Natural convection around a horizontal solid cylinder wrapped with a layer of fibrous or porous material

Heat losses in duct flow and heat transfer enhancement are investigated through an analysis of natural convection about a horizontal cylinder with a porous or fibrous coating. The porous substrate may be used for two purposes. According to its properties, it may be employed as an insulating material or as a means to surface augmentation. An optimization study is then carried out in order to find the best conditions that allow good thermal insulation or heat transfer rate improvement. The flow motion and heat transfer coefficient are predicted for various conditions. The results show that an efficient insulation which means less than 10% in heat losses is obtained for a porous layer thickness of 0.8× tube diameter and a permeability corresponding to Da ⩽ 10−7. Nevertheless, there is a Darcy number limit above which convection must be accounted for. Porous or fibrous materials may also be used as a heat transfer augmentation technique. To achieve this goal, porous media with high permeability and/or high effective thermal conductivity must be selected.