CFD-DEM Simulations of Heat Transfer inside Fluidized Beds at Elevated Temperatures
For many different applications the good heat transfer rates in gas-solid fluidized beds is key to the overall performance. However, the formation of densified zones near confining walls or submerged surfaces, or the formation of clusters of particles, or even agglomerates, significantly affect the fluidization behaviour and the heat transfer inside the fluidized bed and the heat exchange with the walls (a.o. Patil et al., 2015; Li et al., 2016). In addition, the operating temperature influences the particle collisional properties and inter-particle forces, altering the fluidization behaviour and consequently the heat transfer processes.
Inter-particle forces are known to cause detrimental effects in industrial drying processes for example for the production of polyolefins, where the formation of particle agglomerates causes reduced particle mixing, which affects the solids circulation patterns and may cause hot spot formation. Recent experiments have shown that at higher temperatures inter-particle forces become more pronounced, indicating a temperature dependency of the Hamaker constant (Campos Velarde et al., 2016). Moreover, also the particle collisional properties are known to strongly depend on the particle temperature.
In this work, the effects of the inter-particle forces and particle properties on the heat transfer processes inside a gas-solid fluidized bed are investigated, employing an in-house developed Discrete Particle Model (DPM), which is an Euler-Lagrange model with a discrete description of the solids phase and a continuous description of the gas phase. Particle-particle collisions are dealt with deterministically, using a soft-sphere collision model, which allows multiple simultaneous contacts between several pairs of particles. The motion of each individual particle is tracked and described with Newtonâs second law of motion. Contrary to commonly used studies, where the particle restitution coefficients are set to fixed constants for all particles, in our DPM we let the restitution coefficients depend on the particle impact velocity and particle temperature using the correlations by Thomton and Ning (1998) and Dong et al. (2014). The van der Waals forces were described using the equations by Hamaker (1937) and Israelachvili (1992). The gas-particle heat transfer coefficient was evaluated with Gunnâs correlation. Special attention was paid to the numerical implementation of the inter-particle forces.
The extended DPM code was used to investigate in detail the effects of the inter-particle forces and collisional properties on the minimum fluidization velocity, the bed porosity at minimum fluidization, and cluster size and the formation of dead zones at different fluidization conditions. In this work we extend this study to characterise the influence of inter-particle forces and change in restitution coefficients on the heat transfer processes in the fluidized bed.
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