CFD Study of Ejector Nozzle Models

Abstract

This paper presents a CFD study of thrust-augmenting ejector nozzles for small turbojet engines, motivated by the theoretical prediction of Alperin and Wu that a substantial thrust increase is attainable at moderate subsonic flight speeds via a so-called “second solution” in which the flow at the exit of the mixing chamber is supersonic. The input parameters were derived from Alperin’s 1D theory; an axisymmetric baseline nozzle model was constructed and validated against a component-based thermodynamic model using the Grid Convergence Index; and 61 ejector nozzle configurations with area ratios α * = 4 and 5 were computed in ANSYS Fluent 2024R1 at M _∞ = 0.6. Fields of Mach number, static pressure and static temperature, together with pathline patterns and integral thrust parameters, are presented and discussed. In none of the configurations tested, spanning annular and nose air intakes, varied mixing chamber geometries, and explicit specification of the secondary flow rate, could the second solution be reached: the flow at the mixing chamber exit remained subsonic in every case, corresponding to Alperin’s first solution, for which the thrust augmentation ratio is below unity. The contraction of the common exhaust unit was identified as the factor suppressing the secondary flow. These results indicate that ejector nozzles of the type studied cannot increase the thrust of a small turbojet at moderate subsonic flight speeds, and they cast doubt on the physical realizability of Alperin’s second solution in this regime.