2 inlets, 2 outlets. Assessment of flow rate ratio.

Jiricbeng · May 21, 2018, 06:47

Hello,

Lets assume a domain with two inlets A and B and two outlets C and D.
Flow rate is set on A and B, static pressure is set on C and D.

How to assess the flow rate ratio A/B through C and analogically A/B through D (how much of the flow rate from A goes through C and D and how much of the flow rate from B goes through C and D)? Is this even possible in single phase steady state analysis to assess?
Thanks a lot.

ghorrocks · May 21, 2018, 07:28

Yes, this can be done single phase and is quite easy to do. Set up an additional variable with a convection equation. Set the value of the additional variable to 1 at A and 0 at B. The value of the additional variable at the outlets will then tell you the ratio of A/B at each outlet.

Jiricbeng · May 21, 2018, 08:13

Thank you very much for the answer, however could you please describe that more detailed, please?

1) Which convection equation? How shall the expression for the additional variable look like?
2) How can I set 0 at a surface A, B? By "if" statement in the expression..?

Please explain it more detailed, I have no idea yet how to carry that out.

ghorrocks · May 21, 2018, 08:28

I can't see a tutorial example to hand which shows how to set it up unfortunately. Have a look on the ANSYS customer webpage and see if you can get one from there. I won't have time to do a tutorial for you, if you can't find one you will have to read the documentation and work it out for yourself.

Jiricbeng · May 21, 2018, 10:30

Thank you very much for recommendation. I went through the tutorials and set the additional variable in CFX Pre. I set 1 on A, 0 on B. On C the value is 0.91, on D the value is 0.41. Obviously the sum C+D is not 1. Is it correct? What is the meaning of the numbers and what if I had ten inlets (A1, A2, ...A10), which numbers would I set ?

Gert-Jan · May 21, 2018, 13:02

Did you check that the imbalances of your add.variable are within 1%? If so, then "what goes in" equals "what goes out". If this is the case, then the values should sum up to 1 (as long as you use Mass Flow Average quantities).

You can create a plot in the solver manager that shows you the imbalances. Most convenient. Alternatively you read the output file. In the bottom part, you can find the balances. Probably you are way off. Don't be mislead by the residuals. These are completely useless as long as your imbalances are not wihtin 5%.

My experience is that the additional variables converge much slower than the others. I usually go to Pre and set a specific timescale factor for the aditional variable (Solver Control>Equation Class> Additional Varaible >Timescale Factor =10 (or 100).

ghorrocks · May 21, 2018, 20:13

Jiri, you are mistaken, C+D does not sum to 1. The values at the outlets simply say the proportion of fluid from A compared to the total fluid at that outlet. If you want to check the conservation of the additional variable you need to work out the mass flow rate of A at the inlet and outlet and check that balances.

Gert-Jan's comments about slower convergence on additional variables is correct, you should check the imbalances to check for this.

Jiricbeng · May 22, 2018, 04:33

Thank you both for the reply.
Well my question about C+D assumed converged solution of course (I know imbalances monitoring etc).
Yes you are right Glenn, the C+D does not sum to 1. C+D sums to 1 if defined inlet flow rate A and B are equal.

Please, what if I had three inlets, or more, such as A1, A2, A3 and two outlets B1 and B2? Which values of additional variable to set on the inlets A1, A2, A3? Shall it be 0, 1, 2 ??? Then, how to determine the flow rate ratio?

Gert-Jan · May 22, 2018, 05:24

If you use a single additional variable, with 1, 2, 3, etc, on each inlet, mixing of these numbers will make it impossible to determine how much of each fluid goes to outlet B1 or B2. You need to set a new additional variable for each inlet. So ten inlets means ten additional variables (or maybe n-1). Only then you can trace where which fluid goes.

Jiricbeng · May 22, 2018, 06:29

Thank you very much indeed Mr. Jan-Gert !

May 21, 2018, 06:47	2 inlets, 2 outlets. Assessment of flow rate ratio.	#1
Jiricbeng Senior Member Jiri Join Date: Mar 2014 Posts: 221 Rep Power: 13	Hello, Lets assume a domain with two inlets A and B and two outlets C and D. Flow rate is set on A and B, static pressure is set on C and D. How to assess the flow rate ratio A/B through C and analogically A/B through D (how much of the flow rate from A goes through C and D and how much of the flow rate from B goes through C and D)? Is this even possible in single phase steady state analysis to assess? Thanks a lot.

