Porous media as a granular region
 

Hello,
How can i set the diameter of particles in porous media?

I think you are asking the wrong question. Otherwise, you can't set the particle diameter. In the theory of homogeneous media... the media is homogeneous so that there is no such thing as a particle with any shape.

You can use the heat transfer coefficient and drag coefficients from two-phase Eulerian formulations to construct the appropriate porous media models. Your volume fraction, particle diameter, etc. will translate into porous media parameters. Just assume the solids velocity is zero, so the relative velocity in those formulae are just the gas velocity.