Air Volume Fraction Research Articles

Hypersonic spectroscopy of porous silicon for acoustic devices

I will review our work on porous silicon (pSi) presenting achievements while highlighting underlying physical questions that remain to be answered. pSi is produced by the electrochemical etching of crystalline silicon. It is typically mesoporous, having pores of 10–30nm diameter. The etching current density determines the final porosity, the volume fraction of air, with a wide range of porosities, 25%–95%, achievable. For wavelengths much greater than the pore size, pSi gives a tunable effective medium for light and sound waves. We have characterized pSi acoustic properties using transmission spectroscopy with matched transducer pairs working at 0.5–2.5 GHz. The results for velocity, v, have fitted to a general law of v=v0(1−φ)k, where v0 is the velocity in bulk silicon, φ is porosity, and k is the fitting parameter. We have investigated the variation of k with the direction of propagation and the etching conditions used to extract the dependence of the elastic constants on porosity. The measurement of velocity has enabled us to produce and characterize pSi Bragg mirrors and rugate filters that have a smoothly varying acoustic impedance. This has demonstrated the potential use of pSi in acousto-optic phoxonic crystal devices that have both phononic and photonic bandgaps.

펌프 섬프장내 자유표면 보텍스에 의한 공기흡입 현상의 가시화

In this study the change of free surface vortex is expressed through the time volume fraction using multiphase unsteady condition in sump, because in previous studies of the pump sump did not represent the behavior of the free surface vortex exactly due to the reason it was calculated using single phase and steady condition. The reliability of the computational analysis is verified through comparing experimental results with that of present numerical analysis. Homogeneous free surface model is used to apply interactions of air and water. The results show that the free surface vortex can be identified on the isotropic surface at air volume fraction 1%~5%. The vortices make an air column from the free surface to the sump intake and are created and destroyed repeatedly. The behavior of free surface vortex at numerical analysis is quite similar to experimental test. The result of vortex motion according to time, works on a cycle.

Numerical Simulation of the Flow of Gases in a Large-Scale Electroslag Furnace

This paper reports a CFD modeling study on the flow of gases in an electroslag furnace during Protective Gas Electroslag Remelting(PESR) process by a commercial code FLUENT. The Realizable k−ε turbulence model, Do radiation model and Mixture model are amply used in FLUENT code to compute the flow of gases in the furnace under different argon gas flow, and the influence of argon gas flow on air volume fraction in the furnace is obtained. Finally, a suited suggestion on argon gas flow during PESR process is presented.

Rheological characterisation of cake batters generated by planetary mixing: Elastic versus viscous effects

Studies of cake batter rheology have focused on viscous behaviour. We demonstrate that elastic effects dominate at the shear rates used in commercial mixers. The development of batter structure was investigated for two flour types using two bench-scale planetary mixers with known shear rate profiles (Kenwood-KM250, maximum 100 s −1; Hobart-N50, 500 s −1). These wet foams (air volume fraction 0.39–0.45) showed shear-thinning behaviour at low shear rates (0.1–10 s −1), with apparent viscosity dependent on air volume fraction. Simple shear thinning behaviour ceased, for foams, above 10–20 s −1: for slurries (air volume fraction, 0.11–0.15) the limit approached 100 s −1. Elastic effects, predominantly arising from the bubble phase, therefore dominate cake batter behaviour at the shear rates experienced in commercial mixers. Filament thinning extensional rheometry confirmed the VE behaviour of batters. These results indicate that visco-elastic analyses are likely to be the most appropriate probe of microstructure in cake batters.

Rheological characterisation of cake batters generated by planetary mixing: Comparison between untreated and heat-treated wheat flours

