Wide Range Of Solid Fractions Research Articles

Retention Forces for Drops on Microstructured Superhydrophobic Surfaces.

Accurate models of retention forces between drops and superhydrophobic (SH) surfaces are required to predict drop dynamics on the surface. This retention force is, in turn, useful in modeling heat transfer rates for dropwise condensation on a SH surface. Drop contact angle distribution and base area on SH surfaces are essential factors for predicting retention forces. The present work measures the contact angle distribution and base area shapes of various drop sizes over a wide range of solid fraction for inclined microstructured SH surfaces at the point of drop departure. Base area shape was found to be well approximated using two ellipses with different aspect ratios, and the contact angle distribution was found to be best fit by a sigmoid function. At an incline near the roll-off angle, drop base area for surfaces with solid fraction close to 1 and close to 0 were found to be nearly circular, whereas the base area of drops on surfaces with an intermediate solid fraction deviated from circular behavior. In this work, maximum advancing and minimum receding contact angles were found as a function of solid fraction and used to calculate retention forces. Contact angle distribution and base area shapes are then used to calculate retention forces between drops and SH surfaces. These calculations are compared with the component of measured drop weight acting parallel to the plane on a tilted surface for validation. Previous retention force studies that investigate base area shape and contact angle distribution for smooth surfaces are not applicable for microstructured SH surfaces. The work shows that using a sigmoid contact angle distribution and modified base area shape yields retention forces that are on average 50% better than previously reported methods. Retention forces for smooth and SH surfaces calculated in this study were used to suggest retention force factor values for varying solid fraction surfaces.

Hydride breathing and its consequence on stresses applied to containers: A review

This review addresses the hydride swelling and shrinking phenomenon, named hydride breathing, when absorbing and desorbing hydrogen. Two aspects are particularly examined: the evolution of the powder bed volume during cycling, and the stress applied to containers by the hydride bed. Vertical and horizontal containers are distinguished. The literature on the subject is not very exhaustive. Consequently, this article makes a rather complete review of it. It reveals quite heterogeneous behavior between different hydrides, and different types of evolutions can be observed during cycling. Concerning the evolution of the powder bed volume under low loading, a wide range of solid fraction is noted. Concerning the stress applied to containers, the behavior is particularly influenced by two factors: the slenderness ratio and the solid fraction. Results are mapped according to these factors. There seem to be a critical value for the slenderness ratio around 1, but it is difficult to give a universal critical value for the solid fraction. Modelling attempts are also exposed. Some comfort these conclusions, but no model is sufficiently predictable for a use in dimensioning hydride tank. The review finishes with empirical and practical recommendations for a safe design of hydrides containers.

Role of the local stress systems on microstructural inhomogeneity during semisolid injection

High pressure metal die casting is an extremely dynamic process with widely ranging cooling rates and intensifying pressures, resulting in a wide range of solid fractions and deformation rates simultaneously existing in the same casting. These process parameters and their complex interplay dictate the formation of microstructural solidification defects. In this study, fast synchrotron X-ray imaging experiments simulating high pressure die casting of aluminium alloys were conducted to investigate the effect of solid fraction, loading conditions and semisolid flow on local microstructural inhomogeneity. While most of the existing literature in this field reports speeds up to 10 µm/s for in situ deformation, the present work captures much faster filling and solidification, at speeds closer to 100 µm/s and at different solid fractions. Semisolid deformation of low solid fractions reveals two typical microstructural features: (i) coarser grains in the middle and finer ones near the walls, and (ii) remelting near the solid-liquid interface due to Cu enrichment in the liquid by the flow. Ex situ scans and digital image correlation analysis of the higher solid fraction samples reveal a porosity formation mechanism based on the local state of stresses, microstructure and feeding. Four different characteristics were identified: (i) plug flow, (ii) dead zone (densified mush), (iii) shear and (iv) bulk zones. These insights will be used to develop zone-specific strategies for the numerical modelling of defect formation during die casting.

Connection between pore-scale and macroscopic flow characteristics of recirculating wake behind a porous cylinder

The wake structure behind a porous square cylinder is numerically investigated by using both pore-scale and macroscopic approaches. The pore-scale simulations (PSSs) concern about the steady flow through and around square arrays of multiple circular cylinders with a wide range of solid fraction. The macroscopic porous media model (PMM) employed is the generalized equation, where the dimensionless permeability Dam is assigned based on the macroscopic permeability Das estimated from PSS via Darcy’s law. The connection between pore-scale and macroscopic flow properties is studied in terms of the flow pattern, the geometric parameters, and the occurrence of the recirculating wake behind the array. It is found that the consistency between PSS and PMM is highly dependent on the ratio of Das and Dam. Discussions in terms of the scale analysis of PMM, the discrepancy between Dam and Das, and the effects of stress-jump parameters are also provided.

