Cascade With Shock-Induced Corner Stall

An experimental and numerical study of the transonic flow through a linear compressor cascade with endwalls was conducted. The cascade with a low aspect ratio of 1.34 was tested at an inlet Mach number of 1.09 and a Reynolds number of 1.9 310 6 . Detailed flow visualizations on the surfaces and five-hole probe measurements inside the blading and in the wake region showed clearly a three-dimensional boundary layer separation on the blade surface and the sidewall, and a severe corner stall induced by a strong 3-D shock system at blade passage entrance. The experimental data have been used to validate and improve the 3-D Navier-Stokes code TRACE. Results showed an excellent resolution of the complex flow field. Surface pressure distributions on the entire blade surface and the endwalls, flow angle and total pressure contours within the blade passage and the wake are compared with the experimental results. An analysis of the secondary flow of this highly staggered cascade did not show the classical corner vortex. Instead, a severe flow deviation and partly reverse flow near the walls is seen. The flow solver helped to identify a weak ring vortex that originates from the passage sidewall. Surface oil flow pictures on the blade contour and the sidewall are in qualitatively good agreement to numerical surface streaklines. A considerable improvement of the numerical results could be achieved by a gradual grid refinement, especially in the corner region and by successive code development.@DOI: 10.1115/1.1460913#

Influence of Blade Fillets on the Performance of a 15 stage Gas Turbine Compressor

In the modern process of the aerodynamic design of multistage compressors and turbines for jet engines as well as for stationary gas turbines, 3D-CFD plays a key role. Before building the first test rig several designs have been investigated using numerical simulations. To understand the characteristics of the individual components it is necessary to simulate their behavior in a multistage simulation and investigate for example, the single stage maps of the compressor in order to understand how the load is divided between the different parts of the compressor during throttling. Increasing computing resources allow ever more details to be incorporated in a 3D simulation. In former times only single blade rows were investigated with a high resolution of the boundary layers, whereas in multistage configurations wall functions were state of the art. Today we are able to apply Low Reynolds resolution even for multistage configurations, so the designer is required to include more and more geometrical details into the simulation. One important such feature is the fillets of rotor and stator blades. Fillets reduce the flow deflection at the endwalls and therefore the loading of the downstream blade rows. This effect is accumulated in a multistage simulation. In this paper a 15-stage compressor with additional inlet and outlet guide vane designed for a stationary gas turbine was investigated with a modern CFD tool by using a real gas approach for two speedlines. Two simulations were done: first a clean configuration with tip and hub clearances but without blade fillets; in the second simulation all rotor blades and the cantilevered stator blades were additionally modeled with fillets. The comparison of the overall global values with measurement data shows a better performance of the simulation with fillets, especially by throttling the compressor. A deeper look into the compressor shows different loads for a considerable number of single stages. The analysis of the steady multistage simulations shows that the numerical stability is reached in different regions of the machine.Copyright

Design, Optimization, and Analysis of a High-Turning Transonic Tandem Compressor Cascade

As a feasibility study for a stator guide vane a highly loaded transonic compressor stator blade row was designed, optimized, and tested in a transonic cascade facility.The flow entering the turning device with an inlet Mach number of 1.06 has to be turned by more than 60° and diffused extremely to leave the blade row without swirl. Therefore, the basic question was: Is it feasible to gain such a high amount of flow turning with an acceptable level of total pressure losses?The geometric concept chosen is a tandem cascade consisting of a transonic blade row with a flow turning of 10° followed by a subsequent high-turning subsonic cascade. The blade number ratio of the two blade rows was selected to be 1:2 (transonic: subsonic).Design and optimization have been performed using a modern Navier-Stokes flow solver under 2D assumptions by neglecting side wall boundary-layer effects. In the design process it was found to be necessary to guide the wake of the low turning transonic blade near the suction surface of the subsonic blade. Furthermore, it is advantageous to enlarge the blade spacing of the ‘wake’ passage in relation to the neighbouring one of the high turning part.The optimized design geometry of the tandem cascade was tested in the transonic cascade windtunnel of the DLR in Cologne. At design flow conditions the experiments confirmed the design target in every aspect. A flow turning of more than 60°, a static pressure ratio of 1.75, and a total pressure loss coefficient of 0.15 was measured. The working range at design inlet Mach number of 1.06 is about 3.5° in terms of the inlet flow angle. A viscous analysis of various operating points showed excellent agreement with the experimental results.Copyright

Improved Compressor Exit Diffuser for an Industrial Gas Turbine

The aerodynamical design process used for the optimization of a centrifugal compressor stage is described. Advanced design tools for blade generation and 3D aerodynamic calculation methods were used. The manufacturing procedure and experimental verification is described.

