Learning Forum

[mandeep_shetty]

Hello,

What is the difference between a 'solid/bulk species' and a 'site species' when using the 'Wall surface reaction' Species Transport model in Fluent?

I have read from the manual "Solid species are species that are deposit to, or etch from, wall boundaries or discrete-phase particles (e.g., Si(s)) and do not exist as fluid-phase species. If you are modeling wall surface reactions with site balancing, where species adsorb onto the wall surface, react, and then desorb off the surface, the Selected Site Species list will show all of the site species in the mixture."  This would mean understanding the difference between Solid and Site species entails understanding the difference between 'Deposition' and 'Adsorption'. But I could not find any concrete difference between deposition and adsorption (so how are they different?).

Ex: I read about a surface reaction where,after the reaction, Carbon is released as a Solid species. Does this mean it is released as a discreet particle !?. Why is it not 'desorption' of Carbon into the fluid phase?

[mandeep_shetty]

I would be really grateful for any info regarding this. Thank you.

[mandeep_shetty]

Still looking for any info on the difference between how the species behaves when set as a Site species and when set as Solid species. Like for example, Ar can be set as a site species which can adsorb onto a surface and desorb into the fluid phase...or..Ar can deposit as a Solid species. So how do deposition and adsorption differ and when to use which type of species?

[Rob]

The first bump was borderline, given we work across multiple time zones the second was unnecessary and potentially meant you'd have been waiting until tomorrow (my time) for a response. Consider this a friendly suggestion to look up "patience".
Given those links are also breaking copyright I'll delete those: all current Ansys documentation can be found by clicking on Help.

A solid is the bit where we only model heat transfer, and on occasion, bending with intrinsic FSI. It's the material assigned to walls in the boundary condition panel. Technically a granular material is a fluid, but it's state is solid. Similarly particles are "solid" but aren't in the solid properties, they're a DPM material.
A surface site species is used to react with the fluid phase and it's quantity may be limited.
Absorption, accretion, deposition can mean different things depending on which models are used.

[mandeep_shetty]

[mandeep_shetty]

Hey Rob Thank you for your time. I am sorry for the bumps, I am pressed for time to learn this.
So when we assign a material (say As created under fluid material) to a 'solid species' when running a surface reaction, and the material is modeled as given out as an etching ( say As is a product of the said surface reaction) then the resultant Arsenic etching (solid species) is simulated as a DPM?
When the "Species Transport" model with wall reaction is used what is the difference in behavior between Adsorption/Desorption (for site species) and Deposition/Etching (when using solid species)?

[Rob]

It depends on what models you've got switched on. I can use reactions to create a new species, remove material, change phase etc. What happens is entirely dependent on what you tell the solver to do. For chemical etching you're most likely converting a fluid+surface species to fluid species, however, that will remove reactant from the surface, it won't move the surface. If you want that you need to add moving mesh, but also be careful to tell Fluent how much surface material is remaining.
Setting the solver up is fairly simple, just follow the models, boundary conditions etc from top to bottom of the tree. The hard part is figuring out what to set up based on what you need to know, what you already know, how much time you have and whether you have enough cpu/resource to run the resulting case.

[mandeep_shetty]

[mandeep_shetty]

Hey Rob Again, thank you so much for helping me understand these concepts. I still have some questions about them. So I will take the help reaction that I found in a tutorial guide for Gallium and Arsenic deposition to put forth my queries. The model has Species Trasport and it's a Finite rate reaction with no turbulence effect on the rate of the reaction. 'Mass Deposition Source' is on.
The reaction mechanism is made of 2 reactions.
AsH3+Ga_s -> Ga+As_s+1.5H2 ...(1)
Ga(CH3)3+As_s->As+Ga_s+3CH3 ...(2)
Here Ga_s and As_s are set as Gallium and Arsenic site species, Ga and As are set as solid Species, and the rest are set as selected/fluid phase species.
Here are my questions
i) In eq 1 the As_s in the product is the Arsenic that is produced due to reaction but is not desorbed or floated away. It is sticking/adsorbed at the surface, right?
ii)In eq 1 the Ga on the product side is a solid species deposited at the surface. How is this Ga different than Ga_s since both are present at the surface? How does the 'deposited' Ga behave differently than the 'adsorbed' Ga_s?
iii)Theoretically, If I wanted to liberate Arsenic to float away with the flow then I would have to add a new species under the 'selected species' (as As_gas) and give the reaction as
AsH3+Ga_s -> Ga+As_gas+1.5H2 ?
If it was desorption then the reaction would be something like As_s ->As_gas ?

[Karthik Remella]

Hello Chemkin actually has 3 types of species for surface reactions: Gas-phase species, site or surface species, and bulk (solid) species. The difference between site and solid species is that the site species sits and reacts from a "site" on the bulk or solid species.Surface chemistry usually describes the reaction between a surface and the gas phase and can include only site species, only bulk species, or bothsite and bulk species. This link in the Chemkin doc gives a good description of the interaction site, bulk (solid), and gas-phase species:
4.1. Atomic vs. Open Site Reaction Formalism (ansys.com)
Hopefully, this answers your question.
Karthik

[mandeep_shetty]

Thank you so much for the info and the link @Karthik.