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43901-AC9
Effect of Particle Size Distribution on Drag Force in Fluid-Particle Suspensions

Sankaran Sundaresan, Princeton University

The overall objective of the three year project funded by ACS-PRF is to construct accurate drag force correlations for fluid-particle systems, allowing for the existence of particle size distribution. In this project, we focus first on developing drag laws for binary mixtures of particles moving in a fluid under low Reynolds number conditions and then extend the results to moderate Reynolds number. We also seek to generalize the results for binary mixtures to systems with several different particle sizes.

In the first year of this project, we have successfully completed the task of characterizing fluid-particle drag forces in binary suspensions (in the low Reynolds number regime) containing particles of same size but different densities (therefore different velocities relative to the interstitial fluid) and proposed a drag correlation based on simulation data. We have then generalized this drag law for multicomponent systems. In our simulations, we found that the particle-particle drag due to hydrodynamic interaction is a very important part of the net drag force. This contribution is typically ignored in virtually all the simulations of fluidized beds and our study has exposed the importance of this interaction. A manuscript based on this work is under preparation. A paper based on this study will be presented at the 2007 Annual Meeting of the AIChE in November 2007.

We have also made considerable progress in simulating low Reynolds number flows involving binary suspensions containing particles of different sizes. Additional simulations are underway and they will be completed in the first half of the upcoming second year of the project and form the basis for a second manuscript.

During the course of the first year of this project, we have identified and tested an efficient method to perform simulations of binary suspension flows at moderate Reynolds number. This computational approach, which we refer to as a frozen particle method, allows particles to have non-zero velocities, but remain frozen in their positions; through such fictitious simulations, we can specify desired local-average relative velocities between particles, which allows us to probe individually the different elements of the friction coefficient matrix. Using this approach, we have generated a first set of results on the effect of moderate Reynolds numbers in a binary mixture of equally sized particles. This work is continuing and will be completed in the course of this project.

We have also performed a few test Lattice Boltzmann simulations involving freely evolving binary suspensions to investigate the nature of inhomogeneities that develop in sedimentation. These simulations suggest gradual evolution of columnar structures, which will be examined in greater detail in the remaining duration of the project.

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