Multiphase+Multicomponent mass fractions

NPat · February 24, 2013, 09:57

Hi,

I'm a new user of ANSYS FLUENT...

I'm trying to model a steady-state multiphase+multicomponent problem in ANSYS FLUENT 14.0. There is phase change in the model, so the composition of each phase changes along the domain. I have a question about the boundary conditions for mass fractions...

The inlet mass fractions are easy to understand and the inputs are straightforward... but, what about the mass fractions at the outlet?

FLUENT ask me to input mass fractions for each species in both phases at the outlet, but that is in fact what I'd expect it could calculate for me... Anyone has some ideia of how could I treat this problem? What could I input in the outlet BCs if I have no ideia about the expected value...

Best Regards.

beer · February 24, 2013, 12:43

Hi NPat
the mass fraction condition you set at the outlet boundary is the backflow mass fraction. If you have no backflow, you have nothing to worry about.

Greetings

NPat · February 24, 2013, 16:02

Thank you! Could I assure that there is no backflow in my problem setting up a pressure outlet boundary condition to zero?

Regards.

beer · February 24, 2013, 16:22

I don`t know what you mean with zero, zero pressure or zero mass fraction? You can't assure that there is noch backflow on a pressure oulet. One oppurtunity is, you could use a mass or velocity outlet and a pressure inlet instead. Or, and that is the best solution in my opinion, you could extend your mesh to a point where you can be sure not to generate backflow or it does't matter anymore. If you just don't want to have a backflow of one species you can set this species to zero, which is easy but unfortunatly not physical.
Greetings

NPat · February 24, 2013, 17:01

I meant zero (gauge) pressure at the outlet as a condition to avoid backflow. Thank you for your reply, Marcus.

beer · February 24, 2013, 18:04

You're welcome

NPat · February 25, 2013, 07:40

I have some additional questions about backflow and outlet mass fractions...

FLUENT tells me that I have backflow in my calculation when it displays the message "reversed flow at the outlet", right? So, let's say I can prevent backflow extending my domain and so on... in this case doesn't matter which value the species mass fractions are assigned... so could I put zero mass fractions for all species mass fractions (assuming no backflow)?

Best Regards.

February 24, 2013, 09:57	Multiphase+Multicomponent mass fractions	#1
NPat New Member Join Date: Sep 2012 Posts: 11 Rep Power: 14	Hi, I'm a new user of ANSYS FLUENT... I'm trying to model a steady-state multiphase+multicomponent problem in ANSYS FLUENT 14.0. There is phase change in the model, so the composition of each phase changes along the domain. I have a question about the boundary conditions for mass fractions... The inlet mass fractions are easy to understand and the inputs are straightforward... but, what about the mass fractions at the outlet? FLUENT ask me to input mass fractions for each species in both phases at the outlet, but that is in fact what I'd expect it could calculate for me... Anyone has some ideia of how could I treat this problem? What could I input in the outlet BCs if I have no ideia about the expected value... Best Regards.

February 24, 2013, 12:43		#2
beer Member Join Date: Dec 2012 Posts: 92 Rep Power: 14	Hi NPat the mass fraction condition you set at the outlet boundary is the backflow mass fraction. If you have no backflow, you have nothing to worry about. Greetings NPat likes this.

February 24, 2013, 16:22		#4
beer Member Join Date: Dec 2012 Posts: 92 Rep Power: 14	I don`t know what you mean with zero, zero pressure or zero mass fraction? You can't assure that there is noch backflow on a pressure oulet. One oppurtunity is, you could use a mass or velocity outlet and a pressure inlet instead. Or, and that is the best solution in my opinion, you could extend your mesh to a point where you can be sure not to generate backflow or it does't matter anymore. If you just don't want to have a backflow of one species you can set this species to zero, which is easy but unfortunatly not physical. Greetings NPat likes this.

February 24, 2013, 18:04		#6
beer Member Join Date: Dec 2012 Posts: 92 Rep Power: 14	You're welcome NPat likes this.

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February 24, 2013, 16:02		#3
NPat New Member Join Date: Sep 2012 Posts: 11 Rep Power: 14	Thank you! Could I assure that there is no backflow in my problem setting up a pressure outlet boundary condition to zero? Regards.

February 24, 2013, 17:01		#5
NPat New Member Join Date: Sep 2012 Posts: 11 Rep Power: 14	I meant zero (gauge) pressure at the outlet as a condition to avoid backflow. Thank you for your reply, Marcus.

February 25, 2013, 07:40		#7
NPat New Member Join Date: Sep 2012 Posts: 11 Rep Power: 14	I have some additional questions about backflow and outlet mass fractions... FLUENT tells me that I have backflow in my calculation when it displays the message "reversed flow at the outlet", right? So, let's say I can prevent backflow extending my domain and so on... in this case doesn't matter which value the species mass fractions are assigned... so could I put zero mass fractions for all species mass fractions (assuming no backflow)? Best Regards.