The Influence of Flow Leakage Modelling on Turbomachinery Blade Forcing Predictions

Sammanfattning: Vibrations in turbomachinery engine components are undesirable as they put the structural integrity of the components at risk and can lead to failure during the lifetime of the turbomachinery engine. Vibrations arising from aerodynamic forces and stability of turbomachinery blades is assessed in the discipline of aeromechanics. Ultimately, aeromechanical considerations limit turbomachinery designs and impose constraints on innovative aerodynamic designs with highly loaded light-weight components. Besides, aeromechanical assessment of blade vibration is done at a late stage of the design process and the number of iterations in the design loop is limited. Aeromechanical calculations can have large uncertainties in the prediction accuracy, especially when made a-priori without test data or without comparable design experience to tune the analysis methods. Therefore, large safety margins are required in the design, given that only a small set of prototype engines of a chosen design can be manufactured for testing. This may result in unnecessarily conservative engines and inhibit efficient or cost-effective design.Accurate prediction methods together with a reliable estimate of the accuracy and sensitivity of the calculations will allow designers to push the limits and to design machines with highly efficient components. Efficiency directly translates into savings in terms of operational cost, capital cost as well as reductions in emissions when fuels are used.In the presented work the sensitivity of aerodynamic forcing to the geometry features of a tip gap, hub cavity, tip-shroud cavity and inlet guide vane partial gaps has been investigated by the means of URANS CFD computations. The results indicate that sensitivity is both feature and case dependent, and that the detailing features can significantly alter the aerodynamic forcing function. The work shows, that the features should be included in high-fidelity aerodynamic models used for aeromechanics and highlights the mechanisms in which the features affect the aeromechanic forcing.Investigations were performed for a subsonic model steam turbine configuration in 1.5 stage simulations, for a transonic turbine stage and for a 1.5 stage transonic research compressor in a 5 row investigation. Computations were performed using time domain simulations on scaled sectors of the blade rows. Results are analysed in terms of generalised modal force, and differences in the flow-field between the investigated detailing configurations are highlighted, marking the influence of the detailing features.