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sameer mohrir August 9, 2000 23:41

impeller of pump
 
What will happen to flow complexity if number of impellers on pump shaft are increased.

John C. Chien August 10, 2000 00:26

Re: impeller of pump
 
(1).It will first increase the efficiency. (2). Then, the blade thickness will take away the available space for the flow. The mass flow will be reduced. (3). Even when the blade thickness is reduced, the total surface area will increase with the number of blades, and the loss will also increase. (4). Eventually, you don't have space left for the flow. (I think, 13 is a good number. 19 is upper limit. 3 or 5 is fine.)

Daniel Jaeggi August 24, 2000 07:09

Re: impeller of pump
 
In terms of flow features, increasing the number of blades reduce the blade loading and the slip factor.

In other words, the velocity distribution across the blade passage is more uniform. This has two effects.

Firstly, the chance of separation on the pressure surface is reduced (this comes from an inviscid analysis of the velocity distribution across the passage).

Secondly, as the Kutta condition is enforced at the end of the blade, similar velocities either side of the blade measn the resulting adverse pressure gradient is reduced. The boudary layers will be thinner and more likely to stay attached.

Hope this helps.

dan

sameer mohrir August 24, 2000 10:04

Re: impeller of pump
 
Hi dan

If possible can you clarify this

"In terms of flow features, increasing the number of blades reduce the blade loading and the slip factor".

in detail

regards


John C. Chien August 24, 2000 20:44

Re: impeller of pump
 
(1). He is talking about the condition which deviates from the idealized inviscid solution model. (in most cases, it is 2-D inviscid, with infinite number of blades having zero thickness. (2). In other words, with finite number of blades, the flow at the exit, or the trailing edge will no longer follow the geometry of the blade. (slip factor).(3). You will have to study the pump theory to understand it.

Daniel Jaeggi August 25, 2000 04:18

Re: impeller of pump
 
Hi sameer,

Hope this helps!

In the impeller, by fixing the mass flow rate, the rotaional speed and the inlet and exit angles, you fix the amout of work done. This comes direstly from Euler's work equation which is derived from the momentum equation applied to a control volume round the blade.

This work has to come from somewhere - the somewhere being an average force on the blades * distance moved. The distance moved is fixed (from rotational speed) and so is the energy. So the force provided by ALL the blades is fixed. Increase the number of blades and you reduce the individual force on each blade.

Where does this force come from? As ever with fluids - pressure. In order to get a pressure difference on either side of the blade, the velocities on either side must be different. So across the blade passage, the velocity must also vary.

The slip factor is a measure of the variation in flow angle at exit across the passage. I forget the exact definition - look in any decent turbomachine text and you should find it. But the slip factor has the effect of reducing the effective exit angle.

dan

sameer mohrir August 25, 2000 05:51

Re: impeller of pump
 
Hi dan

This will help. Thanks

sameer

Daniel Jaeggi August 25, 2000 09:26

Re: impeller of pump
 
A final note!!

There is an excellent treatment of centrifugal impellers in "Compressor Aerodynamics" by N Cumpsty.


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