Jochen Gier

The Computation of Adjacent Blade-Row Effects in a 1.5-Stage Axial Flow Turbine

This paper deals with the numerical simulation of flow through a 1.5-stage axial flow turbine. The three-row configuration has been experimentally investigated at the University of Aachen where measurements behind the first vane, the first stage, and the full configuration were taken. These measurements allow single blade row computations, to the measured boundary conditions taken from complete engine experiments, or full multistage simulations. The results are openly available inside the framework of ERCOFTAC 1996. There are two separate but interrelated parts to the paper. First, two significantly different Navier-Stokes codes are used to predict the flow around the first vane and the first rotor, both running in isolation. This is used to engender confidence in the code that is subsequently used to model the multiple blade-row tests; the other code is currently only suitable for a single blade row. Second, the 1.5-stage results are compared to the experimental data and promote discussion of surrounding blade row effects on multistage solutions.

Interaction of Shroud Leakage Flow and Main Flow in a Three-Stage LP Turbine

Endwall losses significantly contribute to the overall losses in modern turbomachinery, especially when aerodynamic airfoil load and pressure ratios are increased. In turbines with shrouded airfoils a large portion of these losses are generated by the leakage flow across the shroud clearance. Generally the related losses can be grouped into losses of the leakage flow itself and losses caused by the interaction with the main flow in subsequent airfoil rows. In order to reduce the impact of the leakage flow and shroud design related losses a thorough understanding of the leakage losses and especially of the losses connected to enhancing secondary flows and other main flow interactions has to be understood. Therefore, a three stage LP turbine typical for jet engines is being investigated. For the three-stage test turbine 3D Navier-Stokes computations are performed simulating the turbine including the entire shroud cavity geometry in comparison with computations in the ideal flow path. Numerical results compare favorably against measurements carried out at the high altitude test facility at Stuttgart University. The differences of the simulations with and without shroud cavities are analyzed for several points of operation and a very detailed quantitative loss breakdown is presented.

Influence of Rim Seal Purge Flow on the Performance of an Endwall-Profiled Axial Turbine

Nonaxisymmetric endwall profiling is a promising method to reduce secondary losses in axial turbines. However, in high-pressure turbines, a small amount of air is ejected at the hub rim seal to prevent the ingestion of hot gases into the cavity between the stator and the rotor disk. This rim seal purge flow has a strong influence on the development of the hub secondary flow structures. This paper presents time-resolved experimental and computational data for a one-and-112-stage high work axial turbine, showing the influence of purge flow on the performance of two different nonaxisymmetric endwalls and the axisymmetric baseline case. The experimental total-to-total efficiency assessment reveals that the nonaxisymmetric endwalls lose some of their benefit relative to the baseline case when purge is increased. The first endwall design loses 50% of the efficiency improvement seen with low suction, while the second endwall design exhibits a 34% deterioration. The time-resolved computations show that the rotor dominates the static pressure field at the rim seal exit when purge flow is present. Therefore, the purge flow establishes itself as jets emerging at the blade suction side corner. The jet strength is modulated by the first vane pressure field. The jets introduce circumferential vorticity as they enter the annulus. As the injected fluid is turned around the rotor leading edge, a streamwise vortex component is created. The dominating leakage vortex has the same sense of rotation as the rotor hub passage vortex. The first endwall design causes the strongest circumferential variation in the rim seal exit static pressure field. Therefore, the jets are stronger with this geometry and introduce more vorticity than the other two cases. As a consequence the experimental data at the rotor exit shows the greatest unsteadiness within the rotor hub passage with the first endwall design.

Designing LP Turbines for Optimized Airfoil Lift

In the last 10 to 15 years substantial effort has been spent on increasing the airfoil lift especially in aero engine low pressure turbines. This has been attractive, since increased airfoil lift can be used for airfoil count decrease leading to weight and hardware cost reduction. The challenge with this effort consequently has been to keep the efficiency at high levels. Depending on the baseline level of airfoil lift, an increase of 20% to 50% has been realized and at least partly incorporated in modern turbine designs. With respect to efficiency there is actually an optimum level of airfoil lift. Airfoil rows at a lift level below this optimum suffer from an excessive number of airfoils with too much wetted surface and especially increasing trailing edge losses. Airfoils at lift levels above this optimum suffer from growing losses due to high peak Mach numbers inside the airfoil row, higher rear diffusion on the airfoil suction sides and increasing secondary flow losses. Since fuel cost have been rising significantly as has been the awareness of the environmental impact of CO2, it becomes more and more important to design LP turbines for an optimal trade between efficiency and weight to achieve the lowest engine fuel burn. This paper summarizes work done recently and in the past to address the main influences and mechanisms of the airfoil lift level with respect to losses and efficiency as a basis for determination of optimal airfoil lift selection.© 2008 ASME

Improving Efficiency of a High Work Turbine Using Nonaxisymmetric Endwalls—Part II: Time-Resolved Flow Physics

