# What is proper boundary condition to model Pump inlet in a multiphase simulation?

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 June 24, 2017, 12:56 What is proper boundary condition to model Pump inlet in a multiphase simulation? #1 Senior Member   sluzzer Join Date: Jul 2014 Posts: 129 Rep Power: 5 A vented tank is connected to a scavenge pump (gear pump). If there is no liquid or insufficient liquid, then the pump will suck the air also. I don't want to model the pump. Instead I like to give an appropriate boundary condition at the end of pipe which is connected to the pump. What boundary condition I should apply here? Mass flow rate or Pressure or any other method? Thanks

 June 25, 2017, 19:05 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,881 Rep Power: 107 Sounds like a constant volume flow rate would be a suitable boundary condition. You can impose this using a defined velocity at the boundary.

 June 26, 2017, 00:16 #3 Senior Member   sluzzer Join Date: Jul 2014 Posts: 129 Rep Power: 5 During testing of the pumps, I observed if we change the diameter of inlet (auction) pipe, the outlet flow rate is varying. So I think constant flow rate boundary condition is not the right choice to simulate the actual condition. My understanding is that the flow in the suction pipe is due to the pressure difference between the pressure at tank and the pressure at the inlet of pump. However, the flow rate in the outlet of pipe is constant and it does not varies with change in pipe diameter. However I don't know how it behaves when it turns into multiphase. I have the test data of inlet pressure when the pump is tested only in single phase (oil). Can I use the same inlet pressure value for the multiphase simulation? Sent from my iPhone using CFD Online Forum mobile app

 June 26, 2017, 19:15 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,881 Rep Power: 107 The basic rule of boundary conditions is: You have to know the flow conditions at a boundary to define a boundary condition there. If you do not know the flow conditions at a boundary then you can't put a boundary there - so you need to move the boundary somewhere you do know the flow conditions, or you need to get more data until you know the boundary condition.

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