Multiphase fuild and streamlines in CFX

Michael VF · November 2, 2020, 17:36

Hi.

I need to simulate tube with fluid, which is mix from water and gas (which is ejected by ejector in water). I don't care about ejection for now, so I selected multiphase fluid in Ansys CFX. When I try to build 3D streamlines from inlet based on Water.Velocity they ends (breaks) right near the inlet. If I make the same simulation for pure water streamlines works well.

And one more question, which is more related to physics:

In real world if the flow rate of the supplied water is a liter/sec, and the flow rate of air supply to the ejector is b liters / sec, then after ejection, if you measure the flow rate of the compound fluid with a flow meter, will the flow rate of mixture be approximately equal to a + b or if it is far more complicated, who it can be estimated?

ghorrocks · November 2, 2020, 19:33

Q1: Make sure you know the difference between "phase.Velocity" and "phase.Superficial Velocity". If this does not help you please post an image showing the problem.

Q2: It all depends on what assumptions you make. If water and air are incompressible and unreactive then mass conservation and volume conservation are the same thing (as density is constant), so the flow rate will be a+b. But if any phase is compressible, or reacts, or dissolves or one of many other things then the volume flow rate will not be a+b.

November 2, 2020, 17:36	Multiphase fuild and streamlines in CFX	#1
Michael VF New Member Mikhail Fokanov Join Date: Jan 2013 Posts: 8 Rep Power: 13	Hi. I need to simulate tube with fluid, which is mix from water and gas (which is ejected by ejector in water). I don't care about ejection for now, so I selected multiphase fluid in Ansys CFX. When I try to build 3D streamlines from inlet based on Water.Velocity they ends (breaks) right near the inlet. If I make the same simulation for pure water streamlines works well. And one more question, which is more related to physics: In real world if the flow rate of the supplied water is a liter/sec, and the flow rate of air supply to the ejector is b liters / sec, then after ejection, if you measure the flow rate of the compound fluid with a flow meter, will the flow rate of mixture be approximately equal to a + b or if it is far more complicated, who it can be estimated?

November 2, 2020, 19:33		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	Q1: Make sure you know the difference between "phase.Velocity" and "phase.Superficial Velocity". If this does not help you please post an image showing the problem. Q2: It all depends on what assumptions you make. If water and air are incompressible and unreactive then mass conservation and volume conservation are the same thing (as density is constant), so the flow rate will be a+b. But if any phase is compressible, or reacts, or dissolves or one of many other things then the volume flow rate will not be a+b. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.