Externally Induced Excitation Characteristics Dominated by Interstage Interaction in a Vaneless Counter-Rotating CompressorOA

By analyzing the unsteady interstage interference phenomena and interaction laws between rotors,the induction mechanisms and spectral characteristics of externally induced excitation affecting the upstream and downstream rotors of a vaneless counter-rotating axial compressor(CRAC)are clarified.The results show that as the throttling effect gradually increases,the unsteady fluctuations in the interstage and internal flow fields of the upstream and downstream rotors all increase.Under near-stall point,although the leading-edge shock wave of the upstream low-pressure rotor(R1)detaches,the unsteady disturbances within it are relatively weak,and the instability of the vaneless CRAC is dominated by the downstream high-pressure rotor(R2).The upstream rotor R1 is subjected to the periodic sweeping effect of the extended shock wave from the downstream rotor R2,and the spectral characteristics of unsteady fluctuations in its flow field are represented by one-time blade passing frequency(1 BPF)and its harmonic frequencies.The downstream rotor R2 is directly subjected to the dual effects of the downward propagation of disturbances in the R1 passage and the outlet backpressure of R1,leading to a significant increase in the complexity of its internal flow field and a substantial increase in entropy.The leading-edge region of R2 is characterized by the dominant frequency of its own tip leakage flow(0.8173 BPF),while the blade passage of R2 evolves to be dominated by the 2-times disturbance frequency(2 BPF)derived from the unsteady fluctuations of the two-stage rotors.In addition,by comparing the spectral characteristics of the externally induced excitation(i.e.,unsteady disturbances)acting on the two-stage rotors with the rotational frequencies of the rotors at different vibration modes,it is found that under near-stall operating conditions,the externally induced excitation frequencies of the upstream and downstream rotors deviate significantly from the blade rotational frequencies,and are thus insufficient to induce large forced resonance of the R1 and R2 blades.