The Validation of Flutter Prediction in a Linear Cascade of Non-Rigid Turbine Blades

Abstract

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.

Experimental and Numerical Study of Controlled Flutter Testing in a Linear Turbine Blade Cascade

“…Each moving coil has its feedback control system in order to set a precise blade motion. Further details about the experimental setup are described in [7].…”

“…The total time duration of unsteady simulations was specified using the total number of periods per run. It was investigated that 6 periods per run were sufficient enough to calculate the required aerodynamic work [7,12]. To quantify convergence criteria of steadystate simulations, the residual values were used where the RMS (root mean square) residual type was chosen and 1 • 10 −5 residual target value was defined.…”

“…Therefore, no other boundary condition adjusting would be necessary. Further details of the differences caused by experimental approach are commented in [7].…”

“…Such methodologies are usually classified in time-linearized, harmonic balance and non-linear approaches both in frequency and time domain and require high quality experimental data for the validation and tuning, especially in off-design conditions. The stability results are evaluated by aerodynamic work which describes the exchange of energy between blade and flow [7,8].…”

Profile Aerodynamics of an Oscillating Low-Pressure–Turbine Blade

“…As a result, such models exhibit deficits in predicting the production and dissipation rates of turbulent kinetic energy under cyclic strain [2]. Likewise, the prediction of more turbomachinery-specific flows, i.e., in vibrating cascades, as well as the resulting prediction of aero-elastic quantities, using unsteady RANS (URANS) or time-linearised RANS exhibits non-negligible discrepancies from experimental validation data, e.g., Refs [3][4][5].…”