Reinhard Niehuis

Clocking Effects in a 1.5 Stage Axial Turbine—Steady and Unsteady Experimental Investigations Supported by Numerical Simulations

The interaction between rotor and stator airfoils in a multistage turbomachine causes an inherently unsteady flow field. In addition, different relative circumferential positions of several stator rows and rotor rows, respectively, have an influence on the flow behavior in terms of loss generation, energy transport and secondary flow. The objective of the presented study is to investigate the effects of stator airfoil clocking on the performance of a 1-1/2 stage axial cold air turbine. The investigated axial turbine consists of two identical stators. The low aspect ratio of the blades and their prismatic design leads to a three-dimensional outlet flow with a high degree of secondary flow phenomena. Nevertheless, the small axial gaps between the blade rows are responsible for strong potential flow interaction with the radial wake regions in the measurement planes. Consequently, parts of the wakes of the first stator are clearly detected in the rotor outlet flow. To give an overview of the time-averaged flow field, measurements with pneumatic probes are conducted behind each blade row at ten different clocking-positions of the second stator. Further, an optimized clocking position was found due to a minimum in pressure loss behind the second stator. The unsteady measurements are carried out with hot-wire probes for three selected stator-stator positions. Animations of selected flow properties show the influence of different circumferential positions of the second stator on the unsteady flow behavior and secondary flow field. In addition and compared with experimental results three-dimensional unsteady viscous flow computations are performed.

A Study on Impeller-Diffuser Interaction—Part I: Influence on the Performance

The interaction between impeller and diffuser is considered to have strong influence on the flow in centrifugal compressors. However the knowledge about this influence is still not satisfying. This two-part paper presents an experimental investigation of the effect of impeller-diffuser interaction on the unsteady and the time averaged flow field configuration in impeller and diffuser and the performance of these components. The flat wedge vaned diffuser of the investigated compressor allows an independent adjustment of diffuser vane angle and radial gap between impeller exit and diffuser vane inlet. Attention is mainly directed to the radial gap, as it determines the intensity of the impeller-diffuser interaction. Part I deals with the integral flow losses and the diffusion in impeller diffuser and the entire compressor. An extensive test series with steady probe measurements at impeller exit and diffuser exit was performed at 10 different diffuser geometries and different operating points. The results show that in most cases smaller radial gaps are leading to a more homogeneous flow field at diffuser vane exit and to a higher diffuser pressure recovery resulting in a higher compressor efficiency. On the other hand, impeller efficiency is hardly affected by the radial gap. In Part II, measurements with a laser-2-focus velocimeter are presented illuminating the reasons for the effects found. The experimental results are published as an open CFD test case under the name Radiver .

Clocking Effects in a 1.5 Stage Axial Turbine: Steady and Unsteady Experimental Investigations Supported by Numerical Simulations

The interaction between rotor and stator airfoils in a multistage turbomachine causes an inherently unsteady flow field. In addition, different relative circumferential positions of several stator rows and rotor rows, respectively, have an influence on the flow behaviour in terms of loss generation, energy transport and secondary flow. The objective of the presented study is to investigate the effects of stator airfoil clocking on the performance of an 1-1/2 stage axial cold air turbine. The investigated axial turbine consists of two identical stators. The low aspect ratio of the blades and their prismatic design leads to a three-dimensional outlet flow with a high degree of secondary flow phenomena. Nevertheless, the small axial gaps between the blade rows are responsible for strong potential flow interaction with the radial wake regions in the measurement planes. Consequently, parts of the wakes of the first stator are clearly detected in the rotor outlet flow.To give an overview of the time-averaged flow field, measurements with pneumatic probes are conducted behind each blade row at ten different clocking-positions of the second stator. Further, an optimised clocking position was found due to a minimum in pressure loss behind the 2nd stator. The unsteady measurements are carried out with hot-wire probes for three selected stator-stator positions. Animations of selected flow properties show the influence of different circumferential positions of the second stator on the unsteady flow behaviour and secondary flow field. In addition and compared with experimental results three-dimensional unsteady viscous flow computations are performed.Copyright

