Site Species

Diger · April 25, 2016, 17:55

Hello all,
I've been looking into this for hours now and couldn't find it.
When enabling the wall surface reactions under species some fluid species reacts with the site species at the wall to some bulk species (not any further reacting species).
Now I have to define the site (or bulk) species somewhere...
Defining the fluid species was no problem, but I thought the site species would be some species I define under solid...
But doing this it doesnt show up in the template
mixture-template -> mixture species (Edit...)
where you add and remove materials, site sepcies and solid species.
So where do I create them????
THX in advance

Diger · April 26, 2016, 05:41

Ok, sry I just saw I have to define them as fluids...
Though I dont understand why for example in this tutorial:
http://www.engr.uconn.edu/~barbertj/...2012/tut16.pdf
one additionally needs to define Ga_s and As_s as site species when the entire process is due to thermal decomposition on the surface, am I right?
For example the first reaction is written as:
AsH3 + Ga_s -> Ga + As_s + 1.5 H2

??
Why do I need Ga_s as a reaction partner?
Also the As_s which is created in this reaction is then consumed in the second reaction?!
Ga(CH3)3 + As_s -> As + Ga_s + 3CH3

Why do I at all need these intermediate site species As_s and Ga_s (which are then consumed in the other reaction immediately) when in the end I'm only interested in Ga and As anyway ?!?!???

PS: What is a reasonable site density ?!

LuckyTran · April 26, 2016, 08:19

Quote:

Originally Posted by Diger

Why do I need Ga_s as a reaction partner?

It's a surface reaction with Ga_s as the surface species participating the reaction. Do you want a surface reaction or not?

Quote:

Originally Posted by Diger

Why do I at all need these intermediate site species As_s and Ga_s (which are then consumed in the other reaction immediately) when in the end I'm only interested in Ga and As anyway ?!?!???

This is a multi-step reaction with intermediate steps. Each individual elementary reaction has its own set of products and reactants. If you want correct detailed chemistry then you must provide all the individual elementary steps in the global reaction.

You can do a single step global reaction, but that is no where near as accurate as multi-step reaction. Real reactions are not single step! The individual steps in the reactants all involve different reactants and produce different products and each occur are different reaction rates, some are catalyzed others are not. The intermediate species might be consumed immediately, but they may not be produced immediately and they may not be produced at the same rate. Also, nothing says a reaction must be complete.

You choice of modelling the reaction is up to you; but you should not be surprised that reality is different than your modeled view of the universe. If you are doing single step reactions, then you shouldn't be using Fluent in the first place (at least a two-step reaction). You can already calculate the solution by hand if your reaction is a single step and do not need Fluent for this.

Diger · April 26, 2016, 09:24

How can I calculate it by hand!?!?
The process for example I want to be modelled is a loss process of some highly reacting species say OH reacting with some wall species...
If I do a singlestep reaction
OH + Killer -> Products
then how should I get more accurate results, when I try this in two steps !?!
Are you saying that do to the discrete nature of the solver a single step might be not as good??
I'm just interested in the OH concentration in the fluid anyway and how it gets lost. I doubt I will be able to calculate this loss rate by hand, since diffusion and the fluid motion itsself are already highly complicated + the reaction of the species in each cell at the boundary!?!
Do you have a quick example if apparently u can calculate it by hand?
Also:
What is the origin of a multistep being more accurate than a single step...
I see u need some species at the wall ur fluid species reacts with.
But in this example above I believe the dominating process is AsH3 hitting the wall and (maybe dependent on temperature) then splits up into 1.5H2 and As...I would think it should be enough to tell Ansys, that AsH3 has a rate (in the cells next to the wall) with which it reacts to the products...Independent of Ga_s or do u really need that stuff in reality?!?!?
Maybe I got it wrong, but that is not how I understood it...

Diger · April 26, 2016, 10:15

Quote:

Originally Posted by LuckyTran

This is a multi-step reaction with intermediate steps. Each individual elementary reaction has its own set of products and reactants. If you want correct detailed chemistry then you must provide all the individual elementary steps in the global reaction.

You can do a single step global reaction, but that is no where near as accurate as multi-step reaction. Real reactions are not single step! The individual steps in the reactants all involve different reactants and produce different products and each occur are different reaction rates, some are catalyzed others are not. The intermediate species might be consumed immediately, but they may not be produced immediately and they may not be produced at the same rate. Also, nothing says a reaction must be complete.

Yes I know. That I was aware of. I was just pointing out that I'm not aware if this decomposition AsH3 -> As + 1.5 H2 is elementary or not, because if its just thermal decomposition it should be, or?
The reaction with Ga_s I thought was just artificially introduced.

Diger · April 26, 2016, 11:58

Another quick question.
At the moment I get
Warning: mass imbalance in OH_LOSS stoichiometry.
Warning: mass imbalance in OH_LOSS stoichiometry.
Warning: mass imbalance in OH_LOSS stoichiometry.
Warning: mass imbalance in OH_LOSS stoichiometry.
Warning: mass imbalance in OH_LOSS stoichiometry.

