Simulação do rendimento térmico de um sistema recuperador de calor gás-gás operado em regime de leito fluidizado borbulhante para aquecimento de ar

Abstract

Fluidized Bed Heat Exchangers (TCFL) stand out for offering a high suspension-tosurface heat transfer coefficient, which can reach several times the value achieved by single-phase gas-gas convection using similar fluidizing gas velocities. In view of identifying gaps in previous studies, particularly those related to gas-gas heat recuperators, this research conducts a theoretical study to evaluate, primarily, the heat transfer rate and the effectiveness achieved by a pilot-scale heat recuperator system operated in a bubbling fluidized bed, aimed at heating atmospheric air from hightemperature industrial waste effluents. The analyses served three purposes: determining the influence of the number of heat recuperator tubes using the classic gas-solid suspension renewal model; comparing the results obtained from four semiempirical models while maintaining the same number of tubes; and quantifying the inclusion of twisted tapes inside the tubes to improve thermal performance under laminar flow conditions. The results obtained by varying the number of tubes showed that, within the explored operating range, the heat transfer rate, as well as the thermal effectiveness of the heat exchanger, may undergo behavioral inversions when the "cold" gas flow regime changes from transition to laminar, since the thermal resistance of this flow is largely dominant. Thus, the discrepancies observed in the suspensionto-tube heat transfer coefficient values, obtained from simulations with the chosen models, had little influence on the thermal performance variables of the heat recuperator. Finally, by considering the inclusion of twisted tapes inside the tubes to reduce the flow's thermal resistance, a gain of 35% to 136% in heat transfer rates and thermal effectiveness was obtained. This result places the gas-gas FBHE at a performance level similar to that achieved by liquid-gas FBHEs, which are widely used in industrial settings.