About the use of fibrous materials in compact heat exchangers

Abstract

One way of increasing the exchange surface area of a conventional exchanger is to replace the fins by a porous structure. Under these conditions, the exchange surface area can reach very high values. However, this increase amplifies the pressure drop of the fluid circulating in the porous matrix. It is therefore necessary to determine the optimal parameters of the porous medium in order to maximize the heat transfer with regard to the pressure drop. The characteristics of two kinds of fibrous materials are discussed: randomly stacked fibers and metallic foams that are used in industrial systems. The randomly stacked fibrous materials present variations of porosity and permeability depending on fiber diameter and aspect ratio. The metal foams are commercial products made with aluminium that present very high porosities and permeabilities due to the arrangement of the solid matrix. The transport properties are analyzed for both materials: permeability, friction factors and effective thermal conductivity. Local and global Nusselt numbers as well as the entrance length are derived from experimental data obtained in rectangular sintered fibrous channels and in cross-flow heat exchangers. Finally, several aspects of two-phase flow case (i.e. liquid–vapor) are discussed.