Understanding the Exit Pressure of Nozzles in Thermodynamics

[scottymo]

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

 

[cjl]

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.

 

[scottymo]

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.

 

[cjl]

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...

 

[scottymo]

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.

 

[cjl]

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?