The rheological behaviour of high ratio cake batters prepared with untreated and heat-treated wheat flours was analysed at different stages of the manufacturing process, namely slurry, (on aeration) foams and (with fat addition) aerated emulsions, featuring air volume fractions up to 0.50. Both steady shear and viscoelastic behaviours were studied. All materials exhibited shear-thinning behaviour at 20 °C over the shear rate range studied (0.07–10 s −1). The generalised Cox–Merz rule could be applied to all samples. Materials prepared with heat-treated flours exhibited greater stability, as indicated by slurry thixotropy and cohesive energy, and the change in apparent viscosity and air content of foams and aerated emulsions on extended mixing. Foams and aerated emulsions showed significant elastic behaviour with G′∼ G″. The temperature dependency of aerated emulsions was studied by oscillatory shear testing from 20 to 100 °C and indicated three regimes in temperature dependence: below 40 °C G′and G″ were insensitive to temperature; between 40 and 70 °C the complex viscosity exhibited Arrhenius-type behaviour, while above 70 °C G′ and G″ increased as expected for gelatinisation and foam setting. The weak gel model for foods was used to analyse the latter data sets and confirmed that the gel network generated in aerated emulsions prepared with heat-treated flours was significantly stronger than those made with unheated flours. The differences between flour types were also observed in tests on un- and heat-treated flours obtained from a second and third harvest. The impact of these quantifiable differences in rheology on performance during baking is discussed.

Measurement of dissolved CO2 transport by snowmelt runoff in paddy fields

The quantity of CO2 carried away by snowmelt during the thawing of a deposited snow layer was measured in an area subject to seasonal accumulation of snow. The concentration of dissolved CO2 was measured in snowmelt discharged from the outlet of a paddy field with heavy clay soil in Joetsu City, Niigata Prefecture, Japan. Samples were taken every few hours over a period of three days. The CO2 flux Fwater transported by the snowmelt was estimated from changes in the concentration of dissolved CO2, with the CO2 concentration continuously measured at the bottom of the deposited snow layer, and the quantity of snowmelt estimated from heat balance measurements. The ratio of the CO2 concentration, Csnow, to that dissolved in the snowmelt, Cwater, gradually decreased as the quantity of snow melted per unit time increased. Therefore, assuming this ratio to be proportional to the inverse of the snowmelt flow speed, a regression was carried out, based on the fact the flow speed is proportional to the quantity of melted snow per unit time raised to a power of 2/3. To assess the validity of the estimated Fwater, we studied the balance of CO2 in the deposited snow layer by using the CO2 store in the deposited snow. The quantity value of the soil respiration flux estimated from the balance of CO2 in the deposited snow layer was close to the average soil respiration flux during the snow-covered period estimated by the CO2 concentration gradient and the volume fraction of air in the snow layer previously deposited in the same paddy field.

Controllability of Radiative Heat Flux across Mould Flux Films by Cuspidine Grain Size

Apparent reflectivities and transmissivities have been measured as functions of cuspidine grain diameter for mould fluxes having constant degrees of crystallinity. Samples used were two types of synthesised mould flux with the basicity of 1, one of which samples contained 1 mass% of Fe2O3, and the grain diameter was varied in the range 1–3.5 μm. The optical measurements were carried out in the wavelength range 300–2600 nm at room temperature using a spectrophotometer with an integrating sphere. With increasing grain diameter, the apparent reflectivity tended to increase and the apparent transmissivity tended to decrease at higher wavelengths for iron oxide free mould fluxes: it seemed that the apparent reflectivity showed a maximum value and the apparent transmissivity showed a minimum value in the grain diameter range 2–3 μm. In contrast, there was less significant dependence on grain size for mould fluxes containing iron oxides. The total radiative heat flux which may reach the mould from the steel shell has been evaluated using apparent reflectivity and transmissivity data on the basis of an optical process model. It has been found that the total radiative heat flux would be smallest in iron oxide free mould fluxes having the highest apparent reflectivity and the lowest apparent transmissivity at higher wavelengths. Effects of grain size on the radiative heat flux are smaller for mould flux containing iron oxides. Comparison of the total radiative heat flux with the total heat flux including conductive contribution suggests that control of cuspidine grain diameter would lead to reduction of the total heat flux by 7–8% for iron oxide free mould fluxes. In addition, the air gap layer would affect the total heat flux more efficiently where the volume fraction of air in the layer exceeds 85%.

