RÉSUMÉ

To establish a theory for finding methods of convective heat transfer enhancement actively not passively, one must connect convective heat transfer with energy dissipation in flow locally. Fluid flow consumes the same mechanical energy, but produces different convective heat transfer intensities. This is the reason that different thermal boundary conditions yield different convection intensities. Based on the transport equation of heat flux, in this paper, we explain the above phenomenon through comparisons of the differences between convective heat transfers through a tube at different thermal boundary conditions in terms of the roles of velocity gradient and velocity. The results show that the convection of flux not only depends on velocity and velocity gradient, but also depends on heat or mass flux and their gradient; if the velocity gradient makes a larger contribution to the convection of heat flux in one case rather than in another, the former will have a higher convection intensity (that is, the strength of convection, which is specified generally by the Nusselt number on the wall surface), even though the same energy is dissipated by fluid flow. The method used in this paper is useful in finding methods to enhance heat transfer.