Korea-Australia Rheology Journal

Kim, Noori;Koo, Sangkyun

doi: 10.1007/s13367-018-0019-5pmid: N/A

Abstract We examine scaling theories to estimate microstructural parameters of fractal aggregates in a colloidal suspension. The scaling theories are based on fractal theories and rheological properties of the colloidal suspension. Rheological measurement in oscillatory and steady shear modes is performed for colloidal suspensions of 56 nm carbon black particles in Newtonian ethylene glycol at the particle volume fractions φ ranging from 0.005 to 0.05. Elastic modulus G’ of the colloidal suspension at φ = 0.02-0.05 in the state of colloidal gel is used to estimate fractal dimension df of the aggregates. Steady-shear measurement gives yield stress τy as a function of φ. Shear dependence of the aggregate radius R is given by a power-law scaling, i.e., R∝S−m, where S is the shear rate. The power-law exponent m is estimated from df and a scaling relation between τy and φ. The estimation gives df = 2.14 and m = 0.33. The parameters df and m which can be determined by either direct measurement or theoretical calculation are used to establish a microrheological model for predicting shear viscosity of aggregated suspension as a function of φ and S. Both the concentration dependence and the shear dependence of aggregates are combined to obtain an expression for the shear viscosity. Hydrodynamic interaction effect among the aggregates is roughly considered in calculating average shear stress on the aggregate. It is found that this consideration critically contributes to behavior similarity with experimental result. It is shown that the predictions by the model reasonably agree with the experimental result.

Kim, See Jo

doi: 10.1007/s13367-018-0018-6pmid: N/A

Abstract It is of great importance to understand and predict Residence Time Distribution (RTD) during a singlescrew extrusion process for extruder design and operation optimization purposes. RTD depends on the extruder geometry, operating conditions, and material properties of non-Newtonian fluids. This paper presents the proper dimensionless parameters and RTD for the three-dimensional circulatory flow. These dimensionless parameters were expressed in terms of physical and geometrical parameters such as flow rate, pressure gradient along down channel direction, aspect ratio, helix angle, and power-law index. RTD computations and analysis were based on the finite element method. This dimensionless parameter study is found to be very useful for extruder designers to understand and to predict the RTD systematically.

Vishal, Badri;Ghosh, Pallab

doi: 10.1007/s13367-018-0015-9pmid: N/A

Abstract Aqueous foams are dispersions of gas bubbles in water, stabilized by surfactant, and sometimes particles. This multiphasic composition gives rise to complex rheological behavior under deformation. Understanding this behavior is important in many applications. Foam shows nonlinear rheological behavior at high deformation, which can be investigated by the large amplitude oscillatory shear (LAOS) experiments. In the present work, we have performed a systematic LAOS study of foam stabilized by 0.1 mol m−3 hexadecyltrimethylammonium bromide and 0.5 wt.% silica nanoparticles. The Lissajous-Bowditch curves and stress waveforms were analyzed at various strain amplitudes. These curves were fitted by Fourier transform rheology and Chebyshev polynomials to understand the contribution of the higher harmonic terms in LAOS. The intracycle LAOS behavior was explained based on the sequence of physical processes. The foam exhibited intracycle strain-hardening and shear-thinning at high deformation. Shear-thickening behavior was observed at moderate deformations.

Son, Kwon Joong

doi: 10.1007/s13367-018-0020-zpmid: N/A

Abstract High-speed planetary mixers with no impellers have been used to blend various types of industrial powders and viscous fluids rapidly and uniformly by means of centrifugal agitation resulting from the combined rotating and revolving motion of a mixing vessel. However, there is still a lack of in-depth understanding of their mixing dynamics at both meso and macro scales. This paper presents a discrete element simulation of the effect of cohesive powder rheology on mixing performance in a blade-free planetary mixer at particle scale. The interparticle cohesive forces were computed using the Johnson-Kendall-Roberts (JKR) model where the surface energy of each particle is a control parameter for the rheological behavior of the bulk powder. A series of mixing simulations were conducted for cohesive and non-cohesive particles in a planetary mixer. The degree of mixing in each simulation was evaluated with the performance parameters such as the mixing index, mixing time, and relative particle fraction. The simulation results showed that cohesion retards the particle mixing rate, but an increasing cohesion does not deteriorate mixing performance in a high-speed planetary mixer. The simulation method adopted in this study can be used in optimizing the design and operation conditions of a planetary mixer.

