Blower fitted with De Laval Nozzle

[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.