Elif Temiz, 030070195, 8th Week Definitions-Part 2

3-Effective Thermal Conductivity

New Definition (Better) (Material Property)

For a porous media filled (saturated) by a stagnant fluid, non reactive, and for an imposed temperature gradient when thermal regime reaches a steady state, there is uniform thermal flux. Thus, it's possible to define an effective, or equivalent thermal conductivity k_eff. Heat transfer is defined as a Fourier law:

Several authors have developed models to predict the effective thermal conductivity of foams in a purely conductive regime. These models are based on the association of the thermophysical properties of both solid matrix and fluid on the one hand, and certain structural parameters of the foam on the other hand (for example the ratio between the average strut length and the size of the strut-junction nodes). Bhattacharya et al. present a comprehensive analytical and experimental investigation for the determination of the effective thermal conductivity of high porosity metal foam. It is a based on a geometrical model of foam consisting on a two dimensional array of hexagonal cells where the fibers form the edge of the hexagon. Commercially available metal foams form a complex array of interconnected fibers . Depending on manufacturer, an irregular lump of metal may be found at strut intersection. Decker et al. provided a detailed experimental characterization and a numerical modeling of the heat and mass transport properties of highly porous media for solar receivers and porous burners. Pan et al. determined experimentally the effective thermal conductivities of aluminium and silicon alloy foams. Writz. et al. Presented a semi-empirical model for the combined conduction and convection heat transfers in a thin porous wall. Bastawros provided experimental measurements and modelling of the thermal and hydraulic aspects of cellular metals subjected to transverse airflow. Kim et al., also investigated numerically the anisotropy of permeability and effective thermal conductivity and its effect on the performance of an aluminium heat sink.

(J. M. P. Q. Delgado, Heat and Mass Transfer in Porous Media, p.230)

Previous Definition

A microstructure-sensitive property that mesures the rate at which heat is transferred through a material. Proper processing using sintering additives and the formation of desirable second phases and control of texture, high thermal conductivity AIN, SiC, Si3Ni4 have been developed.

(The Science And Engineering Of Materials, Donald R. Askeland, P.P. Phulé, pages: 801-802)