Jung, Hae In;Jung, Seon Yeop;Kang, Tae Gon;Ahn, Kyung Hyun

doi: 10.1007/s13367-018-0022-xpmid: N/A

Abstract In this paper we investigated numerically the flow and mixing characteristics of the barrier-embedded partitioned pipe mixer (BPPM) in non-creeping flow conditions. Numerical simulations are conducted for three mixing protocols of the BPPM, co-rotational, mirrored co-rotational, and counter-rotational protocols in the range of the Reynolds number (Re), 0.1≤Re≤300, focusing on the effect of the Reynolds number, the barrier height, and the mixing protocols on the mixing in the BPPM. Each mixing protocol creates two crosssectional flow portraits with crossing streamlines. Poincaré sections were plotted to investigate the flow system affected by the Reynolds number and the barrier height. Mixing in a specific BPPM is characterized using the intensity of segregation in terms of the compactness and the energy consumption. The dependency of the barrier height and the Reynolds number on the final mixing state of the BPPMs was identified by mixing analyses. The co-rotational protocol results in an efficient mixing in the creeping flow regime. Meanwhile, mirrored co-rotational and counter-rotational protocols, which lead to poor mixing in the creeping flow regime, turned out to be efficient protocols in the higher Reynolds number regime.

Huilgol, R. R.

doi: 10.1007/s13367-018-0021-ypmid: N/A

Abstract From the lattice Boltzmann equation, it is possible to derive the continuity equation and Cauchy’s equations of motion for a compressible medium, when one uses the Bhatnagar-Gross-Krook (BGK) - Welander approximation. From this, one can obtain the equations relevant to incompressible fluids. However, these require that the pressure be proportional to the density and the viscosity be dependent on the collision relaxation time. Clearly, these restrictions on the pressure and the viscosity are unacceptable in modelling the flows of Newtonian or non-Newtonian, incompressible fluids. In order to overcome these inherent problems, new models for the particle distribution functions are needed which are as general as possible. The motivation for the development of these models is driven by the history of nonlinear continuum mechanics, which shows that this subject evolved at the level of utmost generality, whether the specific topic was finite deformations in isotropic elasticity or the flows of viscoelastic fluids; or, the formulations of constitutive equations; and, the kinematics of flows. In order to maintain this generality in deriving the continuity equation and the equations of motion in Cartesian, cylindrical and spherical coordinates for all fluids using particle distribution functions, their evolution equations are written in a divergence form applicable in three dimensions. From this set, it is shown that the equations of continuum mechanics for Newtonian and non- Newtonian fluids can be derived in the three coordinate systems, when additional source terms are added to the equations of evolution in the latter two coordinate systems. If the body forces are present, a new set of source functions is required in each coordinate system and these are described as well. Next, the energy equation is derived by using a separate set of particle distribution functions. Modifications of the relevant equations to be applicable to incompressible fluids are described. The incorporation of boundary conditions and the description of the numerical scheme for the simulation of the flows employing the new approach is given. Validation results obtained through the modelling of a mixed convection flow of a Bingham fluid in a lid-driven square cavity, and the steady flow of a Bingham fluid in a pipe of square cross-section are presented. Finally, some comments on the theoretical differences between the present approach and the existing formulations regarding Lattice Boltzmann Equations are offered.

Kwon, Young Il;Song, Young Seok

doi: 10.1007/s13367-018-0016-8pmid: N/A

Abstract In this study, we investigated the injection molding (IM) process for the production polymeric parts with thin walls. The injection molding process was numerically modelled, and thin-walled polymeric parts were injection-molded experimentally. In the case of polymeric parts with thin walls, it is critical to understand flow and heat transfer behavior in the mold during the process. Injection-compression molding (ICM) was adopted to fabricate the parts, and the resulting residual stress and warpage were evaluated. In addition, birefringence of the molded part was analyzed to validate the numerical results.