One question about dynamic pressure

[pranj5]

It's possible, but depends on the specific conditions and conservation of energy will apply.

There is no question of violation of conservation of energy. It's the internal heat of the gas that will be converted into higher pressure.

 

[russ_watters]

It's the internal heat of the gas that will be converted into higher pressure.

Well, that's something you can't have: in order for the temperature to drop in a process that does work, the pressure must go down.

As always, you'll do a lot better being specific about your scenarios rather than doing vague handwaving.

 

[FactChecker]

In this discussion, it may be wise to specify what type of pressure you are talking about, total, static, or dynamic. I must assume that the original post was asking about using a venturi to make it easier to get air of lower total pressure to flow into an area of higher total pressure. Assuming that there is initially no air flow, the total pressure equals static pressure on both sides. It will require the amount of work necessary to raise the lower static pressure up above the high static pressure to get the air will to flow in the desired direction. Till then, the air would flow in the reverse direction.
The effect of the venturi and release of internal heat within the venturi will not change that.

 

[pranj5]

Assuming that there is initially no air flow, the total pressure equals static pressure on both sides.

Not at all. There is a blower that is adding velocity of the air at low pressure. The nozzle will further enhance the velocity at the cost of enthalpy (both pressure an temperature) and after that a diffuser is used to convert that velocity into pressure.
What I can assume is that the fall in pressure and temperature inside the venturi is adiabatic but when the diffuser converts the velocity into pressure again, it's isothermal. Why? Because there is already an amount of gas at higher pressure in stock at that point that will stabilise the temperature during compression. In fact, the lower the temperature of the gas at higher pressure, lesser energy will be required to do the job.
What the nozzle will do is to convert a part of internal heat into motion which later will be converted to pressure by the diffuser. But, during the conversion process (from velocity to pressure), the temperature wouldn't rise as much like the adiabatic process.

 

[FactChecker]

Sorry, I should have said zero dynamic pressure initially rather than "no air flow". So are you saying that the static pressure on the fan side is lower but the total pressure is higher due to the fan? The total pressure must be higher to get air flowing the way you want it to. That requires a certain amount of work from the fan to get the required dynamic pressure to make the total pressure high enough. That is all independent of the shape of the venturi.

 

[russ_watters]

Not at all. There is a blower that is adding velocity of the air at low pressure. The nozzle will further enhance the velocity at the cost of enthalpy (both pressure an temperature) and after that a diffuser is used to convert that velocity into pressure.
What I can assume is that the fall in pressure and temperature inside the venturi is adiabatic but when the diffuser converts the velocity into pressure again, it's isothermal. Why? Because there is already an amount of gas at higher pressure in stock at that point that will stabilise the temperature during compression. In fact, the lower the temperature of the gas at higher pressure, lesser energy will be required to do the job.
What the nozzle will do is to convert a part of internal heat into motion which later will be converted to pressure by the diffuser. But, during the conversion process (from velocity to pressure), the temperature wouldn't rise as much like the adiabatic process.

So what you are saying is that you can have air behave adiabatically in expansion and then isothermally in compression...and do compression in a diffuser? Please provide a reliable source that says that is possible and define, specifically, the properties of the air along the process; because it seems to me that you can't do compression isothermally without extracting heat from it...and do all this in an airstream where you aren't doing any work on it. Otherwise this would seem to be a clear-cut violation of conservation of energy...or perhaps two of them.

 

[pranj5]

Sorry, I should have said zero dynamic pressure initially rather than "no air flow". So are you saying that the static pressure on the fan side is lower but the total pressure is higher due to the fan? The total pressure must be higher to get air flowing the way you want it to. That requires a certain amount of work from the fan to get the required dynamic pressure to make the total pressure high enough. That is all independent of the shape of the venturi.

Wrong again. The power supplied by the blower will be increased by the nozzle. The blower alone can't do the job. The nozzle is a crucial part here as it converts heat into motion.

So what you are saying is that you can have air behave adiabatically in expansion and then isothermally in compression...and do compression in a diffuser?

Are you telling that it's theoretically impossible? I have clearly said that as the fluid injected is mixed with an amount of air that is already present there and at lower temperature. I hope you know that something has to be theoretically verified before making a practical example.

it seems to me that you can't do compression isothermally without extracting heat from it

Have you noticed that the compression is done along with an amount of gas already present at higher pressure level. The heat produced during the compression is released into that previously available high pressure gas.

and do all this in an airstream where you aren't doing any work on it.

If it's an air stream, that means it already has some kinetic energy stored in it. The nozzle will convert heat into motion and that will add to the kinetic energy. How can the question of violation of conservation of energy can arise?

 

[russ_watters]

I have clearly said that as the fluid injected is mixed with an amount of air that is already present there and at lower temperature.

No, you haven't clearly stated anything. Your OP doesn't specify any conditions, doesn't tell us if the air is compressible or not and doesn't say anything about mixing with another airstream.

If you had specified, for example, a starting velocity of 150m/s, then we would have known that the airstream was compressible without having to guess what you were after.

Are you telling that it's theoretically impossible? ...I hope you know that something has to be theoretically verified before making a practical example.

That isn't good enough. In order to know if *something* is theoretically possible, that *something* must be specifically described. Please do so now or this thread will be locked for being substandard.

 

[FactChecker]

Wrong again. The power supplied by the blower will be increased by the nozzle. The blower alone can't do the job. The nozzle is a crucial part here as it converts heat into motion.

There is no motion through the venturi except from the higher total pressure to the lower total pressure. Suppose the total pressure remains lower on the fan side. Then if the venturi does anything, it will be helping the flow in the reverse direction.

 

[pranj5]

No, you haven't clearly stated anything. Your OP doesn't specify any conditions, doesn't tell us if the air is compressible or not and doesn't say anything about mixing with another airstream.

I have said it mixing that with another amount of air at higher pressure, not mixing with any kind of stream. In case of injectors, it's obvious that something is taken from a poll of lower pressure and added that to another poll at higher pressure. It's common sense. In my starting thread, I just have mentioned fluid that means both compressible and non-compressible.

There is no motion through the venturi except from the higher total pressure to the lower total pressure. Suppose the total pressure remains lower on the fan side. Then if the venturi does anything, it will be helping the flow in the reverse direction

I hope you know about valves. And you have forgot that after the nozzle, there is a diffuser that will convert that velocity into pressure and at the end the pressure level will be higher than that where it will be injected.

 

[russ_watters]

Thread locked for the reason stated in my previous post.

I may have locked it sooner if not for the fact that I didn't notice the thread until it was on page 2 and didn't want to waste everyone's replies. To everyone who replied, I thank you for your efforts, but suggest in the future that reporting a substandard post/thread may be better than wasting your time trying to answer it...though I do recognize that it can sometimes be difficult to see how deep the rabbit hole goes.