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

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. In this simulation method, 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 spool 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 multispool compressor engines because the short intercompressor 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 the simple coupling factor introduced in this paper. There are two fundamentally different reasons for the change in engine 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 the actions of the control system. 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 the 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 recirculating bleed air for increasing the surge margin of a compressor affects the performance of the engine negatively. Two examples show clearly that the pros and cons of recirculating bleed can only be judged with a full system simulation; looking at the surge line improvement alone can be misleading.