Ligament Diameter Research Articles

Ligament-type liquid disintegration by a spinning wheel

In this paper, liquid disintegration by a spinning wheel was investigated experimentally. The mechanism of ligament formation on a spinning wheel was studied using photographs taken by a high-speed camera. Three different liquids with Newtonian properties were used at various flow rates and the wheel rotational speed was varied in a wide range. The atomization process was found to be significantly different from the spinning disc and cups atomization due to the highly non-uniform circumferential ligament distribution and the absence of the direct drop formation mode. Nevertheless, the dependence of ligament number and diameter on the input process parameters is similar as with other types of centrifugal atomizers due to the same underlying hydrodynamic instabilities.

Effect of catalyst and substrate on the moisture diffusivity of silica-aerogel-coated metal foams

Silica aerogel desiccants have good adsorption and desorption characteristics and commonly are deployed on metallic or non-metallic substrates as thin coatings in typical dehumidifying applications. The dehumidifying performance of desiccant coated on substrates often depends on the micro-structure of desiccants, as well as on the characteristics of substrates, such as surface area. The current study focuses on the preparation and adsorption/desorption performance evaluation of various types of silica aerogel coatings on different metal foam substrates. The silica aerogel coated metal foam samples have been prepared by dip coating process using different basic and acidic catalysts, followed by the super-critical drying. The microstructures of aerogel coating obtained by scanning electron microscopy, are compared to porous structures of solid desiccant blocks prepared using the same method (catalyst). A new automatic dynamic vapor sorption method is used to determine the mass diffusion coefficient of silica aerogel coated foam samples. SEM image analysis is used to determine the geometrical parameters (pore diameter, ligament diameter) of coated and uncoated foams of 5, 10 and 20PPI for data reduction. The impact of substrate type and microstructure of the coating, which depends on the catalyst used in sol–gel process on the mass diffusivity, has been evaluated. The results of this study can be used in the development and performance evaluation of various dehumidification applications containing silica aerogels deployed as thin coatings.

Primary and Secondary Dealloying of Au(Pt)-Ag: Structural and Compositional Evolutions, and Volume Shrinkage

Systematic study of the dealloying of Au-Ag and Au(Pt)-Ag alloys shows that the dealloying occurs by two processes: a primary dealloying process that selectively dissolves Ag from the parent alloy and creates a nanoporous (np) structure, and a secondary dealloying process that occurs behind the corrosion front and further dissolves the residual Ag from the nano-ligaments. The secondary dealloying can occur during coarsening, and/or when a more anodic potential is applied. With suppressed np structure coarsening in Pt-containing samples, we found that the intrinsic np structure created by the primary dealloying contains small ligament diameter (3–7 nm) and high concentration of residual Ag (∼50 at.%), irrespective of the dealloying potentials. Dilatometry experiments show that the volume shrinkages are rather small during primary dealloying and are large during secondary dealloying. The primary dealloying can be explained by the percolation dissolution mechanism. Although the mechanism of the secondary dealloying (without coarsening) remains unclear, we point out that the kinetics of the secondary dealloying is decisive to some important characters of np metals, such as the crack formation and the final residual Ag concentrations.

Vortex shedding in a highly porous structure

Highly porous media (with a porosity ≥85%), like metal foams, fins or designed porous inserts are widely used in industrial flow applications. Motivated by the use of highly porous inserts in continuous chemical reactors, the time dependent behavior of flows through a highly porous structure is investigated experimentally and numerically for Reynolds numbers ReD=33 and 330 (based on the ligament diameter and the bulk velocity). The experimental studies are based on the Particle Image Velocimetry (PIV), whereas the numerical studies are conducted by using the Large Eddy Simulation (LES) approach with the dynamic Lagrangian subgrid closure. The vortex shedding phenomena is observed for both values of Reynolds number. It is demonstrated that the length of the wake and the orientation of the shedded vortices are dependent on the position within the highly porous structure.

A Theoretical Model with Experimental Verification to Predict Hydrodynamics of Foams

An experimentally validated theoretical model, based on hydraulic resistance network and scale analysis at the pore level, is developed to predict the pressure drop for flow through foams. The complex microstructure of the foams is modeled as a matrix of interconnected solid ligaments forming simple cubic arrays of cylinders. New correlations for permeability and form drag (inertia) coefficient are presented as functions of the mean pore and ligament diameter as well as the foam porosity. The present model makes it possible to conduct parametric studies. Results obtained from the proposed model are successfully compared with our experimental data as well those found in the literature to observe good agreement.

