Single Spherical Grain Research Articles

On energetic evaluation of robotic belt grinding mechanisms based on single spherical abrasive grain model

A tentative study from the perspective of abrasive grain geometry in this paper is conducted to investigate the specific energy and energy efficiency for clarifying the robotic belt grinding mechanisms. The energy efficiency model is established based on the friction coefficient model of the single spherical grain, then the experiments and simulation are implemented to energetically evaluate the microscale material removal mechanisms from the specific energy contributions. It has been demonstrated that the specific plowing energy is more predominant than both the specific cutting and sliding energy in robotic belt grinding, resulting in the energy efficiency ranges between 17 and 41 %. Both the large grain size and normal contact force can be taken as optimization strategies to maximize the energy efficiency for material removal.

Optimal oscillation parameters of vibrating screens

Optimal oscillation parameters of vibrating screens are determined using analytical methods by investigating movement of a single spherical grain particle in the vibrating sieve’s non-inertial reference frame. Dimensionless parameters determining the efficiency of screening process are defined. Effect of interactions between multiple numbers of particles moving on the screen is analyzed using discrete element method. By comparing the analytical and numerical results, it can be seen, that the analytical evaluation of screening efficiency can be used until a given contaminant size unlimitedly in the examined range of screen angle. Additionally in case of this minimal contaminant size there is a maximal screening angle value. Above this angle it seems, that the use of our discrete element based evaluation of screening efficiency is unavoidable.

Simulation of water sorption dynamics in adsorption chillers: One, two and four layers of loose silica grains

This paper presents a mathematical model of coupled heat and mass transfer in multi-layers of loose adsorbent grains under realistic conditions of adsorption heat transformation (AHT) cycle. The model allows a simulation of the adsorption dynamics in the adsorbent layer which consists of a low number of loose adsorbent grains (1≤n≤4).Firstly, the model was validated by comparison with the kinetics of isothermal water adsorption on a single spherical grain, initiated by a small pressure drop, for which an analytical solution is well-known.Afterwards, the model was applied to simulate non-isothermal water dynamics for adsorbent-heat exchanger configurations of one, two and four layers of loose grains of Fuji silica type RD. The grains are located on a metal support subjected to a fast variation of temperature as it takes place during isobaric phases of AHT cycle. The system of partial differential equations was solved by using the COMSOL Multiphysics® simulation environment.The calculated sorption dynamics is in a good accordance with the experimental data obtained for n=1, 2 and 4 under the same boundary conditions. Moreover, the model was used to simulate the ad/desorption process for different grain sizes (0.45, 0.85 and 1.7mm).The input parameters which ensure the best data fitting were compared with those experimentally determined for Fuji silica type RD as well as with the input data of other heat and mass transfer models presented in the literature. The developed model gives a powerful tool for accurate simulation of dynamic features for the AHT units with practically interesting configuration of the adsorber/heat exchanger which utilizes a low number of loose adsorbent grains. Moreover, useful information about radial and axial distributions of the temperature and vapour concentration can be obtained for both gas and solid phases. Finally, the specific cooling power of AHT cycle was estimated and recommendations on the cycle dynamic optimization were made.

A stochastic model for initial movement of sand grains by wind

AbstractA stochastic model for entrainment of sand grains by wind is presented through analysis of the forces exerted on a single spherical grain, coupled with fluctuations of wind velocity and the change in grain position on the surface. The structure of the stochastic model is consistent with experimental data in the literature. The probability of initial motion increases first, and then decreases, with grain size. It reaches a maximum at diameters of about 0·9 mm. Some sand grains are still in motion at less than the conventional threshold velocity, even at very low velocities. The probability of sand grain movement reaches unity at twice the conventional threshold velocity. Considerable discrepancies amongst conventional threshold formulae may result from the different probabilities of initial movement implied in these formulae. Copyright © 2008 John Wiley & Sons, Ltd.

Significance of straining in colloid deposition: Evidence and implications

Considerable research suggests that colloid deposition in porous media is frequently not consistent with filtration theory predictions under unfavorable attachment conditions. Filtration theory was developed from an analysis of colloid attachment to the solid‐water interface of a single spherical grain collector and therefore does not include the potential influence of pore structure, grain‐grain junctions, and surface roughness on straining deposition. This work highlights recent experimental evidence that indicates that straining can play an important role in colloid deposition under unfavorable attachment conditions and may explain many of the reported limitations of filtration theory. This conclusion is based upon pore size distribution information, size exclusion, time‐ and concentration‐dependent deposition behavior, colloid size distribution information, hyperexponential deposition profiles, the dependence of deposition on colloid and porous medium size, batch release rates, micromodel observations, and deposition at textural interfaces. The implications of straining in unsaturated and heterogeneous systems are also discussed, as well as the potential influence of system solution chemistry and hydrodynamics. The inability of attachment theory predictions to describe experimental colloid transport data under unfavorable conditions is demonstrated. Specific tests to identify the occurrence and/or absence of straining and attachment are proposed.