Experimental And Numerical Verification of an Optimization of a Fast Rotating High Performance Radial Compressor Impeller

Outgoing from a well-proven radial compressor design which has been extensively being tested in the past known as SRV4 impeller (Krain impeller), an optimization has been performed using the AutoOpti tool developed at DLR’s Institute of Propulsion Technology. This tool has shown its capability in several tasks, mainly for axial compressor and fan design as well as for turbine design. The optimization package AutoOpti was applied to the redesign and optimization of a radial compressor stage with a vaneless diffusor. The numerical results of this optimization were presented by Voss et al. [1] and by Raitor et al. [2]. The optimization was performed for the SRV4 compressor geometry without fillets using a relatively coarse structured mesh in combination with wall functions. The impeller geometry deduced by the optimization had to be slightly modified due to manufacturing constraints. In order to filter out the improvements of the new so-called SRV5 radial compressor design, two work packages were conducted: The first one was the manufacturing of the new impeller and its installation on a test rig to investigate the complex flow inside the machine. The aim was, first of all, the evaluation of a classical performance map and the efficiency chart achieved by the new compressor design. The efficiencies realized in the performance chart were enhanced by nearly 1.5 %. A 5 % higher maximum mass flow rate was measured in agreement with the RANS simulations during the design process. The second work package comprises the CFD analysis. The numerical investigations were conducted with the exact geometries of both, the baseline SRV4 as well as the optimized SRV5 impeller including the exact fillet geometries. To enhance the prediction accuracy of pressure ratio and impeller efficiency the geometries were discretized by high resolution meshes of approximately 5 million cells. For the blade walls as well as for the hub region the mesh resolution allows a low-Reynolds approach in order to get high quality results. The comparison of the numerical predictions and the experimental results shows a very good agreement and confirms the improvement of the compressor performance using the optimization tool AutoOpti.

Investigation of Unsteady Flow Effects in an Axial Compressor Based on Whole Annulus Computations

Flows in the first stages of axial compressors are subject to a number of unsteady effects which are not generally taken into account in standard investigations, e.g. transition under the influence of impinging wakes as well as that of downstream potential effects, or the influence of unsteady blade row interactions. For numerical studies of these phenomena, usually, simplifications are made to the test case in question. These simplifications include domain scaling, quasi-unsteady modeling, or modeling assumptions regarding boundary layer development, like wall functions; and their impact on the final result can be quite high. This paper is concerned with the investigation of unsteady blade row interactions in a 4.5 stage research compressor without any of these simplifications. To this end, the whole annulus of the first two stages of the compressor was meshed and unsteady RANS simulations were carried out at two different operating points. Spatial and temporal resolutions were fine enough to allow the investigation of transitional phenomena on the blade surface using an integral multi-mode transition model. For each operating point, one revolution was recorded after computations reached a periodic state. Examination of the boundary layer parameters shows that transition plays a considerable role for the first two compressor stages which were investigated in more detail. It was evident that the development of the blade boundary layer depends not only on incoming wakes from upstream blade rows; the precise transition location is also highly dependent on the potential effect of downstream blade rows.Copyright

On the Simulation of Unsteady Turbulence and Transition Effects in a Multistage Low Pressure Turbine: Part III — Comparison of Harmonic Balance and Full Wheel Simulation

This is the third part of a series of three papers on the simulation of turbulence and transition effects in a multistage low pressure turbine. The third part of the series deals with the detailed comparison of the harmonic balance calculations with the full wheel simulations and measurements for the two-stage low-pressure turbine. The harmonic balance calculations were taken into account turbulence and transition either only once in the 0th harmonic and the other times in all harmonics. The same Wilcox’s two-equation k − omega turbulence model along with Menter and Langtry’s two-equation  Gamma− Re_Theta transition model is used in the Harmonic Balance simulation as in the full wheel simulations. The measurements on the second stator of the low-pressure turbine have been carried out separately for downstream and upstream influences. Thus, a dedicated comparison of the downstream and upstream influences of the flow to the second stator is possible. In the Harmonic Balance calculations, the influences of the not directly adjacent blade, i.e. the first stator, were also included in the second stator. In the first analysis, however, it was shown that the consistency with the full wheel configuration and the measurement in this case was not as good as expected. From the analysis of the full wheel simualtion, we found that there is a considerable variation in the magnitude of the unsteady values in the second stator. In a further deeper consideration of the configuration, it is found that modes are reflected in downstream rows and influences flow in the second stator. After the integration of these modes into the harmonic balance calculations, a much better agreement was reached with results the full wheel simulation and the measurements. The second stator has a laminar region on the suction side starting at the leading edge and then transition takes place via a separation or in bypass mode, depending on the particular blade viewed in the circumferential direction. In the area of transition, the clear difference between the calculations without and with consideration of the higher harmonics in the turbulence and transition models can be clearly seen. The consideration of the higher harmonics in the turbulence and transition models brings about an improvement in the consistency.

On the Simulation of Unsteady Turbulence and Transition Effects in a Multistage Low Pressure Turbine: Part II — Full-Wheel Simulation

This is the second part of a series of three papers on the simulation of turbulence and transition effects in a multistage low pressure turbine. In this second part, the investigated two-stage low pressure turbine is described and results of a nonlinear full-wheel time-domain simulation are presented, analyzed and compared with the available experimental data. Furthermore, recent improvements to the CFD solver TRACE are described in brief that lead to significantly reduced wall-clock times for such large scale simulations. The utilized models match those used in the Harmonic Balance (HB) based simulations that are presented in the third paper of this series, such that the full-wheel result can be utilized to validate the HB result. Transition, flow separation and wall pressure fluctuations on the stator blades of the second stage are analyzed in detail. A strong azimuthal Pi-periodicity is observed, manifesting in a significantly varying stability of the midspan trailing edge flow with a quasi-steady closed separation bubble on certain blades and highly dynamic partially open separation bubbles with recurring transition and turbulent reattachment on other blades. The energy spectrum of fluctuating wall quantities in that regime shows a high bandwidth and considerable disharmonic content, which is challenging for the HB based simulations.