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Abstract: A hydrodynamic model describing gas–solid two phase flow has been used to numerically study the effect of wall on the bubble shape, size, detachment time and the bubble trajectory in a two-dimensional fluidized bed. Using the numerical model, the influence of the wall is analysed by moving the jet from the centre of the bed towards the wall. The bubble characteristics are compared for different nozzle locations, thus presenting the effect wall has on asymmetrical injection as compared to symmetrical injection. A 30% increase in the bubble detachment time for an asymmetric injection (jet velocity of 10m/s) is found when the nozzle is displaced from the centre of the bed towards the wall (80mm offset from wall). In addition, the bubble evolution reveals an asymmetric wake formation during detachment indicating an early onset of mixing process. The wall forces acts tangentially on the bubble and has a significant impact on the bubble shape, neck formation during detachment, holdup time near the wall and its trajectory through the bed. [Copyright &y& Elsevier]

Abstract: Fluidised beds are characterised by excellent thermal and chemical uniformity and have a wide application range including heat and surface treatment, ore roasting and catalyst production. However, compared to other gas-based systems, to fluidise a particulate mass, a significant quantity of gas is required. To conserve gas there is potential to operate the fluid bed under low-pressure conditions. It is also observed that heat transfer remains constant with reduction in pressure. The present work has numerically studied the nature of hydrodynamics in fluidised bed at sub-atmospheric conditions and a new drag law is proposed to account for the increased mean free path of the fluid. A wide range of sub-atmospheric pressures were considered such that slip flow regime, which is characterised with Kn∼1, is applicable. An open source code (MFIX) is used to numerically solve the multiphase problem of a jet in the fluidised bed column with an immersed surface at vacuum pressure conditions. Bubbling fluidisation in shallow and deep beds are also solved. The new drag model takes into consideration the effect of slip flow to model drag force on the particles and the results of velocity distributions in the column and around the submerged surface is presented. The results of velocity distributions from the slip flow model are compared with the existing Gidaspow’s model. Significant differences were observed in the simulation results of velocity distributions and flow structure in the fluidised bed under vacuum conditions. [Copyright &y& Elsevier]

• Multistage free-falling particle receiver design evaluated for radiation absorption. • Repeated stable falling particle curtain enhances solar radiation absorption. • Reflection losses at part-load operation reduces by over 50%. A novel multi-stage free-falling particle receiver design is proposed to improve the simple free-falling concept by enhancing the hydrodynamic stability and improving the radiation absorption of the particle curtain. The multi-stage design arising from repeated re-initialisation of the particle curtain by using intermediate troughs in the receiver results in an increased average volume fraction and residence time of the particles. The present work numerically solves the mass, momentum and radiative transfer equation for an isothermal two dimensional Eulerian–Eulerian particle–gas multiphase flow equations to estimate the absorption characteristics of the particle curtain. The multi-stage receiver concept significantly improves the absorptance of the curtain and reduces the reflection losses by over 50%. The reflection losses are seen to be insensitive to increase in size of the receiver making the multi-stage concept highly scalable. [ABSTRACT FROM AUTHOR]

• An indirectly-irradiated packed-bed solar thermochemical reactor is developed. • The reactor is experimentally tested under simulated high-flux solar irradiation. • A CFD model coupling reaction kinetics, heat transfer and fluid flow is developed. • The maximum solar-to-chemical energy conversion efficiency reaches 9.3%. The reduction of iron–manganese oxide particles in a high-temperature packed-bed solar thermochemical reactor is investigated using an advanced transient three-dimensional heat and mass transfer model. The model couples the reaction kinetics and fluid flow to conductive, convective, and radiative heat transfer. A reactor prototype that features a reaction tube confining the packed particles and a surrounding diffuse reflective cavity is tested under simulated high-flux solar irradiation to validate the model. The numerically predicted temperature profiles and oxygen generation rates are in good agreement with the experimental data. The validated model is applied to evaluate the thermochemical performance of the reactor. The calculated temperature profiles indicate that uniform temperature distribution in the reactive packed particles is achieved from the onset of the reaction. An energy rate balance analysis shows the instantaneous peak solar-to-chemical energy efficiency reaches 9.3%. [ABSTRACT FROM AUTHOR]