A quasi-one-dimensional critical-throat acoustic boundary condition for thermally choked dual-mode ramjet nozzles

Thermally choked nozzles, where choking is induced by heat addition rather than a geometrical throat, are a promising solution for dual-mode ramjet transitions to hypersonic speed. Despite their relevance, thermal throat boundary conditions for quasi-one-dimensional acoustic modelling are not derived in the state-of-the-art. This study introduces a generalized critical-throat acoustic boundary condition applicable to both geometrically and thermally choked flow configurations. A dedicated one-dimensional linear acoustic solver is formulated to incorporate this condition and is validated against two-dimensional Euler simulations. Particular attention is paid to the impact of entropy and acoustic waves at the critical throat. The results show that the new boundary condition improves the prediction of acoustic reflection, entropy noise production, and transmission coefficients, especially under thermally choked conditions where the commonly used quasi-steady assumption fails. For both the thermal-throat and geometric-throat configurations, the deviation in the acoustic transmission coefficient between the linear acoustic model using the proposed boundary condition and the simulations remains below 2%, while the deviation in the entropy-noise transmission coefficient remains below 5%, demonstrating the robustness of the proposed boundary condition.