Decoupling of Combustion Mode Transition in Scramjets Under Fluid–Thermal–Structural Interaction

To elucidate the governing mechanism of fluid–thermal–structural interaction (FTSI) on combustion mode transition in rectangular scramjets, this study establishes a three-dimensional hypersonic FTSI analytical framework and performs numerical simulations over a 100 s operational duration. The FTSI is decomposed into structural deformation and wall temperature rise to qualitatively analyze its relative contributions. Results show that the combustion mode experiences a three-stage evolution under FTSI: cavity shear layer combustion, cavity-assisted jet wake combustion, and jet wake combustion. The core heat release region migrates upstream from the downstream divergent section to the cavity and isolator. Structural deformation dominates the mode transition by inducing a throat-compensation mechanism near the cavity trailing edge that elevates backpressure. Meanwhile, wall temperature rise synergistically promotes upstream flame propagation by elevating the fuel–air mixture temperature and suppressing heat loss. The dominant heat release regime shifts from early subsonic premixed lean combustion to late subsonic diffusion combustion, with the premixed combustion heat release fraction decreasing from 86.52% to 31.05%. Notably, in the cavity-assisted jet wake combustion mode, competition for combustion dominance between the cavity shear layer and the jet wake causes fluctuations in both combustion efficiency and specific impulse.