# Mass flow rate

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 April 5, 2006, 18:04 Mass flow rate #1 eddie Guest   Posts: n/a Hello All, I have a mass flow rate (0.0468 kg/s) outlet result for a 2D model and need to apply this to a 3D model for inlet mass flow rate boundary. What does this mass flow rate means in term of 3D? How can I use this result and apply to the 3D model? Thank you

 April 6, 2006, 00:36 Re: Mass flow rate #2 carno Guest   Posts: n/a Was your 2D model axisymmetric? If yes then the mass flow rate is real. Apply directly to the 3D model.

 October 13, 2010, 23:45 contrast case #3 Member   Dokeun, Hwang Join Date: Apr 2010 Posts: 71 Rep Power: 7 hello~ everyone I have a another question about this post What if I have mass flow rate for 3d and i want to apply that flow rate to 2d axisymmetric casek for reducing computational cost?

 October 14, 2010, 01:33 #4 Super Moderator     Maxime Perelli Join Date: Mar 2009 Location: Switzerland Posts: 2,992 Rep Power: 30 Same date for axisymmetric and 3d. But you can also use velocity inlet, and check the massflow rate (report/) __________________ In memory of my friend Hervé: CFD engineer & freerider

 October 17, 2010, 05:54 #5 Member     Marta Lazzarin Join Date: Jun 2009 Location: Italy Posts: 69 Rep Power: 8 Dear All, I have simulated a combustion flow in CFX some time ago. It was possible to define a surface which was a wall, but from which some reacting species were inserted into the domain. This boundary was a wall+source, so that I was able to specify both mass fractions of the various species and the mass flow rate coming in; it was also possible to have a certain heat flux at that boundary. It was one of the parameters calculated during the simulation. Is there a similar condition in Fluent? 'cause i saw just traditional walls and mass flow inlet boundaries. If not, is there any other way to make Fluent do what i need? (which is basically introducing a certain amount of mass from the boundary, with specified mass fractions and calculating the heat flux without imposing it at the beginning). Thanks a lot Marta

 July 30, 2013, 14:42 Negative mass flow rate in everywhere except inlet #6 Senior Member   Tanjina Afrin Join Date: May 2013 Location: South Carolina Posts: 161 Rep Power: 4 Hi all, In my 2D model, I found negative mass flow rate in everywhere except in the inlet. What does it mean? Please enlighten me . Thanks in advance.

 July 30, 2013, 16:03 #7 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,107 Rep Power: 19 It means that the flow exits the computational domain.

July 30, 2013, 16:52
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Tanjina Afrin
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Quote:
 Originally Posted by flotus1 It means that the flow exits the computational domain.
Thank you flotus for your answer. Can you be a little bit more specific, I didn't understand what are you trying to mean, unfortunately .

 July 30, 2013, 17:27 #9 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,107 Rep Power: 19 Boundary faces like an inlet have an orientation, indicated by the normal vector of the face. For example the normal vector of the inlet face points towards the computational domain. If the flux vector is pointing in the same direction, the mass flow rate is positive (the flow enters the computational domain). For faces where the normal vector and the flux vector are pointing in opposite directions, the mass flow rate is negative. For an outlet this indicates that the flow is leaving the domain. rgd likes this.

July 30, 2013, 17:50
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Tanjina Afrin
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Quote:
 Originally Posted by flotus1 Boundary faces like an inlet have an orientation, indicated by the normal vector of the face. For example the normal vector of the inlet face points towards the computational domain. If the flux vector is pointing in the same direction, the mass flow rate is positive (the flow enters the computational domain). For faces where the normal vector and the flux vector are pointing in opposite directions, the mass flow rate is negative. For an outlet this indicates that the flow is leaving the domain.
Thank you Flotus for explaining this. That means for this model normal vector of the outlet face points outwards of the domain. Then it should be negative for all the model where flow is allowed to leaving the domain, right ?

 July 28, 2014, 22:39 negative mass flow rate at outlet #11 New Member   Join Date: Dec 2012 Posts: 23 Rep Power: 4 I am facing the same problem of negative mass flow rate & I don't quite understand what's meant by the fluid's leaving the domain. Isn't the direction of my flow supposed to be the same at all positions? and that's what I'm getting when I display the velocity vectors, then how come my mass flow rate is negative? Please help me understand this.

July 29, 2014, 04:17
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Quote:
 Originally Posted by Raghdinia I am facing the same problem of negative mass flow rate & I don't quite understand what's meant by the fluid's leaving the domain.
Suppose you are doing a simulation of water flowing through a pipe, flowing from left to right. At the left side, the water comes into your simulation, and Fluent will show a positive mass flow rate. At the right side, water goes out of your simulation, and Fluent will show a negative mass flow rate.
Quote:
 Isn't the direction of my flow supposed to be the same at all positions?
In general: no. There is no reason why your flow can not change direction (unless you are simulating a trivial situation).

 August 12, 2014, 09:15 #13 New Member   Join Date: Dec 2012 Posts: 23 Rep Power: 4 Thanks a lot Pakk, that was really helpful

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