Size Of Medium Research Articles

Numerical investigation of homogenized Stokes–Nernst–Planck–Poisson systems

We consider charged transport within a porous medium, which at the pore scale can be described by the non-stationary Stokes–Nernst–Planck–Poisson (SNPP) system. We state three different homogenization results using the method of two-scale convergence. In addition to the averaged macroscopic equations, auxiliary cell problems are solved in order to provide closed-form expressions for effective coefficients. Our aim is to study numerically the convergence of the models for vanishing microstructure, i. e., the behavior for \(\varepsilon \rightarrow 0\), where \(\varepsilon \) is the characteristic ratio between pore diameter and size of the porous medium. To this end, we propose a numerical scheme capable of solving the fully coupled microscopic SNPP system and also the corresponding averaged systems. The discretization is performed fully implicitly in time using mixed finite elements in two space dimensions. The averaged models are evaluated using simulation results and their approximation errors in terms of \(\varepsilon \) are estimated numerically.

Numerical modelling of the self-heating process of a wet porous medium

In this research, the shortcomings of some advanced models for heat and fluid flow and moisture transport within and around a reactive porous medium (a coal stockpile) are numerically investigated based on a local thermal non-equilibrium approach and results are compared with experimental data. A full account of the problem is presented, including transient variation of the maximum temperature in the medium and the time required to vaporize the water content. The former is the main parameter used in the study of spontaneous self-ignition of coal stockpiles and the latter is an indication of the last step of self-heating toward self-ignition. Correlations are developed for the maximum dimensionless temperature inside the pile when the moisture content is near zero and the time that it takes from the beginning of self-heating until this time as functions of porosity, permeability, moisture content and the overall size of the porous medium.

Spectral singularities of a complex spherical barrier potential and their optical realization

The mathematical notion of a spectral singularity admits a physical interpretation as a zero-width resonance. It finds an optical realization as a certain type of lasing effect that occurs at the threshold gain. We explore spectral singularities of a complex spherical barrier potential and study their realization as transverse spherical electromagnetic waves emitted by a gain medium with a spherical geometry. In particular, for a typical dye laser material, we obtain a lower bound on the size of the gain medium for the occurrence of this kind of spectral singularities.

Optimum heat storage design for heat integrated multipurpose batch plants

Heat integration to minimise energy usage in multipurpose batch plants has been in published literature for more than two decades. In most present methods, time is fixed a priori through a known schedule, which leads to suboptimal results. The method presented in this paper treats time as a variable, thereby leading to improved results. Both direct and indirect heat integration are considered together with optimisation of heat storage size and initial temperature of heat storage medium. The resulting model exhibits MINLP structure, which implies that global optimality cannot generally be guaranteed. However, a procedure is presented that seeks to find a globally optimal solution, even for nonlinear problems. Heat losses from the heat storage vessel during idling are also considered. This work is an extension of MILP model of Majozi (2009), which was more suited to multiproduct rather than multipurpose batch facilities. Optimising the size of the heat storage vessel as well as the initial temperature of the heat storage fluid decreased the requirement for external hot utility for an industrial case study by 33% compared to using known parameters.

Effect of Top Dielectric Medium on Gate Capacitance of Graphene Field Effect Transistors: Implications in Mobility Measurements and Sensor Applications

We have carried out Hall measurement on back-gated graphene field effect transistors (FET) with and without a top dielectric medium. The gate efficiency increases by up to 2 orders of magnitude in the presence of a high κ top dielectric medium, but the mobility does not change significantly. Our measurement further shows that the back-gate capacitance is enhanced dramatically by the top dielectric medium, and the enhancement increases with the size of the top dielectric medium. Our work strongly suggests that the previously reported top dielectric medium-induced charge transport properties of graphene FETs are possibly due to the increase of gate capacitance, rather than enhancement of carrier mobility.

