Estimation of effective thermal conductivity enhancement using foam in heat exchangers based on a new analytical model

Abstract

Thermal performance of open-cell metal foam has been investigated under low Reynolds number by comparing the heat transfer coefficient and thermal conductivity for the flow through a packed channel of high porosity metal foam to that of an open channel. In the case of Al-Air at porosity 0.971, the ratio of heat transfer coefficients is estimated to be 18.5 when the thermal conductivity ratio of foam matrix to fluid conductivity is 130. This demonstrates that the use of foam in the structure of conventional air coolers increases effective thermal conductivity, heat transfer coefficient and thermal performance considerably. To overcome the drawbacks of previous models, a new model to describe the effective thermal conductivity of foam was developed. The model estimates effective thermal conductivity based on a non-isotropic tetrakaidecahedron unit-cell and is not confined only to isotropic cases as in previous models. Effective thermal conductivity is a function of foam geometrical characteristics, including ligament length (L), length of the sides of horizontal square faces (b), inclination angle that defines the orientation of the hexagonal faces with respect to the rise direction (Θ), porosity, size, shape of metal lump at ligament intersections and heat transfer direction. Changing dimensionless foam ligament radius or height (d) from 0.1655 to 0.2126 for Reticulated vitreous foam -air (RVC-air) at Θ=π 4 and dimensionless spherical node diameter (e) equal to 0.339 raises effective thermal conductivity by 31%. Moreover, increasing Θfromπ/4 to 0.4π for RVC-air at d=0.1655 and e=0.339 enhances effective thermal conductivity by 33%.