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No Net Force on a Centrifugal Impeller? Really Puzzling

No Net Force on a Centrifugal Impeller? Really Puzzling

No Net Force on a Centrifugal Impeller? Really Puzzling

(OP)
Hi everyone!

I have seemingly a very simple question regarding centrifugal fans/blowers and really thought that you here might be able to answer considering your experiences in the area.

We all know a centrifugal fan/blower has a very low pressure in the center where suction occurs: (Example: a vacuum cleaner impeller may have 20Kpa of suction pressure which would translate to 5kg of suction pressure force DIRECTLY above a typical 5cm diameter inlet of the impeller!)

My question is, is there a NET unidirectional propulsive force on the centrifugal fan UPWARDS due to this extreme low suction pressure at the center? -(of course we assume here an enclosed impeller where we cover the impellerĀ“s outer radial high static pressure area and also prevent the high pressure ejected air from recirculating back into the inlet. Only the inlet is opened to the atmosphere where the atmospheric pressure can do its job.)- ..

Now, would there be a lift, Bernoulli sais there should be if I understand this correctly. If there is no net lift on the impeller, that would be interesting.So why not? Would you explain why please?? This question has and is still really bothering me as I find no answer. All I find on the web is analysis of velocities and pressure-distributions on the impeller which I already know about. The question is, whether there is a net force on the impeller itself resulting from the center suction pressure..

Thank you all a lot for taking your time to read my question
Best Regards
key200

RE: No Net Force on a Centrifugal Impeller? Really Puzzling

So there is an axial flow of air into the impeller, and a radial flow out of it?

Mr Newton will be of more help than Mr Bernouilli. Plus his name is much easier to spel.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: No Net Force on a Centrifugal Impeller? Really Puzzling

(OP)
Hello!

I am not sure if I understand the point you are trying to make? I am aware of both gentlemen :) and you are absolutely correct, Newton has a much easier name to spell hehe. Also, Newton always apply everywhere. How it applies is of course depends on many factors.

Anyway, perhaps the following words may be a waste of space but just to make sure that we are on the same grounds. Any parallel airflow along a surface which was a result of acceleration from a previous lower air velocity in the same streamline will produce a lift force perpendicular to the surface. Reason being that the energy of the air mass must be conserved, so if the air mass was somehow naturally accelerated (like from a venturi effect, or a rushing in to fill a lower pressure void /section) without direct big body interference, the air massĀ“ static pressure must decrease as the case is in the center area of a centrifugal impeller where the air first has been centrifugally pushed out creating a central partial void of lower pressure and inrush of air towards the center from above.

At the center of an impeller, there is a very low static pressure and air rushing very quickly vertically from above flowing down and parallel to the center surface. So again, I wonder if there is any net force acting on the impeller or not? I just cant find any information on that point, and if there is no force, what is the reason?

Thanks

RE: No Net Force on a Centrifugal Impeller? Really Puzzling

I don't understand your definition of up or down since you have not defined any orientations. However, if the outlet from the impeller is closed and there is no recirculation, then there is no air flow and the pressure in the center of the impeller will be at ambient. The only difference in forces acting on your impeller and one operating in vacuum will be viscous forces between the spinning impeller and the stationary case. The pressure around the impeller is actually caused by the weight of the air spinning with the impeller in its high centripetal acceleration, which behaves like a high gravitational field.

RE: No Net Force on a Centrifugal Impeller? Really Puzzling

(OP)
Hi, I uploaded a picture,


The outlet is not closed. It is just away from the inlet as to not interfere or recirculate with the inlet flow. The assumed centrifugal fan is rotating just like in any centrifugal blower like in the picture, thereby a strong suction pressure/low static pressure occurs as we all know at the inlet.

My question thus is if there is a net force - in the direction of the big red arrow - on the blower due to the low pressure above the suction inlet? If there is no force, Would you explain why not?

http://files.engineering.com/getfile.aspx?folder=f...

I hope my point is clear :)

Thank you for taking your time reading this


RE: No Net Force on a Centrifugal Impeller? Really Puzzling

Your questions make no sense. Your picture is simply that of a blower. A simple force balance on a free-body diagram will tell what forces are acting on the blower. Lift is not one of them. This website is for engineering professionals. Students are not allowed because of questions like this.

RE: No Net Force on a Centrifugal Impeller? Really Puzzling

Quote:

My question thus is if there is a net force - in the direction of the big red arrow - on the blower due to the low pressure above the suction inlet? If there is no force, Would you explain why not?
There is a net axial force, within the blower. For air, it is usually small enough to be dealt with by a modest thrust bearing.
If you examine centrifugal liquid pumps, you will find vanes and other funny business on the 'back' side of the impeller, intended to control or balance thrust forces.

Mike Halloran
Pembroke Pines, FL, USA

RE: No Net Force on a Centrifugal Impeller? Really Puzzling

You can run the math. Assume that it's an aerodynamic drag problem. Assume, say, 10 mph inlet airspeed. Assume 3-in diameter opening. That results in about 50 milli-newton of force, which is about the weight of 5 grams.

TTFN
I can do absolutely anything. I'm an expert!
homework forum: //www.engineering.com/AskForum/aff/32.aspx
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers

RE: No Net Force on a Centrifugal Impeller? Really Puzzling

(OP)

Hi again

Yes it is a blower, used as an example. My question relates to centrifugal impellers in general. It seems my question is misinterpreted.

I am not a student. My question as said earlier dealt with the lower pressure created by the increased velocity on inlet surface. The question was whether the lower static pressure on the center inlet surface did produce a net force perpendicular to the impeller surface.

My wondering behind a force production is because of the low pressure created on the inlet center surface

Check the other new picture here: http://files.engineering.com/getfile.aspx?folder=9...
to understand the reasoning behind my question regarding force production.

Air outside of impeller is at atmospheric pressure, the air is accelerated sucked in and sped up along the center surface radially as shown in the picture, Bernoulli says that if an air streamline has changed its velocity, its pressure must change, thus we have a lower pressure at the center of the impeller. therefore I thought there should be a net force perpendicular to the impeller surface? Right or not, I am not sure.

Thanks

RE: No Net Force on a Centrifugal Impeller? Really Puzzling

(OP)
IRstuff, thanks!

So it appears that I can apply the force calculation in this situation. Good, well, the thing however is that tests carried out on impellers did barely show any net forces at all(i.e. perpendicular net force to the impeller surface as seen in the previous pictures uploaded).

During a long session of series of experiments carried a long time ago I used semi-open impellers and closed impellers (where only the inlet and outlet is exposed to outside air, just basically a blower as I showed in the pictures earlier) to test the expected suction pressure NET forces(like that big red arrow in the earlier picture).

Impellers varied in size between 6" to 8" in diameters, rpms reached above
14 000. Inlet air speeds were in access of 430mph. Calculated forces would be several pounds or an average of 8 lbs for some of the impellers: the force at least should be easily measurable. However, there were barely any force measurable during the tests, or at most, ironically as IRstuff exemplified, only few millinewton were actually detected, which indicates that something is wrong here thereby my original question.

Thanks

RE: No Net Force on a Centrifugal Impeller? Really Puzzling

I think it's required to integrate over the entire surface of the impeller. While the pressure is low in the middle, it is high at the periphery and of unknown quantity on the reverse side.

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