Probing Prokaryotic Social Behaviors with Bacterial “Lobster Traps”

Bacteria are social organisms that display distinct behaviors/phenotypes when present in groups. These behaviors include the abilities to construct antibiotic-resistant sessile biofilm communities and to communicate with small signaling molecules (quorum sensing [QS]). Our understanding of biofilms and QS arises primarily from in vitro studies of bacterial communities containing large numbers of cells, often greater than 108 bacteria; however, in nature, bacteria often reside in dense clusters (aggregates) consisting of significantly fewer cells. Indeed, bacterial clusters containing 101 to 105 cells are important for transmission of many bacterial pathogens. Here, we describe a versatile strategy for conducting mechanistic studies to interrogate the molecular processes controlling antibiotic resistance and QS-mediated virulence factor production in high-density bacterial clusters. This strategy involves enclosing a single bacterium within three-dimensional picoliter-scale microcavities (referred to as bacterial “lobster traps”) defined by walls that are permeable to nutrients, waste products, and other bioactive small molecules. Within these traps, bacteria divide normally into extremely dense (1012 cells/ml) clonal populations with final population sizes similar to that observed in naturally occurring bacterial clusters. Using these traps, we provide strong evidence that within low-cell-number/high-density bacterial clusters, QS is modulated not only by bacterial density but also by population size and flow rate of the surrounding medium. We also demonstrate that antibiotic resistance develops as cell density increases, with as few as ~150 confined bacteria exhibiting an antibiotic-resistant phenotype similar to biofilm bacteria. Together, these findings provide key insights into clinically relevant phenotypes in low-cell-number/high-density bacterial populations.

Effects of concentration of dispersions on particle sizing during production of fine particles in wet grinding process

Stirred media milling is a prospective technology for producing colloidal dispersions by means of wet grinding process. In the past, many researchers have studied the effects of different operating parameters such as size, shape, nature and quantity of grinding medium, the speed of agitator in grinding chamber, the feed rate of dispersions, etc. in stirred media mills. However, it is still less known how particle sizing which generates valuable information of particle size of the product to interpret, control and optimize the grinding process, is influenced by the concentration of the dispersion during stirred media milling where particles change their size from micron to colloidal range rapidly. One of the reasons of this lack had been our incapability in the past to study the particle size distribution of dispersions without dilution. The recent advent of acoustic attenuation spectroscopy is known to be capable of studying dispersions without dilution, under real process conditions and on line. The study employs acoustic attenuation spectroscopy to investigate the effects of concentration of dispersions of CaCO 3 on its particle sizing during size reduction process in a stirred media mill (LabStar manufactured by NETZSCH). The dispersions of CaCO 3 at 5%, 10%, 20% and 30% (m/m) were studied about six hours under a selected set of operating conditions. Contrary to the existing knowledge obtained through other techniques of particle sizing that are based on the principle of dilution, acoustic attenuation spectroscopy shows that, under certain grinding time at given operating conditions, increase in concentration of dispersion results in better grinding results yielding smaller particles. The causes behind the differences in results of acoustic attenuation spectroscopy and dynamic light scattering have been thoroughly investigated. We find certain limitations of acoustic attenuation spectroscopy in particle sizing. A typical phenomenon which causes misleading trends in particle sizing is multiple scattering in acoustic measurements. Multiple scattering, particularly, influences acoustic results when particles approach to fine size range during size reduction process.

