flow rate impeller

guillaume74 · September 21, 2015, 10:13

Hi all,

I am simulating a flow through an axial pump with a given total inlet pressure at the inlet and a mass flow rate at the outlet. My impeller is a rotating domain and my inlet and diffuser are both stationary domains. My simulation is steady state.

My flow rate is very low , the order of 0.1 kg/s and the angular velocity is something like 4000 rev/min.

The simulation runs fine with low residuals and low imbalances but when I post process, i am quite dubious of the results of the streamlines. It appears that the streamlines are kind of stuck at the start of the impeller. When i increase the flow rate (about 10 kg/s), the results of the streamlines make sense and the fluid reaches the outlet but for the low flow rate, it is quite weird.

I am wondering if it is not because the flow rate is too low compared to the angular velocity of the impeller...

What are your thoughts?

ghorrocks · September 21, 2015, 17:34

When the device runs off the design speed there will be big separations and other complex flow things happening. So you would expect to see a very different flow field.

Please post some images of what you are seeing.

guillaume74 · September 22, 2015, 08:16

Hi Glenn,

Thanks for your answer.

Here is a screenshot of streamlines. As you can see they are stuck in the impeller for this flow rate. If i set a larger flow rate, the streamlines will reach the outlet.

The weird things is that the simulation converges correctly. So i wonder if it is a visual thing or not... basically if i can trust the results or not...

ghorrocks · September 22, 2015, 08:24

I do not understand what your image shows. You will have to show more detail than just a wireframe.

Also an image showing what the result you like shows would help as well.

guillaume74 · September 22, 2015, 08:41

Thanks for your time glenn,

below are two images: one is the fluid domains in cfx and the other is the same run with a larger flow rate.

ghorrocks · September 22, 2015, 20:01

I would expect that if the flow rate was too low in that thing the rotor would just wizz around and not do anything. The streamlines will then just go round as well.

Also I note your exit mesh is very coarse. You are not going to get any useful results with a mesh as coarse as that.

QIAN06 · September 24, 2015, 06:57

Quote:

Originally Posted by guillaume74

simulating a flow through an axial pump with a given total inlet pressure at the inlet and a mass flow rate at the outlet.

Did you try to set a mass flow rate at the inlet and a total pressure at the outlet?

guillaume74 · September 25, 2015, 06:30

yes i tried with a different configuration. I was using total pressure at the inlet because it is generally how you set up a turbomachinery problem. However, i really think that ghorrocks is right by saying that the design is not made to deal with such a low mass flow rate. The velocity at the outlet is something like 0.02 m/s...

Well anyway, thanks for your help!

September 21, 2015, 10:13	flow rate impeller	#1
guillaume74 New Member Join Date: Sep 2015 Posts: 17 Rep Power: 10	Hi all, I am simulating a flow through an axial pump with a given total inlet pressure at the inlet and a mass flow rate at the outlet. My impeller is a rotating domain and my inlet and diffuser are both stationary domains. My simulation is steady state. My flow rate is very low , the order of 0.1 kg/s and the angular velocity is something like 4000 rev/min. The simulation runs fine with low residuals and low imbalances but when I post process, i am quite dubious of the results of the streamlines. It appears that the streamlines are kind of stuck at the start of the impeller. When i increase the flow rate (about 10 kg/s), the results of the streamlines make sense and the fluid reaches the outlet but for the low flow rate, it is quite weird. I am wondering if it is not because the flow rate is too low compared to the angular velocity of the impeller... What are your thoughts?

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September 21, 2015, 17:34		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	When the device runs off the design speed there will be big separations and other complex flow things happening. So you would expect to see a very different flow field. Please post some images of what you are seeing.

September 22, 2015, 08:24		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	I do not understand what your image shows. You will have to show more detail than just a wireframe. Also an image showing what the result you like shows would help as well.

September 22, 2015, 20:01		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	I would expect that if the flow rate was too low in that thing the rotor would just wizz around and not do anything. The streamlines will then just go round as well. Also I note your exit mesh is very coarse. You are not going to get any useful results with a mesh as coarse as that.

September 25, 2015, 06:30		#8
guillaume74 New Member Join Date: Sep 2015 Posts: 17 Rep Power: 10	yes i tried with a different configuration. I was using total pressure at the inlet because it is generally how you set up a turbomachinery problem. However, i really think that ghorrocks is right by saying that the design is not made to deal with such a low mass flow rate. The velocity at the outlet is something like 0.02 m/s... Well anyway, thanks for your help!