One question about dynamic pressure

[pranj5]

As per wikipedia, dynamic pressure is nothing but the expression of kinetic energy of unit volume of a fluid in motion. That's applicable to gases too. Question is, when a fluid is passed through a convergent and/or convergent-divergent nozzle, then its speed towards a specific direction increases. Does that mean that its dynamic pressure also increases in that direction too?
And, does this dynamic pressure can be converted into static pressure by simply passing through a diffuser?

 

[FactChecker]

As per wikipedia, dynamic pressure is nothing but the expression of kinetic energy of unit volume of a fluid in motion. That's applicable to gases too. Question is, when a fluid is passed through a convergent and/or convergent-divergent nozzle, then its speed towards a specific direction increases. Does that mean that its dynamic pressure also increases in that direction too?

Yes.

And, does this dynamic pressure can be converted into static pressure by simply passing through a diffuser?

Yes.
Static pressure has no preferred direction and dynamic does. They sum to total pressure, which remains constant if no energy is added or taken away.

 

[pranj5]

Well, then as per your answer, we can inject a low pressure gas into higher pressure with less energy consumption by using a convergent and/or convergent-divergent nozzle. Right? As the nozzle means higher velocity at the entry, that means the conversion into pressure is also higher. That's as per the answer you have given.

 

[FactChecker]

Well, then as per your answer, we can inject

So you are adding energy to inject?

a low pressure gas into higher pressure

Do you mean total pressure?

with less energy consumption by using a convergent and/or convergent-divergent nozzle. Right? As the nozzle means higher velocity at the entry, that means the conversion into pressure is also higher.

In the narrow part of a venturi the dynamic pressure is higher, the static pressure is lower, and the total pressure is unchanged.

That's as per the answer you have given.

Are you saying that it is easier to force more air into a high pressure container through a small opening than through a large opening? That sounds correct, but I am not sure that it is a good application of Bernoulli's principle. I think that you would still have to add enough energy to raise the total pressure of the outside air above that of the air in the container.

 

[pranj5]

So you are adding energy to inject?

Certainly. It's impossible to inject air/gas without energy input. What I want to mean is that whether the nozzle can do some kind of multiplying effect or not. No energy means multiplying something with zero.

Do you mean total pressure?

Actually, when the diffuser will convert the dynamic pressure into static pressure, then what will be left at the end is static pressure alone. I want to mean the pressure after the diffuser converting the dynamic pressure into static pressure.

In the narrow part of a venturi the dynamic pressure is higher, the static pressure is lower, and the total pressure is unchanged.

Not only the pressure, but also the temperature is lowered and this increased velocity come at the expense of both the pressure and temperature.

Are you saying that it is easier to force more air into a high pressure container through a small opening than through a large opening? That sounds correct, but I am not sure that it is a good application of Bernoulli's principle. I think that you would still have to add enough energy to raise the total pressure of the outside air above that of the air in the container.

I specifically want to use a convergent and/or convergent-divergent nozzle for injecting lower pressure air/gas to higher pressure because the nozzle will convert internal enthalpy (both pressure and temperature) into motion i.e. dynamic pressure.

 

[FactChecker]

I specifically want to use a convergent and/or convergent-divergent nozzle for injecting lower pressure air/gas to higher pressure because the nozzle will convert internal enthalpy (both pressure and temperature) into motion i.e. dynamic pressure.

I don't believe that will work. If anything I said implied that, then I didn't mean it that way.

 

[pranj5]

Kindly just read this thread. It's a fact that convergent and/or convergent-divergent nozzles can convert internal heat into motion.

 

[Nidum]

It's a fact that convergent and/or convergent-divergent nozzles can convert internal heat into motion

If that were actually true you could build a jet engine with no moving parts .

 

[FactChecker]

I think we have all witnessed that air escaping through a nozzle from a high pressure tank is cold. But that is air flowing from high pressure to lower pressure. That is how it will always flow. Your question was whether a venturi would help you to get air to flow from low pressure to higher pressure. The answer is no. You will have to raise the pressure of the air source higher than the pressure of the tank you want it to flow into. Any restriction like a venturi will not help; it will make it harder.

 

[pranj5]

If that were actually true you could build a jet engine with no moving parts .

Ramjet and Scramjet don't need any kind of moving parts.

I think we have all witnessed that air escaping through a nozzle from a high pressure tank is cold. But that is air flowing from high pressure to lower pressure. That is how it will always flow. Your question was whether a venturi would help you to get air to flow from low pressure to higher pressure. The answer is no. You will have to raise the pressure of the air source higher than the pressure of the tank you want it to flow into. Any restriction like a venturi will not help; it will make it harder.

The picture from wikipedia clearly shows how both pressure and temperature falls with the increase in velocity. Inside a nozzle, the speed of fluid increases with decreasing radius of the pathway and in case of a convergent nozzle, the speed is at the peak at the throat. In case of convergent-divergent nozzle, velocity increases even after exit from the throat. It's also a known fact that both temperature and pressure decreases inside a nozzle as soon as it reaches the throat. This basic phenomenon is totally different from the scenario what you have described. in your described case, cooling comes because the compressed air has done some work by increasing its volume and that reduces its temperature.