Microfluidic phase change valve with a two-level cooling/heating system

A phase change (PC) microvalve with an integrated two-level cooling/heating system is developed for microfluidic applications in this article. This PC microvalve utilizes the liquid–solid PC of a small portion of the working medium in a microchannel to switch on/off the flow in the microchannel. The size of the working medium for the PC microvalve is 5-mm long, 50-μm high, and 80-μm wide (50 μm × 80 μm is the cross-sectional area of the channel) in this study. The switch is actuated by using a two-level cooling/heating system integrated on the chip. The first-level cooling/heating unit keeps the working medium in the valve area in the temperature range of supercooling state. Based on the supercooling state, the second-level cooling/heating unit either heats up or cools down the medium in the valve area to trigger its PC between liquid and solid for valving purposes. The proposed microfluidic PC microvalve is characterized experimentally in microfluidic chips. The thermal impact of one PC microvalve in one particular microchannel on its adjacent channels is discussed by establishing a preliminary analytical model and a numerical model. In addition to no leakage and no moving element, this PC microvalve with a two-level cooling/heating system can achieve a very short cooling time (i.e., 2.72 s).

Friction Model for Strip Rolling

To improve the accuracy and speed of finite element calculation, a new friction model was proposed, in which a layer of thin friction element between the roller and workpiece was introduced to consider the contact friction. This model was used in the three-dimensional elastic-plastic finite element model of strip rolling process. Taking the geometrical size and physical characteristics of intermediate medium between roller and workpiece into consideration, the stiffness of the friction element was corrected by two factors, i. e., the thickness of friction element and β factor. Without any assumption about the value and direction of friction force and neutral point position, the rolling process was simulated by using this model. The results obtained by this model were compared with the experimental results and the feasibility of the model was verified.

High-density lipoprotein subclasses are a potential intermediary between alcohol intake and reduced risk of cardiovascular disease: The Rancho Bernardo Study

We conducted a cross-sectional study of NMR-derived HDL subclasses and alcohol intake among 2171 community-dwelling older adults with a large proportion of daily or near-daily alcohol consumers (44 %). We aimed to assess whether, in addition to increasing total HDL, alcohol may induce a beneficial shift in HDL particle size distribution. Participants were categorised based on reported alcohol intake (g per week) and on frequency (none, < 3 times/week, 3-4 times/week, ≥ 5 times/week). The association between alcohol intake and lipoprotein fractions was examined using sex-specific linear regression models adjusted for age, BMI, diabetes, current smoking, exercise and hormone therapy in women. There was a stepwise gradient with the highest weekly alcohol consumption associated with the highest total HDL size and greatest number of medium and large HDL particles, as well as higher total HDL concentrations (all P < 0.001); total small HDL did not differ. Alcohol-HDL size associations were similar in both sexes and did not differ by use of hormone replacement therapy in women. In conclusion, regular alcohol consumers had a higher number and percentage of large HDL particles than non-drinkers. These results suggest that one way that alcohol may decrease CVD is through potentially favourable changes in lipoprotein subclass composition.

Water Quality and Well Injectivity: Do Residual Oil-in-Water Emulsions Matter?

Summary The present work is a part of a thorough and systematic laboratory study of oil-in-water emulsion flow in porous media that we have undertaken recently to investigate the mechanisms of oil-droplet retention and its consecutive effect on permeability. One of our main objectives was to see how the in-depth propagation of produced- water (PW) residual dilute emulsion could impair the permeability during PW reinjection (PWRI). During this casework, we used granular packs of sharp-edged silicon carbide grains and stable and dilute dodecane-in-water emulsions. The flow experiments were performed under well-controlled conditions, and we studied the effect of most of the relevant parameters, including flow rate, salinity, droplet size, and permeability of the porous medium. A careful monitoring of the salinity and the jamming ratio (JR) allowed us to consider and work separately on the two main mechanisms of droplet capture (i.e., surface capture and straining capture). In a previous paper (Buret et al. 2008), we reported on the effect of salinity and flow rate on emulsion flow through porous media where the pore-size/droplet-size ratio (JR) was very high, ensuring that only droplet capture on pore surface is operative. This paper reports on the effect of salinity and JR on both mechanisms, with the main focus being on the induced permeability impairment. We demonstrated that surface capture could induce significant in-depth permeability losses even at a high JR. The maximum reached permeability loss is very sensitive to salinity and flow rate (shear-thinning effect). This maximum is always lower than a limiting value dictated by the surface-coverage jamming limit of random sequential adsorption (RSA) theory. This limiting value increases while decreasing the JR, according to a simple formula extracted from Poiseuille's law with a mean hydrodynamic thickness of the deposited layer close to the droplet diameter (monolayer deposition). Regarding the straining capture, we determined a critical JR of 7 for this mechanism to occur. Preliminary results using only two JR values and one flow rate are presented. Compared to surface capture, the results show that straining capture induces more severe plugging with a lower rate of propagation. The lower the JR is, the more severe the plugging is and the lower the propagation rate is. However, more investigations are still required, notably using various JRs and flow rates to characterize this important mechanism better.

