Experimental proof of Venturi Effect

[T C]

As per the post from cji, a non-ducted fan can generate more flow than a ducted fan especially when fitted at the throat of a convergent nozzle shaped duct. You are saying the opposite. Which one should I consider to be correct?

 

[russ_watters]

As per the post from cji, a non-ducted fan can generate more flow than a ducted fan especially when fitted at the throat of a convergent nozzle shaped duct. You are saying the opposite. Which one should I consider to be correct?

I don't see where he said that, but regardless, "flow" usually means volumetric flow rate, not velocity.

You have a bad habit of trying to connect unconnected things.

 

[T C]

and allow it to operate much closer to its open air performance.

That clearly means its ducted performance is worse than open air performance.

 

[russ_watters]

That clearly means its ducted performance is worse than open air performance.

"Performance". That word evidently doesn't mean what you think it does.

 

[T C]

Then kindly explain it.

 

[russ_watters]

Then kindly explain it.

I did!

 

[boneh3ad]

In case of a wind tunnel, IMO the main purpose is to create a flow at a specific speed. If a open air blower can generate more speed in comparison when fitted at the throat of a convergent nozzle shaped duct, why should one need that?

For one, a wind tunnel has a lot of other considerations other than just a specific speed. You have to worry about flow uniformity, angularity, and free-stream fluctuations.

 

[T C]

I just want to know whether the velocity will be same or less in case of flow through design or open air design.

 

[russ_watters]

I just want to know whether the velocity will be same or less in case of flow through design or open air design.

It depends on the fan. If the fan is capable of generating substantial static pressure the velocity through a duct can be higher than without.

But the volumetric flow rate will never be higher with a duct (for an axial fan, with the possible exception of a shroud/inlet cone).

 

[russ_watters]

Here's that second set of results in the draw-through configuration:

Notes:

• I tried to read the velocity on the short side of the large box and got zero (which is really just <50 fpm). The transition is too steep between the small and large box on the short side, and the flow separates from the side of the box, leaving a dead spot up against the side. Shape of the tunnel matters a lot.
• Shutoff pressure was 0.06", about the same as in the blow-through configuration. I didn't attempt to limit re-circulation at the tips. I suspect this wouldn't help much due to the noticeable rpm noise drop when closing the variable geometry inlet. In other words, while the fan shape is bad, the torque requirement for the asynchronous motor is a bigger problem. Most real-world HVAC fans or others with big motors are constant RPM and the torque increases as load increases (or the motor dies trying).
• Notice that in this test I was able to achieve a higher velocity in Box 2 than with the bare fan. But just barely. With a better fan, capable of generating static pressure when restricted, the velocity could be much higher in a small duct than at the fan outlet for a bare fan. But airflow (volumetric) will pretty much always be lower in a duct than on a bare fan.

 

[T C]

In this video, the proof of Venturi Effect is much clear and ratio is far better.

 

[russ_watters]

In this video, the proof of Venturi Effect is much clear and ratio is far better.

I don't see anything in that video about the Venturi Effect or the area/velocity ratio, with the possible exception of an out-of-focus/unreadable CFD snapshot. What results, exactly are you referring to? What ratio are you seeing?

Anyway, I wasn't trying to optimize accuracy, I was optimizing effort and cost. I'm quite certain you could produce better results than I did if you choose to put some effort into it, but your attempt was a far superior optimization of effort than mine was, so I doubt you will. I'm genuinely confused as to what you are after here (and I don't just mean this subject/thread, I mean your entire effort on PF).

 

[T C]

I don't see anything in that video about the Venturi Effect or the area/velocity ratio, with the possible exception of an out-of-focus/unreadable CFD snapshot. What results, exactly are you referring to? What ratio are you seeing?

Just look at the RPM with and without the flaps, that will be good example of velocity increase because RPM will increase in direct proportion to velocity. IMO, the best way get a proper venturi effect demonstration is using plastic or metal cones.

 

[russ_watters]

Just look at the RPM with and without the flaps, that will be good example of velocity increase because RPM will increase in direct proportion to velocity. IMO, the best way get a proper venturi effect demonstration is using plastic or metal cones.

It's just so disappointing that after all this effort trying to explain it to you that you think that has anything directly to do with the Venturi Effect. I feel like either you aren't trying at all or you are messing with us here.

 

[T C]

My main motto was to understand why my homemade experiment is unsuccessful and what is necessary to make it successful.

 

[russ_watters]

My main motto was to understand why my homemade experiment is unsuccessful and what is necessary to make it successful.

In that case you should re-read this thread from the start and put some real effort into learning the concepts. And for experimenting yourself, I showed you what could be done with a bigger fan, two boxes, a roll of duct tape and virtually no money or effort. If you put in even a small amount of effort and money you could surely do vastly better.

Or maybe better, you should probably enroll in an introductory fluid dynamics class, because you seem completely incapable of self-directed or forum-assisted learning. You may need an absolutely rigid structure that forces you to focus and doesn't allow you to wander off track. Here we just close threads and issue infractions. Maybe failing a test would help you focus.

