Wide Range Of Aspect Ratios Research Articles

Cold flow experiments in an entrained flow gasification reactor with a swirl-stabilized pulverized biofuel burner

Short particle residence time in entrained flow gasifiers demands the use of pulverized fuel particles to promote mass and heat transfer, resulting high fuel conversion rate. The pulverized biomass particles have a wide range of aspect ratios which can exhibit different dispersion behavior than that of spherical particles in hot product gas flows. This results in spatial and temporal variations in temperature distribution, the composition and the concentration of syngas and soot yield. One way to control the particle dispersion is to impart a swirling motion to the carrier gas phase. This paper investigates the dispersion behavior of biomass fuel particles in swirling flows. A two-phase particle image velocimetry technique was applied to simultaneously measure particle and gas phase velocities in turbulent isothermal flows. Post-processed PIV images showed that a poly-dispersed behavior of biomass particles with a range of particle size of 112–160µm imposed a significant impact on the air flow pattern, causing air flow decelerated in a region of high particle concentration. Moreover, the velocity field, obtained from individually tracked biomass particles showed that the swirling motion of the carrier air flow gives arise a rapid spreading of the particles.

CFD simulations and experimental measurements of flow past free-surface piercing, finite length cylinders with varying aspect ratios

In this work, the flow past surface-piercing cylinders with free lower ends is studied numerically with computational fluid dynamics (CFD) at varying aspect ratios (AR = H/D) from around 1 to 19, and compared against experimental measurement across the same range. Numerical and experimental work is carried out at Re = 2900 and Fr = 0.65. The aspect ratio of the cylinder is varied by raising and lowering the cylinder into the water. The resulting flow field and loads on the cylinder as a result of changing the cylinder’s depth of submergence are investigated in terms of drag, lift and frequency content. First, the effects of changing the aspect ratio are examined. CFD simulations demonstrate a reduced drag coefficient, consistent with the experimental measurements, as compared to a fully-submerged, infinite cylinder at the same Reynolds number. A significant reduction in the drag coefficient is observed at an aspect ratio of 2. For aspect ratios 3 and below, Karman vortex shedding is completely suppressed due to the dominance of free surface effects. With increasing aspect ratio the Strouhal number is found to approach 0.2. Second, the flow regime is investigated as a function of cylinder length for a cylinder with aspect ratio of 12. Different regions of vortex shedding are observed along the length of the cylinder, as demonstrated by lift forces, Strouhal numbers and CFD flow visualizations. Strouhal numbers below 0.2 were observed for regions near the free surface. Additionally, the drag coefficient was found to decrease near the free surface. This study over a wide range of aspect ratios exhibits the effects of a free surface, free end and also the length of the cylinder on both the flow regime and loading.

Tuning the electrical percolation threshold of polymer nanocomposites with rod-like nanofillers

Abstract In this communication, a stochastic approach based upon Komori and Makishima’s work has been used for predicting the electrical percolation threshold of cylindrical nanofillers with three-dimensional (3D) spatial orientations in a typical nanocomposite system. Specifically, the proposed model was able to predict the volume fraction based percolation threshold of nanofillers with a wide range of aspect ratios (10–1000), which was substantiated with a variety of experimental data sets obtained from the literature. The anisotropic behavior of 3D aligned nanofillers was successfully introduced via in-plane and out-of-plane orientation distributions. The percolation threshold values of nanofillers were also found to be comparable with those obtained from the stochastic model that has incorporated the excluded volume effect.

Lid-driven cavity flow of viscoelastic liquids

The lid-driven cavity flow is a well-known benchmark problem for the validation of new numerical methods and techniques. In experimental and numerical studies with viscoelastic fluids in such lid-driven flows, purely-elastic instabilities have been shown to appear even at very low Reynolds numbers. A finite-volume viscoelastic code, using the log-conformation formulation, is used in this work to probe the effect of viscoelasticity on the appearance of such instabilities in two-dimensional lid-driven cavities for a wide range of aspect ratios (0.125≤Λ=height/length≤4.0), at different Deborah numbers under creeping-flow conditions and to understand the effects of regularization of the lid velocity. The effect of the viscoelasticity on the steady-state results and on the critical conditions for the onset of the elastic instabilities are described and compared to experimental results.

