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-   -   Mass flow rate (http://www.cfd-online.com/Forums/fluent/40460-mass-flow-rate.html)

eddie April 5, 2006 18:04

Mass flow rate
 
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

carno April 6, 2006 00:36

Re: Mass flow rate
 
Was your 2D model axisymmetric? If yes then the mass flow rate is real. Apply directly to the 3D model.

dokeun October 13, 2010 23:45

contrast case
 
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?

-mAx- October 14, 2010 01:33

Same date for axisymmetric and 3d.
But you can also use velocity inlet, and check the massflow rate (report/)

Marta October 17, 2010 05:54

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

Tanjina July 30, 2013 14:42

Negative mass flow rate in everywhere except inlet
 
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.

flotus1 July 30, 2013 16:03

It means that the flow exits the computational domain.

Tanjina July 30, 2013 16:52

Quote:

Originally Posted by flotus1 (Post 442910)
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 :( .

flotus1 July 30, 2013 17:27

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.

Tanjina July 30, 2013 17:50

Quote:

Originally Posted by flotus1 (Post 442919)
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 ?

rgd July 28, 2014 22:39

negative mass flow rate at outlet
 
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.

pakk July 29, 2014 04:17

Quote:

Originally Posted by Raghdinia (Post 503506)
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).

rgd August 12, 2014 09:15

Thanks a lot Pakk, that was really helpful :)


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