Analytical and Numerical Modeling of Fluid Flow and Heat Transfer through Open-Cell Metal Foam Heat Exchangers

摘要

In this thesis analytical and numerical investigations of fluid flow and heat transfer through open cell metal foam heat exchangers are presented. Primarily, different representative unit cell approximations, i.e, tetrakaidecahedron, dodecahedron and cubic are discussed. By applying the thermal resistance analogy, a novel formulation for evaluation of the effective thermal conductivity of metal foams is proposed. The model improves previous models based on cubic or hexagonal cells. By using computer tomography images of a nickel foam sample a realistic 3D geometry is created and the foam's geometrical properties (i.e., porosity and surface area to volume ratio) and effective thermal conductivity are obtained. By using the experimentally found values of permeability, Forchheimer coefficient and solid-fluid interfacial convection coefficient, mathematical models for fluid flow and heat transfer in metal foams are developed. Two different assumptions: local thermal equilibrium (LTE) and local thermal non-equilibrium (LTNE), are used. LTNE yields more accurate results.;A three-dimensional computational fluid dynamics (CFD) model of a metal foam is made and validated against the experimental data for a square cross sectional nickel foam heat exchanger channel heated from the side walls while cooling air passes through the foam. The simulations are carried out for constant temperature or heat flux and different foam materials with pore densities of 10 and 40 pores per inch. The results show that the bonding of the foam to the walls has a considerable impact on the heat transfer rate. Convective heat transfer coefficients in terms of Nusselt number as functions of Reynolds number are also obtained.;The design and CFD modeling of metal foam cross flow heat exchangers are also discussed. The results indicate both effectiveness and number of transfer units (NTU) for the metal foam heat exchangers are higher than those of a hollow channel, however, the effectiveness-NTU curves are identical.;The design and simulation of a metal foam heat shield for high temperature applications are presented. By locating the regions with maximum temperatures, through the CFD simulations, the design of the heat shield is optimized for both water-cooled and air-cooled models.