SimScale CAE Forum

Axial Pump


#1

Hi SimScale,

I am working on my final monograph and it is about an axial water pump simulation. The problem is that my experimental result delivers a Head of 0,26 [m] while the SimScale result shows 29 [m] for the Head (like x10 above). Please, what is wrong with my simulation?

The project link is here and the values there for volumetric flow and rotational speed are right in the setup. The run is called 6.75 - 100% > Real run.

Thanks in advance!!

Roberto Nazareno.


#2

Hi Roberto!

Have you checked the dimensions of your geometry? A factor of 10x,100x sometimes indicate that there is a scaling issue. Now the height of your component is approximately 12.5 meters, is that correct?

Also do you have any reference data except the 0.26 [m] mentioned?

Cheers,

Jousef


#3

Hi Jousef!

Yeah, my geometry is OK. It’s a really large pump and the dimensions are right. All the data that I have is from my research working with that pump here at my home university, and that value 0.26 [m] (experimental data) sounds OK for an axial pump. Please help me… :disappointed_relieved:

Thanks for replying!

Best,

Roberto Nazareno.


#4

Sorry, I am no expert here but wouldn’t you need some boundary layers on your no-slips walls and your rotor?

Not sure they would make up that much difference though…

Also, I do not see where your rotor faces are even used in the simulation, but perhaps I just don’t know where to look :slight_smile:


#5

Hi @DaleKramer,

The rotor faces are here as highlighted. Just embedded within the rotational zone.

And yes you did point out correctly that boundary layers are needed for a more accurate simulation at the rotor and the side walls, but since the results are off by around 99.1%, setting boundary layers will not be enough.

@robnaz

Fundamentally, something is wrong with the setup. With reference back to your first copy of “Copy of 6.75-100%” where your outlet was set to gauge pressure 0, what was the Head value?

Cheers.

Regards,
Barry


#6

Hi everyone,

It was 10.8 [m] of Head, and that value is showed because I am considering a fixed pressure at the inlet which is not real in that run.
Backing to the real run, those are my experimental data, look:

Above you can see that I am having an inlet pressure of 201320 [kgm-1s-2] and an outlet pressure of 204158 [kgm-1s-2]. The main difference of the simulation and the experimental data is here, I guess. Please, let me know what you suggest me to do.

Guys, really thanks for responding!!

Best,

Roberto Nazareno.


#7

@Get_Barried

Here is the MRF mesh:

The circular faces of that zone are very confusing, that image shows it as basically a cup, not a cylinder.

Also do those longitudinal streaks on the surface of the zones cylinder cause a problem?

What am I missing?

I also see that SimScale imports the Fan component as a solid but when I import the geometry into Rhino, the fan is not a solid (perhaps due to small face edge deviations somewhere).

And the mesh has almost 100 illegal faces as shown in the meshing log…

Dale


#8

Hi @DaleKramer,

I see the issue. Looked fine on my end. I will need to double check the geometry on my end but you may be on to something.

Hi @robnaz,

We probably have to fix your geometry first before we proceed with anything else. Get back to you on this.

Cheers.

Regards,
Barry


#9

@robnaz

This project has piqued my interest in that it appears that you have some significant experimental results that will allow us to determine how closely a CFD analysis can be made to match experimental results.

In that vein, I would like to investigate the accuracy of your CAD geometry and I have a few questions?

  1. Was the CAD model ‘axial’, drawn from engineering drawings?
  2. How many sections (ribs) were the blades lofted from? I am concerned since I do not think that the blades of this fan would have been built with this surface waviness:
  3. Are there more details available for the fan that are not represented in your CAD, for instance, there is no ‘afterbody’ spinner on the downstream side of the fan.
  4. Could you provide the details of the fan support structure that passes through the duct?

I will try to modify the current CAD file to work within the limitations of this CFD environment and try to get some close results.

Right now I think that the Snappy mesh algorithm is not meshing the MRF nicely due to the fact that it is only ~4mm larger than the fan blade diameter, first I will try to trim the fan blades a few mm smaller diameter and fix the fan edges that are not joined in Rhino.