Maximally dense random packings of spherocylinders

A spherocylinder, a convex 3D object consisting of a cylinder with two hemispherical ends, can possess a wide range of aspect ratios and is one of the most studied non-spherical particle shapes. Previous investigations on disordered spherocylinder packings yielded inconsistent packing characteristics, especially the packing density φ, indicating that density alone is not sufficient to uniquely characterize a packing. In this paper, we delineate in the density–order–metric diagram (i.e. order map) the border curve that separates geometrically feasible and infeasible packings of congruent spherocylinders via the geometric-structure approach, i.e., by generating and analyzing a large number of packing configurations with a diversity of densities and degrees of order generated via a relaxation algorithm. We find that the border curve possesses a sharp transition as the packing density increases, i.e., the initial increase of φ is not associated with any notable increase of the degree of order, while beyond a threshold φ value, the increase of density is strongly positively correlated with the increase of order. This allows us to propose the concept of maximally dense random packing (MDRP) state for spherocylinders, which corresponds to the transition point in the boarder curve and characterizes the on-set of nontrivial spatial correlations among the particles. It can also be considered as the maximally dense packing arrangement of spherocylinders without nontrivial spatial correlations. The degree of order of a spherocylinder packing is quantified via the nematic order parameter (S) and the local order metric (SLocal), which respectively characterize the level of global orientational order and local order in particle clusters. The latter metric SLocal, which measures the average order in the neighborhoods of particles with the second Legendre polynomial, is a new order metric for spherocylinder systems. We find that the packing density of the MDRPs initially increases with the increase of the aspect ratio w from 0, and reaches the maximal value of 0.725 when w=0.5, then drops with further increase of w. The MDRP at w=0.5 is verified to be jammed via a Monte Carlo jamming-testing algorithm and thus, should also represent the maximally random jammed (MRJ) packing state as well.

Numerical study of driven flows of shear thinning viscoelastic fluids in rectangular cavities

In this work, we present a numerical study of flow of shear thinning viscoelastic fluids in rectangular lid driven cavities for a wide range of aspect ratios (depth to width ratio) varying from 1/16 to 4. In particular, the effect of elasticity, inertia, model parameters and polymer concentration on flow features in rectangular driven cavity has been studied for two shear thinning viscoelastic fluids, namely, Giesekus and linear PTT. We perform numerical simulations using the symmetric square root representation of the conformation tensor to stabilize the numerical scheme against the high Weissenberg number problem. The variation in flow structures associated with merging and splitting of elongated vortices in shallow cavities and coalescence of corner eddies to yield a second primary vortex in deep cavities with respect to the variation in flow parameters is discussed. We discuss the effect of the dominant eigenvalues and the corresponding eigenvectors on the location of the primary eddy in the cavity. We also demonstrate, by performing numerical simulations for shallow and deep cavities, that where the Deborah number (based on convective time scale) characterizes the elastic behaviour of the fluid in deep cavities, Weissenberg number (based on shear rate) should be used for shallow cavities.

A Simple Free Energy for the Isotropic-Nematic Phase Transition of Rods

A free energy expression is proposed that describes the isotropic-nematic binodal concentrations of hard rods. A simple analytical form for this free energy was yet only available using a Gaussian trial function for the orientation distribution function (ODF), leading, however, to a significant deviation of the predicted binodals. The new free energy proposed here is based upon a rationalized correction to the orientational and packing entropies when using the Gaussian ODF. In combination with Parsons-Lee theory or scaled particle theory, it enables describing the isotropic-nematic phase coexistence concentrations of rods accurately using the simple Gaussian ODF for a wide range of aspect ratios.

Conditions for blister formation during thermal cycles of Al–Si–Cu–Fe alloys for high pressure die-casting

The mechanism of blister formation during thermal cycle in a High Pressure Die Cast Al–9Si–3Cu–Fe alloy (EN 46000) was investigated by means of FEM simulation of a pressurized subsurface defect, considering the high-temperature elastoplastic behavior experimentally derived. The effect of parameters related to initial defect geometry, location and maximum temperature was analyzed. Blister formation was considered to occur as the maximum surface displacement after a thermal cycle exceeded 0.005mm. It was shown that, for a given reference gas pressure, pore volume and depth, as well as temperature, blisters were developed from pores below a critical aspect ratio, as a result of high plastic strain accumulation in localized regions laying between the pore and the outer surface (ligament). Similarly, critical ligament thickness was identified and correlated to temperature and reference gas pressure. The development of blisters at temperatures lower than 400°C from pores with reference pressure 90MPa was predicted for a wide range of aspect ratios and ligament thickness. The possible occurrence of blisters in conventional HPDC components during heat treatment cycles at 350°C was modeled for different pore pressures in the case of surface-near defects, mainly from lamina-shaped pores and was experimentally confirmed.

