Effects of Inlet Flow Distortion on the Performance of Aircraft Gas Turbines

Abstract

The main problem with aircraft engine inlet flow distortion is its effect on the stability of the compression system. However, distortion does also influence the performance delivered by the propulsion system. There are two fundamentally different reasons for the change in performance: First there is the impact of the flow distortion on the component efficiencies and thus the thermodynamic cycle and second there are performance changes due to actions of the control system. This paper describes how the fundamental effects of inlet flow distortion on the performance of gas turbines can be evaluated with any engine performance program that employs an integrated parallel compressor model. This sort of simulation is a valuable tool for evaluating the basic effects of complex flow phenomena on the performance of a gas turbine. It delivers fundamentally correct answers since even the most complex flow structures obey the laws of mass and energy conservation and that’s all what the overall system simulation is about. In the parallel compressor model both pressure and temperature distortions are quantified with coefficients which relate the pressure (respectively temperature) in the spoiled sector to the value in the clean sector. In single compressor engines the static pressure at the exit of the clean sector equals that of the distorted sector. This hypothesis does not hold true with multi-compressor engines because the short inter-compressor ducts, which often contain struts or vanes, do not allow the mass flow transfer over the sector borders which would be required for balancing the static pressures. The degree of aerodynamic coupling of compressors in series can be described in the performance simulation program by a coupling factor. From the engine system simulation results it becomes clear why inlet flow distortion has only a minor impact on the thermodynamic cycle if the comparison of the two operating conditions (with clean and distorted inlet flow) is made at the properly averaged engine inlet conditions. For each compressor the parallel compressor theory yields two operating points in the map, one for the clean sector and one for the spoiled sector. The performance loss due to distortion is small since the efficiency values in the two sectors are only a bit lower than the efficiency at a comparable operating point with clean inlet flow. However, the control system of the engine can react to the inlet flow distortion in such a way that the thrust delivered changes significantly. This is particularly true if a compressor bleed valve or a variable area nozzle is opened to counteract compressor stability problems. Especially using re-circulating bleed air to increase the surge margin of a compressor affects the performance of the engine negatively. Two examples show clearly that the pro and cons of re-circulating bleed can only be judged with a full system simulation, looking at the surge line improvement alone can be misleading.