Spontaneous imbibition of liquid in glass fiber wicks,Part II: Validation of a diffuse‐front model

In Part I (Zarandi MAF, Pillai KM. Spontaneous Imbibition of Liquids in Glass‐Fiber Wicks. Part I: Usefulness of a Sharp‐Front Approach. AIChE J, 64: 294–305, 2018), a model based on sharp liquid‐front was proposed where a good match with the experimental data was achieved. However, the model failed to account for partial saturations in the wicks. Here, Richard's equation to predict liquid saturation is tried where the equation is solved numerically in 3D using COMSOL and analytically in 1D using Mathematica for glass‐fiber wicks after treating them as transversely‐isotropic porous media. As a novel contribution, relative permeability and capillary pressure are determined directly from pore‐scale simulations in wick microstructure using the state‐of‐the‐art software GeoDict. The saturation along the wick length is determined experimentally through a new liquid‐N₂ based freezing technique. After including the gravity effect, good agreements between the numerical/analytical predictions and experimental results are achieved in saturation distributions. We also validated the Richard's equation based model while predicting absorbed liquid‐mass into the wick as function of time. © 2017 American Institute of Chemical Engineers AIChE J, 63: 306–315, 2018     

Investigating liquid fronts during spontaneous imbibition of liquids in industrial wicks. Part I: Experimental studies

A large set of industrial wicks made from sintered polymer beads and fibrous materials were studied. Three types of liquid fronts were observed: sharp, semi-sharp, and diffuse. The bead-like microstructure with lower porosity is found to support sharp fronts, while the aligned-fiber microstructure with higher porosities leads to a smudging/diffusing of the fronts. The previously developed sharp-front model is found to predict the front height as a function of time quite well in the sintered-bead wicks and the two “semi-sharp” fibrous wicks. The liquid-front height calculated from the liquid mass-gain data was validated through visual observations for all but the two fibrous wicks generating the diffuse fronts. These latter cases, observed to start off as a sharp fronts, faded with time.     

Flow along and across glass‐fiber wicks: Testing of permeability models through experiments and simulations

A rigorous test of important theoretical models for permeability of glass‐fiber wicks, backed by numerical simulations, is conducted using a novel small‐scale experiment. The models include those for flow along and across aligned fibers and for flow through random fibers. The domains for numerical simulations were created by randomly distributed parallel fibers in a cube‐like unit‐cell using Geodict. Two separate simulations were considered: (1) Stokes‐flow solution using GeoDict, (2) Whitaker's closure‐formulation solution using COMSOL. The falling‐head parameter was adapted to measure the permeability along and across the fibers. Multiple measurements were conducted for each of the wicks to establish repeatability and estimate scatter. The permeabilities obtained through experiments matched with those from the theoretical and numerical methods. But numerical permeabilities for the longitudinal flow were exceptionally accurate. Also, the specialized models for longitudinal and transverse flows were more accurate than the random‐fiber models. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3491–3501, 2018     

Investigating liquid-fronts during spontaneous imbibition of liquids in industrial wicks. Part II: Validation by DNS

To validate the experimental results of Part-1, we conducted a two-phase flow simulation of imbibition of a wetting liquid through 2D microstructures made of ellipses of varying aspect ratios. The flow simulation in the particulate microstructures, characterized by low (ellipse) aspect ratio, produced somewhat even micro-fronts, thus replicating the sharp fronts at the visual (macroscopic) scale observed in Part-1. Whereas simulations in the fibrous microstructures produced highly uneven micro-fronts, suggesting the formation of semi-sharp or diffuse visual fronts. Increasing the porosity from 50% to 70% resulted in solid-phase clustering and led to further increase in the unevenness of micro-fronts, pointing to purely diffuse visual fronts. The evolution of the saturation plots along the flow direction, obtained from area-averaging of fluid-distribution plots, pointed to diffusing of sharp fronts with time. The predictions matched our previous experimental and numerical observations, that is, the particulate media create sharp fronts while the fibrous media create semi-sharp/diffuse fronts.     

Wicking and evaporation of liquids in porous wicks: A simple analytical approach to optimization of wick design

Wicking and evaporation of volatile liquids in porous, cylindrical wicks is investigated where the goal is to model, using simple analytical expressions, the effects of variation in geometrical parameters of a wick, such as porosity, height and bead‐size, on the wicking and evaporation processes, and find optimum design conditions. An analytical sharp‐front flow model involving the single‐phase Darcy's law is combined with analytical expressions for the capillary suction pressure and wick permeability to yield a novel analytical approach for optimizing wick parameters. First, the optimum bead‐radius and porosity maximizing the wicking flow‐rate are estimated. Later, after combining the wicking model with evaporation from the wick‐top, the allowable ranges of bead‐radius, height and porosity for ensuring full saturation of the wick are calculated. The analytical results are demonstrated using some highly volatile alkanes in a polycarbonate sintered wick. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1930–1940, 2014     