Comparative Study Between Various Antireflective Coatings For Solar Cells Applications

The incessant progress in thin layers technology, especially inhomogeneous materials with variable index allows the fabrication of antireflective coating (ARC) less sensitive to the thickness and the incidence angle. Accordingly, our work is devoted to study the optical behaviour of graded refractive index ARC made from: silicon oxynitrides (SiOxNy), mixture of silicon and titanium oxides (SiO2-TiO2) and porous silicon (P-Si), for solar applications. The refractive index of the studied ARC varies versus thickness according to Fermi profile variation law. The physical parameters of such coating are its: thickness, form factor, profile inflection point and silica or air (porosity) volume fraction variation at the two interfaces (air/ARC and ARC/substrate). The optimisation of the studied films performance shows that best results are obtained with P-Si ARC, which reduce weighted reflectance till to 1.86% (between 300-1100 nm) leading to an improvement in photogenerated current of 51%.

Numerical simulation of the plug discharge under aerated condition

In Fluent, 3-D RNG k–ɛ mathematical model of water and air mixtures is employed to compute plug discharge under aerated condition. When flow enters the plug section, the gradient of air volume fraction increases much and varies drastically, and cavitation number decreases sharply and soon reaches the minimum. When flow enters the sudden expansion, air volume fraction and cavitation number increases respectively. With the increase of sudden expansion length, air volume fraction appears large at the top and small at the bottom and cavitation number increases to recover eventually. Cavitation number increases with the increase of air volume fraction.

Low-frequency sound scattering from microbubble distributions.

Backscatter from the sea surface is governed by the roughness of the surface and subsurface micro bubble distributions. At low frequencies, due to the paucity of large bubbles, scattering results primarily from coherent and/or collective scatter from bubbles entrained by the subsurface vorticity or carried to depth by the Langmuir circulation and thermal convection. Theoretical treatments using the work of Mallock and Wood show that scattering from compact regions is a function of the volume fraction of air and to first order can be described by a Minnaert formula modified with the volume fraction. Experiments to test this theory were conducted in a lake and large tank facility. Back scattering from a submerged bubble cloud produced results consistent with our theory when parameters independently measured, such as bubble size distribution, volume fraction, and the cloud size, were used to calculate the resonant frequency with the modified Minnaert formula. This theory was further tested by backscattering experiments on bubbly liquid filled tubes, bubbly gel filled cylinders, and voided polyurethane spheres. Again the resonant frequency and backscatter measurements were found consistent with theoretical expectations; however, the effective dampening was found to depend on the nature of the material containing the microbubbles.

CFD simulation and optimization of effective parameters for biomass production in a horizontal tubular loop bioreactor

The focus of the current study was to perform an experimental investigation and computational fluid dynamic (CFD) simulation of flow hydrodynamics in a forced-liquid horizontal tubular loop bioreactor for the production of biomass. The simulations were performed using the FLUENT commercial CFD package, a segregated unsteady solver and a two-phase Eulerian multiphase model. To validate the simulation results, several experiments were performed in a pilot bioreactor. In addition, the design of experiments methodology using a Taguchi orthogonal array (OA) was applied to evaluate the influence of four factors on the hydrodynamic behavior of the bioreactor. The effective parameters considered for optimization were air inlet velocity, liquid inlet velocity, bubble diameter, and viscosity. An L9 OA was used to conduct the Taguchi experiments to study the significance of these parameters and the possible effects of any two-factor interactions. The optimum conditions and most significant process parameters affecting the hydrodynamic behavior were determined using an analysis of variance model. The results showed that the liquid inlet velocity had the most influence on the air volume fraction in the bioreactor. A subsequent confirmatory test demonstrated that the results were within the confidence interval.

The study on the air volume fraction of electrospun nanofiber nonwoven mats

Electrospinning is an efficient method to produce polymer fibers with a diameter range from nanometers to a few microns using an electrically driven jet. Electrospun nanofiber nonwoven fabrics can be applied into different areas with higher air volume fraction, especially applied into textile materials with good warmth retention property. In this article, the air volume fraction in nonwoven mats made of electrospun nanofibers was verified by studying fiber volume fraction in the mats. Then the relationship between fiber volume fraction and fiber diameter was derived, and the fiber volume fraction is in direct ratio to the square of fiber radius. By experimental verification, to get electrospun PAN nanofiber nonwoven mats with high air volume fraction about 99 %, it can fix the polymer concentration on 8 %. The voltage fixed on 20 kV, the tip-to-collector distance on 15 cm. The experiment is in accordance with the theory excellently.

