Multi-Bypass Matching and Flow Mechanisms in a Variable Cycle Compression System

Adaptive cycle engines regulate the bypass ratio through variable geometry to reconcile low fuel consumption during subsonic cruise with high specific thrust during supersonic operation. This study investigates a two-shaft, two-bypass variable cycle compression system (VCCS) using an experimentally validated three-dimensional numerical method. Throttling characteristics and internal flow mechanisms are analyzed for three throttling paths under representative low- and high-bypass ratio conditions. During throttling, upstream components move toward their instability boundaries, whereas downstream components move toward choke. Throttling either bypass decreases the total bypass ratio, while core throttling increases it. First-bypass throttling produces the largest bypass ratio variation at the low-bypass ratio condition, whereas core throttling dominates at the high-bypass ratio condition. The component that governs the final instability limit depends on the throttling path; the front fan, rear fan, and high-pressure compressor dominate during second-bypass, first-bypass, and core throttling, respectively. At the high-bypass ratio condition, excessive closure of the rear fan inlet guide vane and compressor radial diffuser causes internal mismatch and large-scale separation, reducing system stability. These results clarify cross-component matching and instability mechanisms and provide guidance for VCCS control-strategy design and stability assessment.