Kinetic model and effect of surface impurities on the crack healing of BK7 glass

When BK7 glass material is heated under appropriate conditions, surface cracks heal spontaneously under the action of viscous flow and capillary forces. In this paper, based on the theories of viscous fluid and capillary flow, a kinetic model for crack healing is established. A series of thermal healing experiments with different parameters are conducted on Vickers indentation radial cracks on BK7 glass, and the effects of surface impurities, heating temperature, crack size, and ambient humidity are studied. The results show that the surface impurities and humidity can promote crack healing. Upon combining the experimental results and relevant theories, the initial kinetic model is modified to accurately predict the healing process of the BK7 glass and describe the relationships among the crack healing length and these factors. In addition, the proposed model can be used for the healing of other materials with the mechanism of viscous capillary flow.     

A coupled thermo-hygro-chemical model for characterising autogenous healing in ordinary cementitious materials

Experimental work has demonstrated that cracks can be healed in ordinary cementitious materials in the presence of water. The primary healing mechanisms are hydration of the unreacted nuclei of cement particles and the long-term formation of calcite. A mathematical model for simulating early-age autogenous healing of ordinary cement-based materials is proposed, which employs a coupled thermo-hygro-chemical (THC) framework and which uses a reactive water transport component to predict the movement of healing materials. A single concentration variable is employed for the healing component of the model that is derived directly from the quantity of unreacted cement and computed using a generalised cement hydration model component. The hydration component is directly linked to an expression for capillary porosity and for the porosity of the material within a healed crack. The results from a series of model simulations are in good general agreement with experimental data from tests on autogenous healing.     

Wall slip of concentrated suspension melts in capillary flows

The effects of wall slip of concentrated suspension melts in capillary flows were investigated at elevated temperature. The modeled material is a mixture of polymer EVA (Ethylene Vinyl Acetate) and non-colloidal spherical powder (glass microspheres) with mean particle size within 53∼63 μm. The effect of particle concentration on wall slip was studied experimentally in a capillary rheometer. For suspensions with different particle loadings (35%, 40%, and 45% by volume), the slip velocity V_s increased with an increase of particle concentration at the same testing temperature. A master slip curve can be obtained by plotting slip velocity versus the product of wall shear stress and square root of particle concentration. As such, a new particle concentration-dependent slip model is proposed. A theoretical approach coupled with the new slip model and flow equation is employed to characterize the flow behavior of concentrated suspension in a capillary rheometer, with reasonable agreement obtained with experimental observations.     

Fragmentation and granular transition of ceramics for high rate loading

The paper presents a numerical model to study the transition of brittle materials from a cracked solid to a granular medium under impact loading. The model addresses competitive crack coalescence in the transition regime and provides insight into the onset of comminution and the initial conditions for subsequent granular flow. Crack statistics obtained from initial flaws using a wing crack growth-based damage model have been used to discretely model elliptical cracks in three dimensions. These discrete cracks are either generated randomly in space or with a constraint that minimizes the intersection between neighboring cracks. These cracks are then allowed to coalesce with nearby cracks along with favorable directions and the output fragment statistics are predicted. A simple statistical model is proposed that suggests a transition criterion resembling the one obtained from the numerical model. Initial fragments are power-law distributed similarly to experimental observations and particle-based models. A generalized form of a microstructure-dependent granular transition criterion based on a threshold measure of crack lengths has been proposed. This model can be implemented in numerical codes to activate granular physics and calibrate the initial conditions of granular flow, such as fragment size and morphology.     

Determination of pressure drop for concentrated suspension in a capillary flow

The rheological behavior of concentrated suspension melts in a capillary die is investigated. Particle migration and wall slip are two major factors affecting the flow behavior. A numerical model is proposed to describe the coupling effect of particle migration and wall slip in a capillary tube flow, incorporating a power‐law model for binder viscosity and a concentrated suspension viscosity model proposed by Krieger. Wall slip of a non‐Newtonian concentrated suspension is characterized by a modified Mooney method for which the conventional Mooney method is not applicable. We characterized the flow behavior of a concentrated suspension of a non‐Newtonian binder, EVA 460 (ethylene vinyl acetate), mixed with spherical glass beads of 40% by volume. Predicted results were compared with experimental observations, with good agreement. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

Capillary driven flow in wettability altered microchannel

The capillary driven flow of water inside a microchannel with altered wettabilities is experimentally investigated and modeled theoretically. The surfaces of the PDMS made microchannel are exposed to oxygen plasma, rendering the surfaces increasingly hydrophilic, which provides the driving force for the flow. The plasma treated surfaces are characterized using topography and phase imaging of AFM scanning, as well as nano‐indentation, to correlate the distinct structural changes to the hydrodynamic profiles of the advancing meniscus. The experimental results are further analyzed using a newly proposed slip velocity model. The aim is to obtain a qualitative relationship between the surface properties and the flow parameters, namely the advancing meniscus velocity and pressure drop inside the channel. The insights are of fundamental importance in diverse fields, such as enhanced oil recovery, microfluidic devices, cell separation, and pathology. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4616–4627, 2017     

