The effective thermal conductivity of packed beds of metal powder

Abstract

In this thesis an investigation into the dependence of the thermal conductivity of a powder on its particle size is described. Measurements of the conductivity of six aluminium powders with interstitial air pressure from atmospheric to 10(^-5) cm. Hg, are reported. The conductivity of a powder is constant at higher pressures until the mean free path of the molecules of the gas becomes comparable to the size of the pore spaces of the powder. The conductivity then decreases with decreasing pressure to a small asymptotic value at low pressures. These results accord with those of previous workers. Measurements of conductivity with varying temperature are also reported and it is concluded that the heat transfer is due to conduction and that convection and radiative heat transfer are not important. The differences in conductivity of the six powders are explained by considering the mean particle size and the porosity of the powders. A finer powder tends to have a low conductivity because a temperature discontinuity can exist at each solid-gas interface. A powder which packstto: at high porosity also tends to have a low conductivity because the thermal resistance of a powder is duo to the presence of the voids. It is shown that the porosity of a packed powder depends on its particle size distribution and it is concluded, therefore, that the conductivity of a powder depends on both the mean particle size and the size distribution. This hypothesis is confirmed by further measurements on mixtures of two of the aluminium powders.