Blower fitted with De Laval Nozzle

[russ_watters]

Here is a selection for the highest pressure blower you're likely to find:

180 inches of water gauge is 6.5 psi or an available pressure ratio of 0.69. That's a lot better than I would have expected, but still quite a bit above the maximum required for choked flow of <0.5. And it requires 170 horsepower for just 200SCFM of airflow. That's like a midsized car engine at full throttle to run a bathroom fan.

 

[jack action]

Here is a selection for the highest pressure blower you're likely to find:
View attachment 224961
180 inches of water gauge is 6.5 psi or an available pressure ratio of 0.69. That's a lot better than I would have expected, but still quite a bit above the maximum required for choked flow of <0.5. And it requires 170 horsepower for just 200SCFM of airflow. That's like a midsized car engine at full throttle to run a bathroom fan.

Oh! Is that what HVAC guys call a "blower"?

Try a Roots-type blower, you can achieve 2:1 pressure ratio easily:

Funny how we see different things from the same statement.

 

[russ_watters]

Oh! Is that what HVAC guys call a "blower"?

Try a Roots-type blower, you can achieve 2:1 pressure ratio easily:

View attachment 224973​

Funny how we see different things from the same statement.

Wow, thanks, I hadn't heard of that. And it fits nicely into the nomenclature overlap, being referred to as a blower, a pump and a compressor in the same article! I would say that's confusing, but it really is in an overlap range: the flow is definitely compressible even though the pressure is below what is common for "compressors".

 

[boneh3ad]

I'd like to point out that 100 m/s could be considered compressible depending on the local speed of sound (which depends on temperature). That's unlikely here since the air is likely warm. The flow through the blower is very possibly compressible, though.

Also, the requirements to choke a CD nozzle are typically greater than the common 0.528 pressure ratio ($p_b/p_0$). Running it without any shocks so the exit velocity is supersonic requires a lower ratio. The 0.528 number is for converging nozzles only.

 

[jack action]

Wow, thanks, I hadn't heard of that. And it fits nicely into the nomenclature overlap, being referred to as a blower, a pump and a compressor in the same article! I would say that's confusing, but it really is in an overlap range: the flow is definitely compressible even though the pressure is below what is common for "compressors".

Actually, it is a simple "air displacer". The air gets into a chamber and the chamber rotates towards the outlet. If you put it in front a restrictor (ex.: an engine with a smaller defined volumetric flow), then the pressure will accumulate between the two, i.e. a compression. So it is in fact a «blower».

There is also a screw type compressor which looks a lot like a roots type, but it actually compresses the air within (the chamber decreases in size as it moves), so it is a compressor. That is why they are more efficient than Roots type.

Here's the cute history of its origin:

The Roots device is a positive displacement pump, used as an air pump or fluid pump. It moves air, but does not compress. The basic design was patented in 1860 by Philander Higley and Francis Marion Roots (hence the name) of Connersville, Indiana, as an air pump for mines, grain elevators, blast furnaces, and other industrial applications. As an air pump, it pulls air by trapping it with meshing lobes against the outer side of the blower case and pushes air from the intake side to the exhaust side. In an engine application, when pumped into a confined space such as an intake manifold, positive pressure is developed, along with an increase in temperature of the pressurized air (not a good thing). An intercooler can be used to help cool the charged air.

 

[JBA]

While we are listing the available types of compressors it would be unfair not to include the sliding vane type as well.

 

[cjl]

Here is a selection for the highest pressure blower you're likely to find:
View attachment 224961
180 inches of water gauge is 6.5 psi or an available pressure ratio of 0.69. That's a lot better than I would have expected, but still quite a bit above the maximum required for choked flow of <0.5. And it requires 170 horsepower for just 200SCFM of airflow. That's like a midsized car engine at full throttle to run a bathroom fan.

A centrifugal compressor can push a pressure ratio of 4:1 or more per stage, with very high airflow. You could easily size a blower that would start a C-D nozzle, and maintain the flow rate to keep it operating, although the power rating would necessarily be extremely high. As an extreme example, multistage axial jet engine compressors can flow hundreds of kg/s of air with a pressure ratio of 40 or 50 to 1, but they require powers on the order of tens of megawatts to drive.

