Page 72 - The Jet Engine

Exhaust system

6. With the convergent type of nozzle a wastage of with the exit proper now being at the end of the flared
energy occurs, since the gases leaving the exit do divergent section. When the gas enters the
not expand rapidly enough to immediately achieve convergent section of the nozzle, the gas velocity
outside air pressure. Depending on the aircraft flight increases with a corresponding fall in static pressure.
plan, some high pressure ratio engines can with The gas velocity at the throat corresponds to the
advantage use a convergent-divergent nozzle to local sonic velocity. As the gas leaves the restriction
recover some of the wasted energy This nozzle of the throat and flows into the divergent section, it
utilizes the pressure energy to obtain a further progressively increases in velocity towards the exit.
increase in gas velocity and, consequently, an The reaction to this further increase in momentum is
increase in thrust. a pressure force acting on the inner wall of the
nozzle. A component of this force acting parallel to
7. From the illustration (fig. 6-3), it will be seen that the longitudinal axis of the nozzle produces the
the convergent section exit now becomes the throat, further increase in thrust.

Fig. 6-5 High by-pass ratio engine exhaust systems.

62