Accurate buckling loads of plates with different boundary conditions under arbitrary edge compression

Analytical procedure for critical load determination of plates with different boundary conditions under arbitrary in-plane compression loads is presented. Ritz energy technique with implemented exact stress distributions within plates and deflection functions in form of double Fourier series which satisfy all boundary conditions, term by term, provides results of high accuracy.The essence and value of this paper are in presentation of numerous examples and data which were used during the convergence and preciseness control of each part of very complex analytical procedure for critical load determination.Hence, through numerous examples of plates with wide range of aspect ratios and different boundary conditions under some classical as well as some not so classical compressive loads, applicability and accuracy of the presented analytical approach are proved. Results here obtained are reaffirmed by numerical finite element runs and also compared with still very few, until now, existing analytical solutions.

Modeling of the experiments on the Marangoni convection in liquid bridges in weightlessness for a wide range of aspect ratios

Hydrodynamic stability of a two-dimensional steady thermocapillary flow under weightlessness in a high-Prandtl number liquid bridge is studied by means of three-dimensional numerical modeling for a wide range of aspect ratios. We suggest an explanation of the findings of a series of microgravity experiments on Marangoni convection in liquid bridges. Stability of the flow with heat transfer through the interface, modeled by the classical Fourier law, is compared with the stability of the same system under adiabatic conditions. Cooling the interface may significantly shift the threshold of hydrothermal instability as soon as the Biot number deviates from zero. It may also affect the structure of the basic Marangoni flow and the mode of the supercritical flow. We demonstrate that the heat loss has a destabilizing effect for the aspect ratios (ratio of radius to height) below 2.4 (with the exception of a region between 1.6 and 1.8), and for the longer liquid bridges the prevailing effect is stabilizing. The heat transfer coefficient as a function of the length of the liquid zone is theoretically calculated using a model of heat transport for laminar forced convection. Comparison of the results of the modeling with the experimental data shows that an incorrect assessment of the heat transfer may lead to wrong conclusions concerning both the critical parameters of the flow and its structure.

Microparticle image velocimetry (μPIV) study of microcavity flow at low Reynolds number

Fluid flows in microchannels with microcavities at low Reynolds number are increasingly used in microfluidic applications such as trapping and sorting of cells and particles. For optimizing the microcavity configuration and better controlling the microenvironment in the microcavities, it is important to thoroughly understand the flow behaviors in the microcavities. Hence, using microparticle image velocimetry (μPIV), we investigated quantitatively the flow characteristics of rectangular microcavities with a wide range of aspect ratio (λ = 0.25–3) and Reynolds numbers (Re = 0–100). Depending on the control parameters (Re and λ), a flow regime map in microcavities has been constructed, including three different flow patterns: attached flow, separated flow, and transitional flow. The critical parameters for the transform of flow patterns were determined. Only a single central microvortex appears in the microcavities, and the evolution and characteristics of the microvortex were investigated in detail. The results revealing the flow mechanism of different flow patterns in the rectangular microcavities can provide useful design guidelines of microfluidic-based devices, as well as a map to help microfluidic users in their design of application-driven microcavities.

Instability of thermocapillary convection in long liquid bridges of high Prandtl number fluids in microgravity

This paper reports experimental results on the instability of thermocapillary convection in long half-zone liquid bridges of high Prandtl number fluids (Pr=67, 112 and 207 for 5, 10 and 20 cSt silicone oils, respectively). The experiments were carried out in microgravity on the International Space Station, which allowed sufficiently long waiting period for the development of instability. Critical temperature differences were measured for liquid bridges of 30 and 50mm diameters and up to 62.5mm length. The resultant critical Marangoni numbers (Mac) were obtained for a wide range of aspect ratio (=height/diameter), AR, up to AR=2.0. Linear stability analyses for Pr=67 were also carried out to obtain numerical data for comparison. The present experimental results for Pr=67 indicate 5.0×103<Mac<2.0×104 for large AR (AR>1.25) and they are in good agreement with the present linear stability analysis result. In contrast, the present results are considerably smaller than the previous data (Pr=74) taken in the Space Shuttle experiments. It is shown that this difference is due to the effect of heating rate of the liquid bridge. The data for oscillation frequency and azimuthal mode number are also presented. The non-dimensional oscillation frequencies as well as Mac for Pr=67 have shown a sudden decrease at around AR=1.25, suggesting the bifurcation of neutral stability curves.

