Question regarding converging-diverging nozzles

  • Thread starter Amaelle
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In summary, the conversation discusses the converging-diverging nozzle and how the mass flow rate can be expanded by reducing the downstream pressure. The question is raised about how to reach the sonic condition in the throat when the downstream pressure is too high. The answer is that without the required pressure differential, the flow cannot be accelerated enough. However, it is possible to achieve the required pressure by either lowering the downstream pressure or increasing the upstream pressure.
  • #1
Amaelle
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TL;DR Summary
Good day!

I have a question regarding the converging-diverging nozzle (look at the image)

Normally, in order to expand the mass flow rate, we reduce the downstream pressure

my question is how can we reach the sonic condition in the throat when the downstream pressure is considerably high(does not reach the critical pressure?

many thanks in advance
Good day!
I have a question regarding the converging-diverging nozzle (look at the image)
Normally, in order to expand the mass flow rate, we reduce the downstream pressure
my question is how can we reach the sonic condition in the throat when the downstream pressure is considerably high(does not reach the critical pressure?
many thanks in advance
convergent divergent nozzle.png
 
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  • #2
In short: you can't. If you don't have the required pressure differential, you don't have the appropriate motive force to accelerate the flow that much.
 
  • #3
To be picky, if you have a considerable pressure downstream, you need a higher pressure upstream to achieve the choking condition. But the downstream pressure will always have to be the critical pressure (either by lowering downstream pressure or increasing upstream pressure) in order to get choking.
 

FAQ: Question regarding converging-diverging nozzles

What is a converging-diverging nozzle?

A converging-diverging nozzle is a type of nozzle used in fluid mechanics to accelerate a fluid, such as gas or liquid, to a high velocity. It consists of a converging section, where the cross-sectional area decreases, followed by a diverging section, where the cross-sectional area increases. This design allows for efficient and controlled acceleration of the fluid.

How does a converging-diverging nozzle work?

A converging-diverging nozzle works by converting the pressure energy of a fluid into kinetic energy. As the fluid enters the converging section, its velocity increases due to the decreasing cross-sectional area. Then, as the fluid passes through the throat of the nozzle, it reaches its maximum velocity. Finally, in the diverging section, the fluid expands and its velocity decreases, resulting in a high-speed, high-pressure jet of fluid.

What are the applications of converging-diverging nozzles?

Converging-diverging nozzles have a wide range of applications, including in rocket engines, gas turbines, and supersonic aircraft. They are also used in industrial processes such as water jet cutting and sandblasting. Additionally, converging-diverging nozzles are used in wind tunnels to simulate supersonic flow.

What factors affect the performance of a converging-diverging nozzle?

The performance of a converging-diverging nozzle is affected by several factors, including the shape and length of the nozzle, the fluid properties, and the inlet pressure. The angle of the diverging section also plays a significant role in the nozzle's performance, as it affects the expansion of the fluid and the resulting exit velocity.

How is the efficiency of a converging-diverging nozzle measured?

The efficiency of a converging-diverging nozzle is typically measured by the thrust coefficient, which is the ratio of the thrust produced by the nozzle to the pressure of the fluid at the inlet. The higher the thrust coefficient, the more efficient the nozzle is at converting the pressure energy of the fluid into thrust. Other measures of efficiency include the mass flow rate and the specific impulse of the nozzle.

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