Increasing Packing Fraction Research Articles

Entropy generation analysis of heat and water recovery from flue gas by transport membrane condenser

Abstract The transport membrane condenser (TMC) has been used for heat and water recovery from coal-fired power plant flue gas. The capillary condensation of water vapor in membrane pore structure is the main gas separation mode. A lumped parameter model was established to study the heat and mass transfer in TMC. Recovered water and heat flow rates, water recovery ratio, heat recovery efficiency, and pressure drops were calculated. The temperature and humidity ratio distributions were displayed. The influences of structural parameters and operating conditions on the water and heat recovery performances were analyzed. In addition, the entropy generation model was proposed to calculate entropy variations and entropy generation components. The aim is to provide insights into TMC parameter selection and operation optimization. Moreover, the relationship between entropy generation components and TMC performances were confirmed. The results show that increasing packing fraction, or decreasing the membrane inner diameter or membrane pore size can improve the heat and water recovery performances. Besides, high water flow rates and low water temperatures have advantages in the operation. Increasing the mass/heat transfer driving force can enhance heat transfer performance, but the heat transfer entropy generation rate also increases. The maximum mass transfer entropy generation rate often corresponds to the best water recovery performance.

Ultrahigh-pressure form of SiO2 glass with dense pyrite-type crystalline homology

High-pressure synthesis of denser glass has been a longstanding interest in condensed-matter physics and materials science because of its potentially broad industrial application. Nevertheless, understanding its nature under extreme pressures has yet to be clarified due to experimental and theoretical challenges. Here we reveal the formation of $\mathrm{OS}{\mathrm{i}}_{4}$ tetraclusters associated with that of $\mathrm{Si}{\mathrm{O}}_{7}$ polyhedra in $\mathrm{Si}{\mathrm{O}}_{2}$ glass under ultrahigh pressures to 200 gigapascal confirmed both experimentally and theoretically. Persistent homology analyses with molecular dynamics simulations found increased packing fraction of atoms whose topological diagram at ultrahigh pressures is similar to a pyrite-type crystalline phase, although the formation of tetraclusters is prohibited in the crystalline phase. This critical difference would be caused by the potential structural tolerance in the glass for distortion of oxygen clusters. Furthermore, an expanded electronic band gap demonstrates that chemical bonds survive at ultrahigh pressure. This opens up the synthesis of topologically disordered dense oxide glasses.

Rheology and structure of polydisperse three-dimensional packings of spheres

We use three-dimensional contact dynamics simulations to analyze the rheology of polydisperse packings of spherical particles subjected to simple shear. The macroscopic and microstructural properties of several packings are analyzed as a function of their size span (from nearly monodisperse to highly polydisperse). Consistently with previous two-dimensional simulations, we find that the shear strength is independent of the size span despite the increase of packing fraction with size polydispersity. At the grain scale, we analyze the particle connectivity, force transmission, and the corresponding anisotropies of the contact and force networks. We show that force distributions become increasingly broader as the size span increases. In particular, stronger forces are captured by large particles, which are also better connected creating the so-called granular backbone. Throughout this backbone friction mobilization is rare and compressive forces control the stability of such structure. In return, small particles create an important population of rattlers discarded of the strength and granular structure analysis. As a consequence, the contact anisotropy declines with size span, whereas the force and branch anisotropies increase. These microstructural compensations allow us to explain the independence of the shear strength from particle size polydispersity.

Finding Topological Charges in Confined Ellipses

The structure of a two-dimensional fluid confined in a circular cavity bounded by a hard wall is investigated using Monte Carlo simulation in NVT ensemble. The fluid consists of hard ellipses with two different aspect ratios. Average density and order parameter are calculated locally to show if any topological charge is formed. The magnitude, location, number of these topological charges, and also their dependence on the radius of the circular cavity are studied here. Different phases are formed by increasing packing fraction of the fluid and it has claimed that the existence and the kind of topological charges both depend on the ellipse’s interaction with the wall (planar or homeotropic), the radius of the circular cavity, and the elongation of particles.

