Thermodynamic cycle analysis of the fuel precooled multi-mode turbine engine mode transition process: Why? When? How?

The fluctuation of engine thrust during the mode transition process is a critical concern that poses a significant risk to aircraft safety. In order to address this issue, a thermodynamic model for mode transition in a dual fuel precooled multi-mode turbine engine is established, and three key issues in the mode transition process are comprehensively examined from the perspectives of steady-state analysis and dynamic process optimization: Why? When? How? Firstly, the basis for mode transition under high Mach number conditions is proposed for the first time from exergy efficiency. The engine exergy efficiency and combustion exergy efficiency in the turbofan mode decrease as the Mach number increases, while the turbojet mode exhibits superior combustion exergy efficiency. To achieve efficient operation, it is imperative to switch from the turbofan mode to the turbojet mode. Subsequently, a novel approach is proposed for determining the mode transition range based on bidirectional cyclic correction, which effectively addresses the limitation of large thrust fluctuations at the mode transition point determined by the conventional maximum state thrust continuous method. The mode transition envelope that can achieve a smooth transition between thrust and airflow rate is determined. Finally, a mode transition method is newly put forward by integrating directional adjustment and offline optimization. Throughout the entire mode transition envelope, the maximum thrust fluctuation during the process remains below 1 %, while the maximum airflow rate change stays within 3 %, which exhibits excellent efficacy in achieving smooth transitions.