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Application of Turbofan Power Ratio to Turbojet Engine with Variable Exhaust Geometry
Károly Beneda

Last modified: 2023-06-29


In commercial and military aviation typically gas turbine engines are utilized for propulsion. Although variable exhaust geometry is conventionally used in military engines due to the requirement of thrust augmentation, commercial designers have an increasing interest in this technology in the recent years, in order to improve flexibility and efficiency of their engines. Control of turbine engines is generally based on rotor speed, Engine Pressure Ratio or Turbofan Power Ratio. Rotor speed has a wide operating range but thrust is not linearly dependent. Engine Pressure Ratio has better linearity, but its range is significantly reduced. In contrast to the previous solutions, Turbofan Power Ratio (TPR) offers both linear correlation and extensive value limits. However, in case of variable exhaust geometry, this parameter shows significant dependence on the actual nozzle position. The aim of this paper is to investigate how the thrust-to-TPR correlation changes under such circumstances, what kind of transformations would be required in order to construct a similar compound thermodynamic parameter to TPR but implementing the effect of variable nozzle as well. To reach this goal, the investigations are first performed on a one-dimension mathematical model of a turbojet engine. Although this type of engine is not very widely used currently, the author has the possibility of validation on a test bench utilizing such equipment. Thus, the preliminary theoretical results can be supplemented by experimental data. Previous assessments were already performed in the two extreme positions of the variable exhaust nozzle, and a simplified model was developed, in which TPR was replaced with a transformed Variable Nozzle Thrust Coefficient (VNTC). Nonetheless, detailed studies showed that the response of TPR is not linear, even if the available range of nozzle area is narrow. Thus, the present investigation focuses on the nonlinear behavior and establishes the correct transformation which covers the entire range of nozzle area and VNTC can supply a fully proportional value to thrust output at all throttle and nozzle conditions. The results of this study can be exploited in gas turbine control and diagnostics, where the obtained parameter VNTC can provide a better insight into the operating processes of the machine resulting in increasing efficiency of the service and introduce additional means of condition monitoring.