Angela Giebmanns

Compressor Leading Edge Sensitivities and Analysis with an Adjoint Flow Solver

Based on the results of a prior study about fan blade degradation, which state a noticeable influence of small geometric changes on the fan performance, an adjoint computational fluid dynamics method is applied to systematically analyze the sensitivities of fan blade performance to changes of the leading edge geometry.As early as during manufacture, blade geometries vary due to fabrication tolerances. Later, when in service, engine operation results in blade degradation which can be reduced but not perfectly fixed by maintenance, repair and overhaul processes. The geometric irregularities involve that it is difficult to predict the blade’s aerodynamic performance. Therefore, the aim of this study is to present a systematic approach for analyzing geometric sensitivities for a fan blade.To demonstrate the potential, two-dimensional optimizations of three airfoil sections at different heights of a transonic fan blade are presented. Although the optimization procedure is limited to the small area of the leading edge, the resulting airfoil sections can be combined to a three-dimensional fan blade with an increased isentropic efficiency compared to the initial blade.Afterwards, an adjoint flow solver is applied to quasi-three-dimensional configurations of an airfoil section in subsonic flow with geometric leading edge variations in orders representative for realistic geometry changes. Validations with non-linear simulation results demonstrate the high quality of the adjoint results for small geometric changes and indicate physical effects in the leading edge region that influence the prediction quality.© 2013 ASME

Analyzing and optimizing geometrically degraded transonic fan blades by means of 2D and 3D simulations and cascade measurements

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.Copyright

A Method for Efficient Performance Prediction for Fan and Compressor Stages With Degraded Blades

Over service time fan and compressor blades vary in their geometry. Due to the geometric irregularities it is difficult to reliably assess the fan or compressor performance. Therefore, in the present study, a method that allows an efficient and accurate estimation of the performance for fan and compressors with degraded blades is established.In a first step, the blade regions mainly exposed to geometry variations are identified. Then, a simple parameterization that enables to easily describe the blade geometry as well as to independently represent the variations of all relevant geometric parameters is defined. For each geometric parameter (e.g. leading edge shape, rotor clearance gap height), the shifts of the operating points in the performance map due to the geometric variations are calculated. This is done with an adjoint flow solver which simultaneously provides the shifts for all extents of geometric parameter variations. As the geometric parameters are assumed to be independent of each other, the combination of variations in several geometric parameters is estimated by simply adding the shift vectors determined for the individual parameters.The results for a subsonic compressor stage show a significant influence of the variations in the geometric parameters on the compressor performance. As validation with few nonlinear simulations demonstrates, this influence is well predicted by the method presented in this study. The relative deviation of the performance map parameters in the operating points around peak efficiency is less than 0.5 %. Compared to the nonlinear simulations, the numeric effort to determine the influence of seven geometric parameters on the performance map with the method based on adjoint simulations is considerably reduced by a factor of 35. In summary, the results demonstrate the feasibility of the method and encourage its application in an industrial context.Copyright

Measurements of the 3D Flow Field Downstream of an Aircraft Engine Fan

Flow field measurements have been performed in a jet engine fan. As a part of a sensitivity study on the influence of geometry changes on the performance of the compressor the experiments provide a data base for the verification of extensive numerical simulations covering the fan blades. These investigations help to improve the understanding of how changes in component geometry due to wear affect the engine.The experimental investigations were carried out down-stream of the fan of a jet engine on the test cell at Lufthansa Technik AG.The main focus of the measurements was to provide highly resolved flow field data downstream of the OGV. A pneumatic five-hole probe was applied in order to obtain 3D information of the flow properties. In order to detect and to resolve the wakes of the OGVs and of the midspan shrouds of the rotor blades the measurements had to be performed along the blade height and had to cover more than one OGV spacing. Therefore, it was necessary to position the probe at various radial and circumferential locations. The radial positioning of the probe was performed by a standard traversing unit. The different circumferential locations were achieved by a specially developed traversing mechanism on a modified engine fan case, which allowed a continuous positioning in the range of two OGV spacings.The experiments were carried out for two different operating points of the jet engine. For each of these operating points, the engine was operated with nominal and with increased tip clearance above the fan rotor in order to simulate different wear conditions.The paper explains the necessary modifications to the jet engine and the set up for the experiments. Furthermore, measured results are shown and compared to the results derived from the CFD simulations which were performed prior to the experimental investigations.© 2014 ASME