Propelling Nozzle - Convergent Nozzles

Convergent Nozzles

Almost all nozzles have a convergent section because it increases the pressure in the rest of the engine- potentially yielding more thrust by acting on the forward sections. It is important to note that convergent nozzles end with this convergent section, and in general, narrower convergent nozzles give lower thrust and higher exhaust speed, but wider convergent nozzles give lower exhaust speed and higher thrust.

Simple convergent nozzles are used on many jet engines. If the nozzle pressure ratio is above the critical value (about 1.8:1) a convergent nozzle will choke, resulting in some of the expansion to atmospheric pressure taking place downstream of the throat (i.e. smallest flow area), in the jet wake. Although jet momentum still produces much of the gross thrust, the imbalance between the throat static pressure and atmospheric pressure still generates some (pressure) thrust.