Mass Flow Rate does not match to experimental data

RybinDenis · January 13, 2020, 07:32

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

Gert-Jan · January 13, 2020, 09:44

You text leads to confusion. Please provide a table what you have on your boundary conditions (with absolute pressures!) in reality, Fluent and CFX.

RybinDenis · January 13, 2020, 23:23

Quote:

Originally Posted by Gert-Jan

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!

Gert-Jan · January 14, 2020, 05:38

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...

January 13, 2020, 07:32	Mass Flow Rate does not match to experimental data	#1
RybinDenis New Member Novosibirsk Region Join Date: Jan 2020 Posts: 2 Rep Power: 0	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(gammamu/RT0)^0.5Area orificef(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

January 14, 2020, 05:38		#4
Gert-Jan Senior Member Gert-Jan Join Date: Oct 2012 Location: Europe Posts: 1,928 Rep Power: 28	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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January 13, 2020, 09:44		#2
Gert-Jan Senior Member Gert-Jan Join Date: Oct 2012 Location: Europe Posts: 1,928 Rep Power: 28	You text leads to confusion. Please provide a table what you have on your boundary conditions (with absolute pressures!) in reality, Fluent and CFX.