Composites of Nanoporous Gold and Polymer

Experimental investigations of the strength of small objects – such as micropillars or nanowires – often point towards a trend of increasing strength with decreasing dimension,1–3 approximating the theoretical shear strength when the size drops to the lower nanometer region.1–4 The observation of theoretical strength in defect-free crystals, such as whiskers, irrespective of their size exemplifies that the trend of smaller is stronger is related to the defect structure.5–7 The interaction of dislocations with the surface is another important factor, as is evidenced by in situ observation of large recoverable flow-stress changes during interfacial charging or electrosorption.8 Irrespective of its microscopic origin, the high strength at small size suggests a search for design strategies that yield high-strength materials exploiting the mechanical properties of metal nanostructures. A key challenge, namely assembling many (1018 for 1 cm3 of material with a 10 nm structure size) nanoscale objects into a macroscopic body, can be overcome by synthesis via dealloying.9–11 The process provides millimeter- or centimeter-sized monolithic samples consisting of a homogeneous network structure of nanoscale “ligaments” with uniform size that can be controlled down to well below 10 nm.12, 14 Investigations by transmission electron microscopy, focused ion beam imaging, and electron backscatter diffraction have established that nanoporous metals prepared in this way are polycrystalline with a grain size of 10–100 μm.15, 16 Each micrometer-sized grain is nanoporous, so that neighboring ligaments share the same crystal lattice. In other words, the local structure in volumes of, say, 1 μm3, is that of a single crystal containing a contiguous nanoscale pore network. The mechanical behavior of these materials obeys scaling equations derived for foams with macroscopic porosity, and the local strength of the ligaments follows the same3, 17–19 or similar16, 20 trends as individual nanowires. The material, and in particular nanoporous gold (npg), has thus been studied as a model system for size-effects on the plasticity of nanostructures.

Effect of thermal coarsening on the thermal conductivity of nanoporous gold

The thermal conductivities of nanoporous gold (NPG) microwires annealed at different temperatures have been measured in the temperature range from 100 to 320 K. Considering the electron-surface scattering, the thermal conductivity is expected to increase with the increase of ligament diameter. However, the thermal conductivity of NPG microwire is found to decrease after thermal coarsening, and has a maximum value at around 250 K for the as-dealloyed sample. We suggest that the defects accumulating at a relatively high temperature and the reduction in defect spacing may cause these temperature behaviors of thermal conductivity. Taking into account the electron scattering on ligament surfaces and defects, a modified theoretical model for the thermal conductivity of nanoporous metal is proposed to agree with our experimental results.

J051045 プレフィルミングェアーブラストアトマイザーから放出される液膜の分裂モデル

Prefilming air blast atomizer is mainly used in the gas turbine engine of an airplaine. Annular liquid sheet flows on a solid surface called prefilmer and is broken into small droplets by aerodynamic force. In this study, a new model for breakup of liquid sheet from air blast atomizer is proposed. In this model, longitudinal wavelength λ_1, and transversal wavelength λ_2 are predicted based on hydrodynamic instability. Liquid sheet thickness h is also predicted theoretically. Given that volume of liquid lamp λ_1,λ_2 ,h equals to volume of liquid ligament 2(πd_l2^/4)λ_1+(π/2)( πd_l^2/4)λ_2 , ligament diameter di is determined. Ligament diameter then gives droplet diameter through Weber's theory. Finally, Droplet diameter D after secondary breakup is calculated using TAB model in which air velocity at atomizer exit U_<Aexit> determined in numerical simulation is used as characteristic velocity. The predicted parameters, λ_1,λ_2 ,h,d_1,U_<Aexit> and D, arel compared with experimental results. The predicted results show good agreement with experimental results.

Effect of heat treatment of Mn-Cu precursors on morphology of dealloyed nanoporous copper

Nanoporous copper with nano-scale pore size was synthesized by dealloying Mn-Cu precursor alloy using a free corrosion method. The effects of heat treatment of Mn-Cu precursors on alloy phase, morphology and composition of the resultant nanoporous copper were investigated. It is revealed that the compositions distribute homogeneously in the bulk Mn-Cu precursors, which consequently results in a more fully dealloying for forming nanoporous copper. The alloy phase changes from Cu0.49Mn0.51 and Cu0.21Mn0.79 of non-thermally treated precursor to Cu0.33Mn0.67 of heat treated alloy. The residual Mn content in nanoporous copper is decreased from 12.97% to 2.04% (molar fraction) made from the precursor without and with 95 h heat treatment. The typical pore shape of nanoporous copper prepared by dealloying the precursor without the heat treatment is divided into two different zones: the uniform bi-continuous structure zone and the blurry or no pore structure zone. Nanoporous copper is of a uniform sponge-like morphology made from the heat-treated precursor, and the average ligament diameter is 40 nm, far smaller than that from the non-thermally treated precursor, in which the average ligament diameter is estimated to be about 70 nm.