Influence of Ge4+ substitution for Zr4+ on the microwave dielectric properties of Mg(Zr1-xGex)Nb2O8 (0 ≤x ≤ 0.4) ceramics

Dense Mg(Zr1-xGex)Nb2O8 (x = 0, 0.1, 0.2, 0.3, 0.4) ceramics were successfully synthesized through traditional solid state reaction route. The relations between microstructure and microwave dielectric properties were studied. The formation of single MgZrNb2O8 phase with the monoclinic wolframite structure was verified by XRD patterns. Experimental dielectric constant was found to be closely related to bond polarizability, and theoretical dielectric constant calculated based on P-V-L theory shows the same tendency with the experimental dielectric constant when the substitution amount of Ge4+ changes. The increase in Q×f and |τf| were mainly attributed to the increase in packing fraction and the Nb-site bond valence, respectively. Excellent dielectric properties were obtained for Mg(Zr1-xGex)Nb2O8 (x = 0.2) ceramic sintered at 1300 °C for 4 h: εr= 24.26, Q×f= 80,492.8 GHz, τf= -51.95 ppm/°C.

Rheology and shear jamming of frictional ellipses

Understanding and predicting dense granular flows is of importance in geology and industrial applications. Still, most theoretical work has been limited to flows and packings composed of discs or spheres, a narrow subset of all possible packings. To advance our understanding of more realistic flows we here study the granular rheology of ellipses in steady-state flow with a focus on the effects of elongation and interparticle friction. We carry out novel numerical simulations of amorphous granular flows in a shear cell under confining pressure, at constant shear rate and at various aspect ratios. Both frictionless and frictional particles are considered. The various rheological curves follow the semi-empirical constitutive relations previously found for granular flows composed of discs or spheres. At the shear jamming point one finds well-defined packings, all characterised by their own set of critical parameters such as critical packing fraction, effective friction, etc. Packings composed of frictionless or almost frictionless particles are found to have a non-monotonic dependence of the macroscopic friction but a monotonic increase in packing fraction as the aspect ratio increases. For packings composed of particles with high interparticle friction the reverse is found. While frictionless packings are found to be hypostatic (except in the disc limit) frictional packings are remarkably close to the isostaticity point of having three contacts per particle. Both frictional and frictionless packings are found to have an increasing nematic ordering as the aspect ratio increases. The onset of a rolling, rather than sliding, motion between very frictional particles diminish this nematic ordering substantially. These findings put new and previously unknown bounds on the packing ratios and yield criteria for these amorphous packings at shear jamming.

Force-chain evolution in a two-dimensional granular packing compacted by vertical tappings.

We experimentally study the statistics of force-chain evolution in a vertically-tapped two-dimensional granular packing by using photoelastic disks. In this experiment, the tapped granular packing is gradually compacted. During the compaction, the isotropy of grain configurations is quantified by measuring the deviator anisotropy derived from fabric tensor, and then the evolution of force-chain structure is quantified by measuring the interparticle forces and force-chain orientational order parameter. As packing fraction increases, the interparticle force increases and finally saturates to an asymptotic value. Moreover, the grain configurations and force-chain structures become isotropically random as the tapping-induced compaction proceeds. In contrast, the total length of force chains remains unchanged. From the correlations of those parameters, we find two relations: (i) a positive correlation between the isotropy of grain configurations and the disordering of force-chain orientations, and (ii) a negative correlation between the increasing of interparticle forces and the disordering of force-chain orientations. These relations are universally held regardless of the mode of particle motions with or without convection.

Bulk dynamics of Brownian hard disks: Dynamical density functional theory versus experiments on two-dimensional colloidal hard spheres.

