Blade Vibration Phenomena of HPC Bliscs Considering Manufacturing Effects and Strain Gauge Application

Abstract

Compared to conventional bladed disc assemblies with heavy blade-disc connections, the integrated design (blisc) of modern aero engine compressors results in reduced rotor masses, facilitates higher maximum rotational velocities and improved pressure ratios (increased efficiency factors). Apart from these advantages a number of disadvantages occur. Some of them are for example a very low structural damping and a higher sensitivity to manufacturing- and material tolerances (mistuning). This results in localized vibration modes, higher displacement amplitudes, critical stress levels and reduced high cycle fatigue (HCF) of several blades due to inauspicious aerodynamic excitations. From this, an optimum choice of blades to be instrumented with strain gauges (s/g) is the basis of a convincing vibration monitoring system during rig tests. Based on an extensive measurement campaign and numerical investigations an overview of all major manufacturing effects (blisc milling, rotor tip grinding, welding the single bliscs together, compressor assembly) and also the strain gauge application itself on the vibration behavior of real mistuned bliscs will be presented for a multi-stage high pressure research compressor (HPC) in detail (see Fig. 1). The investigations show, that the original blade mistuning deviations resulting from the initial blisc milling procedure remain until the last manufacturing process of the aero engine and still dominate the blisc forced response behavior during aerodynamic excitation. In contrast to this effect, rotor tip grinding procedures as well as the assembly of the rotor drum only cause an even frequency shift of every blade, which does not significantly change the blisc behavior. Complementary to the influence of manufacturing-caused blade mistuning on the whole rotor balance, the effects of an application of high-temperature strain gauges on HPC-blades will be discussed regarding the modification of the blisc vibration modes. Finally different methods will be introduced aiming at a compensation of the strain gauge induced blade stiffening depending on the blade dimensions to ensure the best possible vibration monitoring results.