Abstract
The effect of grid coarsening on the predicted total drag force and heat exchange rate in dense gas–particle flows is investigated using Euler–Lagrange (EL) approach. We demonstrate that grid coarsening may reduce the predicted total drag force and exchange rate. Surprisingly, exchange coefficients predicted by the EL approach deviate more significantly from the exact value compared to results of Euler–Euler (EE)-based calculations. The voidage gradient is identified as the root cause of this peculiar behavior. Consequently, we propose a correction algorithm based on a sigmoidal function to predict the voidage experienced by individual particles. Our correction algorithm can significantly improve the prediction of exchange coefficients in EL models, which is tested for simulations involving Euler grid cell sizes between 2d_p and 12d_p. It is most relevant in simulations of dense polydisperse particle suspensions featuring steep voidage profiles. For these suspensions, classical approaches may result in an error of the total exchange rate of up to 30%.
Highlights
Particle–gas systems are extensively used in various processes such as chemical, petrochemical and pharmaceutical industries
Our preliminary work showed that utilizing such a correction for voidage fields that are characterized by relatively low gradients yields an artificial increase in the total drag force
The result of particle unresolved EL approach (PU-EL) simulations demonstrated that coarsening the CFD grid size reduces the total drag force, and the total heat exchange rate in these flows
Summary
Particle–gas systems are extensively used in various processes such as chemical, petrochemical and pharmaceutical industries. Due to the complexity of such systems, as well as their opaqueness, numerical tools have been widely exploited This includes simulations to better understanding phenomena originating from (i) particle–particle interactions (e.g., cohesive forces), as well as (ii) particles and the interstitial flow (e.g., elutriation of fines from fluidized beds). This insight can be achieved through detailed local information from two- or three-dimensional simulations, which have become valuable tools for engineers and researchers. For numerical investigations of gas–particle flow using the PU approach, i.e., employing a closure for the drag force, generally two approaches can be employed: (i) the Euler– Euler (EE) approach, in which gas and particles are treated as interpenetrating continua. PU-EL avoids the need for the often prohibitively substantial number of Computational Particle Mechanics (2018) 5:607–625 fluid grids and offers comparably fast predictions that can account for, for example, intra-particle effects (e.g., diffusion and chemical reactions within porous particles)
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