[rajcoep.88]

Hi,

I tried to search about it before posting my doubt. But, I could not find anything which would clarify the things that are bugging me. It might seem like stupid question. But, I am new to CFD field.

When one does simulation of internal incompressible flow, then at inlet one should assign Mass flow rate or volume flow rate or velocity and gage pressure 0 at outlet. Lets say I want to assign Velocity at inlet. So this velocity is Standard or Actual Velocity

I have done test of pneumatic check valve. Flow meter gives reading of 6 Standard LPM (Standard Conditions written on flow meter are 20deg C and 1 Atm) and Pressure Drop across valve is 50 hPa. I want to verify same using Simulation. From Q=AV, I can calculate velocity. Pipe Internal Diameter is 4mm. But, people say standard and actual flow rates are different in case of air flow from what I have read. So which flow rate should I use to calculate Velocity so that I can give it as inlet boundary condition?

Thanks in advance!

Best Regards,
Rajdeep Rajput

[ghorrocks]

Why not just specify the inlet condition with the parameters you know. If you use a Mass Flow rate inlet you can use some CEL to set the MFR to 6 [l/min]*1.225[kg m^-3]

You will need to change the density to the density you wish to use for your conditions.

When you use your input parameters directly like this it is clearer what you are doing and you get less mistakes.

[rajcoep.88]

Thanks for the quick reply! Please see the attached images to understand the things I am gonna talk about.

I am using Autodesk CFD for simulation. There we have one option "Environment Scenario". There we have to set reference. The Flow meter I am using has P=1Atm, T=20deg C and density=0.001204g/cc written on its glass body. So, I set Pressure and Temperature as 101325Pa and 293.15K. Then, applied air material to fluid domain and asked to use reference for density to vary. In the box, density updates to 1.20412E-6g/mm3.

Now, for inlet BC I did calculations like this:
Standard Flow Rate:6Nl/min , Density at STD:1.20412E-6g/mm3
So using above data Mass flow rate: 0.12041g/s

I put it as inlet BC outlet BC P=0 gage. I did simulation. Pressure drop almost matches to experimental data. But, when I check back the volume flow rate, mass flow rate and density at inlet. They are not equal to what I have given. Density changes to 1.29228E-6g/mm3 (in the software one gets it at 0deg C). Temperature in overall fluid domain is 0degC. Mass flow rate is calculated using this density. So, it does not matches the input value and flow rate comes out to be 5.6l/min.

Now, in CFD softwares is it supposed to happen like this? Am I doing everything right? Or am I missing something here?

Please comment. I am really confused from last two days. I cannot get past this thing.

Best Regards,
Rajdeep Rajput

[turbo]

First of all, you posted the question in the wrong forum.

Think about physical meaning of the outlet BC you set as 0 gauge pressure under your expectations at inlet based on STD.

[rajcoep.88]

Sorry for posting in this forum. But I thought people using Cfx might know the answer or can help me. Outlet BC is static gage. Many tutorials have set outlet like that if one wants to check pressure drop. When I did the test, outlet pipe was open to atmosphere. That's why I am setting outlet BC as 0 static gage.

[turbo]

Try another run with the outlet BC as static pressure smaller than atm.

[rajcoep.88]

OK. Thanks for the reply. I will run the simulation and would let you know. It might take a while as model is big and takes a while to finish till convergence.