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Mass Flow Rate does not match to experimental data

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Old   January 13, 2020, 07:32
Smile Mass Flow Rate does not match to experimental data
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Hi guys!!! I have a such problem in CFX: i need to evaluate mass flow rate through the orifice with diameter equal 2 mm. In our experiment we have a total pressure 2.15 atm on inlet and 1 atm on the outlet. Our flow meter and analytical model of gas flow gives us values about 1.6 g/msec. And difference between experimental and analytical methods is about of 3-5%. But, when we trying realize flow in CFX and Fluent we have a values 1.3 g/msec. And my first question is - why? Maybe anyone can help me advice what to do next?

In Fluent i use compressible laminar flow and BC: Total Pressure on inlet 2.15 atm, Operating pressure 1 atm, Outlet - 0 atm. Gas - air - ideal gas/sutherland.

In CFX i use Air ideal gas, laminar flow or turbulent (it doesn't matter), total Pr on inlet 2.15 or Static Pr 1.15, outlet - opening BC, and reference pressure 0 atm.

The following formula is used in the analytical model: MF=P0*(gamma*mu/R*T0)^0.5*Area orifice*f(y) where P0 - Total pressure, Pa; gamma=Cp/Cv - adiabatic exponent for Air; R - universal gas constant; T0 - temperature, K; mu - molar weight, kg/mol; Area orifice, m^2; and f(y)=(2/(gamma+1))^(0,5*(gamma+1)/(gamma-1))

With bet regards, Denis!
Hope for your help
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Old   January 13, 2020, 09:44
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You text leads to confusion. Please provide a table what you have on your boundary conditions (with absolute pressures!) in reality, Fluent and CFX.
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Old   January 13, 2020, 23:23
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Quote:
Originally Posted by Gert-Jan View Post
You text leads to confusion. Please provide a table what you have on your boundary conditions (with absolute pressures!) in reality, Fluent and CFX.
Gert-Jan, ok, no problem. I attached file of BC in Excel file.

With best wishes!
Attached Files
File Type: xlsx BC Table.xlsx (9.4 KB, 4 views)
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Old   January 14, 2020, 05:38
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This is not what I asked. I asked Absolute Pressures. Not Gauge, Static or Total Pressures. Gauge pressure is also not objective. The value depends on how you measure it (Bernoulli....).

Nevertheless, could it be that in CFX you set Static Pressures whereas in FLuent you have Total Pressure. I would always set Total Pressure on the inlet in CFX.

Other source of difference can be anywhere. Is your case Laminar? What about grid, Energy model, etc, etc...

Last edited by Gert-Jan; January 14, 2020 at 17:05.
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