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Centrifugal Pump Simulation Problem!

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Old   September 20, 2014, 01:24
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Abbas
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Hi Boris,

After making some changes in the rotating region, I got the desired result. Now the head value is very close to the experimental result.

Now I want to calculate efficiency of Impeller and overall pump.
I tried using the torque component for calculation of shaft power but the efficiency calculated w.r.t. shaft power is almost double.

I have used the below formula for calculating efficieny:
Overall Efficiency = (Q x Pressure differenc)/ (R.P.M in rad/s x Torque)

The torque value calculated from flow sim is lower than actual value.
Do you know what might be the reason behind it? or is there any other way to calculate it?

Regards,
Abbas
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Old   September 21, 2014, 10:07
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Bilal Waseem
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i am working on CFD simulation of flow through the impeller of centrifugal pump. I am not considering the volute. My problem is that I don't have a clear idea about how much pressure rise should occur in the impeller. I mean, it is just giving kinetic energy to the fluid so that means, static pressure should not rise too much. But in my case, the results are:

Inlet BC: 1.99 bar pressure
Outlet BC: 332 kg/s flow rate
Wall BC: Shroud, hub and blades

From simulation, I get
Outlet Pressure: About 9 bar

My question is that isn't it too high for an impeller of 48 cm diameter running at 1480 rpm .. ?
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Old   September 21, 2014, 16:46
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Hi Billy,

I cannot tell if 9 bar is wrong but in general for a radial pump or compressor such pressure ratios are realistic. I just cannot tell if it is correct for your blade geometry and RPM.
Try to do some hand calculations if possible in order to check the results.

Also I'm not sure if the BC are correct. You say you have Wall BC on the blades?
What type of wall BC?

Boris
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Old   September 21, 2014, 16:56
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Hi Abbas,

If you use Pa for Pressure, rad/s, Nm for the torque and m^3/s for the flow rate then the equation should be correct.
Have you tried if the result of the equation makes sense if you use the measured torque?
Try also to set the reference pressure which is used for force calculation but it should still give correct results.
Has the result converged? Sometimes it takes some more iterations until the fluctuations have really gone. Especially in rotating models.

Boris
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Old   September 22, 2014, 00:32
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Hi Boris,

I have used the same units you mentioned.
I had saw on a tutorial for calculating the efficiency using this formula.
Is the measured torque in flow simulation correct for the use of calculation of efficiency? or Is there any other parameter which should be used instead of torque? Or is there any other way to calculate shaft power?
The results has also been converged.

Regards,
Abbas
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Old   September 22, 2014, 11:14
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Hi Abbas,

The torque is calculated correctly. I just did a calculation of a fan and the results of Shaft power and efficiency matched very well the measurements.

I would suggest you contact your support to let them have a closer look on the problem. Maybe there is a small mistake you made somewhere which they can find when looking on your model and project setup.

Boris
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Old   September 22, 2014, 16:00
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Bilal Waseem
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Hi Boris,
Thanks alot for your response. Actually, mine is a mixed flow pump. My issues are:
1) I am a little confused as to which pressure I should plot. The options I have are Static pressure, total pressure, total pressure in relative frame and total pressure in stationary frame?
2) If I plot total pressure in relative frame, that includes the effect of both static and dynamic head. I personally think that pressure at impeller outlet is almost all dynamic. Once it will enter the volute, dynamic head will be converted to static head keeping total pressure same. I think the volute only converts dynamic head to static head. Am I right about this?

Thanx in advance.
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Old   September 22, 2014, 16:02
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Hi Boris,
I forgot to mention that I am modeling the blade surface as wall with no slip condition. Kindly comment on this and also please answer the two questions I have asked.

Regards,
Billy
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Old   September 23, 2014, 09:07
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Hi Billy,
What fluid are you using?
In general gasses or liquids don't have slip condition. This usually applies only with non-newtonian liquids. So you wouldn't have to define the blade wall conditions in that way.

1) usually the pressure is plotted which is the static pressure but then that depends on what you want to show/see. The relative frame plays a role due to the dynamic pressure (whihc is part of the total pressure) relative to the rotating reference frame or not as it contains the velocity which is used in this relativity.

2) Well everywhere is pressure loss and the total pressure loss is the one considered if you want to really see how much energy is lost du to bad geometry design you could say. A volute should also ideally not create any total pressure losses, of course that is impossible. The volute however is not there to convert anything in particlular but to direct the flow mainly. If you have a radial pump you would receive an increase in energy due to the feed of energy by driving the pump rotor. So puttung energy into the flow. But without a volute the flow would leave radial in 360 around the rotor which is something you cannot work with if you need it going into a pipe. For that you would need a volute. Here the volute is basically an intake for the pipe which gathers the flow that leaves the rotor 360 around it into that pipe and ideally with a minimum in losses. Of course you want to have it mostly in dynamic pressure as you want to have some flow in the pipe.
The pump is designed to generate dynamic pressure as it is moving of course and with that creating flow. And that's the main reason for a pump, to drive the fluid or in some cases keep a high pressure for the case the pump works at its curve maximum for the delta P with zero flow. Like filling a gas canister to its maximum or better said to the maximum of the pumps curve. If the canister can store 70 bar of pressure then you would want to design the pump in the way that it can reach 70 bar pressure difference before there is 0 flow in order to not kill the pump after some fillings already. You wouldn't want to go to its limits.

I hope this helps,
Boris
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Old   September 23, 2014, 10:27
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Hi Boris,
I am using water as the fluid.
And I didn't really understand what type of pressure I should use. I want the total pressure rise in impeller, so I think I should use Total Pressure in Relative Frame. Because if I use static pressure at the impeller outlet, that would not include the velocity effects i.e the dynamic pressure rise.
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