This paper is the second part of a two part paper that reports on the improvement of efficiency of a one and a half stage high work axial flow turbine. The first part covered the design of the endwall profiling, as well as a comparison with steady probe data; this part covers the analysis of the time-resolved flow physics. The focus is on the time-resolved flow physics that leads to a total-to-total stage efficiency improvement of 1.0%±0.4%. The investigated geometry is a model of a high work (Δh / U 2 = 2.36), axial shroudless HP turbine. The time-resolved measurements have been acquired upstream and downstream of the rotor using a fast response aerodynamic probe (FRAP). This paper contains a detailed analysis of the secondary flow field that is changed between the axisymmetric and the nonaxisymmetric endwall profiling cases. The flowfield at the exit of the first stator is improved considerably due to the nonaxisymmetric endwall profiling and results in reduced secondary flow and a reduction in loss at both hub and tip, as well as a reduced trailing shed vorticity. The rotor has reduced losses and a reduction in secondary flows mainly at the hub. At the rotor exit, the flow field with nonaxisymmetric endwalls is more homogenous due to the reduction in secondary flows in the two rows upstream of the measurement plane. This confirms that nonaxisymmetric endwall profiling is an effective tool for reducing secondary losses in axial turbines. Using a frozen flow assumption, the time-resolved data are used to estimate the axial velocity gradients, which are then used to evaluate the streamwise vorticity and dissipation. The nonaxisymmetric endwall profiling of the first nozzle guide vane show reductions in dissipation and streamwise vorticity due to the reduced trailing shed vorticity. This smaller vorticity explains the reduction in loss at midspan, which is shown in the first part of the two part paper. This leads to the conclusion that nonaxisymmetric endwall profiling also has the potential of reducing trailing shed vorticity.

On the Impact of Blade Count Reduction on Aerodynamic Performance and Loss Generation in a Three-Stage LP Turbine

Reducing the number of blades in low pressure turbines is a desirable option for decreasing total operation costs. From an aerodynamical point of view this directly leads to an increased blade load. However, increasing the blade load above a certain level results in viscous effects like separation bubbles and finally full separation. This becomes especially significant for aero engine turbines, which operate at high altitudes and thus low Reynolds numbers. The underlying local flow phenomena and the effect on the aerodynamic performance of such configurations are addressed in this paper.This investigation is based on a three-stage low pressure turbine typical for aero engines. Different setups are employed with different number of guide vanes in certain stages. Furthermore, the Reynolds number is varied within a wide range. These configurations are investigated numerically using a modern steady-state transitional Navier-Stokes solver and experimental results from the same turbine. Based on this information, a detailed analysis of the viscous flow phenomena is performed with focus on the influence of separation bubbles on the loss production after the transition. These results are discussed with respect to blade count reduction.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

Effects of Suction and Injection Purge-Flow on the Secondary Flow Structures of a High-Work Turbine

In high-pressure turbines, a small amount of air is ejected at the hub rim seal to cool and prevent the ingestion of hot gases into the cavity between the stator and the disk. This paper presents an experimental study of the flow mechanisms that are associated with injection through the hub rim seal at the rotor inlet. Two different injection rates are investigated: nominal sucking of -0.14% of the main massflow and nominal blowing of 0.9%. This investigation is executed on a one-and-1/2-stage axial turbine. The results shown here come from unsteady and steady measurements, which have been acquired upstream and downstream of the rotor. The paper gives a detailed analysis of the changing secondary flow field, as well as unsteady interactions associated with the injection. The injection of fluid causes a very different and generally more unsteady flow field at the rotor exit near the hub. The injection causes the turbine efficiency to deteriorate by about 0.6%.

Analysis of Complex Three-Dimensional Flow in a Three-Stage LP Turbine by Means of Transitional Navier-Stokes Simulation

The complex three-dimensional flow field in a highly loaded three-stage LPT is analysed on the basis of a steady three-dimensional flow simulation. The quality of the simulation concerning this configuration is demonstrated by means of a comparison with extensive experimental data gathered in a turbine test rig. For an accurate representation of the transitional character of the turbine flow a modified version of the Abu-Ghannam Shaw transition model is employed in the TRACE_S Navier-Stokes code in connection with a two-equation turbulence model.The flow field of this highly loaded turbine is characterised by complex secondary flow pattern as well as local separation and reattachment zones. The need and applicability of transition modelling is demonstrated by a comparison with a fully turbulent calculation and experimental flow visualisation. The basic flow structure is described in terms of several characteristic quantities and discussed in detail. For further analysis variations of the point of operation and the geometry also based on experiments are included in this investigation.Copyright

Designing Low Pressure Turbines for Optimized Airfoil Lift

In the past 10-15 years, substantial effort has been spent on increasing the airfoil lift especially in aero-engine low pressure turbines. This has been attractive, since increased airfoil lift can be used for airfoil count decrease leading to weight and hardware cost reduction. The challenge with this effort consequently has been to keep the efficiency at high levels. Depending on the baseline level of airfoil lift, an increase of 20-50% has been realized and at least partly incorporated in modern turbine designs. With respect to efficiency there is actually an optimum level of airfoil lift. Airfoil rows at a lift level below this optimum suffer from an excessive number of airfoils with too much wetted suface and especially increasing trailing edge losses. Airfoils at lift levels above this optimum suffer from growing losses due to high peak Mach numbers inside the airfoil row, higher rear diffusion on the airfoil suction sides, and increased secondary flow losses. Since fuel cost have been rising significantly, as has been the awareness of the environmental impact of CO 2 , it becomes more and more important to design low pressure turbines for an optimal trade between efficiency and weight to achieve the lowest engine fuel burn. This paper summarizes work done recently and in the past to address the main influences and mechanisms of the airfoil lift level, with respect to losses and efficiency as a basis for determination of optimal airfoil lift selection.