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Pyrolysis of biomass and organic matter in fluidized bed reactors have become popular in recent years due to the efficient conversion of biomass to bio-fuels. Among others, the presence of mesoscale structures produced by the transient nature of gas solid flow governs the performance of fluidized bed reactors. In the present study, monodispersed system with the particle size of 155 µm and bidispersed system with particle sizes of 128 and 1500 µm are simulated by the multi-fluid model with the kinetic theory of granular flow closure laws. The objective is to investigate the effect of size distribution and solid volume fraction on the hydrodynamics of gas-solid flows. The ANSYS Fluent® software is used for the simulations and the slip velocity, heterogeneity index as well as mixing index are investigated. A two dimensional fully periodic domain is used to investigate the slip velocity at five different solid concentrations in dilute range. The slip velocity, which represents the interaction between solid particles and gas, calculated by simulation is higher than that calculated through Richardson-Zaki's correlation. The mixing index is found to be minimum for a solid volume concentration of 5%. The heterogeneity index is significantly less than 1 for both cases with bidisperesed system having lower value as compared to monodispersed system. The image analysis of solid fraction contours exhibits that mesoscale structures are dense and non-uniform for solid fractions of 0.05 and are small and uniform of solid fractions of 0.025 and 0.075. • Solid particle clusters using Eulerian CFD for bidispersed systems are observed. • H d is for bidisperesed system is much lower than monodispersed system. • Dense, non-uniform clusters are observed for solid fraction of 0.05. • Small, uniform clusters are observed for solid fractions of 0.025 and 0.075. [ABSTRACT FROM AUTHOR]

• The manuscript presents method to characterize solid particle clusters using digital image analysis and Eulerian CFD. • Cluster size increases with the increase in particle dimeter as well as with the increase in initial solid concentration. • 1% concentration shows single peak while 5 & 10% show bimodal cluster distribution. Presence of meso-scale structures such as particle clusters and gas bubbles is ubiquitous in fluidized beds frequently used in pyrolysis of coal and biomass. In recent years, bubbling and circulating fluidized beds have become popular for highly efficient conversion of carbonaceous feedstock into pyrolysis products. The characterization and study of hydrodynamics inside these reactors can provide an insight into the mechanisms controlling the performance of these reactors. Among others, cluster size distribution and mean cluster size are the key parameters defining the hydrodynamics of these reactors. In the current work, Digital Image Analysis (DIA) was applied to computationally obtained instantaneous snapshots during the CFD simulations of gas-solid flows. Transient two fluid modeling (TFM) was applied to three different gas-solid systems in periodic domains with different particulate systems keeping same grid size. The dimensionless axial cluster size was defined in line with literature. The cluster sizes and their distributions are obtained and compared with the available empirical correlations present in the literature based on experimental observations having similar range of operating conditions as well as system characteristics. It is found that the results of current approach agree well with the available data. The methodology followed in current work presents an alternative to expensive experimentation for development of a universal cluster size correlation. [ABSTRACT FROM AUTHOR]