挤出过程中NR／CPB混炼胶壁滑移行为研究：Ⅱ.壁滑移速度

应用Ｍｏｎｓａｎｔｏ加工性能试验机，考察了天然胶／顺丁胶胶料在毛细管挤出过程中的壁滑移速度及其影响因素。发现，在较低的剪切速率下，试样的流动曲线出现不连续；壁滑移速度与壁面剪切应力之间大体上呈指数函数关系；发生壁滑移时的临界剪切应力随着温度的升高而下降。     

Effects of wall slip on ZrO2 rheological behavior in micro powder injection molding

The effects of wall slip on the rheological behavior of ZrO₂ feedstock flowing through various channels were studied. As compared to conventional no-slip condition, a power-law wall slip model was established to simulate the filling behavior of ZrO₂ feedstock flowing through the mold. The effects of wall slip on pressure distribution of the micro spool mandrils during filling was compared with the injection molded micro components. Experimental results verified that the simulation including wall slip yielded better prediction for the pressure gradient as well as crack propagation for the micro components. Increasing mold temperature not only enhanced the feedstock temperature flowing through the micro channels, but also reduced the temperature difference of the micro components and likely the ensuing thermal stress as well as cracks. Powder-binder separation is more sensitive to the mold temperature variation than melt temperature variation. Defect-free micro spool mandrils were injection molded using the optimized parameters.     

Fountain flow revisited: The effect of various fluid mechanics parameters

Numerical simulations have been undertaken for the benchmark problem of fountain flow present in injection‐mold filling. The finite element method (FEM) is used to provide numerical results for both cases of planar and axisymmetric domains under laminar, isothermal, steady‐state conditions for Newtonian fluids. The effects of inertia, gravity, surface tension, compressibility, slip at the wall, and pressure dependence of the viscosity are all considered individually in parametric studies covering a wide range of the relevant parameters. These results extend previous ones regarding the shape of the front, and in particular the centerline front position, as a function of the dimensionless parameters. The pressures from the simulations have been used to compute the excess pressure losses in the system (front pressure correction or exit correction). Inertia leads to highly extended front positions relative to the inertialess Newtonian values, which are 0.895 for the planar case and 0.835 for the axisymmetric one. Gravity acting in the direction of flow shows the same effect, while gravity opposing the flow gives a reduced bulge of the fountain. Surface tension, slip at the wall, and compressibility, all decrease the shape of the front. Pressure‐dependence of the viscosity leads to increased front position as a corresponding dimensionless parameter goes from zero (no effect) to higher values of the pressure‐shift factor. The exit correction increases monotonically with inertia, compressibility, and gravity, while it decreases monotonically with slip and pressure‐dependence of the viscosity. Contour plots of the primary variables (velocity‐pressure) show interesting trends compared with the base case (zero values of the dimensionless parameters and of surface tension). © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Toughening by Crack Bridging in Heterogeneous Ceramics

The toughening of a ceramic by crack bridging is considered, including the heterogeneity caused simply by spatial randomness in the bridge locations. The growth of a single planar crack is investigated numerically by representing the microstructure as an array of discrete springs with heterogeneity in the mechanical properties of each spring. The stresses on each microstructural element are determined, for arbitrary configurations of spring properties and heterogeneity, using a lattice Green function technique. For toughening by (heterogeneous) crack bridging for both elastic and Dugdale bridging mechanisms, the following key physical results are found: (i) growing cracks avoid regions which are efficiently bridged, and do not propagate as self-similar penny cracks; (ii) crack growth thus proceeds at lower applied stresses in a heterogeneous material than in an ordered material; (iii) very little toughening is evident for moderate amounts of crack growth in many cases; and (iv) a different R -curve is found for every particular spatial distribution of bridging elements. These results show that material reliability is determined by both the flaw distribution and the "toughness" distribution, or local environment, around each flaw. These results also demonstrate that the "microstructural" parameters derived from fitting an R -curve to a continuum model may not have an immediate relationship to the actual microstructure; the parameters are "effective" parameters that absorb the effects of the heterogeneity. The conceptual issues illuminated by these conclusions must be fully understood and appreciated to further develop microstructure-property relationships in ceramic materials.     

