An ejector uses the momentum of a high-pressure motive fluid to entrain a second fluid. Having no moving parts it is reliable, and it is used in vacuum generation, steam refrigeration, fuel cell recirculation and ventilation mixing.
Critical and subcritical operation
The ejector characteristic is not linear:
Critical (choked) regime. While the back pressure stays below a certain threshold, the entrained flow rate is independent of it. The secondary flow reaches sonic velocity in the mixing chamber and chokes. Operation is stable.
Subcritical regime. If the back pressure exceeds the threshold, the entrained flow falls rapidly and beyond a point the flow reverses. This is an abrupt transition, not a gradual one.
The aim of the design is to keep the operating point inside the critical regime with a reasonable margin to the threshold.
Modelling requirements
A compressible solution is mandatory. A sonic condition forms at the throat; an incompressible model cannot see the choking and overpredicts the entrained flow. See the compressible flow note.
The shock structure must be resolved. A shock train forms in the mixing chamber and diffuser; its position determines the back pressure threshold. The mesh must be fine there.
Turbulent mixing governs. Ejector performance depends on how well the two streams mix. The resolution of the mixing layer enters the result directly, and the turbulence model choice makes a marked difference here. See the turbulence model note.
Geometric sensitivity
The axial position of the nozzle exit relative to the mixing chamber changes performance markedly for a few millimetres of movement. That is why an ejector is one of the components best suited to parametric optimisation — and one of the worst suited to manual trial and error.
References
- Ferziger, J. H., Perić, M. & Street, R. L. (2020). Computational Methods for Fluid Dynamics, 4th ed. Springer. DOI: 10.1007/978-3-319-99693-6
- Fay, J. A. & Riddell, F. R. (1958). Theory of stagnation point heat transfer in dissociated air. Journal of the Aerospace Sciences, 25(2), 73–85. DOI: 10.2514/8.7517