Axial flow turbomachinery is susceptible to self-excited vibrations known as flutter, primarily affecting high-aspect ratio rotor blades. Experimental studies of blade flutter are essential to understanding the phenomenon. The existing research has shown that the most critical factor in determining the aerodynamic stability of the blade is the mode shape. In this work, measurements of subsonic flutter of a linear turbine blade cascade with various mode shapes and torsion axis locations in a chordwise direction are carried out. The cascade consists of eight blades representing reduced tip sections of the last stage blade of the steam turbine rotor, and central four flexibly mounted blades are forced to vibrate in pure bending and pure torsion modes and a combined motion. Both the travelling wave mode approach and the influence coefficient method are tested. The results show that the pure torsion mode is insensitive to increasing reduced frequency and is aerodynamically unstable for each positive angle of incidence tested, while pure bending and combined modes become less stable with the increased angle of incidence. In addition, high sensitivity of blade behaviour to changes in torsion axis position and phase shift between bending and torsion motions is demonstrated.
The effect of manufacturing geometry deviations on the flow past a NACA 64(3)-618 asymmetric airfoil is studied. This airfoil is 3D printed according to the coordinates from a public database. An optical high-precision 3D scanner, GOM Atos, measures the difference from the idealized model. Based on this difference, another model is prepared with a physical output closer to the ideal model. The velocity in the near wake (0–0.4 chord) is measured by using the Particle Image Velocimetry (PIV) technique. This work compares the wakes past three airfoil realizations, which differ in their similarity to the original design (none of the realizations is identical to the original design). The chord-based Reynolds number ranges from 1.6×104 to 1.6×105. The ensemble average velocity is used for the determination of the wake width and for the rough estimation of the drag coefficient. The lift coefficient is measured directly by using force balance. We discuss the origin of turbulent kinetic energy in terms of anisotropy (at least in 2D) and the length-scales of fluctuations across the wake. The spatial power spectral density is shown. The autocorrelation function of the cross-stream velocity detects the regime of the von Karmán vortex street at lower velocities.
In order to eliminate occurrences of flutter of low pressure turbine blades it is necessary to understand the associated unsteady aerodynamics. For this reason, an experimental and numerical study of controlled flutter (travelling wave mode) in a linear turbine blade cascade oscillating in a torsional as well as translation motion is conducted. Unsteady aerodynamic forces and moments were measured on a subsonic eight-blade turbine cascade rig where central four blades are flexibly mounted each with two degrees of freedom. Thin blades in the cascade represent the tip section of the last stage rotor blades, which defines the turbine overall performance. A commercially available 3D CFD software ANSYS CFX is used to simulate the unsteady aerodynamic loading in the blade cascade. Experimental data and simulations are compared and influence of aerodynamic forces and moments on flutter is analysed.
In low-pressure steam turbines, aerodynamic and structural design of the last stage blades is critical in determining the power plant efficiency. The development of longer last stage blades which are recently over 1 meter in length is an important task for steam turbine manufactures. The design process involves a flutter analysis of last stage blade tip sections where increased unsteady aerodynamic forces and moments might endanger the blade aerodynamic stability. However, numerical design tools must be validated using measurements in test facilities under various operating conditions. In this work, ANSYS CFX is used for flutter prediction of turbine blade tip sections oscillating in a travelling wave mode. Simulations are compared to experimental results obtained from controlled flutter tests in a wind tunnel with a linear cascade of eight turbine blade profiles made of carbon fibre. Central four blades are flexibly mounted each with two degrees of freedom (i.e. bending and torsion motions). Large deflections of thin blade profiles are accounted for the estimation of unsteady aerodynamic forces and moments. A satisfactory agreement between the simulations and experiments is achieved.
Abstract. Last stage blades are a key element of steam turbines and in many ways determine the turbine configuration alongside with the overall turbine performance. The total efficiency of the low pressure turbine section can be increased by extending the last stage blades. The design process of such long blades involves a flutter analysis using CFD tools which have to be validated by measurements in test facilities under various operating conditions. Experimental data obtained from a subsonic wind tunnel with an oscillating turbine blade cascade, which is available at the Department of Power System Engineering at the University of West Bohemia, was compared with simulations in ANSYS CFX currently used in the Doosan Škoda Power. The paper provides a brief summary of experimental rig description, CFD tool setup and the results for the case of a travelling wave mode with the pure torsion motion of amplitude of 0.5°, Ma = 0.2, reduced frequency of 0.38 and angle of attack +5°.
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