Convert 3D Mass flow rate to 2D planner case.

  • Last Post 28 February 2019
  • Topic Is Solved
RajuMath posted this 27 February 2019


I have mass flow rate 100 Nm3/hr through a lance for 3D geometry. How can I convert the given mass flow rate in 2D planner cases. This is required for setting boundary condition. Definitely, in 2D planner case the mass flow rate will be changed. Are there any formula to do it?


Another query is: in 2D planner case, what would the assumption for third dimension. Is it per meter or infinity?

Order By: Standard | Newest | Votes
rwoolhou posted this 27 February 2019

In 2d the model is 1m thick.  Re the mass flow: scale by area.  However, you need to make sure the numbers you're adding into the 2d model are sensible and if the flow is incompressible use a velocity boundary. 

RajuMath posted this 27 February 2019

A nozzle with diameter 50mm is connected with a lance with diameter 200mm. Given that, gas flow rate 2.5 kg/s through the lance with 5 nozzles. As I am considering 1 nozzle, for 1 nozzle gas flow rate is 2.5 kg/s /5 =0.5kg/s.  Now I am calculating the velocity for lance inlet from mass flow rate using

m_dot=rho*A*V; rho=1.138 kg/m3 gives V=0.5/(1.138*0.00785)=55.97m/s.

The same mass will flow through the nozzle exit. If I calculate the velocity at the nozzle exit

m_dot=rho*A*V; rho=1.138 kg/m3 gives V=0.5/(1.138*0.0019)=231.24m/s


In my simulation, I am using incompressible flow and velocity inlet boundary condition for 2D planner case. After converging, I am getting the nozzle exit velocity around 70 m/s.

What is the reason of this difference? Are there anything wrong in my above calculation?

abenhadj posted this 27 February 2019

Check the mass flow reported by Fluent in Flux Report.

Best regards,


RajuMath posted this 27 February 2019

Mass flow rate in fluent is ok and giving the velocity 70 m/s. But my question is how I am getting 231 m/s velocity from my above calculation? I also expect 70m/s velocity from my calculation.



abenhadj posted this 28 February 2019

Now use the reported mass flow rate from Fluent divide by the Area reported by Fluent and density.

My colleague has already said that the depth in 2D planar cases is 1 m. You can change that in the reference values to something else.

Best regards,