Intrinsic thermal conductivity and wall contact resistance effect in granular active carbon generators

摘要

It is acknowledged that the commercial success of adsorption heat pumps or refrigeration systems depend crucially on a good heat and mass transfer in the adsorbent bed. Good heat and mass transfer allow greater specific heating or cooling power which reduces capital costs, producing compact sorption generators, and as well reduces the cycle time of the system. In this study the intrinsic thermal conductivity and thermal contact resistance of vibrated grains and powder samples and binary mixtures of active carbon are investigated and compared using two different thermal conductivity measurement techniques: a flat plate steady state measurement and a hot tube transient measurement. This study draws conclusions on the way that particle size and distribution, binary mixture ratio and bulk density affect the overall thermal performance of the adsorbent. Different carbon grain sizes were tested, ranging from 1.68 mm to fine powder, 1 μm. For each grain size three binary mixtures (2/3 grains - 1/3 powder, 1/2 grains - 1/2 powder and 1/3 grains - 2/3 powder) and grains and powder on their own were tested. The binary mixtures show higher densities (up to 750 kg/m~3) and as a consequence higher thermal conductivities (0.32 W/(mK)) and lower wall contact resistances (equivalent air contact layer thickness of 0.005 mm) than only grains or powder samples.