Experimental investigation of wall-to-bed heat transfer of carbon nanotubes in a fluidized bed

Abstract

Wall-to-bed heat transfer characteristics of multi-walled carbon nanotubes (MWCNTs) were determined in a fluidized bed (0.15 m i.d. by 2.0 m height). Two types of CNTs with different nanotube shapes were used: entangled CNTs (ENCNTs) and vertically aligned CNTs (VACNTs). The flow regimes of MWCNTs were identified as fixed bed, partial fluidization, and complete fluidization from the pressure drop across the bed (ΔP) with gas velocity. In the partial fluidization regime, different gas channeling phenomena occurred depending on the nanotubes shape on the CNT particles, affecting the bed hydrodynamics. The instantaneous surface temperature (Tsi) of the heat transfer probe decreased and its fluctuation increased with the gas velocity proportional to the bed particle movement near the wall. Fluctuations in ΔP and Tsi at the wall had a strong correlation for all MWCNTs, and the wall-to-bed heat transfer was greatly influenced by the movement of particles induced by a preferential flow path of gas in the bed such as gas channeling and bubbles. Fluctuations in ΔP and Tsi of VACNTs were considerably lower than those of ENCNTs because of the formation of a chain-like network structure of the bed. The wall-to-bed heat transfer coefficient (hw) in the CNT fixed bed was considerably higher than that of comparable particles owing to their high conductivity. The average hw of the CNTs increased with gas velocity, and the rate of increase depended on the shape of the nanotubes. The hw of the VACNTs which form easily tangled agglomerates by many long nanotubes, was lower than that of the ENCNTs. Correlations for hw prediction were firstly developed for the partial and complete fluidization regimes. The predicted values well accorded with the experimental results, and the hw changes of the MWCNTs with gas velocity could be predicted well based on the proposed correlation and two-phase model of the bubbling fluidized bed.