Anaerobic treatment of winery wastewater in fixed bed reactors

The treatment of winery wastewater in three upflow anaerobic fixed-bed reactors (S9, S30 and S40) with low density floating supports of varying size and specific surface area was investigated. A maximum OLR of 42 g/l day with 80 +/- 0.5% removal efficiency was attained in S9, which had supports with the highest specific surface area. It was found that the efficiency of the reactors increased with decrease in size and increase in specific surface area of the support media. Total biomass accumulation in the reactors was also found to vary as a function of specific surface area and size of the support medium. The Stover-Kincannon kinetic model predicted satisfactorily the performance of the reactors. The maximum removal rate constant (U(max)) was 161.3, 99.0 and 77.5 g/l day and the saturation value constant (K(B)) was 162.0, 99.5 and 78.0 g/l day for S9, S30 and S40, respectively. Due to their higher biomass retention potential, the supports used in this study offer great promise as media in anaerobic fixed bed reactors. Anaerobic fixed-bed reactors with these supports can be applied as high-rate systems for the treatment of large volumes of wastewaters typically containing readily biodegradable organics, such as the winery wastewater.

Tuneabilities of localized electromagnetic modes in random nanostructures for random lasing

The modal characteristics of localized electromagnetic waves inside random nanostructures are theoretically presented. It is crucial to know the tuneabilities of the localized modes systematically for demonstrating a specific random lasing application. By use of FDTD (Finite-Difference Time-Domain) method, we investigated the impulse response of two-dimensional random nanostructures consisting of closely packed cylindrical dielectric columns, and precisely analyzed the localized modes. We revealed the tuneability of the frequency of the localized modes by controlling the medium configurations: diameter, spatial density, and refractive index of the cylinders. Furthermore, it is found to be able to tune the Q (quality) factors of the localized modes dramatically by controlling simply the system size of the entire medium. The observed Q factors of approximately 1.6×104 were exhibited in our random disordered structures.

Bio-oil production from fast pyrolysis of waste furniture sawdust in a fluidized bed

The amount of waste furniture generated in Korea was over 2.4 million tons in the past 3 years, which can be used for renewable energy or fuel feedstock production. Fast pyrolysis is available for thermo-chemical conversion of the waste wood mostly into bio-oil. In this work, fast pyrolysis of waste furniture sawdust was investigated under various reaction conditions (pyrolysis temperature, particle size, feed rate and flow rate of fluidizing medium) in a fluidized-bed reactor. The optimal pyrolysis temperature for increased yields of bio-oil was 450 °C. Excessively smaller or larger feed size negatively affected the production of bio-oil. Higher flow and feeding rates were more effective for the production of bio-oil, but did not greatly affect the bio-oil yields within the tested ranges. The use of product gas as the fluidizing medium had a potential for increased bio-oil yields.

