Darcy Law using PDE

Hi,

I'm trying to implement the Continuity Equation with Darcy law and the Energy equation using a PDE (base package). The Darcy equation in the continuity is u=-(k/mu)*d(p,x) where k is the permeability, mu is the viscosity, and p is the pressure varying with x. However, when I try to implement u in the Energy equation, the term (rho*Cp*u)dT/dx doesn't not work well. I've seen some examples using u.dl, but I'm not sure what is dl. When I try to add it in the equation, the model doesn't recognize dl.

Does anyone has any clue?

Thanks.

Marco