Investigation of drag properties for flow through and around square arrays of cylinders at low Reynolds numbers

The drag properties of flow through and around square arrays of 10 × 10 evenly spaced circular cylinders with a wide range of solid fraction ϕ from 9.69×10-11 to 0.785 and Reynold number of the array Re from 1 to 50 are investigated numerically. The purpose of the present study is to better understand the phenomena of flow through and around permeable bodies that appear in bioreactors and fluid–solid reactors, for example. Instead of using the semi-empirical porous medium model, the direct numerical simulation is employed in order to investigate the flow mechanism from a microscopic point of view. A peak in drag coefficient is observed for the range of ϕ investigated, which appears at a smaller ϕ as Re increases. Three flow regimes are identified based on the shape effects quantified via the averaged velocity within the array, and the drag property in each regime is analyzed. A novel drag formula is also found by investigating the dependence of drag on ϕ and R (Reynolds number based on the mean velocity within the array). Finally, the dimensionless macroscopic permeability Da is estimated via the Darcy’s law. The drag ratio computed is in a good agreement with the previous results from porous media models.

Effect of the forced flow on the permeability of dendritic networks: A study using phase-field-lattice Boltzmann method

A phase-field-lattice-Boltzmann method, coupled with parallel computing and adaptive mesh refinement (Para-AMR), was employed to study the effect of forced flow on permeability of the mushy zone during solidification. Accuracy of the method was testified by comparing with predictions of conventional models, as well as other numerical investigations in literature. Results showed that the permeability of the mushy zone changed as the forced flow was imposed. For columnar dendritic networks, the permeability decreased exponentially either parallel or normal to the dendritic growth direction due to impinging and interlocking of secondary arms. The permeability of equiaxed dendritic networks decreased significantly when the flow was present. The permeability changed as a result of morphological transformation of the dendritic network, which became more columnar-like under a higher incoming velocity. Based on the numerical results and theoretical analysis, a new model was proposed for predicting permeability under the forced flow. Results showed that the model was suitable for describing the permeability of both columnar and equiaxed dendritic structures under a moderate forced flow, and could be used as a viable alternative to determine the permeability in a wide range of solid fraction during solidification.

A study of the rheology of planar granular flow of dumbbells using discrete element method simulations

Granular materials handled in industries are typically non-spherical in shape and understanding the flow of such materials is important. The steady flow of mono-disperse, frictional, inelastic dumbbells in two-dimensions is studied by soft sphere, discrete element method simulations for chute flow and shear cell flow. The chute flow data are in the dense flow regime, while the shear cell data span a wide range of solid fractions. Results of a detailed parametric study for both systems are presented. In chute flow, increase in the aspect ratio of the dumbbells results in significant slowing of the flow at a fixed inclination and in the shear cell it results in increase in the shear stress and pressure for a fixed shear rate. The flow is well-described by the μ-I scaling for inertial numbers as high as I = 1, corresponding to solid fractions as low as ϕ = 0.3, where μ is the effective friction (the ratio of shear stress to pressure) and I is the inertial number (a dimensionless shear rate scaled with the time scale obtained from the local pressure). For a fixed inertial number, the effective friction increases by 60%-70% when aspect ratio is increased from 1.0 (sphere) to 1.9. At low values of the inertial number, there is little change in the solid fraction with aspect ratio of the dumbbells, whereas at high values of the inertial number, there is a significant increase in solid fraction with increase in aspect ratio. The dense flow data are well-described by the Jop-Forterre-Pouliquen model [P. Jop et al., Nature 441, 727–730 (2006)] with the model parameters dependent on the dumbbell aspect ratio. The variation of μ with I over the extended range shows a maximum in the range I ∈ (0.4, 0.5), while the solid fraction shows a faster than linear decrease with inertial number. A modified version of the JFP model for μ(I) and a power law model for ϕ(I) is shown to describe the combined data over the extended range of I.

