2 component air

Diger · November 23, 2015, 07:03

In CFX there u can simply chose 2 component labeling of ur fluid under the default domain TAB.
Where do I find the corresponding thing in fluent?

Diger · November 24, 2015, 03:34

Sorry but Im not quite familiar with the program and just couldnt find the right option. In my model I have two inlets and the air coming from each inlet I wanted to label differently. It's just a matter of how this is implemented in fluent. As far as I understood multiphase is a different thing concerning 2 real phases, but in my case I do not have 2 phases just 2 (basically identical) gases.

ghost82 · November 24, 2015, 03:38

Hi,
I don't understand well your question.
If you want to have 2 identical fluids (air), named differently, in the material tab copy air two times, so to have identical properties, then rename one of them.

Diger · November 24, 2015, 05:42

Yeah but when running the simulation in the end I want to visualize the 2 different airparts, so that I can see how the 2 parts are mixing or not mixing well...
Is that even possible without fluent running the 2 different airparts seperately (or knowing before that he is supposed to "differ" the 2 fluids) ?
The labeling u were talking about is enough in this sense and later in the results tab I can label everything by what I just defined in the material tab in the solution part of the program? I thought this tab (Materials) was just to define materials/fluids but didnt have any further purpose if I didn't specifically choose the 2. fluid in some other part like multiphase or similar?!

ghost82 · November 24, 2015, 05:51

I think you need a mutiphase approach to visualize the mixing of the 2 gases, but I'm not sure 100%.

shakabrade · November 24, 2015, 15:20

For qualitative indicator of mixing, you can try to use streamlines. You don't need to define new zones or anything. Just use equal spacing at the inlet and see how streamlines go.

In CFD Post, you can define multiple streamlines and if you choose a different color for each, you can make it easier to see.

Diger · November 27, 2015, 05:26

So within the CFD Post I can specify the streamlines coming from a certain inlet to be in some color range and the other streamlines in a different color range?

ghost82 · November 27, 2015, 06:35

Yes, you can.

Diger · December 2, 2015, 12:33

Diger · December 2, 2015, 12:39

Jesus Christ..Why is the text gone after I inserted the pictures????
Cool now I have to write everything again:
To make it short:
These pictures show the 2 different inlets:
the small one has a set massflow of 0.003g/s
the top open inlet has an order of 0.5g/s
Now This result seems weird to me as one would expect mixing along the flow, but the simulation tells me the opposite?
Is this reasonable?
Im wondering if the equations incorporate the mixing or if the "selfdiffusion" has to be accounted for seperately or if these processes do not have anything to do with each other?
What in this sense does multiphase stand for?:
Is there meant to be 2 real phases like liquid and gaseous or can that also mean 2 "similar" gases ?!

brunoc · December 2, 2015, 14:02

Your results seem quite reasonable to me. You have a main inlet with a much higher velocity then you secondary inlet. There is no swirl. I see no reason for the fluids to mix more then they do.

If do want to see two different gases mixing, just do a multicomponent simulation (look for examples in the tutorials). The results should be pretty close to what you've got.

Cheers

Diger · December 3, 2015, 03:58

Thank you for your answer. But doesnt diffusion play a role in mixing?
Is this accounted for in the equations?

brunoc · December 4, 2015, 05:32

It does. But in a simulation with just one fluid, the only diffusion you have is viscosity, since there is no mass fraction gradient. But even so, unless diffusion is really high and/or gas properties really different, it seems like advection will be the main effect in your case.

But them again, you can just do a multicomponent simulation and check it.

Cheers

Diger · December 4, 2015, 12:28

Not quite sure whats wrong.
If I specify a mixture say
1. air
2. n2
in this order in the mixture-template then under the tab species of some inlet under boundary conditions the 2. one doesnt show up...Why is that?
Also in this case, I cant choose "Full Multicomponent Diffusion" ?!

brunoc · December 4, 2015, 12:32

Because you only need to solve the mass fraction of one of the gases. The mass fraction of the second one is 1 minus the other.