Thermal Assessment of Forced Convection Through Metal Foam Heat Exchangers

Using a thermal resistance approach, forced convection heat transfer through metal foam heat exchangers is studied theoretically. The complex microstructure of metal foams is modeled as a matrix of interconnected solid ligaments forming simple cubic arrays of cylinders. The geometrical parameters are evaluated from existing correlations in the literature with the exception of ligament diameter which is calculated from a compact relationship offered in the present study. The proposed, simple but accurate, thermal resistance model considers: the conduction inside the solid ligaments, the interfacial convection heat transfer, and convection heat transfer to (or from) the solid bounding walls. The present model makes it possible to conduct a parametric study. Based on the generated results, it is observed that the heat transfer rate from the heated plate has a direct relationship with the foam pore per inch (PPI) and solidity. Furthermore, it is noted that increasing the height of the metal foam layer augments the overall heat transfer rate; however, the increment is not linear. Results obtained from the proposed model were successfully compared with experimental data found in the literature for rectangular and tubular metal foam heat exchangers.

Evolution of Nanoporous Pt–Fe Alloy Nanowires by Dealloying and their Catalytic Property for Oxygen Reduction Reaction

AbstractThe short life and high cost of carbon‐supported Pt nanoparticle catalysts (Pt/C) are two main problems with proton exchange membrane fuel cells. Porous Pt alloy nanowires have more durability and catalytic activity than Pt/C. Dealloying is a facile way to make nanoporous Pt. However, the process of porosity formation is difficult to control. In this paper, electrospinning and chemical dealloying techniques are used to make long, thin and yet nanoporous Pt–Fe alloy nanowires. The evolution of nanoporosity is observed and studied. It is found that non‐uniform composition in the precursor PtFe5 alloy nanowires helps the formation of nanoporous structure. The overall wire diameter is about 10–20 nm and the ligament diameter only 2–3 nm. These porous long nanowires interweave to form a self‐supporting network with a high specific activity, 2.3 times that of conventional Pt/C catalysts, and also have better durability.

Are Nanoporous Materials Radiation Resistant?

The key to perfect radiation endurance is perfect recovery. Since surfaces are perfect sinks for defects, a porous material with a high surface to volume ratio has the potential to be extremely radiation tolerant, provided it is morphologically stable in a radiation environment. Experiments and computer simulations on nanoscale gold foams reported here show the existence of a window in the parameter space where foams are radiation tolerant. We analyze these results in terms of a model for the irradiation response that quantitatively locates such window that appears to be the consequence of the combined effect of two length scales dependent on the irradiation conditions: (i) foams with ligament diameters below a minimum value display ligament melting and breaking, together with compaction increasing with dose (this value is typically ∼5 nm for primary knock on atoms (PKA) of ∼15 keV in Au), while (ii) foams with ligament diameters above a maximum value show bulk behavior, that is, damage accumulation (few hundred nanometers for the PKA's energy and dose rate used in this study). In between these dimensions, (i.e., ∼100 nm in Au), defect migration to the ligament surface happens faster than the time between cascades, ensuring radiation resistance for a given dose-rate. We conclude that foams can be tailored to become radiation tolerant.

An experimental investigation on the breakup of surfactant-laden non-Newtonian jets

The combined effect of polymers and soluble surfactants on the dynamics of jet breakup, and especially on satellite drop formation, was experimentally investigated. Xanthan gum and Carbopol ® 934 NF were dissolved in water with Sodium Dodecyl Sulfate as the surfactant. Controlled disturbances were imposed at the laminar jet interface using a piezoelectric vibrating nozzle with breakup dynamics recorded using a high-speed camera. Drop and ligament diameters were measured from the digital images. The focus of the work was investigating how bulk and interfacial properties of the prepared fluids influenced ligament and drop evolution. It was found that if the proper concentration of surfactant (close to the critical micelle concentration, CMC) was selected, and if the flow time scales were large enough, Marangoni interfacial stresses may lead to an increase in satellite drop size as previously reported for breakup simulations of shear-thinning jets covered with insoluble surfactant. It was also experimentally confirmed that the introduction of surfactant contributes to a delay in jet breakup.

Wear behaviour of interpenetrating alumina–copper composites

The wear behaviour of a variety of alumina–copper interpenetrating composites was tested as a function of copper ligament diameter and volume fraction of copper. The wear mechanisms of pure copper and pure alumina were adhesive and abrasive wear, respectively. In the composites with 1 μm, 5 μm and 15 μm copper ligament diameters, the wear mechanism was a mixture of adhesive and oxidative; in composites with a 30 μm copper ligament diameter a mixture of abrasive and oxidative. Increasing the amount of copper decreased hardness and thus increased wear, except where cyclic tribolayer behaviour occurred or where the alumina grains were weakly bonded. Increasing the copper ligament diameter decreased wear, although this trend was only clear under a load of 20 N. The composites with the highest wear resistance had the highest copper ligament diameter of 30 μm. This was probably due to the higher heat conductivity and fracture toughness caused by the coarse, fibrous copper network replicating the wool felt used to produce it. This was possibly also because these composites had the smallest alumina grain size.

