# mass flux inlet

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 April 3, 2010, 12:23 mass flux inlet #1 New Member   Lawrence Join Date: Jul 2009 Location: England Posts: 28 Rep Power: 8 Hi all I am trying to use constant mass flux for the pipe inlet. What I am doing is: if the desired mass flux for water is 200 kg/(s m2), then set the water density as 1000, then the velocity as 0.2. After some time steps I checked the inlet mass flux, but it is not 200 but a very small value. Could somebody tell me what is going wrong with it? Thanks a lot! Lawrence

 April 5, 2010, 10:55 #2 Senior Member   Pauli Join Date: Mar 2009 Posts: 182 Rep Power: 8 1) Did you properly account for the inlet area. What you shared implies it is 1m^2. Seems like this is your most likely problem. 2) Did you set the flow switch to mass flux or velocity? Sounds like you want it to be mass flux. On the other hand I believe this switch is only important for compressible flow problems.

April 6, 2010, 12:35
#3
New Member

Lawrence
Join Date: Jul 2009
Location: England
Posts: 28
Rep Power: 8
Quote:
 Originally Posted by Pauli 1) Did you properly account for the inlet area. What you shared implies it is 1m^2. Seems like this is your most likely problem. 2) Did you set the flow switch to mass flux or velocity? Sounds like you want it to be mass flux. On the other hand I believe this switch is only important for compressible flow problems.

Yes, I have set the flow switch to mass flux and also I am simulating compressible idea gas.

The unit for mass flux is kg/(m2 s), so the result from density times velocity should be mass flux. So I think we do not need to consider inlet area. (Please let me know if I am wrong). But there is indeed a possibility, because I am not sure about the unit of mass flux in STAR-CD. Sometimes I plot graphs, the unit for mass flux is kg/s which is for mass flowrate. If the unit for mass flux in STAR-CD is kg/s, then we have to consider inlet area.

Could you please confirm what the mass flux unit in STAR-CD? Thanks a lot!

Lawrence

 April 6, 2010, 13:04 #4 Senior Member   Pauli Join Date: Mar 2009 Posts: 182 Rep Power: 8 The units for mass flux is kg/s. Inlet boundary mass flux is density * velocity * area. You can get inlet area from: bset news inlet area bset Be careful with the geometry scale factor - you need to convert boundary area to m^2. Your first post indicated you have water with density =1000kg/m^3. Then your second post indicates you are using compressible ideal gas. I'm confused. Those two statements are contradictory.

April 6, 2010, 14:55
#5
New Member

Lawrence
Join Date: Jul 2009
Location: England
Posts: 28
Rep Power: 8
Quote:
 Originally Posted by Pauli The units for mass flux is kg/s. Inlet boundary mass flux is density * velocity * area. You can get inlet area from: bset news inlet area bset Be careful with the geometry scale factor - you need to convert boundary area to m^2. Your first post indicated you have water with density =1000kg/m^3. Then your second post indicates you are using compressible ideal gas. I'm confused. Those two statements are contradictory.

Thanks Pauli.

I did mentioned water and compressible ideal gas. What I am doing is air-water two-phase flow simulation. In the pipe there are two phases (compressible air and water).

Lawrence

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