A Study on Impeller-Diffuser Interaction—Part II: Detailed Flow Analysis

The interaction between impeller and diffuser is considered to have strong influence on the flow in centrifugal compressors. However, the knowledge about this influence is still not satisfying. This two-part paper presents an experimental investigation of the effect of impeller-diffuser interaction on the unsteady and the time-averaged flow field in impeller and diffuser and the performance of these components. The flat wedge vaned diffuser of the investigated compressor allows an independent adjustment of diffuser vane angle and radial gap between impeller exit and diffuser vane inlet. Attention is mainly directed to the radial gap, as it determines the intensity of the impeller-diffuser interaction. In Part I it was shown that smaller radial gaps improve diffuser pressure recovery, whereas impeller efficiency is hardly affected. Part II focuses on the reasons for these effects. Measurements with a laser-2-focus velocimeter in the highly unsteady flow field between the impeller exit region and diffuser throat were performed at three different diffuser geometries allowing a detailed flow analysis. Especially the unsteady results show that for a smaller radial gap more impeller wake fluid is conveyed towards the highly loaded diffuser vane pressure side reducing its loading and leading to a better diffusion in the diffuser channel. Concerning the impeller flow, it was found that a smaller radial gap is leading to a noticeable reduction of the wake region at impeller exit. The experimental results are intended to be published as an open CFD test case under the name Radiver.

A Study on Impeller-Diffuser Interaction: Part II — Detailed Flow Analysis

The interaction between impeller and diffuser is considered to have strong influence on the flow in highly loaded centrifugal compressors. However, the knowledge about this influence is still not satisfying. This two-part paper presents an experimental investigation of the effect of impeller-diffuser interaction on the unsteady and the time averaged flow configuration in impeller and diffuser and the performance of these components. The flat wedge vaned diffuser of the investigated stage allows an independent adjustment of diffuser vane angle and radial gap between impeller exit and diffuser vane inlet. Attention is mainly directed to the radial gap, as it determines the intensity of the impeller-diffuser interaction. In part I it was shown that smaller radial gaps improve diffuser pressure recovery, whereas impeller efficiency is hardly affected. Part II focuses on the reasons for these effects. Measurements with a laser-2-focus velocimeter in the highly unsteady flow field between the impeller exit region and diffuser throat were performed at three different diffuser geometries allowing a detailed flow analysis. Especially the unsteady results show that for a smaller radial gap more impeller wake fluid is conveyed towards the highly loaded diffuser vane pressure side reducing its loading and leading to a better diffusion in the diffuser channel. Concerning the impeller flow, it was found that a smaller radial gap is leading to a noticeable reduction of the wake region at impeller exit. The experimental results are intended to be published as an open CFD testcase under the name “Radiver”.Copyright

Numerical Investigation of the Influence of the Tip Clearance on Wake Formation Inside a Radial Impeller

The objective of the presented work is to investigate the influence of the tip clearance on the wake formation inside a radial impeller. The position and size of the wake region does not only depend on the clearance height, but also on the distribution of the clearance gap along the blade chord. In order to examine this influence, several calculations have been performed with a three dimensional Navier-Stokes flow solver on a centrifugal impeller, which was experimentally investigated in much detail at Aachen University. The original clearance gap was 0.7 mm at the leading edge and 0.48 mm at the trailing edge. These values were independently varied in the computations, such that different distributions of clearance heights could be chosen. The wake position of the smallest clearance height at the leading and trailing edge was closest to the pressure side. The calculations show, that a relatively large clearance height at the leading edge combined with a small height at the trailing edge move the wake further to the suction side, which corresponds very well with the experimental results. Reasons for that behavior are discussed in the paper.Copyright

Improving 3D Flow Characteristics in a Multistage LP Turbine by Means of Endwall Contouring and Airfoil Design Modification: Part 2 — Numerical Simulation and Analysis