Darcy's law–based numerical simulation for modeling 3D liquid absorption into porous wicks

Liquid imbibition into polymer wicks, where a clear liquid front can be seen rising during the wicking process, is modeled using the concepts of flow in porous media. The flow of liquid behind the moving liquid front is modeled using the physics of single‐phase flow in a porous medium where the Darcy's law is combined with the continuity equation and a capillary suction pressure is imposed at the liquid front. A novel numerical simulation PORE‐FLOW^© based on the finite element/control volume method is proposed to model such imbibitional flows in wicks of complex shapes. A validation of the simulation is obtained by achieving an excellent comparison of its predictions with an experimental result, an analytical solution, and the Washburn equation for the case of wicking against gravity in a cylindrical wick. The simulation is also used to predict a case of two‐dimensional (2D) wicking in the altered cylindrical wicks with two different cross‐sectional areas. Once again an excellent match is obtained with the experimental results, while analytical solutions for the single and double cross‐section cases along with the Washburn equation fail to predict the 2D wicking. Later, some other types of altered wicks with sharp changes in their cross‐sectional areas were analyzed numerically for their wicking behavior. It was observed that the height of liquid front in a vertical wick as a function of time, which is proportional to the history of liquid imbibed, is strongly dependent on the extent of reduction in the wick cross‐sectional area as well as its location vis‐à‐vis the wick entrance. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

On mixing rules for the hydraulic permeability of fiber mixtures

We perform numerical simulations of Stokes flow through ordered arrays of fibers of square cross-section. The results for the permeability of single-size-fiber arrays are in very good agreement with the well-known correlation of Jackson and James, provided that an effective radius of equivalent cylindrical fibers is used with the same cross-sectional area. Then, we consider arrays of mixtures of different-sized fibers and test several mixing rules, proposed for estimating the composite permeability. The two most well-known, namely, the unweighted- and the volume-averaged resistivity rules give good predictions for not too large size-ratios, yet they systematically overestimate and underestimate the numerical results, respectively. Then, it appears almost self-evident that a combination of the above two mixing rules would provide a better prediction. Indeed, the geometric mean gives excellent agreement with the numerical results over an extended range of size ratios and relative volume fractions.     

Darcy's law for two‐dimensional flows: Singularities at corners and a new class of models

As is known, Darcy's model for fluid flows in isotropic homogeneous porous media gives rise to singularities in the velocity field for essentially two‐dimensional flow configuration, like flows over corners. Considering this problem from the modeling viewpoint, this study aims at removing this singularity, which cannot be regularized via conventional generalizations of the Darcy model, like Brinkman's equation, without sacrificing Darcy's law itself for unidirectional flows where its validity is well established experimentally. The key idea is that as confirmed by a simple analogy, the permeability of a porous matrix with respect to flow is not a constant independent of the flow but a function of the flow field (its scalar invariants), decreasing as the curvature of the streamlines increases. This introduces a completely new class of models where the flow field and the permeability field are linked and, in particular problems, have to be found simultaneously. © 2017 American Institute of Chemical Engineers AIChE J, 2017     

Darcy's law‐based model for wicking in paper‐like swelling porous media

The wicking of liquid into a paper‐like swelling porous medium made from cellulose and superabsorbent fibers was modeled using Darcy's law. The work is built on a previous study in which the Washburn equation, modified to account for swelling, was used to predict wicking in a composite of cellulose and superabsorbent fibers. In a new wicking model proposed here, Darcy's law for flow in porous media is coupled with the mass conservation equation containing an added sink or source term to account for matrix swelling and liquid absorption. The wicking‐rate predicted by the new model compares well with the previous experimental data, as well as the modified Washburn equation predictions. The effectiveness of various permeability models used with the new wicking model is also investigated. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

A new approach for determining the critical cooling rates of nucleation in glass‐forming liquids

Crystallization of a liquid below liquidus temperature is a complex process due to simultaneous nucleation and growth of crystals. Nucleation is the crucial initial step of the crystallization process, and affects the glass‐forming ability, especially when there is a large overlap between nucleation and crystal growth versus temperature curves. From the temperature‐time‐transformation (TTT) diagram, one can estimate the critical cooling rate, , of glass‐formation, however this is time‐consuming. In this paper, we establish a simple approach to determine the using calorimetric and viscometric data. Based on the classical nucleation theory, the correlation between the crystallization onset temperature and cooling rate is described by combining two temperature‐dependent functions. The new approach is applicable to a wide range of glass‐forming systems. This work also gives insight into heterogeneous nucleation and glass formation kinetics.     