Sound propagation in a monodisperse bubble cloud: From the crystal to the glass

We present a theoretical study of the propagation of a monochromatic pressure wave in an unbounded monodisperse bubbly liquid. We begin with the case of a regular bubble array--a bubble crystal--for which we derive a dispersion relation. In order to interpret the different branches of this relation, we introduce a formalism, the radiative picture, which is the adaptation to acoustics of the standard splitting of the electric field in an electrostatic and a radiative part in Coulomb gauge. In the case of an irregular or completely random array--a bubble glass--and at wavelengths large compared to the size of the bubble array spatial inhomogeneities, the difference between order and disorder is not felt by the pressure wave: a dispersion relation still holds, coinciding with that of a bubble crystal with the same bubble size and air volume fraction at the centre of its first Brillouin zone. This relation is discussed and compared to that obtained by Foldy in the framework of his multiscattering approach.

Analysis of creaming and formation of foam layer in aerated liquid

A model for creaming and formation of a foam layer in an aerated system consisting of Newtonian liquid is proposed. The variation of air volume fraction in the dispersion layer is described by hindered creaming which is coupled to syneresis in the top foam layer that is described by flow of liquid through a network of Plateau borders due to gravitational and capillary forces. The present analysis accounts for the compressibility of foam layer by coupling creaming analysis with syneresis in the foam layer. The behavior of the system is described by three parameters: (a) characteristic time scale of creaming of an isolated bubble, (b) hydrodynamic interaction factor, and (c) capillary number, ratio of capillary and gravitational forces in the foam layer. System behavior is shown to be different for four different regions of initial air volume fractions for which the phase diagram and evolution of the profile of air volume fraction for batch dispersion are presented.

Computational fluid analysis of abrasive waterjet cutting head

Waterjet cutting is an appealing technology for cutting thick materials with zones that must not be affected by heat. This paper presents computational fluid dynamics (CFD) and theoretical analyses to optimize the mixing of components by the multi-phase approach. Water, air, and abrasives are mixed in a mixing chamber. This modeling is used to predict the influence of air and abrasives on the mixing at different distances within the mixing tube. At the same time, particle tracking was conducted to monitor the erosion rate density at the nozzle wall. Results show that nozzle length has an effect on the mixing of water, air, and the abrasives, and that the velocity of the waterjet influences the erosion rate at the nozzle wall. The k-ɛ turbulence model was used for simulation of the abrasive coupled with air. This investigation reveals that the erosion in the nozzle body is higher at the initial zone and that as the length of the nozzle length increases, the volume fraction of air increases accordingly. The entrance of the orifice is affected by a highly pressurized water stream (with minimal particulate matter), which causes chipping at the leading edge. To reduce the turbulence inside the mixing chamber, the use of a vacuum assist could be helpful, but precautions should be taken in order that the abrasives do not escape from the mixing chamber.

Modeling of Subgrid-Scale Mixing in Large-Eddy Simulation of Shallow Convection

Abstract This paper discusses an extension of the approach proposed previously to represent the delay of cloud water evaporation and buoyancy reversal due to the cloud–environment mixing in bulk microphysics large-eddy simulation of clouds. In the original approach, an additional equation for the mean spatial scale of cloudy filaments was introduced to represent the progress toward microscale homogenization of a volume undergoing turbulent cloud–environment mixing, with the evaporation of cloud water allowed only when the filament scale approached the Kolmogorov microscale. Here, it is shown through a posteriori analysis of model simulations that one should also predict the volume fraction of the cloudy air that was diagnosed in the original approach. The resulting model of turbulent mixing and homogenization, referred to as the λ–β model, is applied in a series of shallow convection simulations using various spatial resolutions and compared to the traditional bulk model. This work represents an intermediate step in the development of a modeling framework to simulate characteristics of microphysical transformations during entrainment and subgrid-scale turbulent mixing.