I thought the molecular weights in the properties of each species have to sum up:
e.g.
OH + oh_reactant -> oh_product
molecular_weight(OH) + molecular_weight(oh_reachtant) = molecular_weight(oh_product)

But this seems to be not the case or whatelse could it be?

EDIT: Ok I think here the problem was that when changing something in the fluids doesnt change the number in the species taken over by the mixture-template...

Diger · May 3, 2016, 05:36

Do you still read? :-/
I was just wondering how you would think to calculate it by hand? I mean you need all the diffusion and flow which is done by ansys or not?

And can you elaborate a bit more on what you mean with:
OH + reactant -> product
is not a good way to model it?
How should you convert it to a 2 equation model?

April 25, 2016, 17:55	Site Species	#1
Diger Senior Member Join Date: Nov 2015 Posts: 135 Rep Power: 10	Hello all, I've been looking into this for hours now and couldn't find it. When enabling the wall surface reactions under species some fluid species reacts with the site species at the wall to some bulk species (not any further reacting species). Now I have to define the site (or bulk) species somewhere... Defining the fluid species was no problem, but I thought the site species would be some species I define under solid... But doing this it doesnt show up in the template mixture-template -> mixture species (Edit...) where you add and remove materials, site sepcies and solid species. So where do I create them???? THX in advance

April 26, 2016, 05:41		#2
Diger Senior Member Join Date: Nov 2015 Posts: 135 Rep Power: 10	Ok, sry I just saw I have to define them as fluids... Though I dont understand why for example in this tutorial: http://www.engr.uconn.edu/~barbertj/...2012/tut16.pdf one additionally needs to define Ga_s and As_s as site species when the entire process is due to thermal decomposition on the surface, am I right? For example the first reaction is written as: AsH3 + Ga_s -> Ga + As_s + 1.5 H2 ?? Why do I need Ga_s as a reaction partner? Also the As_s which is created in this reaction is then consumed in the second reaction?! Ga(CH3)3 + As_s -> As + Ga_s + 3CH3 Why do I at all need these intermediate site species As_s and Ga_s (which are then consumed in the other reaction immediately) when in the end I'm only interested in Ga and As anyway ?!?!??? PS: What is a reasonable site density ?! Last edited by Diger; April 26, 2016 at 07:41.

April 26, 2016, 11:58		#6
Diger Senior Member Join Date: Nov 2015 Posts: 135 Rep Power: 10	Another quick question. At the moment I get Warning: mass imbalance in OH_LOSS stoichiometry. Warning: mass imbalance in OH_LOSS stoichiometry. Warning: mass imbalance in OH_LOSS stoichiometry. Warning: mass imbalance in OH_LOSS stoichiometry. Warning: mass imbalance in OH_LOSS stoichiometry. I thought the molecular weights in the properties of each species have to sum up: e.g. OH + oh_reactant -> oh_product molecular_weight(OH) + molecular_weight(oh_reachtant) = molecular_weight(oh_product) But this seems to be not the case or whatelse could it be? EDIT: Ok I think here the problem was that when changing something in the fluids doesnt change the number in the species taken over by the mixture-template... Last edited by Diger; April 27, 2016 at 08:13.

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April 26, 2016, 09:24		#4
Diger Senior Member Join Date: Nov 2015 Posts: 135 Rep Power: 10	How can I calculate it by hand!?!? The process for example I want to be modelled is a loss process of some highly reacting species say OH reacting with some wall species... If I do a singlestep reaction OH + Killer -> Products then how should I get more accurate results, when I try this in two steps !?! Are you saying that do to the discrete nature of the solver a single step might be not as good?? I'm just interested in the OH concentration in the fluid anyway and how it gets lost. I doubt I will be able to calculate this loss rate by hand, since diffusion and the fluid motion itsself are already highly complicated + the reaction of the species in each cell at the boundary!?! Do you have a quick example if apparently u can calculate it by hand? Also: What is the origin of a multistep being more accurate than a single step... I see u need some species at the wall ur fluid species reacts with. But in this example above I believe the dominating process is AsH3 hitting the wall and (maybe dependent on temperature) then splits up into 1.5H2 and As...I would think it should be enough to tell Ansys, that AsH3 has a rate (in the cells next to the wall) with which it reacts to the products...Independent of Ga_s or do u really need that stuff in reality?!?!? Maybe I got it wrong, but that is not how I understood it...

May 3, 2016, 05:36		#7
Diger Senior Member Join Date: Nov 2015 Posts: 135 Rep Power: 10	Do you still read? :-/ I was just wondering how you would think to calculate it by hand? I mean you need all the diffusion and flow which is done by ansys or not? And can you elaborate a bit more on what you mean with: OH + reactant -> product is not a good way to model it? How should you convert it to a 2 equation model?