An Experimental Study on the Effects of Non-Uniform Vane Spacing on Forced Response Reduction in a Mistuned Rotor Blisk in a Multistage Axial Compressor

Abstract Stators with non-uniform vane spacing are known to effectively reduce the forced response of adjacent rotors by spreading the primary engine order excitation at a certain speed to a series of weaker excitations at nearby EOs over a wider speed range. The classical estimation method to predict the blade response reduction is to quantify the reduction of the forcing function at a potential resonance crossing by conducting a circumferential Fourier analysis of the asymmetric flow field. However, besides excitation reduction, the blade forced response also depends on other factors, such as damping and blade-to- blade interactions due to mistuning. To study the blade forced response reduction in a realistic environment, an experimental study was conducted in the Purdue 3-stage axial research compressor at three different loading conditions. The vibratory response of the 1st torsion mode forced response of Rotor 2 was measured by strain gages for two different upstream Stator 1 configurations: the symmetric 38-vaned Stator 1 configuration and the NUVS Stator 1 configuration with 18-20 vane halves. There was substantial blade-to-blade variation in blade response for both Stator 1 configurations due to the inherent non- intentional mistuning in the R2 blisk. This, in turn, causes a large blade-to-blade variation in the reduction factors. The measured reduction factors and the peak response EO are not well predicted by the classical estimation method. This indicates that both damping and mistuning effects need to be considered in the NUVS reduction factor prediction.