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Continuity in transport of diluted species

Posted Jun 27, 2017, 7:58 a.m. EDT 5 Replies

Daniel Hernández
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Hi:

I´m working in a 3D model of a pipe cross junction with different velocity conditions, about chlorine distribution throught de cross section (see attached image please). My model only consist of 2 inlets, 1 with clean water an 1 with a constant concentration of 250mol/m^3, and two outlets. When I run the simulation, I check the two outlets, resulting this:

Concentrations

inlet 1 = 0 mol/m3
inlet 2 = 250 mol/m3
outlet 1= 58.7643 mol/m3
outlet 2= 247.905551 mol/m3

A wanto to know if this is correct. I supose that the concentration sum of two inlets must be 250mol/m3, but the two outlets reach 360.66981 mol/m3. There exists other form to verify the continuity of species?

Thank you for reading.

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5 Replies Last Post Jun 29, 2017, 6:14 a.m. EDT
Lasse Murtomäki
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Posted: 7 years ago Jun 28, 2017, 2:53 a.m. EDT
How about the mass balance, i.e. what are the flow rates? I experienced troubles with mass balance until I used a denser mesh.

BR
Lasse
How about the mass balance, i.e. what are the flow rates? I experienced troubles with mass balance until I used a denser mesh. BR Lasse
Daniel Hernández
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Posted: 7 years ago Jun 28, 2017, 8:39 a.m. EDT

How about the mass balance, i.e. what are the flow rates? I experienced troubles with mass balance until I used a denser mesh.

BR
Lasse


Thanks for answer:

The flow rates for this example are these:

**INLETS**
inlet 1 = 6.553 lt/s
inlet 2 = 12.432 lt/s

inlet 1 + inlet 2 = 18.9851 lt/s

**OUTLETS**
outlet 1 = 8.473
outlet 2 = 10.492

outlet 1 + outlet 2 = 18.9647 lt/s

**difference between inlets, outlets = 0.0204 lt/s (0.107% error)

About flowrates, the continuity is reached with very slow error, this happens in all different flow conditions that I am simulating. The mesh in all cases is fine, predefined by comsol for turbulent conditions and the y+ = 11.1, at the same way, this is for turbulent flow conditions.

But I don´t know why the concentration continuity is not developed like this.

Thanks BR Lasse
[QUOTE] How about the mass balance, i.e. what are the flow rates? I experienced troubles with mass balance until I used a denser mesh. BR Lasse [/QUOTE] Thanks for answer: The flow rates for this example are these: **INLETS** inlet 1 = 6.553 lt/s inlet 2 = 12.432 lt/s inlet 1 + inlet 2 = 18.9851 lt/s **OUTLETS** outlet 1 = 8.473 outlet 2 = 10.492 outlet 1 + outlet 2 = 18.9647 lt/s **difference between inlets, outlets = 0.0204 lt/s (0.107% error) About flowrates, the continuity is reached with very slow error, this happens in all different flow conditions that I am simulating. The mesh in all cases is fine, predefined by comsol for turbulent conditions and the y+ = 11.1, at the same way, this is for turbulent flow conditions. But I don´t know why the concentration continuity is not developed like this. Thanks BR Lasse
Lasse Murtomäki
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Posted: 7 years ago Jun 29, 2017, 1:42 a.m. EDT
Your picture looks laminar flow. What is the Reynolds number? What are the dimensions of the picture?

Lasse
Your picture looks laminar flow. What is the Reynolds number? What are the dimensions of the picture? Lasse
Lasse Murtomäki
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Posted: 7 years ago Jun 29, 2017, 2:54 a.m. EDT
Actually your mass balance is quite good:

Inflow = (0.25 mol/L)*(12.432 L/s) = 3.108 mol/s

Outflow1 = (0.0587643 mol/L)*(8.473 L/s) = 0.481 mol/s
Outflow2 = (0.2479 mol/L)*(10.492 L/s) = 2.601 mol/s

Total outflow = 3.082 mol/s

Error is thus only 0.8%.
Actually your mass balance is quite good: Inflow = (0.25 mol/L)*(12.432 L/s) = 3.108 mol/s Outflow1 = (0.0587643 mol/L)*(8.473 L/s) = 0.481 mol/s Outflow2 = (0.2479 mol/L)*(10.492 L/s) = 2.601 mol/s Total outflow = 3.082 mol/s Error is thus only 0.8%.
Daniel Hernández
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Posted: 7 years ago Jun 29, 2017, 6:14 a.m. EDT

Actually your mass balance is quite good:

Inflow = (0.25 mol/L)*(12.432 L/s) = 3.108 mol/s

Outflow1 = (0.0587643 mol/L)*(8.473 L/s) = 0.481 mol/s
Outflow2 = (0.2479 mol/L)*(10.492 L/s) = 2.601 mol/s

Total outflow = 3.082 mol/s

Error is thus only 0.8%.


THANKS VERY MUCH LASSE MUTOMÄKI!!!

This is what i'm talking about, I didn't know the correct way to see my mass balance. I only maked simple sums of concentration, buy what I need to verify is a particle flow depending time, like you did.

Thanks again!

Daniel Hernández Cervantes
[QUOTE] Actually your mass balance is quite good: Inflow = (0.25 mol/L)*(12.432 L/s) = 3.108 mol/s Outflow1 = (0.0587643 mol/L)*(8.473 L/s) = 0.481 mol/s Outflow2 = (0.2479 mol/L)*(10.492 L/s) = 2.601 mol/s Total outflow = 3.082 mol/s Error is thus only 0.8%. [/QUOTE] THANKS VERY MUCH LASSE MUTOMÄKI!!! This is what i'm talking about, I didn't know the correct way to see my mass balance. I only maked simple sums of concentration, buy what I need to verify is a particle flow depending time, like you did. Thanks again! Daniel Hernández Cervantes

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