Using dynamical density functional theory (DDFT), we theoretically study Brownian self-diffusion and structural relaxation of hard disks and compare to experimental results on quasi two-dimensional colloidal hard spheres. To this end, we calculate the self-van Hove correlation function and distinct van Hove correlation function by extending a recently proposed DDFT-approach for three-dimensional systems to two dimensions. We find that the theoretical results for both self-part and distinct part of the van Hove function are in very good quantitative agreement with the experiments up to relatively high fluid packing fractions of roughly 0.60. However, at even higher densities, deviations between the experiment and the theoretical approach become clearly visible. Upon increasing packing fraction, in experiments, the short-time self-diffusive behavior is strongly affected by hydrodynamic effects and leads to a significant decrease in the respective mean-squared displacement. By contrast, and in accordance with previous simulation studies, the present DDFT, which neglects hydrodynamic effects, shows no dependence on the particle density for this quantity.

Sol-gel Auto Combustion Synthesis, Structural and Magnetic Properties of Mn doped ZnO Nanoparticles

Mn2+ doped ZnO nanoparticles were synthesized by sol–gel auto combustion technique. The effects of Mn2+ doping on the structural and magnetic properties of ZnO nanoparticles were investigated. Synthesized nanoparticles were characterized by X-ray diffraction technique (XRD), Fourier transform infrared spectroscopy (FTIR). The hexagonal wurtzite structure with single phase of Mn2+ doped ZnO nanoparticles were confirmed by XRD analysis. The average crystallite size determined by Scherrer’s formula was found in the range 14–17 nm. It is observed that the lattice parameter, volume of unit cell, X-ray density, and atomic packing fraction increase with increasing Mn2+ content. FTIR analysis revealed a slight change in main absorption band of ZnO in the frequency range of 500-1000 cm−1. Magnetic characterization was performed using vibrating sample magnetometer (VSM) and, which evidenced the properties of pure ZnO and Mn doped ZnO nanoparticles showed the diamagnetic and paramagnetic behaviour respectively.

Discrete element analysis of buffering capacity of non-spherical granular materials based on super-quadric method

Granular system commonly encountered in industry or nature is comprised of non-spherical grains. Comparing with spherical particles, high discretization and interlocking among non-spherical particles can effectively dissipate the system energy and improve the buffer capacity. The superquadric element based on continuous function envelop can form the geometric shape of irregular particles accurately, and then contact collision action between particles can be calculated easily. In this paper, we provide a comprehensive introduction to particle-particle and particle-boundary contact collision. In addition, considering different shapes and surface curvatures under various contact patterns between super-quadric particles, the linear contact force model cannot be applied to the accurate calculation of the contact force, and a corresponding non-linear viscoelastic force model is developed. In this model, the equivalent radius of curvature at a local contact point is adopted to calculate the normal contact force, and the tangential contact force is simplified based on the contact model of spherical elements. To examine the validity of the algorithm and this model, we compare the discrete element analytical results with the analytical results for a single cylinder impacting a flat wall and the previous experimental results for spherical granular material under impact load, and this method is verified by good agreement between the simulated results and the previous experimental results. According to the aforementioned method, we study the buffer capacity of non-spherical particles under impact load by the discrete element method, and the influences of granular thickness and particle shapes on the buffer capacity are discussed. The results show that a critical thickness Hc is obtained for different particle shapes. The buffer capacity is improved with increasing the granular thickness when H Hc, but is independent of the granular thickness and particle shapes when H Hc. Moreover, the impact peak and initial packing fraction increase significantly with increasing the blockiness. Rectangular particles account for the highest packing fraction, and the packing fraction of cylindrical particles is higher than the packing fraction of spherical particles. Therefore, Rectangular particles are more likely to form dense face-face contacts and ordered packing structures with high packing fraction. These denser packings prevent the particles from their relatively moving, and thus reducing the buffering capacity of the particles. Furthermore, the impact peak and initial packing fraction decrease with increasing or reducing the aspect ratio of cylindrical particles and the aspect ratio of rectangular particles. The aspect ratio of particle can be used to adjust the dense packing structure and reduce the stability of the system. It means that the particles have more effective buffer capacity for the non-spherical particle system.