 

[jim hardy]

is this any help?

https://en.wikipedia.org/wiki/Injector

A steam injector is typically used to deliver cold water to a boiler against its own pressure using its own live or exhaust steam, replacing any mechanical pump. This was the purpose for which it was originally invented in 1858 by Henri Giffard. Its operation was from the start intriguing since it seemed paradoxical, almost like perpetual motion, but its operation was later explained using thermodynamics.[1] Other types of injector may use other pressurised motive fluids such as air.

Operation
The injector consists of a body containing a series of three or more "cones" containing nozzles along one axis.

It uses the Venturi effect of a converging-diverging nozzle on a steam jet to convert the pressure energy of the steam to velocity energy, reducing its pressure to below atmospheric which enables it to entrain a fluid (eg. water). After passing through the convergent "combining cone", the mixed fluid is fully condensed releasing the latent heat of evaporation of the steam which imparts extra velocity to the water. The condensate mixture then enters a divergent "delivery cone" which slows the jet, converting kinetic energy back into static pressure energy above the pressure of the boiler enabling its feed through a non-return valve.[3] [4]

Most of the heat energy in the condensed steam is returned to the boiler, increasing the thermal efficiency of the process. Injectors are therefore typically over 98% energy-efficient overall; they are also simple compared to the many moving parts in a feed pump.

 

[Nidum]

Ramjet and Scramjet don't need any kind of moving parts.

These both depend on there being airflow induced by the forward motion of the engine .

If what you say about nozzles were true then you could build an engine with no moving parts which produced continuous thrust while sitting static on the ground .

 

[pranj5]

If what you say about nozzles were true then you could build an engine with no moving parts which produced continuous thrust while sitting static on the ground

Simply impossible. What nozzles can to is to multiply the effect of the input flow. Static engine means it has to start from zero. Now, whatever you multiply with zero, the end result would be zero.
Kindly don't put words on me that I haven't said.

 

[Nidum]

Kindly just read this thread. It's a fact that convergent and/or convergent-divergent nozzles can convert internal heat into motion.

 

[Baluncore]

These both depend on there being airflow induced by the forward motion of the engine .

If what you say about nozzles were true then you could build an engine with no moving parts which produced continuous thrust while sitting static on the ground .

I have seen a ram jet generate thrust while being held static in a bench vice. It required a burst of compressed air to open the inlet valves, then it ran continuously without forward motion, or compressed air.

 

[FactChecker]

I have trouble believing that a restriction in the passageway between two different pressures can assist the flow from lower pressure to higher. If anything, it seems that the restriction must require more work to overcome. If it is true, I would like to see an experiment that shows it.

is this any help?

https://en.wikipedia.org/wiki/Injector

This is very interesting. But I think it shows something different from the use of Bernoulli. It uses the latent heat of evaporation (when the water condenses and releases additional heat) to increase velocity and assist in the injection.

 

[FactChecker]

Kindly just read this thread. It's a fact that convergent and/or convergent-divergent nozzles can convert internal heat into motion.

Heat is non-directional. If it is converted to directional motion, I think it would push backward as well as forward and require more work to force the "lower pressure" air through the venturi. I quote "lower pressure" because I believe that the total pressure must be raised above the other side before the air will flow. That requires work.

 

[Nidum]

' Water will flow down hill on it's own . To make it go up hill you need a big man with a big brush ' - Dr.Hooker at one of his famous impromptu drawing board tutorials .

 

[jim hardy]

This is very interesting. But I think it shows something different from the use of Bernoulli. It uses the latent heat of evaporation (when the water condenses and releases additional heat) to increase velocity and assist in the injection.

The diagram he posted makes use of internal heat as well.
From its parent article https://en.wikipedia.org/wiki/De_Laval_nozzle

A de Laval nozzle (or convergent-divergent nozzle, CD nozzle or con-di nozzle) is a tube that is pinched in the middle, making a carefully balanced, asymmetric hourglass shape. It is used to accelerate a hot, pressurized gas passing through it to a higher supersonic speed in the axial (thrust) direction, by converting the heat energy of the flow into kinetic energy. Because of this, the nozzle is widely used in some types of steam turbines and rocket engine nozzles. It also sees use in supersonic jet engines.

We're all so accustomed to using Bernoulli for incompressibles that we forget (well at least I do) it has a term for internal energy.

As an old broken down EE i seldom delve deep into Bernoulli . It always made me feel sympathy for Mechanicals - my Ohm's law is SOOO much simpler than theirs.

http://geosci.uchicago.edu/~moyer/GEOS24705/Notes/Bernoulli.pdf

 

[pranj5]

I have trouble believing that a restriction in the passageway between two different pressures can assist the flow from lower pressure to higher. If anything, it seems that the restriction must require more work to overcome. If it is true, I would like to see an experiment that shows it.

That's the inherent quality of a nozzle. It has been tested quite a long time and already accepted. In this thread, Just read what boneh3ad has said here.