Laminar Free Convection in Bingham Plastic Fluids from an Isothermal Elliptic Cylinder

Free convection from an isothermal elliptical cylinder in Bingham plastic fluids is studied numerically. This work spans a wide range of aspect ratios of the elliptical cylinder () to delineate the effect of shape on the flowfield close to the heated cylinder. The influence of the relevant parameters, namely, Rayleigh number (), Prandtl number (), and Bingham number (), on the momentum and heat transfer characteristics is studied in terms of the yielded/unyielded regions, streamline, isotherm contours, and local Nusselt number for an elliptical cylinder of constant surface area in blunt () and slender orientations (). The Nusselt number shows a positive dependence on the Rayleigh number, and it correlates negatively with Bingham and Prandtl numbers. The blunt orientations () enhance heat transfer as compared to that for a circular cylinder (of equal surface area) while slender orientations () impede it. Using the present numerical results, predictive correlations have been established in terms of the modified Rayleigh number () and Prandtl number (), thereby enabling the prediction of Nusselt number in a new application.

Optimization of high-quality vertically aligned ZnO rod arrays by the response surface methodology

Optimization of the deposition parameters was conducted by the response surface methodology to synthesize high-quality ZnO rod arrays with a high texture coefficient, a large aspect ratio and a narrow bandgap. In addition, mathematical models based on statistical analysis were also developed to predict the texture coefficient, aspect ratio and bandgap of the ZnO rod arrays. With the optimized parameters, all of the three involved responses obtained the desired optimum values. The results show that the texture coefficient can be elevated up to a value of 0.998, which represents an almost perfect value. Moreover, wide range of aspect ratios was obtained for various applications and the obtained maximum value of 21.3 is relatively high value by wet chemical method, especially when no capping agent and no refreshing growth solution in a nearly neutral solution is used.

Direct Numerical Simulation of the Onset of Vortex Shedding for Blunt Elongated Bodies

This research project focuses upon the wake behind a two-dimensional blunt-trailing-edged body. Data are obtained numerically by means of a Direct Numerical Simulation code. The body has an elliptical nose followed by a straight section that ends in a blunt base. The present paper is dedicated to the analysis of the onset of the shedding process. The effort is certainly worthwhile, because, in contrast to the case of the circular cylinder, the boundary layers’s separation points are defined and fixed. This allows a better assessment of the vital influence of the boundary layers upon the wake, in a controlled way. This is not the case for the circular cylinder, because, in this instance, the separation points oscillate in relation to a mean position. In the present analysis, the relationship between the onset of shedding Reynolds number, RehK , and the aspect ratio, AR, is obtained.To this end, a wide range of aspect ratios between 3 and 25, was investigated. The result represented by this relationship is a novelty in the literature. Values of RehK are strongly influenced by the aspect ratio for the case of the short cylinders — for which AR is low. After AR about 9, the curve flattens and the influence of the aspect ratio upon the shedding Reynolds number is very mild. Besides, the paper discusses another very important aspect; the overall stability of the pre-shedding laminar bubble at the base of the body. It is important to stress that the latter study relies on the fact that the boundary layers separation points are fixed.

Synthesis and CT imaging of gold nanostructures with tunable optical absorption spectroscopy

With a slight modification of typical seed-mediated synthesis of gold nanoparticles (GNPs), a wide range of aspect ratios for nanorods, spherical and dumbbell-like GNPs were synthesized. Their unique optical properties such as localized surface plasmon resonance related to the distinct morphologies were investigated. On the basis of the preparation condition for short gold nanorods (GNRs), by varying the amounts of ascorbic acid, the morphological transition from rod to dumbbell occurred and the growth mechanism was proposed. Compared with conventional iodine-based contrast agents, GNPs exhibited preferable x-ray CT imaging effect, and are good candidates for x-ray CT contrast agents in biomedical applications.