 

[T C]

One question just comes to my mind against boneh3ad's post no. 19. If a turbogenerator is used to produce power by using the velocity of the released air/fluid, does that alter the temperature or remain the same? Suppose that the turbine blades can withstand the blow of liquid oxygen droplets.

 

[russ_watters]

One question just comes to my mind against boneh3ad's post no. 19. If a turbogenerator is used to produce power by using the velocity of the released air/fluid, does that alter the temperature or remain the same? Suppose that the turbine blades can withstand the blow of liquid oxygen droplets.

When a gas flows through a turbine, it does work against the turbine, expands and cools.

 

[T C]

I know that and that's why I want to know whether in such case the fall in temperature will be same or more.

 

[russ_watters]

I know that and that's why I want to know whether in such case the fall in temperature will be same or more.

Than without the turbine? The temperature drop should generally be more with the turbine I believe...but you are also reducing the exit speed, so I'm not sure how this fits with what you were trying to do before.

 

[T C]

This is a new question. As the thread is still alive; that's why I have posted that in this thread for quick response. This has nothing to do with the question with which I have started this thread.

 

[T C]

Another question just comes to mind against post 19 here. In case of an ideal/reversible process, can we say that if we have air/gas at 108 k temp and 614.8 m speed and that has been passed through a divergent nozzle, then when the speed is zero then the pressure will be 33.3 bar?

 

[jack action]

Another question just comes to mind against post 19 here. In case of an ideal/reversible process, can we say that if we have air/gas at 108 k temp and 614.8 m speed and that has been passed through a divergent nozzle, then when the speed is zero then the pressure will be 33.3 bar?

Yes, as long as you consider the ideal process. If the speed is reduced by going through a normal shock (usual method in CD nozzle), then there will be a lost that will be reflected in the total pressure (i.e. the pressure when the fluid will be brought to rest).

Finally, the ratio of stagnation pressures is shown below where

is the upstream stagnation pressure and

occurs after the normal shock. The ratio of stagnation temperatures remains constant across a normal shock since the process is adiabatic.

 

[cjl]

Just to clarify, it is not usual for there to be a normal shock in a C-D nozzle unless it is poorly designed or operating off of the expected design conditions. Usually, you would expect it to be fairly close to ideal, so yes, it is reversible. It is quite difficult to slow down the flow without a shock though - much more difficult than accelerating isentropically, so you would be hard pressed to actually demonstrate that in experiment.

 

[T C]

That means if an air/gas stream has 614.8 m/s speed and at 1 barA pressure, then we can raise the pressure to 33.3 barA by using a c/d or divergent nozzle?

 

[jack action]

In theory and ideal conditions, yes.

 

[boneh3ad]

True, a normal shock should not exist inside of a C-D nozzle as long as the pressure ratio is sufficient for the nozzle design. That said, if you want to then slow the resulting supersonic flow down, you almost always have a normal shock somewhere. If you're extremely clever you can maybe avoid that with a borderline magical diffuser. Otherwise, it is nearly impossible to isentropically slow a supersonic flow down to subsonic speeds without a shock. Compression waves simply coalesce too readily into shock waves to make that practical in most cases.

Also, if your gas already had 614 m/s velocity and a pressure of 1 barA, you wouldn't need a C-D nozzle to increase the pressure. You would just need some means of slowing the flow down to zero velocity isentropically. In theory, you could use a C-D diffuser to do it, but in practice, that is wishful thinking and a shock will still likely form.

 

[T C]

Also, if your gas already had 614 m/s velocity and a pressure of 1 barA, you wouldn't need a C-D nozzle to increase the pressure. You would just need some means of slowing the flow down to zero velocity isentropically. In theory, you could use a C-D diffuser to do it, but in practice, that is wishful thinking and a shock will still likely form.

Is there any other way to do the job?

 

[cjl]

Is there any other way to do the job?