Reducing Formation Damage with Microbubble-Based Drilling Fluid: Understanding the Blocking Ability

Abstract Commonly, bridging materials used to reduce formation damage and mud losses in the near wellbore region consist of solid particles. These particles need to be removed after drilling through processes such as acidizing. Microbubble-based drilling fluids utilize gas bubbles to bridge the pores instead of solid particles. These microbubbles can be removed during the initial stages of production, thereby, reducing the costs associated with stimulation processes. Although there has been some work done on the flow of microbubbles through porous media, little is known regarding what conditions (e.g. viscosity, fluid composition, pressure) determine whether the microbubbles will or will not block the pores. Both the microbubble diameter and the rock pore size distribution play roles in determining sealing of the pores. In order to gain an appreciation of the pore blocking mechanism, experiments were conducted using micro-model cells to visually understand the blocking mechanism. The composition of the fluid is varied, as well as the flow rate at which the fluid is injected. Through this, the pressure at which the microbubbles invade the medium for various compositions of the fluid for pore blocking is determined. The average microbubble size at the invasion pressure is compared to the average pore size of the porous medium. By understanding the extent of the microbubble invasion under varying conditions, a greater comprehension of the pore blocking mechanism can be established. As well, the success rate of applying the microbubble system to various types of reservoirs can be evaluated. Introduction Common blocking agents used in the oil and gas industry to reduce formation damage and mud losses while drilling and to block highly permeable streaks in the reservoir during production can be composed of gels, solid particles, emulsions and foams(1). The ability of these fluids to block specific areas of the reservoir is determined through the reduction of permeability in high permeable zones and the ability to place the fluid in the correct area within the reservoir. Specifically, many studies have been conducted on the blocking ability of foams in porous media(2–7). When discussing blocking ability, most of these studies are concerned with blocking the flow of gas(3). Albrecht and Marsden(3) found that, with an increase in surfactant concentration, the blocking effect was greater. The efficiency of foams to block gas propagation in porous media with crude oil was studied by Hanssen and Dalland(4). They tested many different surfactants and found that there was no correlation between gas blocking and interfacial tension and that only a few foams blocked the samples tested. They stated that the foam blocking is a complex phenomenon. Aarra and Skauge(5) and Aarra et al.(8) tested foams for gas mobility control in the North Sea with successful results. Nimir and Seright(6) compared the placement efficiency of foams to gels as blocking agents in various cores ranging in permeability from 7.5 to 900 mD. They used a C14–16α-olefin sulfonate and found that the foam was a better blocking agent when the low permeable zone is less than 7.5 mD and the high permeable zone is more than 80 mD.

Investigation of the Transport and Deposition of Fullerene (C60) Nanoparticles in Quartz Sands under Varying Flow Conditions

A coupled experimental and mathematical modeling investigation was undertaken to explore nanoscale fullerene aggregate (nC60) transport and deposition in water-saturated porous media. Column experiments were conducted with four different size fractions of Ottawa sand at two pore-water velocities. A mathematical model that incorporates nonequilibrium attachment kinetics and a maximum retention capacity was used to simulate experimental nC60 effluent breakthrough curves and deposition profiles. Fitted maximum retention capacities (S(max)), which ranged from 0.44 to 13.99 microg/g, are found to be correlated to normalized mass flux. The developed correlation provides a means to estimate S(max) as a function of flow velocity, nanoparticle size, and mean grain size of the porous medium. Collision efficiency factors, estimated from fitted attachment rate coefficients, are relatively constant (approximately 0.14) over the range of conditions considered. These fitted values, however, are more than 1 order of magnitude larger than the theoretical collision efficiency factor computed from Derjaguin-Landau-Verwey-Overbeek (DLVO) theory (0.009). Data analyses suggest that neither physical straining nor attraction to the secondary minimum is responsible for this discrepancy. Patch-wise surface charge heterogeneity on the sand grains is shown to be the likely contributor to the observed deviations from classical DLVO theory. These findings indicate that modifications to clean-bed filtration theory and consideration of surface heterogeneity are necessary to accurately predict nC60 transport behavior in saturated porous media.