@Get_Barried

What are your thoughts of MRF diameter (currently 1.4777 m) compared to duct diameter (currently 1.481 m diameter) and to the fan blade diameter (currently approximately 1.474 m diameter)?

Is the MRF sized properly with regard to its length and X coordinates of its circular end faces?

Would fan support structure that passes through the MRF affect the results accuracy (I assume they would not as all rotors need to be supported to some structure that passes through the MRF)?

I also assume that MRF’s CFD analysis somehow interact properly with, in this case the surrounding water flow and that any streamlines would not be discontinuous at the MRF boundaries, is that correct? (sorry this is my first experience with MRF’s)…

Dale


#10

Sorry, the MRF only appears as a cup shape when the mouse cursor is on top of it, here it is with mouse away from it:

My concerns about the longitudinal streaks remain :wink:


#11

Hi @DaleKramer,

Great that you are interested in this. It should interesting to work on this together. Great questions to ask, lets see if we can deduce anything else from the geometry from @robnaz’s answers.

I initially saw that the blades are touching the MRF and I believe they are. From the mesh it seems like a cross-section of the blade profile can be seen at the MRF. This is workable but of course, not good. If however, the MRF diameter is larger than the fan blade diameter then it is ok. But it should also be smaller than the duct diameter. Based on what you measured, it should be ok. Though it would be best to give more margin between the MRF and the fan blade diameter as you have correctly pointed out, SnappyHexMesh tends not be happy with small margins like that (4mm).

I do think your geometry fix in Rhino would help the simulation plenty. I might do some modifications myself and re-run the mesh if needed.

The length of the MRF acts similarly to that of the diameter. As long as it is lengthwise longer than that of the length of the rotors then it is fine. With some margin of course.

Yes it would, unless the support is supposed to rotate with the fan or if the support is very thin/insignificant. Then again, you can have the supports inside the MRF. Just have to assign a moving wall velocity 0 in all directions to prevent the MRF from “rotating” the supports.

MRFs provide a rotational value to the geometry within its zone (hence the need for internal meshing). With no geometry, the MRF will not be able to apply a rotation and the flow will not rotate. So in essence, the MRF doesn’t interact with the flow but rather the geometry which then rotates the flow. No worries! I had ran into alot of problems with MRFs from another project. Quite an interesting parameter.

Cheers.

Barry


#12

Agree. This is exactly what I saw. It is very likely the mesh is giving issues. Calculation towards the boundary conditions can be resolved later.


#13

Ok, so we just make sure that the rear circular face of the MRF falls between (with space for Snappy) the rotor and the missing ‘afterbody’ and support members. For now, I would just ignore any CAD that is the shaft which the rotor is mounted on so that we have no structure passing through the MRF.


#14

Hi @DaleKramer,

We can just ignore the support members and have the blades and center rotating cone encased within the MRF. Results can be fine tuned with a geometry update with the supports if needed. Otherwise, magic floating prop will do :sweat_smile:


#15

Remember, that is a diameter difference, so actual clearance is 1/2 that.

What fan rotor diameter should I try with the current MRF diameter of 1.4777m?

Dale


#16

@robnaz

Also, for the sake of completeness, could you confirm that your results chart was actually for a rotor speed of 21.476 radians/sec, inlet flow rate was 6.75 m^3/sec and the outlet pressure was 0Pa?

Thanks,
Dale


#17

We only have a margin of 0.007 (diameter wise) to work with so maybe somewhere in the middle would be a good place to start. So 1.4775? Not much of a difference though, we might need to adjust the meshing settings to try and get snappyhexmesh to not screw around.


#18

I agree, I have made a mesh at rotor diameter of 1.466m and still there are mesh streaks etc.

And still 60 illegal faces…

When I watch the meshing log go by during meshing, it appears to have the most trouble making the level 2 and 3 faces, I find this strange…

I will persevere :slight_smile:


#19

@DaleKramer, is the MRF a solid or just a shell? I assume it is meshed internally as well. The MRF should be a solid in the CAD.


#20

Not sure about beaing a solid or not in the MRF mesh but in CAD yes it is.

I have not been able to get mesh clips working in new WB, so I can not see its internal cells :frowning: