Manipulating variables from 3d to 2d temporal

Hi: I'm working with a coupled heat transfer - momentum equations, but what I need to do is to use a dependent variable previously solved which depends on x, y and z axis in a 3D system, being transformed in a 2D temporal system. I mean first I've got a solved a 3D model variable u(x,y,z) then I need to use it as u(x,y,t) in a second model.

I need to do this in order to calculate accurate Nusselt numbers in the entrance, and I'm having out of memory issues, and using Weak Constraints on my T=cte boundary conditions, convergence issues when using a iterative solver.

Thank In advance.