Vacuum pump simulation boundary conditions

jwillie2000 · October 19, 2016, 11:13

Hi Everyone,

I am simulating a claw vacuum pump using Immersed Solid method in CFX to mimic the rotations of the claws and i am wondering if using an Opening inlet and Opening outlet is the right thing to do? Plus as i monitor the mass flow rate at the inlet & at the outlet the inlet value is negative and outlet value is positive. My gauge pressrue at the inlet is -816mbar and at the discharge it is 6 mbar. I took the reference pressure to be 1 bar.

I have done about 4 revolutions now and the average mass flow rate @ the inlet is -0.011548 kg/s & at the outlet it is 0.0069885 kg/s. After 3 revolutions, it was average mass flow rate at inlet of -0.0073165 kg/s & outlet of -0.0015219 kg/s. Is the trend in the right direction?

Thanks!

Jimmy

ghorrocks · October 19, 2016, 18:06

I would check in the post processor that the leakage flow is what you expect and that the immersed solids are freezing the flow under them as intended. Immersed solids can be "leaky" when the momentum source scaling factor is too small.

jwillie2000 · October 20, 2016, 03:38

Thanks for your suggestion. I am using 10 for the momentum source term factor. This is the value recommended in the CFX user manual. Do you think it can be increased? What value would you recommend?

Thanks!

Jimmy

Quote:

Originally Posted by ghorrocks

I would check in the post processor that the leakage flow is what you expect and that the immersed solids are freezing the flow under them as intended. Immersed solids can be "leaky" when the momentum source scaling factor is too small.

ghorrocks · October 20, 2016, 05:14

The required value is different for each simulation, so you need to determine it for your application. The default of 10 is a good starting point but it is good practise to check it. Repeat the simulation using values of 20 and 40. See if it makes a difference.

jwillie2000 · October 21, 2016, 05:21

Thanks again. I am now doing that. My other question was about why the mass flow rate at the inlet is negative and that at the outlet is positive...Is this what you would expect....is it physical or non-physical? I am having 100 time steps per revolution of the rotors and i am doing average over this time and it is negative at the inlet and positive at the outlet.

Thanks!
Jimmy

ghorrocks · October 21, 2016, 06:05

You defined the pressure at the boundaries so this is the flow which the solver reckons results from those pressures. I have no idea whether this is possible as it depends on the details of what you are modelling and how you are modelling it.

But you should check your time step, convergence criteria and mesh are OK before you trust your results. Do a sensitivity check to be sure they are OK.

October 19, 2016, 11:13	Vacuum pump simulation boundary conditions	#1
jwillie2000 Member James Willie Join Date: Mar 2009 Posts: 81 Rep Power: 17	Hi Everyone, I am simulating a claw vacuum pump using Immersed Solid method in CFX to mimic the rotations of the claws and i am wondering if using an Opening inlet and Opening outlet is the right thing to do? Plus as i monitor the mass flow rate at the inlet & at the outlet the inlet value is negative and outlet value is positive. My gauge pressrue at the inlet is -816mbar and at the discharge it is 6 mbar. I took the reference pressure to be 1 bar. I have done about 4 revolutions now and the average mass flow rate @ the inlet is -0.011548 kg/s & at the outlet it is 0.0069885 kg/s. After 3 revolutions, it was average mass flow rate at inlet of -0.0073165 kg/s & outlet of -0.0015219 kg/s. Is the trend in the right direction? Thanks! Jimmy

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October 19, 2016, 18:06		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	I would check in the post processor that the leakage flow is what you expect and that the immersed solids are freezing the flow under them as intended. Immersed solids can be "leaky" when the momentum source scaling factor is too small.

October 20, 2016, 05:14		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	The required value is different for each simulation, so you need to determine it for your application. The default of 10 is a good starting point but it is good practise to check it. Repeat the simulation using values of 20 and 40. See if it makes a difference.

October 21, 2016, 05:21		#5
jwillie2000 Member James Willie Join Date: Mar 2009 Posts: 81 Rep Power: 17	Thanks again. I am now doing that. My other question was about why the mass flow rate at the inlet is negative and that at the outlet is positive...Is this what you would expect....is it physical or non-physical? I am having 100 time steps per revolution of the rotors and i am doing average over this time and it is negative at the inlet and positive at the outlet. Thanks! Jimmy

October 21, 2016, 06:05		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	You defined the pressure at the boundaries so this is the flow which the solver reckons results from those pressures. I have no idea whether this is possible as it depends on the details of what you are modelling and how you are modelling it. But you should check your time step, convergence criteria and mesh are OK before you trust your results. Do a sensitivity check to be sure they are OK.