Quantum graphs with mixed dynamics: the transport/diffusion case

We introduce a class of partial differential equations on metric graphs associated with mixed evolution: on some edges we consider diffusion processes, and on others transport phenomena. This yields a system of equations with possibly nonlocal couplings at the boundary. We provide sufficient conditions for these to be governed by a contractive semigroup on a Hilbert space naturally associated with the system. We show that our setting is also adequate to discuss specific systems of diffusion equations with boundary delays. (paper)