Heat transfer in metal foams and designed porous media

We present the characterization of heat transfer in commercial metal foam filled tubular reactors in comparison to a designed laser sintered device. The investigations are performed at empty tube Reynolds numbers ranging from 600 to 7600. Volumetric heat transfer performances up to 4.5 MW/(m 3 K) were estimated by means of a simple calorimetric measurement setup, which is 3 orders of magnitude higher compared to conventional batch reactors. The heat transfer was found to increase with the ligament diameter ascribed to the enhanced turbulent kinetic energy induced. The fixed wall connection of the fully sintered device, realized by the applied manufacturing method, leads to 30% improvement of the heat transfer compared to no connection. Selective laser sintering was found to be an efficient tool for the design of continuous heat exchanger reactors covering a wide range of applications by simply adapting the geometry.

Deformation mechanisms in gold nanowires and nanoporous gold

We present a study of the deformation of gold nanowires of diameter 30–70 nm and nanoporous gold specimens with ligament diameter 5–10 nm, both produced by electrodeposition into anodised aluminium oxide templates. The nanowires show extensive surface slip steps and low dislocation densities with a few perfect dislocation loops, Shockley partial dislocations and microtwins observed after deformation. Nanoporous specimens show deformation localised to the nodes between the ligaments of the foamed structure, with high densities of microtwins and Shockley partial dislocations in these regions. Similar dislocation structures are seen in larger nanowires deformed in bending. This is shown to be consistent with a strain gradient plasticity model for the deformation of nanoporous gold, with the strain gradient accommodated by geometrically necessary twins and partial dislocations.

On tuning the morphology of nanoporous gold

This paper concentrates on tuning the morphology of nanoporous metals, in particular Au, as a function of the dealloying potential. Ligament diameter and pore size were evaluated from digital image analysis and from measurements taken directly from scanning electron micrographs. Linear relationships were observed between the dealloying potential and the size of ligaments and pores. At a low dealloying potential the pore size is larger than the ligament diameter but at a high dealloying potential they are comparable.

Strain gradients and the strength of nanoporous gold

The very high strengths that have been reported for nanoporous gold may be related to strain gradients within the deforming porous microstructure. We present a mechanism-based model for the strength of nanoporous foams that is derived from conventional models for the deformation of macroscopic foams and now includes the influence of strain gradients. This model predicts that the strength of the ligaments within the nanoporous gold is proportional to the ligament diameter raised to the power −0.5. We have used the model to analyze experimental data for the strength of nanoporous gold and find excellent agreement with published data.

Liquid Atomization Using a Rotary Bell Cup Atomizer (Influence of Flow Characteristics of Liquid on Breakup Pattern)

This present study investigates liquid flow patterns on the inner surface of a rotary bell cup atomizer and the effect of these flow patterns on the breakup patterns at the edge of the rotary bell cup. It also estimates the atomization characteristics based on the diameter of liquid ligaments issuing from the cup edge. The cup had an outer diameter of 70 mm and the following experimental conditions were used: rotational speed N = 1,000 to 50,000 rpm and liquid flow rate Q = 50 to 300 mL/min. Classification of the breakup pattern at the cup edge revealed that, irrespective of the flow rate, several liquid ligaments were produced at low rotational speeds and fine ligaments, which cause Rayleigh breakup, were generated at high rotational speeds. Classification of the liquid flow pattern on the cup surface revealed that the liquid film on the surface was a smooth film at low rotational speeds and a radial streak film or a partial dry film at high rotational speeds. Good atomization characteristics are expected since the radial streak film and the partial dry film cause fine ligament breakup at the cup edge. It was also demonstrated that the mean droplet size for Rayleigh breakup, which generates uniform droplets, can be effectively estimated by calculations based on the diameter of the ligaments that form on the cup edge.

Synthesis and characterization of nanoporous Pt–Ni alloys

Two nanoporous Pt–Ni alloys were synthesized by dealloying ternary amorphous Si–Pt–Ni precursors. Both foams have nearly the same composition, ligament diameter size, and density. However, their ligament patterns are different, depending on the microstructure of precursors. The difference in morphology is shown to have a profound effect on mechanical properties. The structure with well-aligned long nanoligaments exhibited over 50% higher hardness and stiffness than the structure with short random-oriented nanoligaments. These nanoporous Pt–Ni structures are thermally stable at 300 °C.