Experimental Investigation on Steel Foams Fabricated by Sintering-Dissolution Process

This article describes a new process to manufacture open-cell steel foams. Calcium chloride anhydrous is used as a space holder. By changing the values of the main manufacturing parameters such as volume percentage, and the size and shape of the space holder, we produce different steel foam samples which cover a wide range of solid fraction, pore size, and shape. The effects of space-holder content and sintering condition such as temperature and time on the porosity of steel foam samples are discussed. The microstructure and composition of steel foam samples are observed and analyzed by scanning electron microscope and X-ray diffraction. The compressive curves of steel foams are measured by a universal testing machine. The experiment results show the compressive strength of steel foam samples with porosities between 65% and 85% is in the range of 66.4 ∼ 12.9 MPa. The compressive strength depends mainly on the porosity and pore shape. The absorbed energy per unit volume (W) of steel foams with porosities between 85% and 65% is in range of 6.8 ∼ 31.2 MJ/m3. Under the condition of identical porosity, the absorbed energy per unit volume (W) of steel foam is about three times of aluminum foam. In compression, steel foam specimens show heterogeneous macroscopic deformation.

The effects of manufacturing parameters on geometrical and mechanical properties of copper foams produced by space holder technique

We describe a powder metallurgical space holder method to produce open-cell metallic foams. By changing the values of the main manufacturing parameters such as volume percentage and the particle size of the space holder agent, we produce different copper foam samples which cover a wide range of solid fraction, pore size and cell wall thickness. All the specimens were synthesized based on a series of designed experiments. We demonstrate how the foams’ density, cell size and specific surface area can be accurately controlled using two easily adjustable manufacturing parameters. The three-dimensional structure of these foams was investigated using X-ray micro tomography. The image quality is sufficient to measure local structure and connectivity of the foamed material, and the field of view large enough to calculate material properties. By combining the finite element method with the tomographic images, we calculate the mechanical response of the foams. We show that the foams’ bulk and shear moduli are strongly correlated to their cell size, cell wall thickness and specific surface area. These parameters can be easily controlled during manufacturing.

Calculation of Drag Coefficient for Arrays of Emergent Circular Cylinders with Pseudofluid Model

The emergent vegetation in open-channel flows is usually simulated using arrays of circular cylinders in laboratory experiments. Analysis of recent experimental data reveals that for a given Reynolds number, the drag coefficient of a cylinder in a dense array is larger than that of an isolated cylinder. A new approach is applied to parameterize the drag coefficient and Reynolds number for flows through arrays of emergent cylinders. The approach is developed based on the concept of pseudofluid, for which an analogy is made between the cylinder-induced drag in an open-channel flow and that induced by the cylinder settling in a stationary fluid. With the proposed parameterization, the experimental database is successfully reorganized in such a way that a generalized drag coefficient is related to a generalized Reynolds number by one single curve, which is valid for a wide range of solid fractions and Reynolds numbers. However, it should be mentioned that only rigid circular stems are considered in this study and their induced drag is assumed to be dominant in comparison with the channel bed resistance.

Biochemical characteristics of solid fractions from animal slurry separation and their effects on C and N mineralisation in soil

Solid fractions from separated animal slurry can be used as organic fertilisers on agricultural land. Solid fractions contain variable amounts of inorganic and organic N, so it is important to synchronise their application in the field with crop demand to ensure N availability in the growing season. This study quantified C and N mineralisation for a wide range of solid fractions from slurry separation applied to soil and examined potential correlations between chemical and biochemical characteristics of solid fractions and their C and N turnover. The solid fractions were mixed with soil and incubated at 14°C for 120 days, during which CO2 evolution and inorganic N content of the solid fractions were determined. A two-parameter exponential function fitted to the individual solid fraction C mineralisation patterns explained 98% of the data variation, while a three-parameter Monod-type equation fitted to the net N mineralisation patterns explained 89% of the variation. Between 5% and 45% of initially added C was mineralised within the incubation period, with the largest proportion tending to be mineralised from simple mechanically separated solid fractions (MEC). Nitrogen was initially immobilised by the majority of solid fractions. Solid fractions from decanter centrifuged, anaerobically digested slurry (DEC) and chemically pre-treated and separated slurry (KEM) began to re-mineralise N after 20–40 days, whereas N was continuously immobilised from MEC solid fractions. The carbon mineralisation rate constant was correlated with the C content in the neutral detergent soluble (NDS), hemicellulose and cellulose fractions and the N content in NDS. Net nitrogen mineralisation was correlated with the C/Norg ratio of solid fractions, the N content of NDS and the C content of hemicellulose and cellulose.