Characterization of a Spherical Packing Medium for Biological Nitrogen Removal by Microelectrodes

A novel packing medium comprised of a spherical net-like shell, plastic stuffing, and a perforated hollow duct for biological reactors was investigated. In this study, we examined a biofilm process that employed the packing medium for combined aerobic carbonaceous substances removal, nitrification, and denitrification. As a result, we were able to obtain the average of nitrification, denitrification, as well as the chemical oxygen demand (COD) removal capacities, which correspond to 0.2 kg N/m3 · day, 0.3 kg N/m3 · day, and 5.56 kg/m3 · day, respectively. Moreover, the spatial concentration distribution of chemical species inside the packing medium was studied, using microelectrodes for dissolved oxygen, NH4+, NO3−, and redox potential. Clear evidence was found that aerobic/anoxic spatial stratification was formed inside the packing medium, and that due to microbial oxygen utilization, the aerobic zone was limited to a surface layer of 12 mm, which had a corresponding decrease in redox potentials from 157.4 mV to −99.5 mV. It was also found that nitrification dominated in the top 4 mm of the surface layer, whereas denitrification occurred in the deeper part of the packing medium. Furthermore, a gap zone only for aerobic heterotrophic oxidation was found to be present between nitrification and denitrification. This implied that a high C/N ratio could contribute to a decrease in the effective size of the packing medium. Based on the measured DO and NH4+ depth profiles under steady-state conditions, the effective diffusion coefficients of DO and NH4+ in the packing medium were calculated to be 0.0172 m2/day and 9.7 × 10−5m2/day, respectively. The turbulent diffusion of oxygen ensured the sufficient oxygen supply not only for aerobic carbon oxidation but for nitrification inside the packing medium as well.

The Effect of Etchant Concentration on Surface Morphology Of Porous GaP Produced By Laser-Induced Etching

Porous GaP have studied and fabricated by using an argon-ion laser beam of energy 2.41 eV was to recording the Raman spectra. The peak position shifts in Raman spectra was observed at 398 cm -1 after etching. The Raman line is broad and asymmetric in comparison to the Raman line for crystalline GaP of 402 cm -1 . The weak structure near 350 cm -1 is a forbidden TO phonon, which arises due to structural disorder in the material. The peak that appears near 378 cm -1 is attributed to a surface phonon mode. The surface phonon frequency depends on the nanocrystalline size, shape and dielectric constant of the surrounding medium. We have studied the surface morphology of porous layers obtained by LIE of n-type GaP (100) substrates by using a Scanning Electron Microscope (SEM). The morphology of porous GaP layers changes rapidly with laser power densities and irradiation times. As well as both the size and shape of the structure depend on the nature and concentration of etchant solution.

Performance of horizontal flow roughing filter using limestone media for the removal of turbidity, suspended solids, biochemical oxygen demand and coliform organisms from wastewater

This paper examines and evaluates the removals of turbidity, suspended solids, BOD and coliform organisms from wastewater using different sizes of limestone roughing filter. Results indicated that removal efficiencies depended on the size of the filter medium and applied flow rates. Turbidity, suspended solids, BOD and coliform organisms removals were between 75% and 92%, 79% and 88%, 51% and 67% and 67% and 96%, respectively, in a combination of the three filter media with particle sizes between 1.91 mm and 16.28 mm. Removal efficiency was found to increase with slower flow rates.

Modelling and simulation of fibrous biological tissues via discrete homogenization methods

AbstractThanks to their geometrical organization at the cell level, soft biological tissues can be modelled from the mechanical point of view as multidimensional networks of elastic bars. The length of the bars is supposed to be small with respect to the size of the macroscopic medium. We introduce a detailed description of the overall structure accounting both for the tensions due to the bars and for the moments between pairs of bars. Using quasi‐periodicity hypotheses, we apply a discrete homogenization technique, [1]. We derive a continuous homogenized mechanical law in the large transformation setting. We describe the basic principles of this approach that was first introduced in the cardiac modelling context in [2]. In a last step we present its implementation in a finite element framework. We comment some aspects of our numerical results. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)