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Convergent nozzles accelerate subsonic fluids. If the nozzle pressure ratio is high enough, then the flow will reach sonic velocity at the narrowest point (i.e. the nozzle throat). In this situation, the nozzle is said to be choked. Increasing the nozzle pressure ratio further will not increase the throat Mach number above one.
Exhaust or nozzle — Turbine exhaust gases pass through the propelling nozzle to produce a high velocity jet. The nozzle is usually convergent with a fixed flow area. Supersonic nozzle — For high nozzle pressure ratios (Nozzle Entry Pressure/Ambient Pressure) a convergent-divergent (de Laval) nozzle is used. The expansion to atmospheric ...
Figure 1: A de Laval nozzle, showing approximate flow velocity increasing from green to red in the direction of flow Density flow in a nozzle. A rocket engine nozzle is a propelling nozzle (usually of the de Laval type) used in a rocket engine to expand and accelerate combustion products to high supersonic velocities.
The case of a converging-diverging nozzle allows a supersonic flow to occur, providing the receiver pressure is sufficiently low. This is shown in figure 3 assuming a constant reservoir pressure with a decreasing receiver pressure. If the receiver pressure is equal to the reservoir pressure, no flow occurs, represented by curve A.
This is referred as overexpanded flow because in this case the pressure at the nozzle exit is lower than that in the ambient (the back pressure)- i.e. the flow has been expanded by the nozzle too much. [13] A further lowering of the back pressure changes and weakens the wave pattern in the jet.
A nozzle operates according to the Venturi effect to bring the exhaust gasses to ambient pressure, while forming them into a propulsive jet; if the pressure upstream of the nozzle is high enough, the flow will reach sonic speed . The role of the nozzle in back-pressuring the engine is explained below.