Understanding the Exit Pressure of Nozzles in Thermodynamics

In summary: Ahh. This would be correct if you have a smaller piston with the same force applied. However, think instead if that reduction happened in the fluid itself (so you had a large piston, then the fluid reduced down to a smaller diameter below it). Now the pressure in the small diameter region is the same as the large diameter reason (I'm ignoring gravity here). This is more analogous to the nozzle case, where the reduction happens in the fluid itself. If you have a nozzle with static fluid throughout, the pressure will be the same throughout, just as in this piston case. Make sense so far?
  • #1
scottymo
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Hi There,

Im studying thermodynamics at the moment and there's one statement about nozzles that I just haven't been able to understand. In my mind when a fluid exits a nozzle it would have a higher pressure than the inlet. Could someone please explain in what sense the pressure of a fluid drops as it goes through a nozzle? Are they talking total pressure over an area? my book doesn't explain why just makes that statement.

Thanks in advance
 
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  • #2
Why would the fluid have a higher pressure at the exit than the inlet? The whole point of a nozzle is that it accelerates the flow. In order for the flow to accelerate, the pressure gradient through the nozzle must be such that the fluid feels a force towards the exit, requiring a lower exit pressure than inlet pressure.
 
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  • #3
cjl said:
Why would the fluid have a higher pressure at the exit than the inlet? The whole point of a nozzle is that it accelerates the flow. In order for the flow to accelerate, the pressure gradient through the nozzle must be such that the fluid feels a force towards the exit, requiring a lower exit pressure than inlet pressure.
I don't know why, I'm clearly wrong on that thought so I'm looking for insight to set my thought process straight on the matter. Maybe its just been beaten into my head to long that you increase pressure when you reduce area. What you say about about the flow seeking lower pressure does however make a lot of sense.
 
  • #4
Out of curiosity, in what context did you hear that you increase pressure when you reduce area? It's certainly not generally true. I'm sorry I'm not giving more detailed answers here, but I really don't understand exactly where your confusion is arising from, so it's hard to address it. I'd love to go into more detail though if you tell me what specifically I should expand on...
 
  • #5
cjl said:
Out of curiosity, in what context did you hear that you increase pressure when you reduce area? It's certainly not generally true. I'm sorry I'm not giving more detailed answers here, but I really don't understand exactly where your confusion is arising from, so it's hard to address it. I'd love to go into more detail though if you tell me what specifically I should expand on...
Well say you take a mechanical advantage piston setup or or the footprint of a column, reducing the area on one of the pistons or at the end of the column will cause a rise in pressure at that point. Thats what I'm used to at least. Now in my mind when I picture a nozzle I see a volume of fluid going in at a certain rate, now the cross sectional area reduces as it travels through meaning less volume for that fluid to occupy, it just makes sense in my head because of this that the pressure of the fluid increases as it travels through the nozzle. I am having trouble getting rid of that notion.
 
  • #6
scottymo said:
Well say you take a mechanical advantage piston setup or or the footprint of a column, reducing the area on one of the pistons or at the end of the column will cause a rise in pressure at that point. Thats what I'm used to at least. Now in my mind when I picture a nozzle I see a volume of fluid going in at a certain rate, now the cross sectional area reduces as it travels through meaning less volume for that fluid to occupy, it just makes sense in my head because of this that the pressure of the fluid increases as it travels through the nozzle. I am having trouble getting rid of that notion.

Ahh. This would be correct if you have a smaller piston with the same force applied. However, think instead if that reduction happened in the fluid itself (so you had a large piston, then the fluid reduced down to a smaller diameter below it). Now the pressure in the small diameter region is the same as the large diameter reason (I'm ignoring gravity here). This is more analogous to the nozzle case, where the reduction happens in the fluid itself. If you have a nozzle with static fluid throughout, the pressure will be the same throughout, just as in this piston case. Make sense so far?
 

Related to Understanding the Exit Pressure of Nozzles in Thermodynamics

What is the exit pressure of a nozzle?

The exit pressure of a nozzle is the pressure of the fluid or gas at the outlet of the nozzle. It is a crucial parameter in the design and performance of a nozzle, as it affects the flow rate and thrust produced.

How is the exit pressure of a nozzle calculated?

The exit pressure of a nozzle can be calculated using the Bernoulli's equation, which takes into account the fluid velocity, density, and the change in cross-sectional area of the nozzle. Other factors such as friction and turbulence may also need to be considered for more accurate calculations.

What happens if the exit pressure of a nozzle is too high?

If the exit pressure of a nozzle is too high, it can cause a phenomenon known as "choked flow", where the fluid flow reaches its maximum velocity and any further increase in pressure will not result in an increase in flow rate. This can lead to a decrease in performance and efficiency of the nozzle.

What factors can affect the exit pressure of a nozzle?

The exit pressure of a nozzle can be affected by several factors such as the shape and size of the nozzle, the fluid properties, the operating conditions (such as temperature and pressure), and the design of the nozzle. Changes in any of these factors can alter the exit pressure and ultimately impact the performance of the nozzle.

How is the exit pressure of a nozzle controlled?

The exit pressure of a nozzle can be controlled by adjusting the inlet pressure, the geometry of the nozzle, or by using a divergent section in the nozzle design. Additionally, incorporating control devices such as valves or dampers can also help regulate the exit pressure of the nozzle.

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