Theoretical Study of Non-additive Hard Convex Body Fluid Mixtures

The structure of a dense fluid or liquid is largely determined by the repulsive intermolecular forces, so the hard core potential has been extensively employed to model real fluids and fluid mixtures. The properties of two dimensional hard body fluids and fluids mixtures have aroused a considerably interest in recent years. This is because they are treated as models of real fluids or fluid mixtures absorbed onto surfaces. The 2D hard-body fluids and fluid mixtures like their three-dimensional counter part are useful reference system in framing a perturbation theory for 2D real fluids and fluid mixtures of non-spherical molecules. The 2D hard-body systems are convex body (HCB) systems such as ellipse (HE) and hard discorectangles (HDR) and hard non-convex body systems i.e., fused hard discs (FHD) and planar hard dumbbell (HDB) systems. The equation of state of hard convex body fluid mixtures (HCB) is studied. Analytic expressions are given for the equation of state of the fluid mixture of the non-additive HCB. The numerical results are discussed for the non-additive hard dumbbell (HDB) fluid mixtures under different conditions for different values of non-additivity \(\Delta\). They depend on the conditions, shape parameters \(L^*_{11}\) and \(L^*_{22}\) , mole fractions \(x_1\) and \(x_2\) and \(\Delta\) in general and increase with the increase of packing fraction \(\Delta\).

Computer simulation of confined prolate hard spherocylinder liquids between hard walls

ABSTRACTIn present work, using canonical Monte Carlo simulation method, the packing fraction profiles and order parameters of prolate hard spherocylinder (HSC) fluid between hard walls are calculated. The hard particles and hard particle–wall interactions are considered. The confinement effects on transition packing fraction of HSCs with aspect ratios and 5 are calculated. In slit pore, while the bulk packing fraction increases, an orientationally ordered phase can form. We conclude that the required packing fraction for forming liquid crystal phases will decrease in confinement. Also while the molecular anisotropy increases, the isotropic–nematic transition packing fraction will decrease in confinement. We could not find the exact or simulation results for comparison with packing fraction profiles of confined HSCs. The results in the case of confined are compared with the simulation results of confined hard Gaussian overlap fluid of elongation . The results are corresponded, qualitatively. Also the confinement isotropic–nematic transition packing fractions are compared with the simulation and fundamental mixed measure theory (FMMT) results of bulk prolate HSCs.

Structure analysis and microwave dielectric properties of germanium ion-doped ZnZrNb2O8 ceramics

Germanium ion-doped ZnZrNb2O8 with composition of ZnZr1−xGexNb2O8 (x = 0.10, 0.20, 0.30, 0.35, 0.40) ceramics that exhibited improved Q × f value were first prepared by solid state method. X-ray diffraction (XRD) rietveld refinement was used to analyze the structure variations of the ceramic samples. The XRD patterns showed that all the samples contained the ZnZrNb2O8 phase with monoclinic structure. With the increase of germanium ion content, the dielectric constant decreased which was ascribed to the decrease of ionic polarizability. The improvement of Q × f value was mainly attributed to the increase of packing fraction. And, the emergence of the second phase also had a promoting effect to a certain extent. The temperature coefficient of resonant frequency (τ f ) correlated with the variation of bond valence of B-site. When x value exceeded 0.4, the microwave dielectric properties deteriorated seriously. The good combination microwave dielectric properties of e r = 23.7, Q × f = 89,341 GHz (where f = 8 GHz), and τ f = −65.48 ppm/°C were obtained for ZnZr1−xGexNb2O8 (x = 0.35) ceramics sintered at 1180 °C for 6 h.

Penetration in bimodal, polydisperse granular material.

