The performance and properties of novel desiccant coated heat exchange surfaces for solar air conditioning
Spillmann, Thorsten S.
This work deals with the preparation, thermo-hydraulic characterisation, and performance analysis of silica gel coated highly conductive surface enhancing structures to be used as tube inserts in a prototype of an innovative water-cooled sorption rotor. The candidate inserts under investigation comprise highly porous aluminium foam inserts, twisted-in wire brushes, and flocked structures, that are investigated for their flow impedance, heat transfer performance, and cyclic dehumidification performance. The conducted analysis comprises experimental testing of insert specific pressure drop and heat transfer performance in a purpose built test rig, that led to the preselection of the foam structures and a twisted-in aluminium wire brush insert for desiccant coating and further investigation. Cyclic heat and mass transfer tests were performed in a purpose-built small-scale test rig, that simulated the dehumidification process of a desiccant rotor with and without employing water-cooling. The experimental analysis is complemented by a numerical investigation of the cyclic heat and mass transfer performance of the brush and metal foam type structures, modelled as two-dimensionally axis-symmetric porous media. The geometry based functions of the insert specific flow characteristics are derived from two- and three-dimensional pore scale computational fluid dynamics models, that are calibrated against experimental data. The validity of fundamental modelling assumptions was confirmed by a decent agreement between numerical and experimental steady-state heat transfer results. The heat and mass transfer investigation showed that the investigated structures were capable of eﬀectively removing heat during the dehumidification half-cycle. The thermal mass was shown to be a critical design parameter in achieving acceptable dehumidification performance.