Recall, the energy balance for a control volume is:

Nozzle

Diffuser

Nozzles and diffusers are commonly utilized in jet engines, rockets, and spacecrafts. Any fluid-mechanical device designed to accelerate a flow is called a nozzle and any fluid-mechanical device designed to decelerate a flow is called a diffuser. For subsonic flow (velocity under the speed of sound), a converging duct is a nozzle and a diverging duct is a diffuser.

Nozzle and diffuser are single-stream (one-inlet-one-exit) systems. If the inlet is denoted by subscript 1 and exit is denoted by subscript 2, the mass balance and energy balance for single-stream steady-flow systems become

There are several common assumptions that are made in the energy analysis of nozzles and diffusers:

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  The fluid has high velocity and thus not spending enough time in the device for any significant heat transfer to take place.
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  Nozzles and diffusers are properly shaped ducts and no shaft or electric work can be transferred in or out.
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  As a fluid passes through a nozzle or diffuser, it experiences a large change in its velocity. Hence, the kinetic energy change must be accounted for.
• 
  The fluid experiences small or no change in its elevation as it flows through the nozzle or diffuser.

Summarizing the above analysis, the energy balance for nozzles and diffusers is:

      (h₂ - h₁) + ( v₂² - v₁²)/2 = 0

Throttling Valves

A significant reduction in pressure can be achieved by introducing a restriction into a line through which a gas or liquid flows. This is commonly done by means of an adjustable valve, a porous plug, or a capillary tube. They are called throttling devices.

There are several common assumptions that are made in the energy analysis of throttling devices:

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  Throttling devices are usually small devices. There is neither sufficient time nor large enough area for significant heat transfer to take place.
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  No work is done to or from the devices.
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  The increase in kinetic energy is insignificant.
• 
  The change in potential energy is very small.

Hence, the energy balance for throttling devices is:

      h₂ = h₁

When the flow through a valve or other restrictions is idealized in this way, the process is called a throttling process.