 

[cjl]

In case of a wind tunnel, IMO the main purpose is to create a flow at a specific speed. If a open air blower can generate more speed in comparison when fitted at the throat of a convergent nozzle shaped duct, why should one need that?

I know I'm a little late here, but to clear up the misconception:

The volumetric flow rate, as Russ points out, will nearly always be highest when the fan is out in the open. There might be some way to slightly exceed that with careful design, but really, open air is about as good as you'll get. However, flow velocity will be higher inside a well designed wind tunnel, with a converging nozzle, a test section, and then a diffuser with the fan at the exit of the diffuser. In addition, as boneh3ad correctly states, velocity is not the only goal of a wind tunnel - flow uniformity and straightness, and having an extremely controlled environment are also very important.

 

[T C]

Is there any reason behind it? If so, what's that?

 

[russ_watters]

Is there any reason behind it? If so, what's that?

Conservation of energy, f=ma, etc.

 

[T C]

Conservation of energy, f=ma, etc.

Can you explain how the position of the fan (inlet or exhaust) can affect such laws?

 

[russ_watters]

Can you explain how the position of the fan (inlet or exhaust) can affect such laws?

It doesn't. The laws govern the flow and the addition of certain elements to the system affects the flow in keeping with the laws. Specifics on particular elements/fittings can be complicated, but much of that has already been discussed.

 

[T C]

So far, there is nothing in this thread that can clearly explain why a wind tunnel performs better when the blower is fitted and the exit instead of the the inlet.

 

[cjl]

It doesn't. At least not necessarily. That's more related to keeping minimum turbulence and swirl in the test section, but from an energy and flow velocity standpoint, you could put the fan on either side (but you'd still always want a nozzle and diffuser to maximize performance, not just a fan in a tube).

 

[T C]

Nozzle is a must to get higher velocity at lesser power consumption. I can understand that.

 

[cjl]

Both a nozzle and a diffuser. Without the diffuser, the pressure requirement to drive the nozzle is much higher, dramatically reducing the flow.

 

[T C]

Both a nozzle and a diffuser. Without the diffuser, the pressure requirement to drive the nozzle is much higher, dramatically reducing the flow.

Why?

 

[russ_watters]

Why?

Air doesn't like abrupt direction changes/transitions.

 

[cjl]

Why?

While Russ's statement is true, I don't think it really explains this very well. This really comes from 3 factors. First of all, you have to understand what a nozzle and diffuser do. A nozzle exchanges pressure for velocity, a diffuser does the opposite. So, at the entry of a nozzle, the pressure will be high and velocity low, while at the exit, the pressure will be lower and the velocity higher. On a diffuser, the entry will be at relatively high velocity and low pressure, and then as it slows the flow down, the pressure increases such that the exit conditions are a higher pressure and lower velocity.

The second important fact is that at any point where a wind tunnel is open to the atmosphere, the pressure must be equal to ambient pressure. This is because it is open to the atmosphere (admittedly, there are some minor caveats here, but we can largely ignore them for the moment).

Third, as was already discussed previously, fans flow more air (volumetrically) when the pressure change across the fan disk is as small as possible.

Now, taking those three things, let's think about a wind tunnel with only a nozzle. As I already said, the entrance of a nozzle must have a higher pressure than the exit. However, if the exit is open to the atmosphere after the test section, this means that the pressure just after the fan (just before the nozzle) must be higher than ambient, in order to drive the flow through the nozzle. This means you have to have a significant pressure gradient across the fan, reducing its flow. Having the fan on the exit doesn't help you that much either, since now the exact same logic still applies, except that the pressure just before the fan must be below ambient (since the entry to the nozzle is at ambient), again requiring a significant pressure jump across the fan.

However, now imagine a system with a nozzle and a diffuser. The air enters the nozzle, then loses pressure as it gains velocity. It then traverses the test section, and then in the diffuser, it loses velocity and the pressure rises again, and it is then exhausted to ambient. This means that the exit of the nozzle now doesn't have to be at ambient pressure - it can be below ambient pressure, since the flow will gain pressure in the diffuser. If we're still imagining a setup with a fan on the front, and you have the same pressure ratio across the nozzle, this means that less pressure is needed at the entrance to the nozzle than would be needed without the diffuser, since now the nozzle exit is actually below ambient pressure. Because of this, the pressure ratio across the fan can be much smaller, allowing it to flow more air (or, alternatively, to flow the same amount of air with less power).

This also still applies with an exhaust fan setup - basically, allowing the test section to operate below ambient pressure allows for a much smaller pressure change across the fan disk, improving operation substantially.

 

[paradisePhysicist]

However, now imagine a system with a nozzle and a diffuser. The air enters the nozzle, then loses pressure as it gains velocity.

Seems like it would gain pressure in the forward direction, but maybe lose perpendicular pressure.

 

[boneh3ad]

Seems like it would gain pressure in the forward direction, but maybe lose perpendicular pressure.

Define "perpendicular pressure." Also define "pressure in the forward direction." Given that pressure is a scalar quantity, I have no idea what you mean here.