Numerical simulation of liquid absorption in paper‐like swelling porous media

The work is built on a previous research by Wiryana and Berg, in which wicking into four wet‐formed paper stripes, consisting of cellulose fibers and four different percentages of the powdered carboxymethyl cellulose (CMC) superabsorbent, was studied experimentally. Because of the swelling of cellulose fibers and CMC powder on contact with water, the wicking was accompanied by a swelling of the matrix. A finite element/control volume (FE/CV)‐based computer program is used for the first time to model the wicking in such swelling porous medium. The simulation used a novel form of continuity equation, which included the effects of liquid absorption and matrix swelling, in conjunction with the Darcy's law to model the single‐phase flow behind a clearly defined liquid‐front. A new method of estimating the time‐varying permeability of the paper, based on the absorbed liquid‐mass vs. time plots, is also proposed. Later, this time‐dependent permeability is used in the numerical simulation to change the permeability in elements behind the moving liquid‐front as a function of the time that the element has been wetted by the liquid, since the passage of the liquid‐front. The numerical prediction of the wicking‐front location as a function of time compares well with the reported experimental data. © 2011 American Institute of Chemical Engineers AIChE J, 58: 2536–2544, 2012     

Snap‐off of a liquid drop immersed in another liquid flowing through a constricted capillary

Emulsions are encountered at different stages of oil production processes, often impacting many aspects of oilfield operations. Emulsions may form as oil and water come in contact inside the reservoir rock, valves, pumps, and other equipments. Snap‐off is a possible mechanism to explain emulsion formation in two‐phase flow in porous media. Quartz capillary tubes with a constriction (pore neck) served to analyze snap‐off of long (“infinite”) oil droplets as a function of capillary number and oil‐water viscosity ratio. The flow of large oil drops through the constriction and the drop break‐up process were visualized using an optical microscope. Snap‐off occurrence was mapped as a function of flow parameters. High oil viscosity suppresses the breakup process, whereas snap‐up was always observed at low dispersed‐phase viscosity. At moderate viscosity oil/water ratio, snap‐off was observed only at low capillary number. Mechanistic explanations based on competing forces in the liquid phases were proposed. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

A numerical study of dynamic capillary pressure effect for supercritical carbon dioxide‐water flow in porous domain

Numerical simulations for core‐scale capillary pressure (P^c)‐saturation (S) relationships have been conducted for a supercritical carbon dioxide‐water system at temperatures between 35°C and 65°C at a domain pressure of 15 MPa as typically expected during geological sequestration of CO₂. As the P^c‐S relationships depend on both S and time derivative of saturation ( ) yielding what is known as the “dynamic capillary pressure effect” or simply “dynamic effect,” this work specifically attempts to determine the significance of these effects for supercritical carbon dioxide‐water flow in terms of a coefficient, namely dynamic coefficient (τ). The coefficient establishes the speed at which capillary equilibrium for supercritical CO₂ (scCO₂)‐water flow is reached. The simulations in this work involved the solution of the extended version of Darcy's law which represents the momentum balance for individual fluid phases in the system, the continuity equation for fluid mass balance, as well as additional correlations for determining the capillary pressure as a function of saturation, and the physical properties of the fluids as a function of temperature. The simulations were carried out for three‐dimensional cylindrical porous domains measuring 10 cm in diameter and 12 cm in height. τ was determined by measuring the slope of a best‐fit straight line plotted between (1) the differences in dynamic and equilibrium capillary pressures against (2) the time derivative of saturation (dS/dt), both at the same saturation value. The results show rising trends for τ as the saturation values reduce, with noticeable impacts of temperature at 50% saturation of aqueous phase. This means that the time to attain capillary equilibrium for the CO₂‐water system increases as the saturation decreases. From a practical point of view, it implies that the time to reach capillary equilibrium during geological sequestration of CO₂ is an important factor and should be accounted for while simulating the flow processes, for example, to determine the CO₂ storage capacity of a geological aquifer. In this task, one would require both the fundamental understanding of the dynamic capillary pressure effects for scCO₂‐water flow as well as τ values. These issues are addressed in this article. © 2014 American Institute of Chemical Engineers AIChE J 60: 4266–4278, 2014     

Dynamic effects in capillary pressure relationships for two‐phase flow in porous media: Experiments and numerical analyses