We investigate the impact penetration of spheres into granular media which are compositions of two discrete size ranges, thus creating a polydisperse bimodal material. We examine the penetration depth as a function of the composition (volume fractions of the respective sizes) and impact speed. Penetration depths were found to vary between δ=0.5D_{0} and δ=7D_{0}, which, for mono-modal media only, could be correlated in terms of the total drop height, H=h+δ, as in previous studies, by incorporating correction factors for the packing fraction. Bimodal data can only be collapsed by deriving a critical packing fraction for each mass fraction. The data for the mixed grains exhibit a surprising lubricating effect, which was most significant when the finest grains [d_{s}∼O(30)μm] were added to the larger particles [d_{l}∼O(200-500)μm], with a size ratio, ε=d_{l}/d_{s}, larger than 3 and mass fractions over 25%, despite the increased packing fraction. We postulate that the small grains get between the large grains and reduce their intergrain friction, only when their mass fraction is sufficiently large to prevent them from simply rattling in the voids between the large particles. This is supported by our experimental observations of the largest lubrication effect produced by adding small glass beads to a bed of large sand particles with rough surfaces.

Low temperature sintering of high permittivity Ca-Li-Nd-Ti microwave dielectric ceramics with BaCu(B2O5) additives

In this paper, the influences of Al substitution and BaCu(B2O5) (BCB2) additives on the sintering temperature, microstructures, phase structures and microwave properties were investigated in detail. The results indicated that all Al added Ca0.35Li0.25Nd0.35Ti1-xAlxO3 (CLNTAx) samples were crystallized as orthorhombic perovskite structure in Pnma space group. A proper amount (x ≤ 0.03) of Al substitution for Ti was beneficial to improve the quality factors mainly due to the increase of packing fraction. But the permittivity (εr) and temperature coefficient of the resonant frequency (τf) deteriorated because of the decrease of B-site bond valence. The perovskite structured phase and BCB2 phase were observed in Ca0.35Li0.25Nd0.35Ti0.97Al0.03O3 (CLNTA3) ceramics, when a small amount of BCB2 was added (y ≤ 5 wt%). The sintering temperature was effectively lowered under 950 °C, and τf was modified from 74.8 ppm/°C to 41.2 ppm/°C with the assistance of BCB2 phase and liquid phase. But excessive BCB2 contents (y > 5 wt%) resulted in the BaNd2Ti5O14 and unknown phase, leading to inferior dielectric properties. Therefore, 5 wt% BCB2-added CLNTA3 ceramics sintered at 950 °C for 2 h exhibited excellent comprehensive microwave dielectric properties: εr = 109.5, Q × f = 2979 GHz and τf = 41.2 ppm/°C.

High-speed ultrasound imaging in dense suspensions reveals impact-activated solidification due to dynamic shear jamming.

A remarkable property of dense suspensions is that they can transform from liquid-like at rest to solid-like under sudden impact. Previous work showed that this impact-induced solidification involves rapidly moving jamming fronts; however, details of this process have remained unresolved. Here we use high-speed ultrasound imaging to probe non-invasively how the interior of a dense suspension responds to impact. Measuring the speed of sound we demonstrate that the solidification proceeds without a detectable increase in packing fraction, and imaging the evolving flow field we find that the shear intensity is maximized right at the jamming front. Taken together, this provides direct experimental evidence for jamming by shear, rather than densification, as driving the transformation to solid-like behaviour. On the basis of these findings we propose a new model to explain the anisotropy in the propagation speed of the fronts and delineate the onset conditions for dynamic shear jamming in suspensions.

