Subsonic Stall Flutter of a Linear Turbine Blade Cascade Using Experimental and CFD Analysis

Modern steam turbines have to be often designed for large operating ranges including the turbines with air-cooled condensers where high backpressures occur. For such cases, where unsteady aerodynamic forces might seriously endanger the long last stage rotor blades aerodynamic stability, flutter resistant blades are necessary to be designed. Furthermore, steam turbines are often requested to operate at strong off-design conditions, where a danger of stall flutter can occur. Therefore, each design process involves a numerical flutter analysis which must be validated using measurements on test rigs. In this paper, experimental testing of subsonic stall flutter is presented. It is performed on a linear turbine blade cascade which is installed in an in-draft wind tunnel situated in an experimental lab at the Department of Power System Engineering at the University of West Bohemia. The blade cascade consists of eight blades, which are the models of the tip section of a long last stage rotor blade. The four central blades are flexibly mounted and each has two degrees of freedom (i.e. bending and torsion motions). The model shapes are performed by using four electromagnetic shakers, which are alternately attached at the top and bottom of a supporting frame. In each shaker, two moving coils are partially inserted into larger solenoids (permanent magnets). A phase delay between the electric currents running through two moving coils defines the oscillation mode. Each moving coil has a feedback control system to set a precise blade motion. By using this, bending and torsional modal shapes of the blades are investigated in a travelling wave mode approach for different boundary conditions. There are compared the cases where the flow is fully attached to the blade profiles, which presents the subsonic classical flutter, with the case where the flow is separated, which presents a subsonic stall flutter. For this type of flutter, a numerical tool used for a flutter prediction in the Doosan Skoda Power Company has been not yet compared with the experimental results. It was found out that the aerodynamic work which is transferred from the flow to the blade is greater when the flow separation occurs. It means that the stall flutter can have a more serious effect to the blade than the classical one. Testing results in the form of aerodynamic work are compared with numerical results which are performed by using an advanced in-house procedure. This procedure is based on a commercial numerical code ANSYS CFX where numerical analyses are performed by using a full-scale time-marching 3D viscous model in order to obtain the solution of the URANS equations in the time domain. A relatively good agreement between the experimental and numerical results was achieved. The differences are described and the reasons are clarified.