## Fluids

#### ways to calculate heat release of reaction in fluent.

• Weiqiang Liu
Subscriber

Hi all,

I am doing a new project. it's air flows through porous soot cake and reacts with soot. The local soot concentration is treated as an UDS in fluent. The reaction is very simple as follows:

C+O2-->CO2.

specific heat of carbon is constant and for other gas species, specific heat is temperature-dependent. I need to write UDF to define source term of enthalpy which arises because of carbon oxidation.

I have two ways to calculate reaction enthalpy.

Firstly: I would use C_H(c,t) to access enthalpy in the cell. Because enthalpy in a cell is mass-weighted, I can calculate enthalpy for oxygen and carbon dioxide. For carbon, the enthalpy is just Cp multiplied by ?T. Ultimately, heat release of the reaction is just enthalpy of the product minus enthalpy of reactants.

Secondly: Because I know specific heat of every species in the system. I can calculate enthalpy of formation by the following equation:

?H(0)+∫Cp(t)*dt. I don't need to use C_H(c,t) macro. I just need to use C_T macro to access temperature in the cell.

I wonder which method is correct or both of them are ok?

I need to confirm this and then go further.

PS: I received very kind help on this community and finished two projects.

Thanks very much!

Weiqiang.

• DrAmine
Ansys Employee
If you look in the way Fluent models reaction kinetics then it uses the second formulation. Just have a look into the energy equation in theory guide.

C_H is total enthalpy by the way.
• Weiqiang Liu
Subscriber

Hi Amine,

Do you mean I should use definite integration in UDF to calculate heat release of reaction. Because I know the function of specific heat which varies with temperature. Yes, I'll check theory guide of ansys.

Thanks very much!

Best.

Weiqiang

• DrAmine
Ansys Employee

You can make it simple and use species reaction.

• Weiqiang Liu
Subscriber

But it's not soot model in fluent. It's like fixed bed simulation and local carbon concentration is an UDS.

Best regards.

weiqiang

• DrAmine
Ansys Employee
Just simple species transport without any additional models.
• Weiqiang Liu
Subscriber

yes, but carbon is solid which is fixed in the system. I tried to use species transport to model carbon reaction. However, seems fluent can not choose solid as one of reactants. Because solid consumption is not considered in fluid governing equation I think while in reaction panel, every species are included in solving governing equation.

Best

Weiqiang

• DrAmine
Ansys Employee

Just assume it a fluid specie so that you can describe volume reactions.

• Weiqiang Liu
Subscriber

if I assume carbon as a fluid specie, than it must have convection and diffusion. However, in my model, soot is fixed in space. In other words, no convection and diffusion, just consuming source exists.

Best

Weiqiang