# Modifying the energy equation in Fluent

Hi,

I use non-premixed combustion model and I want to perform some changes in the diffusion term of the energy equation. I know that I have to use a UDF file to do that but I'm facing some difficulties in choosing the required macro to perform these changes. For example, if I write a UDF file using DEFINE_DIFFUSIVITY, Fluent will link this UDF to the UDS transport equation NOT the energy equation. So, Can you please give me some recommendations to modify the diffusion term (or any term) in the energy equation (or any equation) in terms of choosing the required macro.

Thank you.

## Answers

417Fluent will link this to the equation you hook the UDF to

8Thank you @YasserSelima for your reply.

In the UDF manual, there is not at least a hint about how to hook DEFINE_DIFFUSIVITY to the energy equation. If you know how to do it, I will be happy to hear from you.

4178without using the species transport model or defining a UDS transport equation, you cannot hook this Macro because the mass diffusivity option will not be available in the material panel.

417So you need to define your own UDS :(

417Can you use define property and change the thermal properties of your fluid?

8,371By strict definition would energy diffuse? Conduct and convect yes: species diffuse rather than conduct. So, as @YasserSelima comments you may want to look at the material properties.

8@YasserSelima

UDS equation is different from the energy equation.

I got your point regarding the thermal properties and will try this route.

@Rob, I got what you meant and will try this route.

Many thanks both for your replies. :)