Distinct flow instability mechanisms in a variable cycle engine fan: Decoupling analysis of backpressure at the different outlets

Variable cycle engines (VCEs) are the core technology of the next-generation military aero-engines. As a key component of VCEs, the compression system is one of the critical factors restricting the overall performance of the VCEs. To investigate the flow instability mechanisms of the compression system in VCEs under different throttling modes, a dual-flow fan was selected as the research object in this study. By employing a throttling method that fixes the backpressure at one outlet while gradually increasing the backpressure at the other outlet, this study systematically studied the flow instability mechanisms of the dual-flow fan under different throttling modes. The results indicate that the flow instability mechanisms of the dual-flow fan differ significantly depending on the throttling mode. During bypass flow throttling, the flow instability originates in the front fan (F-Fan), whereas during core flow throttling, the initial instability position shifts to the aft fan in the inner bypass. The key criterion for determining whether the initial instability position is located in the F-Fan is whether the static pressure at the F-Fan outlet exceeds “F-Fan instability static pressure” (obtained by calculating the F-Fan separately). Furthermore, the initial instability position in the studied dual-flow fan is independent of the outlet backpressure of the initial operating condition. Specifically, during bypass flow throttling, the flow instability always occurs in the F-Fan, and during core flow throttling, it always occurs in the aft fan. This study further refines the theoretical framework for flow instability in VCE compression systems and provides valuable insights for addressing potential stability issues and designing flow control strategies.