There are quite a few ways, but as boneh3ad said, it's almost impossible to slow a flow down from supersonic isentropically. You can do better than just a normal shock - most high speed aircraft use a series of oblique shocks for their engine intakes, as the loss in an oblique shock is lower, but you'll pretty much always end up at a lower pressure and higher temperature than the isentropic relation would indicate, due to the losses inherent in the shocks.

 

[T C]

What kind of nozzles are more suitable for the job? I mean whether a c/d nozzle or a simple divergent nozzle?

 

[cjl]

Note that a nozzle is specifically something to accelerate a flow. If you're trying to slow it down, you're looking for a diffuser. As stated above, C/D nozzles are very good at isentropically accelerating flow, but to slow it down, you'll want a converging diffuser designed to generate a large number of weak, oblique shocks.

 

[boneh3ad]

If you want to accelerate a flow from subsonic to supersonic, a CD nozzle is the only way to do it. You can remove the C portion but ultimately the separation upstream of the throat creates its own C portion for you with greater losses. Same goes for removing the D portion, where the flow could continue expanding on it's own after leaving the nozzle but will incur greater losses.

 

[JBA]

The type of shock that develops for the discharge of a converging nozzle or flat plate orifice discharging to atmosphere is dependent upon the pressure ratio. At exactly the critical ratio there is a a flat shock at the exit face; then, as the pressure differential is increased the shock transforms into a portion of a spherical dome that, as the inlet pressure increases, grows to form a full hemisphere; and, at above that point a train of shock diamonds results. The number of diamonds and the length of that train grows as the pressure differential further increases. I have observed this process a number of times at a large valve testing and certification flow facility where we were capable of increasing and holding the valve inlet pressure at very small increments.

The same may occur at the exit of C/D nozzle, but I have no direct observations of discharge shock formations of C/D nozzles to confirm that.

As side note, for the radial flow from a annular nozzle there is an initial flat ring shock that then smoothly evolves directly into a wedge shaped ring of growing height as the pressure differential increases.

 

[T C]

Give above is an example that at very high speed, there are diffusers available that can convert that high speed into pressure. This is a technology used by Twister BV, a Netherlands based company for dehydrating natural gas and other gaseous products. At first, high pressure gas is released and that has been given a twist so that a vortex is formed. At the centre, the temperature falls low and all the humidity remaining in the input gas is being liquefied. Due to the centrifugal force, the liquid droplets were thrown to external wall and then extracted out. Whatsoever, it is to be noted that after re-compression, the pressure level will be about 75% of the initial input level.

 

[russ_watters]

Give above is an example that at very high speed, there are diffusers available that can convert that high speed into pressure. This is a technology used by Twister BV, a Netherlands based company for dehydrating natural gas and other gaseous products. At first, high pressure gas is released and that has been given a twist so that a vortex is formed. At the centre, the temperature falls low and all the humidity remaining in the input gas is being liquefied. Due to the centrifugal force, the liquid droplets were thrown to external wall and then extracted out. Whatsoever, it is to be noted that after re-compression, the pressure level will be about 75% of the initial input level.

Where are you going with this/what is your point? Yes, when speed increases pressure drops and when speed decreases again, pressure goes back up. We all know this. So what?

 

[T C]

What I want to mean is that there are diffusers available at present that can convert high speed into pressure without much loss.In the example above, frictional losses are high as there is a swirling motion going on inside.

 

[russ_watters]

What I want to mean is that there are diffusers available at present that can convert high speed into pressure without much loss.

Certainly: if they couldn't, they wouldn't be doing their job! Still not sure why you are pointing this out...

...I'm sensing perhaps that because you jumped right into more advanced topics, you are just now realizing some basic facts/principles. I'd suggest starting from the beginning by reading the wiki articles on Bernoulli's Principle and the Venturi effect.

High speed flow combines fluid dynamics and thermodynamics, so learning each of those separately is really needed before trying to learn their offspring.

 

[T C]

Both boneh3ad and cjl suggested that in reality such diffusers will incur great losses and that's why I want to show that with a properly designed diffuser, at least 75% of the pressure can be recovered.

