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A contribution for accuracy in sub-grid modeling of gas-solid fluidized flows


This project intends to contribute for the development of gas-solid fluidized bed reactors, widely applied in the petrochemical and energy industries. Are of higher interest the large scale applications for catalytic cracking of petrol, and for combustion/gasification of fossil/renewable fuels, both responsible for extreme impact over world's economy. The development of fluidized bed reactors is an empirical science, based on successive gradually scaled demonstration plants, involving extremely high costs and extended implementation times. This scenario has prompted, both in industry as well as in academy, a growing expectation of using computational simulation as an auxiliary or even replacing tool to the expensive empirical developments. Aligned in this context, the current project has an ultimate goal of contributing for the habilitation of CFDs as realistic tools of computational experimentation for optimization, design and scaling of industrial scale fluidized bed reactors. In particular, it is aimed to contribute for the formulation of enhanced mathematical modeling through the proposition of growingly accurate sub-grid closure models. The challenging multiscale nature of the gas-solid fluidized flows has prompted research under a variety of modeling approaches, all of them practiced ultimately searching for accuracy. An approach widely practiced is based on two-fluid modeling, where gas and solid particulates are both treated as interpenetrating continuum phases in thermodynamic equilibrium. This is the line of modeling practiced in this project, following an approach where filtered formulations (fTFM) provide for large-scale descriptions of real full domains (via LSS, large scale simulations), while microscopic formulations (mTFM) provide for meso-scale descriptions of partial domains (via HRS, highly resolved simulations). Results of mTFM simulations are frequently used for the derivation of sub-grid models, which are then provided as closures in fTFM formulations. It is in this exact context that the current project intends to contribute, specially through the search for growingly accurate sub-grid models. Two aspects are brought into perspective: the first, related to the accuracy of the mTFM simulations themselves; the second, related to the correctness and suitability of the independent variables that are assumed in sub-grid model correlation. In previous researches both the aspects were considered: the first, with focus on effects of macro-scale flow conditions, effects of gas phase sub-grid turbulence, and effects of interparticle friction; the second, by assuming independent variables for correlation usual in literature. It is now proposed to extend those studies to more rigorous and comprehensive conditions, by considering significant ranges of numbers of Froude and Stokes, and different combinations of independent variables for correlation. For such a purpose, computational simulations will be advanced under the mTFM of the open source code MFIX (made available by NETL-DOE-USA), properly modified by including interparticle friction effects and gas sub-grid turbulence effects. The major impact intended will be over the state of the art, in practice by contributing to the viability of: i. research tools of higher accuracy, for the scientific community; ii. more realistic work tools for design, optimization and scaling of industrial plants, for engineers and researchers in industry. (AU)

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