Plug nozzle

The plug nozzle is a type of nozzle which includes a centerbody or plug around which the working fluid flows. Plug nozzles have applications in aircraft, rockets, and numerous other fluid flows.

Common garden hose trigger nozzles are a simple example of the plug nozzle and its method of operation. In this example the nozzle consists of a conical or bell shaped opening with a plug on a movable rod positioned in front of the nozzle. The plug looks similar to a poppet valve. The stem of the valve runs back through the body of the nozzle body to a "trigger", normally a long lever running down the back of the nozzle assembly. A spring keeps the valve pressed against the opening under normal use, thereby providing a failsafe cut-off that stops the flow of water when the nozzle is dropped.

When water is supplied to the hose, it flows through the nozzle body to the opening, where it would normally flow straight forward in a stream. Just after leaving the opening it encounters the plug, which deflects the water sideways through an angle. After travelling a short distance the water encounters the outside of the nozzle opening, which deflects it forward again. This two-step process causes the water to be ejected in a ring-shaped pattern, which causes less water to strike any one location, and thereby reduces erosion while also making it easier to water larger areas.

The shaping of the plug and the nozzle opening allows the angle of the ring to be adjusted. Normally this is shaped so that when the plug is pulled back toward the opening it both partially cuts off the water flow, as well as causing it to spread out to the widest possible angle. This can be used for "misting" plants. When the trigger is pushed down further, the plug moves away from the opening, causing less blockage and disruption of the flow, ultimately allowing the water to form back into a stream.

Plug nozzles belong to a class of altitude compensating nozzles much like the aerospike which, unlike traditional designs, maintains its efficiency at a wide range of altitudes.

Similar to the garden hose example, plug nozzles use a shaped rocket nozzle with a poppet-shaped plug to allow the pattern of the rocket exhaust to be changed. This is used to adjust for changes in altitude; at lower altitudes the plug is pulled back to cause the exhaust to spread out, while at higher altitudes the lower air pressure will cause this to happen naturally. An alternative construction for the same basic concept is to use two nozzles, one inside the other, and adjust the distance between them. This pattern has the advantage of better control over the exhaust and simpler cooling arrangements.

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