Understanding the Exit Pressure of Nozzles in Thermodynamics

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Discussion Overview

The discussion revolves around the concept of exit pressure in nozzles within the context of thermodynamics. Participants explore the relationship between fluid pressure and flow acceleration as fluid passes through a nozzle, addressing misconceptions and seeking clarification on the principles involved.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about the statement that pressure drops as fluid exits a nozzle, initially believing that pressure would increase due to reduced area.
  • Another participant argues that for a nozzle to accelerate flow, the exit pressure must be lower than the inlet pressure, challenging the initial assumption.
  • A participant reflects on their misunderstanding, acknowledging that they have been conditioned to think that reducing area leads to increased pressure, and seeks clarification on this point.
  • Another participant questions the context in which the idea of increased pressure with reduced area was encountered, stating that it is not generally true in fluid dynamics.
  • One participant attempts to clarify the analogy of a piston setup, explaining that if the reduction in area occurs in the fluid itself, the pressure remains constant across different diameters, which aligns more closely with nozzle behavior.

Areas of Agreement / Disagreement

Participants do not reach a consensus, as there are competing views regarding the relationship between area reduction and pressure changes in nozzles. The discussion remains unresolved with ongoing confusion and differing interpretations of the principles involved.

Contextual Notes

Participants express uncertainty regarding the definitions and principles of fluid dynamics, particularly in relation to pressure changes in nozzles and the effects of area reduction. There are references to specific setups (like pistons) that may not directly apply to nozzle behavior, indicating a potential limitation in understanding the broader context.

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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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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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.
 
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...
 
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.
 
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?
 

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