[warex]

Hello all,

I am a brand new user to CFD, this is my first attempt at a simulation in fact. I am using Solidworks 2009. I am looking to compare various different impellers in a centrifugal pump application. I was able to follow the tutorial with moderate sucess but i had a few questions/problems.

Here is my setup:

Inlet Boundary Condition: Environment pressure at 101325 Pa
Outlet Boundary Condition: Environment pressure at 101325 Pa
Global rotating reference frame: 210 rad/s (2000 rpm)
Fluid: Water
Walls: Pump housing walls set to Stator.

Goals:
Inlet Static Av. Pressure
Outlet Static Av. Pressure
Torque on Impeller
Surface Flow Rate Out
Equation goal = pressure drop (outlet press - inlet press)
Equation goal = efficiency of impeller (pressure drop*flow rate/angular vel/torque on impeller)

Results of goals:
Inlet Static Av. Pressure = 88687 Pa
Outlet Bulk Static Av. Pressure = 100573 Pa
Torque on Impeller = -0.1898 Nm
Surface Flow Rate Out = 0.005 m^3/s
Equation goal = pressure drop
Equation goal = efficiency of impeller = 1.58 = 158%

As you can see the efficiency is definitely wrong as you would assume it to never be above 1. I do not know if the values of torque or pressures are reasonable as I do not have much experience. The one thing I have done different than the tutorial is my inlet boundary condition is environmental pressure and not a flow rate. I felt that it was strange that you would feed a pump with external flow rate and I would rather evaluate the performance of the impeller with no external help.

I will be evaluating many different impeller geometries so utilizing efficiency as a performance metric would be very useful. I just want to make sure that my boundary conditions and goals are reasonable.

Thanks for everyone's help.

[warex]

Can anyone shed some light?

[Boris_M]

Hi warex,
if your model is more complex than the tutorial model and you use the outlet opening of the real pump and not a ring like opening as in the model you should use "bulk avg." for your goals as the massflow is also considered in the average not just the surface. You would notice the difference as soon as you have a non equally distribution of the velocity on the outlet.

But most important you have to define a volume flow rate for the inlet. This is the way the boundary conditions (BC) for the rotating regions work the most stable and accurate. You have to think of three different zones, the inlet non rotating zone, the rotating zone and the outlet non rotating zone. Each zone receives a flow rate inlet condition and a pressure outlet condition. You cannot control the BC of the rotating region, they are setup automatically with the definition of the feature rotating region. So the outlet of the inlet zone is pressure and the inlet of the rotating region is flow rate and these two are automatically connected.
I assume you want to record the pump curve and with that you can simply run several projects with predefined volume flow rates and get the corresponding pressure difference, efficiency etc.

Try this and it should work, let me know the results. I'm curious :-)

I hope this helps,
Boris

[warex]

Thanks Boris, my model isn't very different from the tutorial. It just has a different impeller but it still has the flow exiting around the circumference, rather than through a volute. I have been using the measurement ring as in the example to measure bulk average static pressure on the outlet.

I see your point on the inlet flow rate boundary condition, i needed to think it over to convince myself but i think i understand now. I have run different projects, as you suggested, with flow rates from 10 gpm to 100gpm in 10gpm increments with a constant 2,000rpm rotation of the impeller. First, I set the inlet volume flow rate and ran all of the projects. I graphed average static outlet pressure vs. flow rate and the graph wasn't what i expected. I expected a graph similar to a characteristic curve where head decreases as flow increases. My graph showed that pressure actually increased as flow increased. I decided to set the flow rate BC to the outlet rather than the inlet and that is when I got a much better graph. The pressure decreased as flow increased. However, my efficiencies were all still above 100% which cannot be accurate.

Here is an example of my goals output when I had the inlet BC set to 0.00315 m^3/s and outlet to environment pressure.
Goal Name
Unit
Value
Inlet Mass Flow
[kg/s]

3.146309726

Outlet Mass Flow
[kg/s]

-3.141098274

SG Av Static Pressure Inlet
[Pa]

87997.56573

SG Bulk Av Static Pressure Outlet
[Pa]

99085.58974

Impeller Torque
[N*m]

-0.26255882

SG Volume Flow Rate Outlet
[m^3/s]

-0.003148776

SG Volume Flow Rate Inlet
[m^3/s]

0.003154

Impeller Force Z Dir
[N]

34.44710258

Av Total Pressure Out
[Pa]

79060.6853

Pressure Drop
[Pa]

11088.02401

Flow out in GPM
[ ]

-49.87661429

Efficiency
[ ]

0.633213315

This looks reasonable, but when i do multiple different flow rates at the same RPM, they do not follow a trend I would expect. When i switched to the outlet flow BC then it started trending much better, but my efficiencies were way off.

As you can see i am still fairly confused

Thanks for the help.

[Boris_M]

Hi warex,
since you are unsing SWFS the first time it would make sense to test your calculations on models that already have been measured, so existing pumps where the pump curve is known and you can validate with them.
The thing with expectations is, they can be wrong.
I know of several companies using SWFS or FloEFD in the pump industry and get great results also with cavitation. This is why I would recommend you to test it on an existing product and compare with known results better than expectations.

I hope you understand. One would expect bumblebees should not be able to fly, but we all know they do :-)

So please try to confirm your calculation on an existing model first and you should see it matches the results very good. It's hard to guess whats wrong from writing here and if that test doesn't work, kontakt your support and let them help you.

Boris

[warex]

I understand, thanks for the advice i'll have to try to get my hands on some existing data. I'll figure it out eventually, this is just a side project anyway so i won't have too much time to work on it. Thanks for all your help!

[ihadi]

hi all

I want to simulate pump impeller,to
can help me anyone,by sending me any tutorial

then i want to adding Fluid-structure interaction into simulating.

thank you for help me.

for.learn@yahoo.com

[Boris_M]

Hello ihadi,

the tutorials for such a case come with the software and there is not much fluid-structure interaction you can do. FloEFD/SW Flow Simulation is a CFD code, no FEM is provided by the developers of that code. There are some interfaces to FEM codes to provide the pressure and temperature loads but there is no feedback from deformed geometry to CFD that you can use directly and create some loop in simulation automatically.

Boris

[ihadi]

hi Boris

I've been working in this field.
But this time I used this method to analyze a pump as close to reality.

Unfortunately I do not have access to Ansys Training Center.

please if you have any filese or linkes send to this Email.
new Email address: for.learning@ymail.com

[Boris_M]

Hi ihadi,

I think you are in the wrong forum. This is about FloEFD/SW Flow Simulation and TloTHERM, not ANSYS products.

Boris