Mechanistic Determinations of Enhanced Heat Transfer Occurring in Stagnant and Flowing Microfibrous Entrapped Catalytic Reactors

Abstract

Microfibrous media (MFM) is a fibrous material made via traditional paper-making techniques resulting in a media that is preferentially oriented within a single plane (but random within that plane). Metal MFMs, particularly copper MFM (Cu-MFM), are used in catalytic beds and are known to exhibit superior heat transfer performance to packed beds. To investigate the mechanisms for this, experiments were conducted in an 1 8-inch alumina pellet packed bed and a Cu-MFM bed composed of 17 mm diameter x 6 mm long fibers and 6 mm diameter x 3 mm long fibers. Heat transfer experiments were performed both with stagnant and flowing gasses as well as under vacuum. Existing literature models were compared to the experimental results and various resistance networks and unit cells are presented for modelling MFM. What was found is that the the presence of a gas within an MFM bed plays a substantial role in its effective thermal conductivity. Additionally, as has been previously shown for packed beds, as the thermal conductivity of the gas within an MFM bed increases, the effective thermal conductivity of the bed increases. It was found that if two fibers are modelled with a gas-filled gap, the length of the gap has a large impact on the effective thermal conductivity of the system. Therefore, it is proposed that the superior heat transfer performance of MFM over packed beds is due in part to the preferential orientation of the fibers as appears in prior literature, and in part due to many short gaps between fibers within the media that, when a fluid is present, provide excellent heat-transfer pathways.