Heat is non-directional. If it is converted to directional motion, I think it would push backward as well as forward and require more work to force the "lower pressure" air through the venturi. I quote "lower pressure" because I believe that the total pressure must be raised above the other side before the air will flow. That requires work

When heat is converted into velocity by a nozzle, it becomes directional.

 

[FactChecker]

That's the inherent quality of a nozzle. It has been tested quite a long time and already accepted. In this thread, Just read what boneh3ad has said here.

When heat is converted into velocity by a nozzle, it becomes directional.

There was work done to put the heat in there in the first place. There is work done to cause the heat to be converted into velocities directed in one direction. Air does not go from low pressure to high pressure just because of the shape of the opening.

 

[pranj5]

There was work done to put the heat in there in the first place. There is work done to cause the heat to be converted into velocities directed in one direction. Air does not go from low pressure to high pressure just because of the shape of the opening.

Heat is the lowest form of energy and doing mechanical work isn't always necessary to heat up something. Moreover, the fluid that will be passed has already its stored heat as internal energy.
Some work is necessary to start the flow, but no extra mechanical work isn't necessary to convert the internal heat into motion. It's the inherent character of nozzle. And, by the way, no one here (including me) is saying that a convergent and/or c/d nozzle can in itself pump up air/gas from low pressure to high pressure. What I want to say is that they can reduce power consumption to do that by converting a part of internal enthalpy of the air/gas into motion. Just like the ramjet example, something is necessary to give a kick start (in this case a blower) and the nozzle will enhance the speed at the expense of internal energy.

 

[FactChecker]

In a typical experiment, the changes of temperature, pressure, velocity, etc. that occur in a venturi are due to the work done to force the air through the venturi. They are consequences of the work and do not reduce the amount of work required.
One exception may be the example of the use of latent heat of transition that happens in @jim hardy 's steam injector example. But that requires energy to produce the steam and is an entirely different principle than Bernoulli's principle.
I would consider providing heat to be equivalent to doing work. Otherwise, the simplest thing would be to warm the low pressure side till its pressure rose above the other side.

 

[russ_watters]

@pranj5, note that your first post references a wiki article on incompressible flow. You would save everyone a lot of time and confusion if you described more accurately and specifically what type of scenario you are referring to.

 

[jim hardy]

Air does not go from low pressure to high pressure just because of the shape of the opening.

Quite so.
Look at a boiler

They take out high energy fluid and use it to pump back in even more low energy fluid.

A piston or turbine driven pump would be intuitive.
But Thermodynamics is where one has to have faith in his math.

I suppose if i'd ever worked on one of those things i'd be less amazed at the thought of it.

 

[jim hardy]

Thank you @Russ watters

 

[Nidum]

Just like the ramjet example, something is necessary to give a kick start (in this case a blower) and the nozzle will enhance the speed at the expense of internal energy.

A very powerful and continuous induced air flow is needed to make a static ramjet function . Remove the induced air flow and the ramjet will just stop functioning .
@Baluncore was referring to a pulse jet .

I suppose if i'd ever worked on one of those things i'd be less amazed at the thought of it.

There are exhaust steam powered injectors as well . They use engine exhaust steam at very low pressure and still get water into the boiler . They are truly amazing things - they seem at first sight to defy all laws of thermodynamics - though of course they do not . They do take some understanding though .

 

[Nidum]

http://www.dukeofgloucester.co.uk/?section=locomotive&page=Exhaust+Steam+Injector

 

[pranj5]

In a typical experiment, the changes of temperature, pressure, velocity, etc. that occur in a venturi are due to the work done to force the air through the venturi. They are consequences of the work and do not reduce the amount of work required.

Wrong! In that case the temperature of the gas will rise instead of falling.

I would consider providing heat to be equivalent to doing work. Otherwise, the simplest thing would be to warm the low pressure side till its pressure rose above the other side.

In that case, the heating should be isovolumetric i.e. it has to be heated in an enclosed container so that its volume can't change. Can't understand what relation such process has to this discussion going on here.

note that your first post references a wiki article on incompressible flow. You would save everyone a lot of time and confusion if you described more accurately and specifically what type of scenario you are referring to

The page contains both compressible and imcompressible fluid equations. As per that page, dynamic pressure is nothing but the expression of kinetic energy of unit volume of gas. Now, we all know that nozzles can increase velocity of gases when passing through it. I simply want to know does that mean increasing dynamic pressure? And it's also a fact that diffusers can convert dynamic pressure into static pressure. And as when a gas will pass through a nozzle and later a diffuser will be used to convert that dynamic pressure into static pressure, does that means we will obtain higher pressure.

 

[russ_watters]

Now, we all know that nozzles can increase velocity of gases when passing through it. I simply want to know does that mean increasing dynamic pressure?

Yes.

And it's also a fact that diffusers can convert dynamic pressure into static pressure. And as when a gas will pass through a nozzle and later a diffuser will be used to convert that dynamic pressure into static pressure, does that means we will obtain higher pressure.

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

 

[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.