Low-temperature fired thermal-stable Li2TiO3–NiO microwave dielectric ceramics

Rock salt-structured (1 − x)Li2TiO3–xNiO (0 ≤ x≤0.5) solid solutions were prepared via a conventional solid-state method. The structure evolution, sintering behavior, and microwave dielectric properties were investigated systematically. The phase transition from an ordered monoclinic to a disordered cubic phase was observed as x increased from 0 to 0.4. Disordered cubic phases in x = 0.4–0.5 samples seemed to conform to the structure of LiTiO2 instead of Li2TiO3. The possible reason was ascribed to both the appearance of Ti3+ in the NiO-modified matrix and the lattice distortion from substituted ions with different radii. With a gradual decline of the structure order degree, τf was found to decrease from +26 to −36 ppm/°C. A greatly improved Q × f value of 103,050 GHz (at 7.65 GHz) could be yielded at x = 0.1 owing to the increased packing fraction. The 0.8Li2TiO3–0.2NiO ceramic exhibited excellent overall microwave dielectric properties of er = 20.4, Q × f = 83,608 GHz (at 7.63 GHz) and τf = 1.97 ppm/°C as sintered at 1250 °C. After the addition of 1 wt% BaCuB2O5, the x = 0.2 ceramic can be well densified at 850 °C and chemically compatible with pure silver, in addition to keeping superior properties of er = 19, Q × f = 62,252 GHz (at 7.84 GHz) and τf = −1.65 ppm/°C. All the above results indicate that the current material system would be a promising candidate for the application of low-temperature co-fired ceramics.

Structural Dependence of Microwave Dielectric Properties of Spinel-Structured Li2ZnTi3O8 Ceramic: Crystal Structure Refinement and Raman Spectroscopy Study

The relationship between the structure and microwave dielectric properties of spinel-structured Li2ZnTi3O8 ceramic has been studied by structure refinement and Raman spectroscopy analysis. The vibration modes of Li2ZnTi3O8 were classified by group-theoretical analysis, and the observed Raman spectra of Li2ZnTi3O8 ceramic assigned. The correlations between bond strengths, packing fraction, the stretch mode of the oxygen octahedron (A 1g(1) mode), and the microwave dielectric properties are discussed. With increase of the A 1g(1) Raman shift, the oxygen octahedron became rigid, thereby decreasing the dielectric constant (e r). With increasing packing fraction and decreasing full-width at half-maximum (FWHM) of A 1g(1) mode, the lattice anharmonic vibration decreased, the damping behavior of A 1g(1) stretch vibration became weaker, and the intrinsic quality factor (Q × f) accordingly increased. The temperature coefficient of resonant frequency (τ f ) improved with the increase of bond strength, irrespective of oxygen octahedron distortion.

The structural origin of the hard-sphere glass transition in granular packing.

Glass transition is accompanied by a rapid growth of the structural relaxation time and a concomitant decrease of configurational entropy. It remains unclear whether the transition has a thermodynamic origin, and whether the dynamic arrest is associated with the growth of a certain static order. Using granular packing as a model hard-sphere glass, we show the glass transition as a thermodynamic phase transition with a ‘hidden' polytetrahedral order. This polytetrahedral order is spatially correlated with the slow dynamics. It is geometrically frustrated and has a peculiar fractal dimension. Additionally, as the packing fraction increases, its growth follows an entropy-driven nucleation process, similar to that of the random first-order transition theory. Our study essentially identifies a long-sought-after structural glass order in hard-sphere glasses.

Structural Evolution of a Granular Pack under Manual Tapping

We experimentally study a two-dimensional (2D) granular pack of photoelastic disks subject to vertical manual tapping. Using bright and dark field images, we employ gradient-based image analysis methods to analyse various structural quantities. These include the packing fraction ($\phi$), force per disk ($F_d$) and force chain segment length ($l$) as a function of the tapping number ($\tau$). The increase in packing fraction with tapping number is found to exponentially approach an asymptotic value. An exponential distribution is observed for both the cumulative numbers of force per disk $F_d$ : $N_{cum}(F_d) = A_F \exp (-\frac{F_d}{F_0})$, and force chain segment length $l$ : $N_{cum}(l) = A_l \exp (-\frac{l}{l_0})$. Whereas the coefficient ${A_F}$ varies with $\tau$ for force per disk, the force chain segment length shows no dependence on $\tau$. The $\tau$ dependence of $F_d$ and $\phi$ allows us to posit a linear relation between the total force of the granular pack ($F_{tot}^*(\tau)$) and $\phi(\tau)$.