The flow field in modern axial turbines is non-trivial and highly unsteady due to secondary flow and blade row interaction. In recent years, existing design-tools like two-dimensional flow solvers as well as fully three-dimensional CFD methods have been validated for the assumption of a quasi-steady flow field. Since the inevitable unsteadiness of the flow field has a direct impact on unsteady loss generation and work transfer, existing design methods stand in need of validation for local unsteady effects within the flow field. In order to clearly separate end-wall losses from those generated by blade row interaction within the blade passage, a two-dimensional core-flow is essential for the investigation. Hence, a new 1.5-stage high aspect ratio low pressure turbine has been designed to determine the intensity of core-flow blade row interaction for different axial gaps. First, inlet and outlet conditions of the test rig are evaluated with regard to homogeneity of the flow parameters in their radial and circumferential distributions. Secondly, the measurement data gained from rig tests have been applied as boundary conditions to time-averaged numerical computations. The flow field analysis for two different axial gaps focuses on the verification of the core flow. The authors show that the new turbine has been successfully verified using both test data and the numerical predictions, serving as a precondition for the validation of the numerical model for unsteady effects within the core-flow.
Numerical and experimental investigations have been performed to determine the effect of a variation of the inter blade row axial gap on turbine efficiency. The geometry used in this study is the 1.5 stage axial flow turbine rig of the Institute of Jet Propulsion and Turbomachinery at RWTH Aachen University. The influence of the blade row spacing on aerodynamics has been analyzed by conducting steady and unsteady RANS simulations as well as measurements in the cold air turbine test rig of the Institute. Both potential and viscous flow interactions including secondary flow were investigated. The results show an aero-dynamic improvement of efficiency and favorable spatial distribution of secondary kinetic energy by reduction of the axial gap. It is shown that this relation tends to become less pronounced for multistage turbines.
This paper presents the results of the analysis of different 3D designs for the first stator and the rotor of a 1.5-stage turbine test rig. A tangential endwall contouring for the hub and the shroud, a bowed profile stacking, and a combination of those have been designed for the first stator. In addition, a tangential endwall contouring has been designed for the hub of the unshrouded rotor. Part I of this two-part paper deals with the design process and the numerical analysis of the results. All designs have been optimized using the stage efficiency as target function. For the design of the 3D stator vanes, the optimization led to an unexpected result: The secondary flow vortex strength increased. However, the secondary flow pattern is rearranged by the 3D-designing, leading to a smoother radial exit flow angle distribution. A subsequent reduction of the rotor losses overcompensates the higher stator losses. In order to understand how the 3D vanes affect the stator secondary flow pattern, a detailed analysis of vortex stretching and vortex dissipation is presented in this paper. With this approach, the various impacts of the 3D designs on the secondary flow vortices' strength can be quantified. In addition, the potential theory effect of the self-induced velocity is introduced here in order to explain the effects of a tangential endwall contouring on the trajectory of the pressure side leg of the horseshoe vortex (HVps). To the best of our knowledge, both approaches are new for the analysis of turbine secondary flows. The impact of the stronger but rearranged stator secondary flow on the rotor secondary loss development is analyzed by means of unsteady simulations. The results show that the rotor secondary flow can be effectively reduced through a proper stator secondary flow pattern. In Part II of this paper, the analysis of extensive experimental results validates and supplements the numerical analysis.
Characterizing the transition process of airfoils can be very challenging and requires often extensive measurement methods. Frequently at low Reynolds numbers the suction side separation often occurs close to the trailing edge so that asserting reattachment of the flow to form a closed separation bubble from the profile pressure distributions becomes uncertain. In the current work the suction side transition process is investigated more precisely with a convenient method to determine the dynamic pressure close to the suction surface using a Preston probe (flattened Pitot tube). Therefore four low pressure turbine airfoils, which show different characteristics of the transition process in the static pressure distribution have been investigated at the High-Speed Cascade Wind Tunnel at the Universität der Bundeswehr München at constant Mach number and under a wide range of Reynolds numbers (40 000 to 400 000). It is shown that this method is appropriate to determine transition start and end as well as the separation and reattachment point of a separated flow as long as the probe height is small enough compared to the boundary layer thickness. The measurement results are compared to profile pressure distributions and hot-wire boundary layer profiles. Also the influence of periodic unsteady inflow conditions on the dynamic pressure near the wall is revealed in the time average. Limitations due to the probe geometry are discussed and a method to estimate the influence of the probe geometry on the measured dynamic pressure coefficient is suggested.
The current work investigates the performance benefits of pulsed blowing with frequencies up to 10 kHz on a highly loaded low pressure turbine (LPT) blade. The influence of blowing position and frequency on the boundary layer and losses are investigated. Pressure profile distribution measurements and midspan wake traverses are used to assess the effects on the boundary layer under a wide range of Reynolds numbers from 50,000 to 200,000 at a cascade exit Mach number of 0.6 under steady as well as periodically unsteady inflow conditions. High-frequency blowing at sufficient amplitudes is achieved with the use of fluidic oscillators. The integral loss coefficient calculated from wake traverses is used to assess the optimum pressure ratio driving the fluidic oscillators. The results show that pulsed blowing with fluidic oscillators can significantly reduce the profile losses of the highly loaded LPT blade T161 with a moderate amount of air used in a wide range of Reynolds numbers under both steady and unsteady inflow conditions.
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