Heat transfer in high porosity open-celled metal foam with interstitial granular material.

摘要

The present basis of this work is a metal hydride application called the Thermal Cycling Absorption Process (TCAP). It consists of a long column containing a granular material called Pd/k (palladium (Pd) on kieselguhr). The effective thermal conductivity of Pd/k, as with most granular materials, is poor. Improving its thermal conductivity has many benefits since the absorption/desorption process must be rapid. High-porosity, open-celled, metal foam has been utilized in recent TCAP designs to enhance the thermal performance of the column with the Pd/k residing interstitially. The metal foam increases the parasitic thermal mass of the reactor bed and reduces the effective density of the active material; therefore, utilizing the metal foam requires optimization. The thermal characterization of the TCAP column has been empirical in nature, requiring tests of full-scale mockups. The overall goal of this research is to obtain a thermal design capability for foam---granular material systems with emphasis on the TCAP materials.; The interfacial heat transfer contact coefficients and surface areas are required in a multiple energy equation analysis. These are not well understood for TCAP and are investigated specifically. To accomplish this, an ideal three-dimensional foam model is presented that is shown to be self-consistent with respect to the foam intensive and extensive geometric properties and the thermal conductance. This was accomplished by comparing the foam model ideal geometry predictions to computed tomography measurements. Thermal conductance predictions using finite element analysis are compared to experimentally determined effective thermal conductivity measurements obtained in this study. The effective thermal conductivity of the column substrate material, kieselguhr, is measured versus pressure and gas type to address limitations in the literature. Correlations of this data indicate a sparsely connected intraparticle structure. The contact conductances of the foam---granular material system are also investigated: the wall-to-foam is investigated numerically based on existing empirical data and the kieselguhr-to-solid boundary is determined experimentally.; The combined heat transfer of the foam - granular material system is then analyzed numerically using data obtained for the individual components in this study. The effectiveness of the foam and local thermal equilibrium is investigated under various operating conditions.