Non-deterministic Appraisal of the Impact of geometric Variability on the Aerodynamic Performance of a Blisk-Manufactured Contra-Rotating Turbo Fan

Kurzfassung

In the context of the European project VITAL, the German Aerospace Center (DLR) designed and coordinated the manufacturing of a contra-rotating turbofan for future high bypass ratio aero engines. The overall effort was led by Snecma, who provided the global specifications, the aerodynamic design of this version CRTF2b was carried out at the Institute of Propulsion Technology of DLR in Cologne. A rig was built and tested at CIAM facilities in Moscow, the measured and calculated performance characteristics will be discussed. The rig comprised of two contra rotating rotors which were manufactured as titanium blisks by CNC controlled five-axis milling machines. To assess the manufacturing quality, the entire first rotor was geometrically measured using a highly accurate optical scanning device. The focus of the present study is to assess and quantify the uncertainty in predicting the aerodynamic performance, here mainly stemming from geometric variations of the manufacturing process. The profile sections were evaluated aerodynamically by applying the 2D coupled Euler/Boundary-Layer Solver Mises. Results in terms of the rotor 1 outflow angle at different inflow incidences will be discussed for all mentioned geometries. Statistical properties of all flow quantities were calculated firstly based on the results of the nine manufactured blades. The corresponding expectancy values along with the standard deviations were then derived and the resulting uncertainty is less than 0.1° at design conditions and rises up to 0.5° at high positive incidence. In a second step the geometric variations were decomposed into their corresponding Eigen forms by means of principal component analysis (PCA). These modes were the basis for carrying out Monte-Carlo (MC) simulations considering only the first six Eigen modes and assuming a normal distribution of these Eigen values, with the prescribed variance being a result of the PCA itself. Since the described approach in itself does not create any additional information related to the global statistic properties, the calculated variances are comparable to the ones derived from the measured blades directly. In contrast, only considering the frequency distributions from MC simulations suggest that the outflow angles are not normally distributed, with increasing skewness towards higher incidence angles. Results from the PCA were exploited regarding manufacturing characteristics and its resulting uncertainty. Furthermore, the sensitivity of the main aerodynamic quantities over the profile’s working range and its propagation will be discussed in greater detail (e.g. Mach number distribution and transition point location). The second part of the paper features 3D-CFD simulations of the entire fan stage for all the nine different and scanned rotor 1 blade geometries. Each of the scanned blades was considered individually and the variation of the overall stage performance parameters due to the geometric variations from blade to blade will be quantified. Apart from the aerodynamic performance, an analysis of the mechanical properties of the rotor 1 blisk in terms of Campbell characteristics and Eigenfrequencies was carried out for all scanned blades, reflecting the frequency scattering of each Eigenmode due to geometric variability.