Patially-mixed ejectors

Ejectors are devices used for noise suppression and thrust augmentation. The ejector is a
shroud placed around a jet exhaust. It absorbs sound and, under certain conditions,
increases thrust. However, in some cases it penalizes thrust, especially if the flight Mach
number is high.
Prevailing analyses assume complete gas mixing inside the shroud, resulting in uniform exit flow.
This is often not true, especially for supersonic engines. A new method for analyzing ejectors
addresses their partially-mixed nature. It is uses a simple, quasi-one dimensional model for each
stream of the ejector, with friction and heat transfer occuring between the two streams. The
results are in good agreement with experimental data in the literature.
Below is one of the results, the thrust augmentation versus fligh Mach number for several ejector
geometries (A/S=axisymmetric, 2D=two-dimensional, L/b=length over radius). It is seen that
the thrust benefit deterierates as the flight Mach number Ma increases. For Ma>0.7, a thrust loss
is anticipated.
Papamoschou, D., "Analysis of Partially-Mixed Supersonic Ejector," AIAA Journal of Propulsion and Power, Vol. 12, No.4, 1996.