Subproject D6 Interaction of combined module variances and influence on the overall system behaviour

Since the performance of an aircraft is directly and inextricably linked to the performance of the propulsion system, efforts are constantly being deployed to increase both the thrust generation and the fuel efficiency of engines. The continuing demands for increased performance result in working very close to the aerodynamic, thermal and structural limits of the propulsion system. At the same time, the operating limits must be selected in such a way as to ensure safe operation even during transient emergency situations. 

However, the deterioration of isolated or combined components leads to two negative effects:

1. The distance to the operating safety limits are reduced
2. Transient loads increase, driving the engine to the limits more quickly.

The objective of this subproject is to evaluate the influence of deterioration and repair on the steady-state and transient operating performance of engines. As an example, Figure 1 shows a repaired (green) and deteriorated (red) map of a compressor with a steady-state and transient operating line between the D and A operating points. The deterioration leads to a shift of the map to the lower left and of the operation line to the upper left. The transient operating line in particular thus comes very close to the stability limit.

In order to investigate this influence as accurately as possible, the ASTOR (AircraftEngine Simulation for Transient Operation Research) performance calculation program is being developed in subproject D6, which discretizes the complete propulsion system using a finite difference method (FDM). For this purpose, the quasi 1D conservation equations for flow and energy transport, the dynamics of rotating machinery, friction and radial gap change due to heat transport in the engine are simulated.  The resulting differential equation system is thus able to calculate the dynamic performance of the engine with interdependent interactions, allowing higher accuracies in the prediction of transient operation (compared to ordinary iterative performance synthesis methods). Figure 2 shows the IFAS research engine in the pseudo-bond graph notation representing the algorithm of ASTOR.

In the upcoming stages of this project, extensive test runs with the IFAS research engine are planned, which are intended on the one hand to validate ASTOR experimentally, and on the other hand to investigate performance degradation caused by specific modifications. Together with subprojects A3 and A6, modifications will be made to the combustor whose influence will be measured at various positions of the engine in order to investigate the influence on the overall engine and to reflect this in ASTOR.

Furthermore, the FDM of the engine in ASTOR allows the simulation of highly dynamic effects, such as the deep rotating stall in the high-pressure compressor and the reaction of the overall system. As an example of this, Figure 4 shows the course of the high-pressure compressor during the deep stall. The characteristic curves shown in green represent the stable range of the high-pressure compressor and the engine up to the stability limit. If the compressor is throttled rapidly because the fuel is added too quickly (acceleration in an emergency), it can enter the unstable range, which would have fatal consequences for the turbofan engine.