Modeling and analysis of windmilling operation during mode transition of a turbine-based-combined cycle engine

Mode transition control is a critical issue of Turbine-Based-Combined Cycle (TBCC) engines when the primary thrust providers change from gas turbine engines to ramjets/scramjets. During mode transition, windmilling operation is inevitable when the fuel mass flow rate of the turbojet engine is decreasing to zero gradually. Existed research on mode transition control of TBCC engines usually neglect this process or simplify this process to a rapid thrust loss due to lacking a windmilling model. However, the absence of the windmilling model might cause critical problems unconsidered at the control system design stage and increase the risk of mission abortion. This paper provides a method of modeling the windmilling operation of a TBCC engine. Sub-idle compressor component maps are obtained by the interpolation method based on a zero-speed line to consider the characteristics that the total pressure ratio is below one. Friction torque is added in the torque balance relationship to achieve the free windmilling operation state when both the compressor and the turbine provide positive torques. The inlet start/unstart status coupled with the operating point of the turbojet engine is also considered. Results show that this model could reflect the characteristics that the total pressure ratio of the compressor is below one, and it provides a positive torque instead of a negative one when the turbojet engine operates at some sub-idle states. Moreover, inlet unstart would happen at some point when the turbojet engine is shutting down to the free windmilling state, under which the turbojet engine would bring in the flight drag instead of a positive thrust. In the descent mode transition when the turbojet engine is accelerating to be restarted, an inappropriate rotating speed of the inlet splitter could result in a fail inlet restart. (C) 2020 Elsevier Masson SAS. All rights reserved.