Wrong mass flow rate in Fluent reports

ciprian · April 22, 2016, 05:22

I'm simulating heat transfer in supercritical nitrogen flowing in a 2mm diameter tube. The setup is 2D planar, steady state using the pressure-based solver. Material properties are defined in a UDF.
The inlet type is velocity-inlet while the outlet is pressure-outlet.
Inlet velocity is set to 1.074 m/s.
Taking into account the following
velocity at inlet = 1.074 m/s
fluid density at inlet = 558.158 kg/m^3
inlet surface area = 3.14e-6 m^2
the mass flow rate should be something like 0.001884 kg/s.

The problem is that under Reports/Fluxes/Mass flow rate/inlet Fluent gives a value of 1.1989 kg/s which is way too high.

Any idea on why is this happening?

I'm sure the UDF is fine because I've used it in similar simulations and the results are good. The mesh should be good as well because I've used something similar in other cases.

ciprian · April 22, 2016, 08:53

The mass flow rate in 2D planar is different from 2D axis-symmetric because of the way Fluent computes the surface area in these models.

Quote:

Originally Posted by ciprian

I'm simulating heat transfer in supercritical nitrogen flowing in a 2mm diameter tube. The setup is 2D planar, steady state using the pressure-based solver. Material properties are defined in a UDF.
The inlet type is velocity-inlet while the outlet is pressure-outlet.
Inlet velocity is set to 1.074 m/s.
Taking into account the following
velocity at inlet = 1.074 m/s
fluid density at inlet = 558.158 kg/m^3
inlet surface area = 3.14e-6 m^2
the mass flow rate should be something like 0.001884 kg/s.

The problem is that under Reports/Fluxes/Mass flow rate/inlet Fluent gives a value of 1.1989 kg/s which is way too high.

Any idea on why is this happening?

I'm sure the UDF is fine because I've used it in similar simulations and the results are good. The mesh should be good as well because I've used something similar in other cases.

LuckyTran · April 22, 2016, 11:39

Just to clarify. The mass flow for an axissymmetric simulation should be correct.

The issue is that, in 2D planar, mass-flow rate is at best ambiguous (really it's meaningless) as there is no 3rd dimension. In 2D, the mass-flow rate is calculated using the reference depth (in the reference values). The default reference depth is 1m.

April 22, 2016, 05:22	Wrong mass flow rate in Fluent reports	#1
ciprian New Member Ciprian Join Date: Oct 2015 Posts: 2 Rep Power: 0	I'm simulating heat transfer in supercritical nitrogen flowing in a 2mm diameter tube. The setup is 2D planar, steady state using the pressure-based solver. Material properties are defined in a UDF. The inlet type is velocity-inlet while the outlet is pressure-outlet. Inlet velocity is set to 1.074 m/s. Taking into account the following velocity at inlet = 1.074 m/s fluid density at inlet = 558.158 kg/m^3 inlet surface area = 3.14e-6 m^2 the mass flow rate should be something like 0.001884 kg/s. The problem is that under Reports/Fluxes/Mass flow rate/inlet Fluent gives a value of 1.1989 kg/s which is way too high. Any idea on why is this happening? I'm sure the UDF is fine because I've used it in similar simulations and the results are good. The mesh should be good as well because I've used something similar in other cases.

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April 22, 2016, 11:39		#3
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,754 Rep Power: 66	Just to clarify. The mass flow for an axissymmetric simulation should be correct. The issue is that, in 2D planar, mass-flow rate is at best ambiguous (really it's meaningless) as there is no 3rd dimension. In 2D, the mass-flow rate is calculated using the reference depth (in the reference values). The default reference depth is 1m.