Diger · December 4, 2015, 15:32

Still 2 components implies diffusion, so why can't I choose "full multicomponent diffusion"

Diger · December 4, 2015, 15:49

Diger · December 4, 2015, 15:50

2 questions regarding the solving:

1. Whats this issue with the Mach number at the pressure-outlet-9???
Do I have to worry about it since the solution seemed to have converged...(The thing at the end is just another new run)

2. Why is he now only solving for air? Since I specified N2 as well shouldnt he also solve for N2 ?

Diger · December 4, 2015, 19:35

Just one other question:
When I specify the Inlet Mass Flow for example, Fluent already assumes that the flow has a stationary profile being zero on the border and higher in the middle....
I guess I could use velocity as boundary condition being fixed to a constant value (I want a plugflow) but then its not assured that the actually wanted massflow is constant. So how can I achieve both?

brunoc · December 7, 2015, 07:48

Quote:

1. Whats this issue with the Mach number at the pressure-outlet-9???
Do I have to worry about it since the solution seemed to have converged...(The thing at the end is just another new run)

Yes, you do have to worry. Lots of things can cause this: bad boundary conditions, bad fluid properties, bad quality mesh. Check your setup.

Quote:

2. Why is he now only solving for air? Since I specified N2 as well shouldnt he also solve for N2 ?

It's math. If

, then you only need to solve for

or

, not both. Also, you should read the documentation. Under section 7.1.1, "Species Transport Equation":

Quote:

When you choose to solve conservation equations for chemical species, ANSYS Fluent predicts the local mass fraction of each species,

, through the solution of a convection-diffusion equation for the $i^{th}$ species. This conservation equation takes the following general form:

$\frac{\partial}{\partial t} \left( \rho Y_i \right) + \nabla \cdot \left( \rho \vec{v} Y_i \right) = -\nabla \cdot \vec{J}_i + R_i + S_i$

where

is the net rate of production of species

by chemical reaction (described later in this section) and

is the rate of creation by addition from the dispersed phase plus any user-defined sources. An equation of this form will be solved for species where is the total number of fluid phase chemical species present in the system. Since the mass fraction of the species must sum to unity, the $N^{th}$ mass fraction is determined as one minus the sum of the solved mass fractions. To minimize numerical error, the $N^{th}$ species should be selected as that species with the overall largest mass fraction, such as

when the oxidizer is air.

Have you tried the tutorials? They're usually really good to help you get going.

Cheers

November 23, 2015, 07:03	2 component air	#1
Diger Senior Member Join Date: Nov 2015 Posts: 135 Rep Power: 10	In CFX there u can simply chose 2 component labeling of ur fluid under the default domain TAB. Where do I find the corresponding thing in fluent?

November 24, 2015, 03:38		#3
ghost82 Senior Member Rick Join Date: Oct 2010 Posts: 1,016 Rep Power: 26	Hi, I don't understand well your question. If you want to have 2 identical fluids (air), named differently, in the material tab copy air two times, so to have identical properties, then rename one of them. __________________ Google is your friend and the same for the search button!

November 24, 2015, 05:51		#5
ghost82 Senior Member Rick Join Date: Oct 2010 Posts: 1,016 Rep Power: 26	I think you need a mutiphase approach to visualize the mixing of the 2 gases, but I'm not sure 100%. __________________ Google is your friend and the same for the search button!

November 27, 2015, 06:35		#8
ghost82 Senior Member Rick Join Date: Oct 2010 Posts: 1,016 Rep Power: 26	Yes, you can. __________________ Google is your friend and the same for the search button!

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November 24, 2015, 03:34		#2
Diger Senior Member Join Date: Nov 2015 Posts: 135 Rep Power: 10	Sorry but Im not quite familiar with the program and just couldnt find the right option. In my model I have two inlets and the air coming from each inlet I wanted to label differently. It's just a matter of how this is implemented in fluent. As far as I understood multiphase is a different thing concerning 2 real phases, but in my case I do not have 2 phases just 2 (basically identical) gases.

November 24, 2015, 05:42		#4
Diger Senior Member Join Date: Nov 2015 Posts: 135 Rep Power: 10	Yeah but when running the simulation in the end I want to visualize the 2 different airparts, so that I can see how the 2 parts are mixing or not mixing well... Is that even possible without fluent running the 2 different airparts seperately (or knowing before that he is supposed to "differ" the 2 fluids) ? The labeling u were talking about is enough in this sense and later in the results tab I can label everything by what I just defined in the material tab in the solution part of the program? I thought this tab (Materials) was just to define materials/fluids but didnt have any further purpose if I didn't specifically choose the 2. fluid in some other part like multiphase or similar?!

