Augmentation or Suppression of Natural Convective Heat Transfer in Horizontal Annuli Filled with Air and Partially Filled with a Porous Matrix Layer

A numerical investigation on natural convection heat transfer is devoted to a horizontal annular region configuration partially filled with a porous material. Insertion of an air gap conducive to heat transfer reduction or insertion of a porous substrate conducive to heat transfer enhancement are considered in the present work. Firstly, a parametric study focuses on the derived economic aspects related to an insulating situation in order to find the optimal conditions in terms of thermal performances. Numerical predictions demonstrate that an air gap separating a concentric porous layer from the annular space may lead to better thermal insulation characteristics when compared to the baseline case of a fully porous annulus. Secondly, when the porous substrate is used as an alternative to surface extension, numerical results demonstrate that substantial heat transfer augmentation can be achieved particularly with the insertion of moderate-to-thick porous materials. As far as economy is concerned, a thin porous layer can yield a better heat transfer improvement under high Rayleigh number conditions.