Numerical results of the determination, by the finite difference method, of the temperature profile in a parallel plates duct containing an incompressible laminar or turbulent fluid in forced convection is presented. Different heat transfer conditions are considered as well as a time-dependent heat flux condition on the top metal plate. The coupling between conduction and convection is carried by the ‘solid-fluid’ interface. The heat transfer coefficient between plate and fluid is found at each point and time by a heat balance equation on the top plate. The results are compared with those found employing a simplified model MS and proved by the infra-red thermography results.