Brief description

During flight operations, an aircraft engine compressor can enter an unstable operating range under certain operating conditions. Such transient conditions can be triggered, for example, by crosswinds, inhomogeneous inflow at the engine inlet, rapid changes in power requirements, excessive fuel supply, or foreign objects in the flow.

The stable operating range of a compressor is limited in the compressor map by the so-called stability limit or surge line. If this limit is not reached, the flow in the compressor can no longer be maintained in a stable manner. Before a complete flow collapse occurs, local flow separations often occur first at the compressor blades. This condition is referred to as rotating stall. This results in areas with greatly reduced or even backward flow, which move as rotating cells along the circumference of the blade row. In this state, the compressor is still basically functional, but significant pressure fluctuations, increased losses, and strong aerodynamic loads on the blades already occur.

If the instability becomes more severe, the pump limit (surge) may be exceeded. In this case, the flow in the entire compressor system collapses. Surge is a global aerodynamic instability in which the entire mass flow through the compressor fluctuates periodically or even reverses direction for a short time. This cycle of pressure build-up, flow collapse, reverse flow, and rebuild repeats periodically and is referred to as compressor pumping.During compressor pumping, strong pressure and mass flow oscillations occur throughout the compressor. These lead to considerable mechanical stress on the structure. The most important consequences include:

• Strong aerodynamic forces on the compressor blades
• Periodically changing bending and torsional loads on the rotor structure
• Possible excitation of natural modes of the blades (aeroelasticity/flutter)

These cyclic loads can cause compressor blades to vibrate or deform elastically. In extreme cases, structural damage can even occur. A particularly critical factor here is that during these unsteady operating conditions, the radial position of the rotor blades relative to the housing can also vary greatly. This can lead to temporary gap closures or even rotor-casing contact.

Project objectives

Current simulation and calculation methods cannot reliably predict changes in the radial rotor-housing clearance during compressor surges. Complex interactions between aerodynamics, structure, and rotor dynamics occur, particularly during compressor instabilities, which cannot be fully captured by the currently established models. 
For this reason, the radial clearance is often designed on the basis of empirical values in practice. To prevent the rotor from running into the housing during unsteady operating conditions, an additional safety margin is taken into account. However, this conservative design can have negative effects. If the safety margin is too large, the increased clearance leads to a loss of compressor efficiency and a reduction in the pump limit distance. This creates a conflict of objectives between mechanical safety and aerodynamic performance.

The aim is therefore to develop a method for calculating radial rotor clearance closures during compressor instabilities. Such a method should make it possible to predict the dynamic change in the radial clearance during transient compressor phenomena and to reduce the necessary safety margin in the clearance design without increasing the risk of rotor-casing contact.