Dynamic surge behavior induced by variable geometry turbine adjustment in a small turbojet engine

Aerodynamic instability in aero engines remains a critical issue, adversely affecting engine performance and structural integrity. While existing studies have primarily focused on component-level testing, there remains limited research on compressor instability under real engine operating conditions. Conventional surge induction methods, such as air injection or fuel-spiking, often introduce significant unsteady disturbances that interfere with surge processes. In this study, modifications were made to the turbine component of a small turbojet engine to enable variable stator functionality. By adjusting the variable-geometry stator, the engine was driven into surge through a quasi-steady process. High-frequency dynamic pressure probes captured the complete pressure history at the compressor inlet and outlet during instability development. The results indicate that mild surge occurs at most tested spool speeds, with stronger surges at certain speeds leading to simultaneous high-frequency instabilities such as rotating stall. This study also outlines the operational constraints of the Variable-Geometry Turbine (VGT) surge induction method across different spool speeds, clarifying its operational envelope for small turbojet engines.