Frequency dependent conductivity of agglomorated spheres having a conducting surface

Charge carrier diffusion in an agglomeration of non-conducting resin grains covered with a layer of small conducting particles occurs on two scales, on single spherical grain surfaces and between different grains. Measurements of the frequency dependent permittivity between 5 Hz and 0.5 THz have been performed. The analysis allows to distinguish between the two contributions to the conductivity of the system.

Effects of Non-Plastic Fines on Minimum and Maximum Void Ratios of Sand

Abstract The behavior of sand is affected by the content of nonplastic fine particles. How and to what degree the fines content affects the minimum and maximum void ratios has been studied in detail. A review is presented of previous theoretical and experimental studies of minimum and maximum void ratios of single spherical grains, packings of spheres of several discrete sizes, as well as optimum grain-size ratios to produce maximum densities. A systematic experimental study is performed of the variation of minimum and maximum void ratios with contents of fines for sands with smoothly varying particle size curves and a large variety of size distributions. It is shown that the fines content plays an important role in determining the sand structure and the consequent minimum and maximum void ratios. It is indicated how the fines content and sand structure affects the compressibility and the static liquefaction potential of the sand.

The Brownian Motion of Dust Particles in the Solar Nebula: An Experimental Approach to the Problem of Pre-planetary Dust Aggregation

Laboratory experiments were performed to study the Brownian motion of μm-sized dust grains and small aggregates under pre-planetary nebula conditions, i.e., in a thin gas atmosphere (Epstein drag regime), in the ballistic limit, and under microgravity conditions. The results of these experiments, i.e., the grain diffusivities, are in quantitative agreement with theoretical predictions for single spherical grains. Deviations from particle sphericity, i.e., in our case aggregates consisting of monodisperse spherical grains, cause only minor deviations between the Epstein drag formula for spheres and our experimental results for equal particle cross section. Thus, we find a quantitative agreement of our measurements with the Epstein relationD∝ 1/σabetween grain diffusivity and geometrical (aerodynamic) cross section.The results of our investigations can be used for the calculation of the gas–grain stopping time τf= ϵ (m/σa) (1/ρgvm) which, in turn, is an important grain characteristic for the calculation of pre-planetary dust aggregation. Here,mand σaare the mass and the aerodynamic, i.e., geometric cross section of the grain, ρgandvmare the mass density of the gas and the mean thermal velocity of the gas molecules, and ϵ is a proportionality factor which we determined to be ϵ = 0.68 ± 0.10. The gas–grain stopping time describes the strength of grain coupling to a given gas motion and its value determines relative velocties and, hence, collision frequencies between dust grains due to sedimentation, drag-induced orbital decay, and gas turbulence in the solar nebula.

Improved permeability bounds for highly polydisperse materials

The influence of polydispersity on the flow of fluid through a medium composed of solid spherical inclusions of many different sizes is studied. Focus is on the case of a medium composed of impenetrable, i.e., nonoverlapping inclusions, which is a good model for flow, e.g., through sand, and also on the case of randomly overlapping inclusions, which provides a well-studied model for flow through porous media. Bounds for the permeability are derived using trial functions of the kind developed by Doi [J. Phys. Soc. Jpn. 40, 507 (1976)] and by Weissberg and Prager [Phys. Fluids 5, 1390 (1962); 13, 2958 (1970)]. It is shown that the bounds for permeability are relatively insensitive to dispersivity of the inclusions or grains composing a material when they are plotted against the proper scaling variable. The porosity is a good scaling variable for materials composed of randomly overlapping inclusions. The mean number of inclusions or grains ρ〈a〉 is shown to be a good scaling variable for permeability bounds on systems of nonoverlapping inclusions. It is shown that the low-density limits of the standard Doi bounds and Weissberg–Prager bounds fail to reproduce the radius-averaged solution of Stokes for flow around a single spherical grain. Also, for extremely disperse media these bounds may become trivial. The improved bounds correct these difficulties.