Well defined experiments and numerical analyses are conducted to determine the importance of dynamic effect in capillary pressure relationships for two‐phase flow in porous media. Dynamic and quasi‐static capillary pressure‐saturation (P^c‐S_w) and, ?S_w/?t‐t curves are determined. These are then used to determine the dynamic effects, indicated by a dynamic coefficient (τ) in the porous domains which establishes the speed at which flow equilibrium (?S_w/?t = 0) is reached. τ is found to be a nonlinear function of saturation which also depends on the medium permeability. Locally determined τ seems to increase as the distance of the measurement point from the fluid inlet into the domain increases. However, the functional dependence τ‐S_w follows similar trends at different locations within the domain. We argue that saturation weighted average of local τ‐S_w curves can be defined as an effective τ‐S_w curve for the whole domain which follows an exponential trend too. © 2012 The Authors. AIChE Journal, published by Wiley on behalf of the AIChE. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. AIChE J, 58: 3891–3903, 2012     

Growth of diopside crystals in CMAS glass‐ceramics using Cr2O3 as a nucleating agent

CaO–Al₂O₃–MgO–SiO₂ (CAMS)‐based glass‐ceramics were prepared using body crystallization method. Adding Cr₂O₃ into the ceramics not only effectively lowered the crystallization temperature, but also led to significant grain refinement of diopside that crystallized in the CAMS glass‐ceramic after crystallization treatment at 900°C for 2 hours. Experimental work verified that the epitaxial growth of the diopside on the spinel particles, which formed during nucleation treatment when fabricating the glass‐ceramics, facilitated the heterogeneous nucleation of diopside on the spinel and refined the diopside. In addition, two energetically favored crystallographic orientation relationships between the epitaxial growth diopside and spinel were experimentally observed. They are //[001]_diopside,////(200)_diopside and //[101]_diopside, (311)_spinel//. These two novel results can be potentially used to develop new glass‐ceramic materials with improved performance.     

The Natural Products Beauveriolide I and III: a New Class of β‐Amyloid‐Lowering Compounds

Attacking Alzheimer's by ACAT : The aggregation of β‐amyloid peptides, especially Aβ₄₂, into senile plaques is a hallmark of Alzheimer's disease (AD). We show that the fungal natural products beauveriolides I and III can potently decrease Aβ secretion from cells expressing human amyloid precursor protein; this offers a potential new scaffold for the development of compounds with proven bioavailability for the treatment of AD.

     

Improving thermo‐oxidative degradation resistance of bamboo fiber reinforced polypropylene composite with antioxidants. Part I: Screening of antioxidants

In this study, the effect of antioxidant application (applied in different combinations, ratios and contents) on the thermo‐oxidative degradation resistance of bamboo fiber polypropylene composites (BFPCs) were investigated, with oxidative induction time (OIT), weight loss, surface color, and flexural mechanical properties as the main indicators for evaluation. The results showed the addition of antioxidants could greatly increase the OIT and reduce the weight loss and color change of the composites after 900 h of thermal weathering. Meanwhile, the flexural mechanical properties were little affected or even slightly improved. The combination of 1076 and DLTP antioxidants at a ratio of 2:1 and 0.2 wt % content was found to exhibit the best thermal oxidation resistance with respect to the OIT, mechanical properties and cost. Our study indicate that the OIT can serve as a simple and quick indicator for the evaluation of thermo‐oxidative resistance of BFPC. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44198.     

Experimental investigation of hysteretic dynamic effect in capillary pressure–saturation relationship for two‐phase flow in porous media

Well‐defined laboratory experiments have been carried out to investigate hysteretic dynamic effect in capillary pressure–saturation relationships for two‐phase flow in homogeneous and heterogeneous (layered) porous media. Conceptually, the dependence of the capillary pressure curves on the rate of change of saturation (dS_w/dt) is defined as the dynamic effect in capillary pressure relationship, which is indicated by a dynamic coefficient, τ (Pa s) and it determines the rate at which two‐phase flow equilibrium is reached, i.e., dS_w/dt = 0 where S_w and t are the water saturation and time, respectively. The dependences of τ on various fluid and porous materials properties have been studied in the context of drainage; but, there is limited study for imbibition and the hysteresis of τ?S_w relationships. As such, the emphasis in this article is on reporting τ?S_w curves for imbibition while also demonstrating the hysteresis in the τ?S_w relationships by comparing τ?S_w curves for drainage (previously reported) and imbibition (this study) in carefully designed laboratory experiments. Homogeneous sand samples composed of either fine (small particle size and lower permeability) or coarse (larger particle size and higher permeability) sand have been used for these experiments. Furthermore, a layered domain made of a find sand layer sandwiched between two coarse sand layers is used as a model of heterogeneous domain. The results of the study confirm that the τ?S_w relationships are hysteretic in nature and, as such, the speed to flow equilibrium should vary depending on whether drainage or imbibition takes place. At a particular water saturation, the magnitudes of the dynamic coefficient (τ) are found to be generally higher for imbibition, which imply that the speed to flow equilibrium at the same saturation will be slower for imbibition. © 2013 The Authors. AIChE Journal, published by Wiley on behalf of the AIChE. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. AIChE J, 59: 3958–3974, 2013