Endwall losses contribute significantly to the overall losses in modern turbomachinery, especially when aerodynamic airfoil load and pressure ratios are increased. Hence, reducing the extend and intensity of the secondary flow structures helps to enhance overall efficiency. This work will focus on secondary flow reduction in typical aero engine low pressure turbines. From the large range of viable approaches, a promising combination of axis symmetric endwall contouring and 3D airfoil thickening was chosen. Aerodynamic design, experimental verification and further analysis based on numerical simulation are described in a two part paper. In the second part the implications of the 3D modifications on the flow structure are analyzed by employing a 3D Navier-Stokes simulation based on the experimental data reported in part one. For obtaining reliable flow simulations at typical LP turbine conditions, it is important to apply a 3D Navier-Stokes solver with proven turbulence and transition modeling to the three-stage LP turbine of the Institute of Aeronautical Propulsion at Stuttgart University. Numerical and experimental results exhibit regions, where the modified design leads to a change in flow pattern in accordance with the design intent, as well as regions with an actual increase in loss production. The flow changes in both regions are evaluated and discussed. It is found that a certain local loss increase phenomenon can also be found in other LP turbine rigs. The reasons for this behavior are analyzed by a comparison with data from other turbine rigs and by an additional variation of the 3D design of the first stage of the investigated turbine.Copyright

Unsteady Navier-Stokes Simulation of a Transonic Flutter Cascade Near-Stall Conditions Applying Algebraic Transition Models

Due to the trend in the design of modern aeroengines to reduce weight and to realize high pressure ratios, fan and first-stage compressor blades are highly susceptible to flutter. At operating points with transonic flow velocities and high incidences, stall flutter might occur involving strong shock-boundary layer interactions, flow separation, and oscillating shocks. In this paper, results of unsteady Navier-Stokes flow calculations around an oscillating blade in a linear transonic compressor cascade at different operating points including near-stall conditions are presented. The nonlinear unsteady Reynolds-averaged Navier-Stokes equations are solved time accurately using implicit time integration. Different low-Reynolds-number turbulence models are used for closure. Furthermore, empirical algebraic transition models are applied to enhance the accuracy of prediction. Computations are performed two dimensionally as well as three dimensionally. It is shown that, for the steady calculations, the prediction of the boundary layer development and the blade loading can be substantially improved compared with fully turbulent computations when algebraic transition models are applied. Furthermore, it is shown that the prediction of the aerodynamic damping in the case of oscillating blades at near-stall conditions can be dependent on the applied transition models.

Numerical Simulation of the Boundary Layer Transition in Turbomachinery Flows

The objective of the presented work is to investigate models which simulate boundary layer transition in turbomachinery flows. This study focuses on separated-flow transition. Computations with different algebraic transition models are performed three-dimensionally using an implicit Navier-Stokes flow solver. Two different test cases have been chosen for this investigation: First, a linear transonic compressor cascade, and second an annular subsonic compressor cascade. Both test cases show three-dimensional flow structures with large separations at the side-walls. Additionally, laminar separation bubbles can be observed on the suction and pressure side of the blades of the annular subsonic cascade whereas a shock-induced separation can be found on the suction side of the blades of the linear transonic cascade. Computational results are compared with experiments and the effect of transition modeling is analyzed. It is shown that the prediction of the boundary layer development can be substantially improved compared to fully turbulent computations when algebraic transition models are applied.Copyright

Parametric Study of Fluidic Oscillators for Use in Active Boundary Layer Control

A parametric study was conducted to identify the main factors influencing the frequency produced by fluidic oscillators with the goal of using the actuator to trigger boundary layer transition through the excitation of Tollmien Schlichting waves. Test bench conditions were chosen to match the static pressure at the actuation position on the candidate blade profile for a cascade exit Mach number of 0.6 and Reynolds numbers from 60,000 to 200,000. The inlet vs. outlet pressure ratio and the position and geometry of the outlet holes were all varied. Additionally, the effect of the oscillator’s scale and the feedback channel geometry were considered. The flow at the exit was measured using a hot wire, while Kulite pressure transducers were used to measure pressure fluctuations within the device. This paper shows that fluidic oscillators can achieve frequencies of 10 kHz and that the parameters considered play an important role in the performance of these devices.Copyright