Analyzing and Optimizing Geometrically Degraded Transonic Fan Blades by Means of 2D and 3D Simulations and Cascade Measurements
Authors: Giebmanns A. , Schnell R. , Steinert W. , Hergt A. , Nicke E. , Werner-Spatz C. 
Published: July 2013

The present study deals with the influence of geometrically degraded transonic engine fan blades on the fan’s aerodynamic behavior. The study is composed of three phases; the first consists of 3D simulations to point out changes in the performance parameters caused by blade degradations. In the second phase, 2D optimizations are carried out to determine the potential of redesigning the blade and in the third phase, measurements on a transonic cascade are used to experimentally verify the numeric results.

During engine operation as well as maintenance processes, geometric variations of the fan blades, and especially of the blades’ leading edges, are observed. They mainly originate from the ambient conditions under which the engine is operated. Though the deformations of the blade differ widely, several typical degradation types can be identified. In advance of the study, these degradation types have been systematized and simplified models representing different degrees of degradation have been built.

In the first phase, the models are aerodynamically analyzed by means of 3D simulations. A high influence on the performance parameters is found for a fan blade exposed to long-term erosion. The model’s characteristics are a blunt leading edge and a reduced chord length. In contrast, the performance parameters of a model representing a re-contoured blade (reduced chord length but reshaped leading edge) are shown to be similar to those of a new fan blade. This leads to the conclusion that an eroded blade may offer almost the initial performance parameters as long as the leading edge is well reshaped.

Since the model of the long-term eroded blade shows great changes in the fan’s performance and the best optimization potential, this has been chosen for the further analysis in the following phases.

In the second phase, 2D optimizations are applied to three airfoil sections at different heights of the blade. The parameterization used is limited to a small area of the leading edge; the shape of the rest of the blade is kept constant. The optimizations lead to loss reduction and demonstrate the potential of the optimization process.

The third phase is carried out in the Transonic Cascade Wind Tunnel of the Institute of Propulsion Technology in Cologne. As the transonic part of the fan blade is the most sensitive to geometric changes, a transonic airfoil with long-term erosion has been chosen. During the tests, the following measurement techniques are applied: Static pressure probes to determine the Mach number distribution, a 3-hole probe to detect exit angle and loss distribution, Schlieren photographs and PIV-measurements to locate the shock system, the L2F method to measure the cascade inflow angle and to resolve the boundary layer distribution and Liquid crystal measurements to observe transition activities. The full analysis of the measurements with PIV, L2F and Liquid Crystals are still in progress, but the evaluation of the loss polar and the Schlieren photographs show increased losses for the degraded blade and a good match with the numeric results.