Continuum Microviscoelasticity Model for Aging Basic Creep of Early-Age Concrete

We propose a micromechanics model for aging basic creep of early-age concrete. Therefore, we formulate viscoelastic boundary value problems on two representative volume elements, one related to cement paste (composed of cement, water, hydrates, and air), and one related to concrete (composed of cement paste and aggregates). Homogenization of the “nonaging” elastic and viscoelastic properties of the material’s contituents involves the transformation of the aforementioned viscoelastic boundary value problems to the Laplace-Carson (LC) domain. There, formally elastic, classical self-consistent and Mori-Tanaka solutions are employed, leading to pointwisely defined LC-transformed tensorial creep and relaxation functions. Subsequently, the latter are back-transformed, by means of the Gaver-Wynn-Rho algorithm, into the time domain. Temporal derivatives of corresponding homogenized creep and relaxation tensors, evaluated for the current maturation state of the material (in terms of current volume fractions of cement, water, air, hydrates, and aggregates; being dependent on the hydration degree, as well as on the water-cement and aggregate-cement ratios) and for the current time period since loading of the hydrating composite material, allow for micromechanical prediction of the aging basic creep properties of early-age concrete.

An experimental study on the influence of fluid flow pattern on microbubble generation

Geometrical properties of generated microbubbles induced by different fluid flow patterns were investigated experimentally. Image processing method has been used to find microbubble size distribution and to determine bubbles’ roundness as well. Three types of flow patterns were produced by changing microbubble generator configuration in order to improve bubbles’ size distribution. These different geometrical configurations of designed microbubble generator were used to generate microbubbles in various air volume fractions. Obtained results confirm direct relation between generated microbubbles size and their distribution with fluid flow patterns. More complex geometry creating high turbulent regions are suggested to increase bubble breaking up especially for high values of void fractions.

Padé approximations for identification of air bubble volume from temperature- or frequency-dependent permittivity of a two-component mixture

The article presents a numerical method developed for identification of information about structural parameters of a two-component mixture from effective complex permittivity measurements. The identification is based on the reconstruction of the spectral function in the analytic Stieltjes representation of the effective permittivity using Padé approximation. The spectral function contains all information about the microgeometry of the mixture, it is used to calculate volume fractions of the components in the mixture. Padé approximation is derived from a constrained minimization problem. Numerical results of recovering volume fraction of air in mixtures of air prolate and oblate spheroidal inclusions in water and in ethanol show good agreement of theoretical and predicted values. The proposed method can be used for estimating volume fractions and other structural parameters using the effective complex permittivity of two-component composite materials.

Concepts in modelling N2O emissions from land use

Modelling nitrous oxide (N2O) emissions from soil is challenging because multiple biological processes are involved that each respond differently to various environmental and soil factors. Soil water content, organic carbon, temperature and pH are often used in models that predict N2O emissions, yet for each of these factors there are concepts that are not fully understood. Though a ubiquitous measure of soil water for models, the application of functions based on water filled pore space across soils that vary in bulk density is not ideal. Diffusion of gases and solutes in soil are controlled by the volume fractions of air and water present. Across soils with different bulk densities, both of these terms vary at constant water filled pore space. Soil organic carbon influences N2O emissions in two ways: as a source of energy for denitrifiers and also by driving biological oxygen demand and the creation of anaerobic zones in the soil. Soil temperature influences N2O emissions through its effect on the activity of microorganisms and enzymes. A variety of temperature response functions have been proposed. The preferred response function should contain a temperature optimum that can be varied in response to climatic conditions to account for microbial adaptation. Soil pH can have direct and indirect influences on rates and product ratios of nitrification and denitrification. The concepts of pH optima and microbial adaptation need to be considered in modelling. Methodological issues such as microsite versus bulk soil measurements and apportioning N2O fluxes to the various N transformation processes remain an impediment to characterising the influence of pH and other factors on N2O emissions. Quantifying the response of N2O emissions to individual factors using regression analysis requires all other factors to be controlled experimentally. Boundary line analysis provides a way of defining the response to a single input variable where other influencing variables are not controlled. Such analyses can aid in the definition of the shape and magnitude of response functions to be incorporated into process simulation models. Process/mechanistic simulation models offer a greater transferability than empirical models but careful consideration of temporal and spatial scale and the availability of data to run these models is critical in developing model structure.