 

[russ_watters]

Both boneh3ad and cjl suggested that in reality such diffusers will incur great losses and that's why I want to show that with a properly designed diffuser, at least 75% of the pressure can be recovered.

Oh, ok. Well neither of them said "great loss", but rather compared the loss between different scenarios relative to each other ("greater loss"). Nor is "great loss" quantified anywhere. So I don't think that necessarily contradicts what they said.

 

[boneh3ad]

What percentage is recoverable is heavy dependent on Mach number. 75% is trivial for a Mach 1.5 but probably almost impossible for Mach 6.

 

[T C]

Do you agree that with a properly designed diffuser, high speed can be converted into pressure? Theoretically there should be no doubt, but in reality.

 

[russ_watters]

Do you agree that with a properly designed diffuser, high speed can be converted into pressure?

Yes: by definition a diffuser - any diffuser - converts speed into pressure (kinetic energy into potential/pressure energy):

A diffuser is "a device for reducing the velocity and increasing the static pressure of a fluid passing through a system”.

https://en.wikipedia.org/wiki/Diffuser_(thermodynamics)

 

[T C]

Yes: by definition a diffuser - any diffuser - converts speed into pressure (kinetic energy into potential/pressure energy):

That's theory and nobody can deny that. What I want to show is that it's practically possible too.

 

[boneh3ad]

I guess I don't understand the question. Of course you can build a supersonic diffuser. The question is whether you can do it without shocks. The answer is "sometimes". It is very difficult and unreliable, but can happen.

 

[T C]

Do you want to mean that such a diffuser may sometime work but not always. Then how the system designed by Twister BV works well?

 

[boneh3ad]

I honestly don't know what sort of answer you are seeking here. A diffuser has to be designed for a specific Mach number, and deviating from that in any way can cause it to fail and cause the whole system to fail to start. Any sort of diffuser is going to be a one-off project for the application in hand and designing one is as much a black art as it is a science.

This is why most supersonic diffusers simply forego the idea of slowing the flow down isentropically and employ a normal shock.

 

[T C]

What I want to mean is that if one company can make and sell such diffusers then it can be done. From your post, it's clear that it's hard but not impossible.

 

[boneh3ad]

At no point did I say it is impossible. Any supersonic flow device has some form of diffuser on it. The question is efficiency.

 

[cjl]

That's theory and nobody can deny that. What I want to show is that it's practically possible too.

Of course it is - nearly every supersonic aircraft uses a diffuser of some kind as its inlet for the engines. One rather prominent and famous example of this is the moving cone inlet on the SR-71, which uses a series of radially symmetric oblique shocks to obtain much better pressure recovery than a simple normal shock or even a 2-d oblique shock (in fact, from what I can find, the SR-71 inlets manage ~80% recovery at mach 3.2, which is very impressive). What's nearly impossible is to obtain an isentropic or near-isentropic supersonic diffuser without shocks.

 

[T C]

The question is efficiency.

If the recovery is 75%, do you consider that efficient or not.

 

[cjl]

Depends on the mach number. As Boneh3ad said earlier, efficient pressure recovery becomes more difficult with increasing mach number, so 90%+ is trivial at a bit over mach 1, but even 75% would be incredibly difficult when hypersonic.

 

[boneh3ad]

If the recovery is 75%, do you consider that efficient or not.

I think you need to study gas dynamics a bit because it is clear from your questions that you are not familiar with the topic and it is making this very difficult to discuss.

Any supersonic flow that started at atmospheric pressure must, at some point, be slowed back down to reach atmospheric pressure. If no diffuser is used, a normal shock typically forms and causes a certain amount of total pressure loss. By fitting various forms of diffuser geometries to the outlet of such a device, we can try to improve upon the performance of a normal shock with varying degrees of success. Therefore, diffuser efficiency is typically measured by comparing it with the efficiency of a normal shock since the pressure recovery is highly dependent on Mach number.

So, if you are asking whether 75% pressure recovery is good, I'd go back to the answer I gave you last time you asked that. That would be pretty great if you had a Mach 5 flow, pretty trivial if you had a Mach 2 flow, and you really just made things worse if you have a Mach 1.5 flow.