November 24, 2015, 15:20		#6
shakabrade New Member Nikola Join Date: Dec 2014 Posts: 6 Rep Power: 11	For qualitative indicator of mixing, you can try to use streamlines. You don't need to define new zones or anything. Just use equal spacing at the inlet and see how streamlines go. In CFD Post, you can define multiple streamlines and if you choose a different color for each, you can make it easier to see.

November 27, 2015, 05:26		#7
Diger Senior Member Join Date: Nov 2015 Posts: 135 Rep Power: 10	So within the CFD Post I can specify the streamlines coming from a certain inlet to be in some color range and the other streamlines in a different color range?

December 2, 2015, 12:33		#9
Diger Senior Member Join Date: Nov 2015 Posts: 135 Rep Power: 10

December 2, 2015, 12:39		#10
Diger Senior Member Join Date: Nov 2015 Posts: 135 Rep Power: 10	Jesus Christ..Why is the text gone after I inserted the pictures???? Cool now I have to write everything again: To make it short: These pictures show the 2 different inlets: the small one has a set massflow of 0.003g/s the top open inlet has an order of 0.5g/s Now This result seems weird to me as one would expect mixing along the flow, but the simulation tells me the opposite? Is this reasonable? Im wondering if the equations incorporate the mixing or if the "selfdiffusion" has to be accounted for seperately or if these processes do not have anything to do with each other? What in this sense does multiphase stand for?: Is there meant to be 2 real phases like liquid and gaseous or can that also mean 2 "similar" gases ?!

December 2, 2015, 14:02		#11
brunoc Senior Member Bruno Join Date: Mar 2009 Location: Brazil Posts: 277 Rep Power: 21	Your results seem quite reasonable to me. You have a main inlet with a much higher velocity then you secondary inlet. There is no swirl. I see no reason for the fluids to mix more then they do. If do want to see two different gases mixing, just do a multicomponent simulation (look for examples in the tutorials). The results should be pretty close to what you've got. Cheers

December 3, 2015, 03:58		#12
Diger Senior Member Join Date: Nov 2015 Posts: 135 Rep Power: 10	Thank you for your answer. But doesnt diffusion play a role in mixing? Is this accounted for in the equations?

December 4, 2015, 05:32		#13
brunoc Senior Member Bruno Join Date: Mar 2009 Location: Brazil Posts: 277 Rep Power: 21	It does. But in a simulation with just one fluid, the only diffusion you have is viscosity, since there is no mass fraction gradient. But even so, unless diffusion is really high and/or gas properties really different, it seems like advection will be the main effect in your case. But them again, you can just do a multicomponent simulation and check it. Cheers

December 4, 2015, 12:28		#14
Diger Senior Member Join Date: Nov 2015 Posts: 135 Rep Power: 10	Not quite sure whats wrong. If I specify a mixture say 1. air 2. n2 in this order in the mixture-template then under the tab species of some inlet under boundary conditions the 2. one doesnt show up...Why is that? Also in this case, I cant choose "Full Multicomponent Diffusion" ?!

December 4, 2015, 12:32		#15
brunoc Senior Member Bruno Join Date: Mar 2009 Location: Brazil Posts: 277 Rep Power: 21	Because you only need to solve the mass fraction of one of the gases. The mass fraction of the second one is 1 minus the other.

December 4, 2015, 15:32		#16
Diger Senior Member Join Date: Nov 2015 Posts: 135 Rep Power: 10	Still 2 components implies diffusion, so why can't I choose "full multicomponent diffusion"

December 4, 2015, 15:49		#17
Diger Senior Member Join Date: Nov 2015 Posts: 135 Rep Power: 10

December 4, 2015, 15:50		#18
Diger Senior Member Join Date: Nov 2015 Posts: 135 Rep Power: 10	2 questions regarding the solving: 1. Whats this issue with the Mach number at the pressure-outlet-9??? Do I have to worry about it since the solution seemed to have converged...(The thing at the end is just another new run) 2. Why is he now only solving for air? Since I specified N2 as well shouldnt he also solve for N2 ?

December 4, 2015, 19:35		#19
Diger Senior Member Join Date: Nov 2015 Posts: 135 Rep Power: 10	Just one other question: When I specify the Inlet Mass Flow for example, Fluent already assumes that the flow has a stationary profile being zero on the border and higher in the middle.... I guess I could use velocity as boundary condition being fixed to a constant value (I want a plugflow) but then its not assured that the actually wanted massflow is constant. So how can I achieve both?