Surge - When the air flows backwards through the engine

The speed of the rotors as well as the throttle position are used to modify the operating state. If both parameters are kept constant, the compressor remains at an operating point in the compressor map. At a fixed rotation speed, the operating point stays on the respective speed line. The higher the rotating speed, the more air accumulates in the compressor and the higher the pressure level.

If the rotating speed is kept constant and only the throttle position is modified, the operating point shifts along the speed line. When throttling, air is accumulated more intensely: the mass flow decreases, but the pressure ratio and the efficiency increase.

At the same time however, the angle of attack of the flow on the blades increases. As with aircraft, the angle of attack cannot be increased arbitrarily, because flow separation might occur. In compressors, flow separation leads to an unstable flow condition. The stability limit is defined by the surge line in the compressor map (Figure 2). When the surge line is crossed, various unstable phenomena might occur. One of these is the so-called surge.

Flutter is a self-excited vibration phenomenon in which the blade vibration amplitude might increase exponentially and cause blade failure. The flutter phenomenon highly depends on the basic flow. This basic flow is much more complex at surge than at normal operating conditions, which makes the prediction of a flutter onset and behavior particularly challenging.

Figure 3 shows the course of a typical surge cycle in the compressor map. During surge, the basic flow changes continuously with a relatively high frequency. During the cycle, various flow states with complex flow conditions such as flow separation, shocks and reversed flow areas occur.

In flight, a margin is defined and prescribed in order to guarantee safe operation of the compressor and to avoid surge line crossings. However, this margin prevents the compressor from operating in the area of maximum efficiency. As a consequence, the full potential of the compressor is never achieved. The aim of the current research is to improve the understanding of the system behavior during surge, to predict it and, in the best case, to be able to control it. For this purpose, experimental and numerical prediction methods are constantly being developed. At the Institute of Aeroelasticity, these predictions contribute to evaluate blade structural loading and to prevent blade row damage during the experiments.

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