Improvements to fan blade maintenance

The components of modern turbofan engines in passenger aircraft are exposed to extreme conditions, and the blades in the fan stage sustain particularly high wear and tear. Thus over time the leading edges are worn away through heavy erosion, the chord lengths of the blades become shorter and the gap between the trailing edges of the blades and the fan casing increases. This leads directly to the loss of thrust and a decline in performance on the affected engines, which ultimately results in higher fuel consumption and CO2 emissions.

Back in 1994 Lufthansa Technik succeeded in developing a procedure for analyzing worn high-pressure compressor blades electronically and then recontouring them with the aid of an Advanced Recontouring Process (ARP) robot. In this way fuel consumption and CO2 emissions can be reduced and the service life of the components significantly extended. But to achieve further improvements in the area of fan blades more detailed, manufacturer-independent information about the engine as a coherent system was needed.

The aim of the LOTUSARP research project, which ran from January 2009 to December 2011 and involved Lufthansa Technik teaming up with four partners – ANSYS Germany, CADFEM, the DLR (the Institute of Propulsion Technology) and the Aachen University of Technology (Institute of Jet Propulsion and Turbomachinery) – was to develop a more detailed understanding of the main variables that influence the fan module and how they interact with each other. The project fell within the framework of the German government's Aerospace Research Program IV and was 50 percent funded by the Federal Ministry of Economics and Technology. 

Flow simulations were also combined with structural mechanics simulations

The impact of different changes of geometry induced as a consequence of the typical manifestations of wear and also as a result of normal repairs was examined on the fan blades with the aid of numerical flow simulation. With the aim of accounting for the deformation and the impact of this on actual flows around the fan blades in operation, some of the flow simulations were also combined with structural mechanics simulations (fluid structure interaction). Further numerical analysis was carried out as a means of examining the interactions that arise in the design of a rotor built from blades of various geometries. Finally a methodology was developed that will make possible significant improvement of the leading edge contour applied during repair to selected profile cross-sections.

Another aspect of the research work was the experimental validation of the results obtained through numerical analysis. Thus extensive two-dimensional analyses were performed on selected blade geometries in a cascade tunnel. Using a specially developed sensor, flow measurements at the fan exit of a real engine were also carried out at the Hamburg engine test facility. The results of the measurements and calculations were largely in agreement, suggesting that the numerical analyses are of sufficient quality to be relied upon.

As a result of the information gained about the influence of different fan blade geometries on operational performance and the effectiveness of the fan stage, new measures can now be developed to extend the service life of the fan blades and to reduce fuel consumption and CO2 emissions. The possible measures include improved methods of repairing blade profiles, especially the leading edge contour (recontouring). The study also considered the issue of what is the optimal point in time at which to perform maintenance work on the fan blades.

Lufthansa Technik has already filed a patent application for Germany and Europe for the process of controlling a repair through numerical simulation. The results from the LOTUSARP research project will serve as the basis for developing a new procedure in the future for the assessment and further development of maintenance measures on compressor blades in engines. Repair methods for other engine modules will also benefit from the new methodology. 

Results of the flow simulation

The distribution of pressure in the airflow (marked in color) and the flow lines (line with arrows) can be seen. Comparison indicates that the compression shock on the eroded blade (figure above) is more pronounced (larger areas in red and yellow) and that behind the sharp edge on the eroded blade close to the component the air is flowing somewhat more slowly (flow lines further apart). This explains why the eroded leading edge results in a loss of thrust and higher fuel consumption.