CFD analysis of flow and heat transfer through natural convection in co-axial cylinders with effect of shape of cylinder

Abstract

Natural convection from isolated sources has been investigated for years, and heat transfer correlations for most basic geometries can be retrieved from heat transfer handbooks. Natural convection in enclosures has also been the subject of intensive research efforts, and its fundamentals are now summarized in monographs and review works. In such a context, major problems are the prediction of the flow regimes and patterns, and the effect of confinement on heat transfer performances. This, in turn, implies the reliability of heat transfer correlations as derived for isolated sources. Buoyancy-induced flows play a central role in a number of practical applications, including environmental thermal control, nuclear design, solar heating, and the cooling of electronic devices. More generally, almost all technologies involving passive heat transfer as the main source of thermal dissipation rely upon natural convection effects. The Boussinesq approximation for buoyancy has successfully been used in analytical and numerical studies of natural convection. By applying this approximation, two dimensionless variables are present in the non-dimensionalized governing equations, i.e. the Grashof (or Rayleigh) and Prandtl numbers. Parametric studies on Grashof and Prandtl-number dependence of steady-state and unsteady features of natural convection can been presented over a wide range of both numbers. Numerical results of natural convection in enclosures can also been extensively documented for various geometries and boundary conditions. Numerical simulations were performed for three values of the ratio between the cylinder radius and the cavity side, the aspect ratio, and four values of the Rayleigh number. The finite-volume method adopted involved the use of structured boundary-fitted quadrilateral meshes, to solve the momentum and energy equations in their steady-state formulation. Flow patterns and thermal fields in all configurations were presented, alongside with profiles of the average Nusselt number in between the enclosure walls and the cylinder surface.