# mass flow rate in fluent vs manual calculation m = rho.v.A

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May 1, 2022, 08:47
mass flow rate in fluent vs manual calculation m = rho.v.A
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New Member

FaridW
Join Date: Oct 2021
Posts: 4
Rep Power: 4
I simulated a simple shape of a pipe with different diameters to understand the mass flow rate between fluent and the formula m = rho.v.A

In the simulation I used the working fluid R141B (NIST Real gas), with a temperature is 120.6 Celsius and 8 bar.
The simulation results show that the density varies at the inlet so that it reaches a value of 4.8 kg/m3. (as shown in picture)
With a mass flow rate is 0.46 kg/s

But if I use the formula , m = rho(density). v . A
Where the density (120.6Celsius, 8bar) I get using the REFPROP application is 38.161kg/m3 (as shown in picture)
velocity V (obtained from the simulation is 298 m/s)
A (cross-sectional area) = 20 mm = 0.000314 m2
So , m = 38.161* 298 * 0.000314
m = 3.57 kg/s

mass flow rate,
fluent result = 0.46 kg/s
calculation result m=rho.v.a = 3.57kg/s

I don't understand this, why it's so different ??
Please, help me.i don't know which one that i can use for my research?
Attached Images
 SS.jpg (59.8 KB, 21 views) SS2.png (16.4 KB, 19 views) SS3.png (108.2 KB, 12 views)

 May 1, 2022, 13:05 #2 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,712 Rep Power: 66 First figure out why the densities are different. Make sure you have the same absolute static pressure in both cases so that you have the same density. If you didn't make a clerical error then it means the profiles over the boundary are not uniform and it is not appropriate to calculate using averages.

 Tags density, mass flow rate, turbine