  We introduce a diffuse interface model describing the evolution of a mixture
of two different viscous incompressible fluids of equal density. The main
novelty of the present contribution consists in the fact that the effects of
temperature on the flow are taken into account. In the mathematical model, the
evolution of the macroscopic velocity is ruled by the Navier-Stokes system with
temperature-dependent viscosity, while the order parameter representing the
concentration of one of the components of the fluid is assumed to satisfy a
convective Cahn-Hilliard equation. The effects of the temperature are
prescribed by a suitable form of the heat equation. However, due to quadratic
forcing terms, this equation is replaced, in the weak formulation, by an
equality representing energy conservation complemented with a differential
inequality describing production of entropy. The main advantage of introducing
this notion of solution is that, while the thermodynamical consistency is
preserved, at the same time the energy-entropy formulation is more tractable
mathematically. Indeed, global-in-time existence for the initial-boundary value
problem associated to the weak formulation of the model is proved by deriving
suitable a-priori estimates and showing weak sequential stability of families
of approximating solutions.
