EXPERIMENTAL STUDY ON IMPELLER BLADE VIBRATION DURING RESONANCE PART 2: BLADE DAMPING

Forming the second part of a two-part paper, the estimation of damping is presented here. Part 1 discusses the experimental approach and the results on blade resonant response measurements. In the study of forced response damping is a crucial parameter which is measured to quantify the ability of a vibrating system to dissipate vibratory energy in response to a given excitation source. The blading of turbomachinery components is particularly prone to forced response excitation which is one of the major causes of high cycle fatigue failure during operation. In turbocharging applications forced response can not be avoided due to a number of factors, i.e. change in speed, inlet bends or obstructions in the flow field. This study aims to quantify the damping parameter for the lightly damped blades of a centrifugal compressor. The impeller geometry is typical of turbocharging applications. As a first step the non-rotating impeller was excited using piezos and the transfer function was derived for a number of pressure settings. Both circle-fit and a curve-fit procedure were used to derive material damping. In the second step, measurements were taken in the test facility where forced response conditions were generated using distortion screens upstream of the impeller. Main blade strain response was measured by sweeping through a number of resonant points. A curve-fit procedure was applied to estimate the critical damping ratio. The contribution of material and aerodynamic damping was derived from a linear curve fit applied to damping data as a function of inlet pressure. Overall, it will be shown that aerodynamic damping dominates the dissipation process for applications with inlet pressure of 1 bar. Damping was found to depend on the throttle setting of the compressor and where applicable CFD results were used to point towards possible causes of this effect.