Random Packing of Hard Spherocylinders

Random packings have been studied using computer simulations for some time, but there is no agreement on the best way to obtain them, especially for nonspherical particles. The present work focuses on random packing of hard spherocylinders. Starting from the mechanical contraction method (Williams and Philipse. Phys. Rev. E 2003, 67, 051301), a modification is introduced that makes it easier to obtain reproducible values, by simplifying the selection of some parameters. Furthermore, it is found that the final packings can be compressed further using Monte Carlo style particle moves. Random packings were generated for a wide range of aspect ratios, 0 ≤ L/D ≤ 40, and it was verified that the final packings do not show positional nor orientational ordering.

Effect of size and aspect ratio on structural parameters and evidence of shape transition in zinc oxide nanostructures

Dependence of structural parameters on the size of nanoparticles is a topic of general interest where the effect of shape is often neglected. We report a comprehensive study on size-dependent structural parameters of ZnO nanostructures (NSs) having a wide range of aspect ratios (AR: length/diameter). With increase in size, ZnO NSs undergo a shape transition from spherical to rod-like morphology that induces a sudden change in the internal parameter (u) which represents the relative position of two hexagonal close-packed sublattices. The change in u is associated with the changes in anion–cation (Zn–O) bond lengths as well as bond angles and thereby bears a linear dependence on the AR. Further, the unit cell volume and microstrain decrease with increase in particle size and show a drastic reduction when flat crystal faces begin to appear at the spherical surface (AR~1.3). The significant change in structural parameters associated with the shape transition arises due to surface dipole-induced electrostatic relaxation that may be further influenced by interaction with the ambient gases as evidenced from the extended X-ray absorption fine structure (EXAFS) measurement. The present study addresses the underlying reasons for shape-induced change in structural and electronic properties of ZnO NSs.

Optimal aspect ratios for non-isothermal flat plate solar collectors for air heating

In the present work, the dimensionless model developed and validated by Baritto and Bracamonte (2012) for thermal behavior of flat plate solar collectors without glass cover was improved by adding the entropy balance equation in a dimensionless form. The model was solved for a wide range of aspect ratios and mass flow numbers. A parametric study was developed and thermodynamically optimal configurations were identified. A set of equations were fitted to reproduce the results and a straight forward method is proposed for the estimation of optimal aspect ratios as functions of the mass flow number and the outlet temperature, which are, in most of cases, requirements for practical applications.

Hard ellipses: Equation of state, structure, and self-diffusion

Despite their fundamental and practical interest, the physical properties of hard ellipses remain largely unknown. In this paper, we present an event-driven molecular dynamics study for hard ellipses and assess the effects of aspect ratio and area fraction on their physical properties. For state points in the plane of aspect ratio (1 ≤ k ≤ 9) and area fraction (0.01 ≤ φ ≤ 0.8), we identify three different phases, including isotropic, plastic, and nematic states. We analyze in detail the thermodynamic, structural, and self-diffusive properties in the formed various phases of hard ellipses. The equation of state (EOS) is shown for a wide range of aspect ratios and is compared with the scaled particle theory (SPT) for the isotropic states. We find that SPT provides a good description of the EOS for the isotropic phase of hard ellipses. At large fixed φ, the reduced pressure p increases with k in both the isotropic and the plastic phases and, interestingly, its dependence on k is rather weak in the nematic phase. We rationalize the thermodynamics of hard ellipses in terms of particle motions. The static structures of hard ellipses are then investigated both positionally and orientationally in the different phases. The plastic crystal is shown to form for aspect ratios up to k = 1.4, while appearance of the stable nematic phase starts approximately at k = 3. We quantitatively determine the locations of the isotropic-plastic (I-P) transition and the isotropic-nematic (I-N) transition by analyzing the bond-orientation correlations and the angular correlations, respectively. As expected, the I-P transition point is found to increase with k, while a larger k leads to a smaller area fraction where the I-N transition takes place. Moreover, our simulations strongly support that the two-dimensional nematic phase in hard ellipses has only quasi-long-range orientational order. The self-diffusion of hard ellipses is further explored and connections are revealed between the structure and the self-diffusion. We discuss the relevance of our results to the glass transition in hard ellipses. Finally, the results of the isodiffusivity lines are evaluated for hard ellipses and we discuss the effect of spatial dimension on the diffusive dynamics of hard ellipsoidal particles.