Experimental research into resonant vibration of centrifugal compressor blades

The objective of this work was to study resonant vibration of high-speed centrifugal compressor blades. Low order resonant response conditions were created in a test facility in order to experimentally investigate four aspects of blade forced response: (1) inlet flow distortion, (2) unsteady blade excitation, (3) damping and (4) resonant response. These aspects were evaluated for resonance cases of the first two blade modes and a number of inlet distortion cases. The major parameters during the study were the inlet pressure and the mass flow. Centrifugal compressors play a key role in processes engineering, propulsion and power generation. Due to the increased demands on aerodynamic performance the designs are pushed towards their mechanical limits. This is in particular the case for high cycle fatigue due to blade vibration. Resonant vibration due to forced response manifests during operation where flow disturbances upstream or downstream of the rotor cause unsteady blade excitation. Generally, aerodynamic non-uniformities are created in the flow field by diffuser vanes or mechanical obstacles in the flow field, i.e. struts or pipe bends. In some applications flow injection generates non-uniformities. Experimental research was conducted in a closed loop test facility driven by a centrifugal compressor. The rotor used in the study featured design and performance criteria typical for turbocharging applications with highly loaded blades and a vaneless diffuser. The flow upstream of the compressor was intentionally disturbed using screens with the number of lobes equivalent to the engine excitation order. The resultant flow field was measured using a fast aerodynamic probe. Pressure sensors and dynamic strain gauges were installed on the blade surface to measure unsteady pressure and strain, respectively. The blade thickness was modified using FEM in order to generate resonant crossing conditions of the first two blade modal frequencies. Modal shapes and frequencies were verified using speckle interferometry and were found to agree very well with the predictions. Due to intentional inlet flow distortion, the total pressure distribution varied in the range of 1% to 4% of the inlet static pressure depending on the mass flow setting. The distortion amplitude represented a case commonly encountered in applications. A one-dimensional model based on loss generation and mass redistribution was developed. It was shown that this model predicts accurately the mean levels of the distortion properties. Depending on the case, the inlet distortion caused unsteady pressure amplitudes on the blade surfaces of 1.5% of the inlet static pressure. For centrifugal compressors, typical strut installations in the inlet cause dominant EO2 and EO3 harmonics which prevail for all screen cases. The intended engine orders, i.e. EO5 or EO6, were of similar amplitude. Overall, in the course of the analysis it was shown that the relatively complex nature of centrifugal compressor flows results in a strong dependence of of the excitation function on the operating point of the compressor. A novel experimental analysis technique has been successfully developed to measure the energy transfer between the vibrating blade and the fluid. The method is generally applicable to any case of blade vibration and was shown to enable the identification of zones on the blade surface where either excitation or damping takes place during resonance. The phase angle between the blade and the unsteady pressure plays a key role in the energy exchange by determining the direction of energy transfer and by scaling the amplitude. Therefore, mass flow variations during operation affect the phase angle distribution and may cause substantial increase in modal force and therefore response amplitudes. The dynamic response characteristic of a centrifugal blade during resonance was proofed to be very well captured by a single-degree-of-freedom (SDOF) model. Based on the SDOF model damping was experimentally shown to be dominated by contribution from aerodynamic damping whereas material damping is by a factor of ten smaller. For centrifugal compressors aerodynamic damping was found to be linearly dependent on inlet pressure. In the present the researched pressure was ranging from 0.2-0.8bar. For the same range of inlet pressure the maximum vibratory amplitude at resonance increases and becomes asymptotic at approximately 1bar. Material damping becomes relevant at pressure levels around 0.3bar and below. For the first eigenmode blade damping was shown to be constant, i.e. to be independent of the operating line or resonance case. Mistuning was experimentally shown to cause a potentially hazardous increase in response amplitude and must not be neglected during design.