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Numerical simulation of bidisperse hard spheres settling in a fluid
구상균 한국화학공학회 2011 Korean Journal of Chemical Engineering Vol.28 No.2
Average settling velocity of non-uniform hard spheres in a viscous fluid is determined by using a largescale numerical simulation that is carried out for over 103 spheres in a periodic unit cell which extends infinitely. An efficient calculation scheme is used for reducing the computation cost which steeply increases with the number of the spheres. The calculation scheme is based on a fast summation method for far-field hydrodynamic interaction among spheres. It is applied in the computation of hindered settling velocity of hard spheres with bidisperse size distribution in a viscous fluid. The simulation results are compared with the theoretical predictions by Batchelor [8] and Davis and Gecol [9]. It is found that the prediction by Davis and Gecol reasonably agrees with the numerical results.
Sedimentation velocity of bidisperse suspensions
구상균 한국공업화학회 2008 Journal of Industrial and Engineering Chemistry Vol.14 No.5
The present study deals with a simple method to predict the sedimentation velocity of suspensions with bidisperse size distribution of particles. The method is developed by modifying the effective-medium theory for monodisperse suspensions. The predictions by the method of the present study are compared with the previous experimental results and the theoretical estimations by Batchelor and Wen [G.K. Batchelor, C.S. Wen, J. Fluid Mech. 124 (1982) 495]. It is shown that the estimations by the effective-medium model of the present study agree well with the experimental results for wider range of concentrations of the suspensions than do those by Batchelor and Wen [G.K. Batchelor, C.S. Wen, J. Fluid Mech. 124 (1982) 495]. However, it is also found that an effective-medium region intrudes the space for the small particle at origin at high total volume fractions near 0.3 when the size ratio of small to large particle is 0.52 and both the volume fractions of the large and small particles are equal. This behavior may be attributed to the segregation of the particles, which has been observed in earlier experiments at the high volume fractions.
구상균 한국화학공학회 2006 Korean Journal of Chemical Engineering Vol.23 No.2
present study deals with the problem of determing drag force acting on spherical particles by slowII, EM-III, and EM-IV are presented to predict the drag on the spheres in random arrays. These predictions are com-pared with numerical simulations. The EM-IV model in which the volume exclusion effect near the representative sphereis taken into account in defining the effective-medium is found to compare very well with the numerical simulationsup to the volume fraction of spheres φ=0.5. In addition, Carman’s correlation is given for comparison. This empiricalcorelation is shown to be in god agreement with the simulation results beyond φ=0.4. Therefore, it is found that se-lective use of EM-IV and Carman’s correlation depending on φof the drag for full range of φ. Finally, the estimations are compared with the previous experimental results for the gaspressure drop across a micropacked bed reactor. The comparison shows a reasonable agreement between the experi-mental results and the estimations by Carman’s correlation.
이보은,구상균 한국공업화학회 2011 Journal of Industrial and Engineering Chemistry Vol.17 No.4
We demonstrate a modified polyol process to synthesize silver nanoparticles on the surface of larger magnetite nanoparticles. The overall process consists of two step reactions. First the magnetite particles of 100–200 nm in diameter are functionalized with a coupling agent 3-mercaptopropyltrimethoxysilane (MPTMS) through a sol–gel reaction. The functionalization of the magnetite surface is confirmed with IR spectra. For the second step silver nitrate is reduced at the site of thiol groups of the functionalized magnetite surface with ethylene glycol (EG) and polyvinylpyrrolidone (PVP). It was observed that silver nanoparticles were formed on the surface of the magnetite particles from transmission electron microscopy images and chemical composition analysis with energy dispersive spectrometer.
Thixotropic behavior and particulate aggregation in a suspension of carbon nanotubes
이다은,구상균 한국유변학회 2023 Korea-Australia rheology journal Vol.35 No.3
The present study dealt with the evaluation of the particulate aggregation in a suspension of multi-walled carbon nanotubes using fractal theory and rheological properties including thixotropy. The multi-walled carbon nanotubes are dispersed in Newtonian glycerol in the concentration range between 0.2 and 0.45 wt%. Rheological measurement was performed for the suspension at various dispersion times up to 300 min. The suspension showed thixotropy, shear-thinning behavior, and yield stress. It also exhibited plateaus of storage modulus in frequency and strain sweep tests. As the dispersion time increases, thixotropy, low-shear viscosities, and yield stress increase, and then their increasing rates slow down. Suspension’s electrical conductivity also showed similar behavior as that of thixotropy with the dispersion time. Viscoelastic behavior was combined with a fractal concept to provide the fractal dimensions of the flocs in the suspension at various dispersion times. The fractal dimension tends to decrease with the dispersion time. Conclusively it is interpreted that as the dispersion proceeds flocs become smaller and chain-like, then the reduced and thinned flocs build the wider range of network structures at rest state.
이후인,구상균 한국유변학회 2016 Korea-Australia rheology journal Vol.28 No.4
Colloidal aggregates in a suspension of carbon black particles are characterized by fractal dimension and their shear dependence. Carbon black particles of 100 nm in diameter are dispersed in Newtonian ethylene glycol with particle volume fraction φ ranging from 0.01 to 0.1. Microstructure of the aggregates is estimated by hydrodynamic transport properties such as average settling velocity and shear viscosity. Scaling analysis is conducted to correlate the hydrodynamic transport properties and the fractal dimension df. The fractal dimension is estimated to be 2.21 from the scaling relation between the settling velocity and the particle volume fraction for φ = 0.01-0.05. The shear viscosity results show shear-thinning behavior of the colloidal suspension. The intrinsic viscosity for the colloidal aggregates is obtained from the data of shear viscosity versus particle concentration. A scaling relation between the intrinsic viscosity and the shear rate gives df = 1.93 at m = 1/3, where m is the exponent defined by a scaling relation between aggregate radius Rg and shear rate S, Rg ∝S−m. Another scaling relation using yield stress data presents df = 1.94, which is nearly equivalent to 1.93 from that by the intrinsic viscosity but quite lower than that from the settling velocity. This discrepancy of the fractal dimension can be attributed to growth or restructuring of the colloidal aggregates by the hydrodynamic stress during long-time settling process.