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 enr_venkat July 22, 2013 07:16

Pump simulation result validation

4 Attachment(s)
In centrifugal pump, pressure rise is expected from 0 Pa at inlet to 100 mBar at outlet. Pressure rise is happening with slight change like 10 mBar. First i need to validate the design operating point i.e. 100 mBar pressure head is expected at 13 liters per minute. So i use the BCs Total Pr @ inlet = 0 Pa and define the mass flow rate @ outlet as 13 kg/min. When i use functional calculator in CFD-POST, i measured the rise in pressure from inlet to outlet.. It is found to be negative pressure at outlet and positive pressure at inlet. Ref pressure is set to 1 bar. Suppose, if inlet pressure is 10 mBar and outlet pressure is -3 mBar, is it correct to interpret that total pressure rise as (10 - (-3 mBar) = 13 mBar) ?

Scenario 1: (Steady state - Frozen rotor)

I used the frozen rotor setup in CFX as shown below:

1. Rotating domain (Impeller speed @ 3000 rpm; with fluid outline and with solid geometry)

2. Stationary domain ( Inlet, outlet and volute casing is defined i.e. as fluid domain)

3. Use the above said BCs

4. Create one fluid solid interface - between stationary and rotating domain.

5. Enable frozen rotor with GGI and pitch change is set to none.

Scenario 2: (Steady state Frozen rotor)

I've tried to run it with a surface of revolution i.e. surface covering the impeller region is set to be rotating and the space outside the impeller including inlet, outlet, wall is set to be stationary. Impeller solid geometry is removed with the subtracted volume. Interface is created between surface of revolution that is rotating and stationary domain.

Please find attached the images for a quick check. I know that something is missing. Mesh is quite ok to get initial results. But at least 70% accuracy is expected.

Do we see pressure and velocity increase at outlet as seen in images ? But when the pressure is high, velocity has to be low as usual.

K-Epsilon turbulence model and Turbulence intensity and viscosity is used in both the cases.

But i'm unable to get valid results in both the cases. Please find attached the screenshots of 2 scenarios.

Any suggestion is highly appreciated. Thanks in advance

 ghorrocks July 22, 2013 07:48

Can you show some images of the entire geometry?

Also, have you read the FAQ on general accuracy: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

 enr_venkat July 22, 2013 08:34

3 Attachment(s)
Quote:
 Originally Posted by ghorrocks (Post 441222) Can you show some images of the entire geometry? Also, have you read the FAQ on general accuracy: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F
Hi,

Thanks for your quick response. Please find attached the two geometries i.e. one without surface of revolution with simple interface and one with surface of revolution. The second image might have protrusion but that is how the fluid gets filled in reality. Please find attached the fluid domain section as well. Will go through the ANSYS Wiki again for accuracy. Awaiting your valuable suggestions.

 ghorrocks July 22, 2013 08:47

What do you mena by "one without surface of revolution with simple interface and one with surface of revolution. The second image might have protrusion but that is how the fluid gets filled in reality."

As you are modelling this with frozen rotor, have you tried it in different rotor positions?

 enr_venkat July 25, 2013 07:06

Please neglect the point - the one that is not created with surface of revolution. Please consider only the geometry with protrusion in the bottom."Fluid gets filled up" means the housing of the pump separates the impeller and surrounding volume from entering into motor zone. The fluid might get leaked down to the protrusion level. Please refer the 3rd attachment showing the fluid domain with the impeller. Guess i should have simplified the model.

 ghorrocks July 25, 2013 07:56

Can you post another image showing what you mean by the protrusion, motor zone and anything else we need to know. I still have no idea what you are talking about.

 enr_venkat July 29, 2013 00:16

1 Attachment(s)
Please find attached a description image for your understanding. All i get is just a pressure drop of 10 mBar in CFD against experimental test results of 100 mBar pressure head. I was wondering if i have missed anything in modeling such as interfaces or setup. Awaiting your response at the earliest. Thanks.

 ghorrocks July 29, 2013 08:11

I have a bit better idea of what you are modelling now, but still your question is unanswerable. The FAQ I previously referred to is the overall guide: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

You can be sure your error is one of the list in that FAQ. Your quest is to find which one. Also check the best practises guide in the CFX documentation for turbomachinery, it is very helpful.

 ARohit January 15, 2014 00:58

finding pressure head of centrifugal pump

Dear sir,

I know how to generate pump impeller report in cfd post and get pump head. but i am doing multiphase flow analysis now and when i try to generste similar report it doesnt happen..

Therefore i want to know some method (traditional ) for knowing pump impeller head..

 ARohit January 15, 2014 01:04

getting pump head without report generation

dear sir,

I am doing something as below by my logic.

I am taking turbosurface at blade TE. and blade LE. then i am finding avg pressure over this surface.

avg pressure based on mass flow rate. (If i am right, i suppose the 'Pressure' variable in cfd post is total pressure which i am considering)

now i am subtracting avg press on LE from TE and i get press head by pump.. which is not good answere. i mean its not matching at all. why.?

so.. isnt wat i am doing logically correct..?