May 21, 2018, 07:28		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	Yes, this can be done single phase and is quite easy to do. Set up an additional variable with a convection equation. Set the value of the additional variable to 1 at A and 0 at B. The value of the additional variable at the outlets will then tell you the ratio of A/B at each outlet. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

May 21, 2018, 08:28		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	I can't see a tutorial example to hand which shows how to set it up unfortunately. Have a look on the ANSYS customer webpage and see if you can get one from there. I won't have time to do a tutorial for you, if you can't find one you will have to read the documentation and work it out for yourself. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

May 21, 2018, 20:13		#7
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	Jiri, you are mistaken, C+D does not sum to 1. The values at the outlets simply say the proportion of fluid from A compared to the total fluid at that outlet. If you want to check the conservation of the additional variable you need to work out the mass flow rate of A at the inlet and outlet and check that balances. Gert-Jan's comments about slower convergence on additional variables is correct, you should check the imbalances to check for this. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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May 21, 2018, 08:13		#3
Jiricbeng Senior Member Jiri Join Date: Mar 2014 Posts: 221 Rep Power: 13	Thank you very much for the answer, however could you please describe that more detailed, please? 1) Which convection equation? How shall the expression for the additional variable look like? 2) How can I set 0 at a surface A, B? By "if" statement in the expression..? Please explain it more detailed, I have no idea yet how to carry that out.

May 21, 2018, 10:30		#5
Jiricbeng Senior Member Jiri Join Date: Mar 2014 Posts: 221 Rep Power: 13	Thank you very much for recommendation. I went through the tutorials and set the additional variable in CFX Pre. I set 1 on A, 0 on B. On C the value is 0.91, on D the value is 0.41. Obviously the sum C+D is not 1. Is it correct? What is the meaning of the numbers and what if I had ten inlets (A1, A2, ...A10), which numbers would I set ?

May 21, 2018, 13:02		#6
Gert-Jan Senior Member Gert-Jan Join Date: Oct 2012 Location: Europe Posts: 1,928 Rep Power: 28	Did you check that the imbalances of your add.variable are within 1%? If so, then "what goes in" equals "what goes out". If this is the case, then the values should sum up to 1 (as long as you use Mass Flow Average quantities). You can create a plot in the solver manager that shows you the imbalances. Most convenient. Alternatively you read the output file. In the bottom part, you can find the balances. Probably you are way off. Don't be mislead by the residuals. These are completely useless as long as your imbalances are not wihtin 5%. My experience is that the additional variables converge much slower than the others. I usually go to Pre and set a specific timescale factor for the aditional variable (Solver Control>Equation Class> Additional Varaible >Timescale Factor =10 (or 100).

May 22, 2018, 04:33		#8
Jiricbeng Senior Member Jiri Join Date: Mar 2014 Posts: 221 Rep Power: 13	Thank you both for the reply. Well my question about C+D assumed converged solution of course (I know imbalances monitoring etc). Yes you are right Glenn, the C+D does not sum to 1. C+D sums to 1 if defined inlet flow rate A and B are equal. Please, what if I had three inlets, or more, such as A1, A2, A3 and two outlets B1 and B2? Which values of additional variable to set on the inlets A1, A2, A3? Shall it be 0, 1, 2 ??? Then, how to determine the flow rate ratio?

May 22, 2018, 05:24		#9
Gert-Jan Senior Member Gert-Jan Join Date: Oct 2012 Location: Europe Posts: 1,928 Rep Power: 28	If you use a single additional variable, with 1, 2, 3, etc, on each inlet, mixing of these numbers will make it impossible to determine how much of each fluid goes to outlet B1 or B2. You need to set a new additional variable for each inlet. So ten inlets means ten additional variables (or maybe n-1). Only then you can trace where which fluid goes.

May 22, 2018, 06:29		#10
Jiricbeng Senior Member Jiri Join Date: Mar 2014 Posts: 221 Rep Power: 13	Thank you very much indeed Mr. Jan-Gert !