[yijecho]

Hi all,

I am using Fluent to model oxidation behaviors of porous graphite by water vapor in 2D. Please see the scheme for the image. A gas mixture of helium and water vapor flows through a channel, which has a porous graphite region on one side. There is a reaction between water vapor and graphite as C(s)+H2O(g)->CO(g)+H2(g).

I used the species model (species transport, reactions) and porous zone in the cell zone condition for the porous graphite. The mixture has helium, carbon (graphite), water vapor, carbon monoxide, and hydrogen as fluid materials. Helium is set as the last species. The reaction type is set to be "wall surface reaction". The surface-to-volume ratio was entered into the reaction tap of the cell zone condition for the graphite. The reaction is only taken place at the graphite.

Here, I have a question about how to introduce the reaction between the porous graphite and the water vapor. As I can add the amount of water vapor from the inlet setting for the flow to have a He-20vol%H2O composition, the flow has water vapor. However, I have no idea about introducing the porous graphite as a solid reactant. I can enter the porosity and viscous resistance for the porous zone, but it seems there is no option for the porous solid reactant.

As a result, I cannot find any carbon in the contour. It seems reasonable because I didn't include any carbon amount into the flow or the porous zone. What I understood is that the surface-to-volume ratio plays a role in increasing the reaction rate like a catalyst loading, not in introducing a presence of graphite in the porous zone. Am I right? Then, how can I define the porous graphite as a solid reactant that reacts with water vapor?

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[vinerm]

Surface-to-Volume ratio is important not in increasing the reaction rate, but providing the total area for wall surface reaction. However, you need to set up a little more

1. Add site and solid species in the material
2. Create reactions for the wall surface
3. Go to Mechanisms, create sites, and provide site densities

If you also want reactions on the wall boundaries, then those have to setup separately by going to wall boundary conditions. As far as reactions on the surface of the porous zone are concerned, just enable reactions in the cell zone, provide surface density value, and select mechanism.

[yijecho]

Hi Vinerm,

I appreciate your comments. I am trying to follow your suggestions but still get stuck. Can you please provide more details about the procedure?

Previously, carbon was set as a selected species. So, I changed it to a site species in Mixture species. I also created a site with the site density and the initial site coverage of 1 in Mechanisms. The reaction type was set as the wall surface. The graphite cell zone was activated with the porous zone and reaction.

With this setup, the result showed that water was present in the graphite zone as intended. However, the reaction products such as carbon monoxide and hydrogen were only observed outside of the graphite zone. It seemed the reaction was taken place at the boundary between the graphite zone and the channel zone. If water is present inside the graphite zone, there should be the reaction between water and graphite (surface of the porous graphite).

I wonder why there are no reaction products in the graphite zone. Did I miss anything in the setup?

By the way, when I set the carbon as a solid species, there was no water in the graphite zone.

I really look forward to hearing from you.

With best regard,
Yije

[yijecho]

I solved this problem by adding the solid carbon as a site species to the reaction. By adding proper a kinetic equation, the reaction will take place inside the porous material. Although a warning message came out as imbalance site species, the flow and reactions were described well.

[Withoutboss]

Originally Posted by yijecho

I solved this problem by adding the solid carbon as a site species to the reaction. By adding proper a kinetic equation, the reaction will take place inside the porous material. Although a warning message came out as imbalance site species, the flow and reactions were described well.

So is it correct that there is an imbalance site species warning?
My chemical formula is Co3O4(s)=3CoO(s)+0.5O2.I don't know how to set the site density value?Is it calculated by the mass of the reactant[Co3O4] or the mass of the product[CoO]?