Rheological behavior of zirconia feedstock flowing through various channels considering wall-slip

The existence of wall slip for ZrO₂ feedstock flow in micro powder injection molding was investigated based on capillary rheometer experiments using dies of three dimensions. A power law function was derived by data fitting to determine the wall slip velocity based on which numerical simulation was carried out to explore the influence of wall slip on micro injection molding. Experimental results indicate that the feedstock is less sensitive to temperature fluctuation at higher shear rates. Power-law model can provide higher accuracy than the modified Cross model to depict the rheological behavior of the feedstock in capillary flows with different channels. Numerical simulation results show that in case of steady flow higher dynamic viscosity of the feedstock and higher pressure losses of the flow appeared when the wall slip boundary was included as compared to no-slip assumption in micro powder injection molding. This is because that when the wall slip boundary was included the shear rate distribution of the feedstock was lower than that of the feedstock assuming no-slip boundary.     

挤出过程中NR/CBR胶料壁滑移行为的研究——Ⅰ.压力振荡现象

聚合物流体流动过程中的壁滑移现象是其粘弹性的重要特征之一。应用Monsanto加工性能试验机考察了NR/CBR胶料在毛细管挤出流动中的壁滑移行为及其影响因素。在简化条件下,应用张量分析方法,导出描述产生壁滑移时的压力变化与挤出过程参变量及试样物性之间关系的数学模型,以及临界滑移距离的计算公式。研究结果表明,临界压力降测量数据与理论预测值软为吻合。     

Dynamic model of temperature rise caused by cementitious materials hydration

This paper presents a dynamic model of temperature rise caused by cementitious materials hydration based on the basic nature of chemical change. There are many models for computation of temperature rise caused by cementitious materials hydration, but the most takes this temperature rise as a function of concrete age only, which does not consider the impact of initial temperature and temperature rise during hydration and thus cannot reflect the real course of temperature rise. This model not only accords with the general law of chemical change, but also coincides well the experimental data and stimulates well the actual course of temperature rise of concrete under any initial temperature.     

田菁冻胶在毛细管及多孔介质中滑移效应研究

本文用实验方法研究了田菁冻胶压裂液在毛细管和多孔介质中的流动特性,求得相应的滑移效应,提出本构方程及C·H.B.模型,并得以验证.     

EXTRUSION AND LUBRICATION FLOWS OF VISCOPLASTIC FLUIDS WITH WALL SLIP

The simplest model flow which approximates the extrusion (shallow screw channels) and lubrication flow is the steady, laminar flow occurring between two infinitely long parallel plates i.e., the generalized plane Couette flow. Here we develop an analytical model of the generalized plane Couette flow of viscoplastic fluids. The deformation and flow behavior of viscoplastic fluids can be realistically represented with the Herschel-Bulkley constitutive equation, which we have utilized as the basis for the development of our analytical model. Furthermore, as also demonstrated here, the deformation behavior of viscoplastic fluids is generally complicated by the presence of wall slip at solid walls, which occurs as a function of the wall shear stress. The wall slip versus the wall shear stress behavior of viscoplastic fluids can be experimentally characterized using viscomelric flows, including steady torsional and capillary flows. Thus determined Navier's wall slip coefficient can then be utilized in modeling of processing flows. In our analytical model of the generalized plane Couette flow of viscoplastic fluids the Navier's wall slip boundary condition was included. This model should be an important engineering tool, which provides design expressions for the extrusion and lubrication flows of viscoplastic fluids, with or without wall slip occurring at the walls. @KEYWORDS:Extrusion, lubrication, flow, viscoplastic, slip.     

基于MUSIG模型的竖直圆管内液氮过冷沸腾数值模拟研究(英文)

Multiple size group （MUSIG） model combined with a threedimensional twofluid model were em ployed to predict subcooled boiling flow of liquid nitrogen in a vertical upward tube. Based on the mechanism of boiling heat transfer, some important bubble model parameters were amended to be applicable to the modeling of liquid nitrogen. The distribution of different discrete bubble classes was demonstrated numerically and the distribu tion patterns of void fraction in the wallheated tube were analyzed. It was found that the average void fraction in creases nonlinearly along the axial direction with wall heat flux and it decreases with inlet mass flow rate and sub cooled temperature. The local void fraction exhibited a Ushape distribution in the radial direction. The partition of the wall heat flux along the tube was obtained. The results showed that heat flux consumed on evaporation is the leading part of surface heat transfer at the rear region of subcooled boiling. The turning point in the pressure drop curve reflects the instability of bubbly flow. Good agreement was achieved on the local heat transfer coefficient aalnst experimental measurements, which demonstrated the accuracy of the numerical model.     

CFD simulation of the high shear mixing process using kinetic theory of granular flow and frictional stress models

In order to enhance process understanding and to develop predictive process models in high shear granulation, there is an ongoing search for simulation tools and experimental methods to model and measure the velocity and shear fields in the mixer. In this study, the Eulerian-Eulerian approach to model multiphase flows has been used to simulate the mixer flow. Experimental velocity profiles for the solid phase at the wall in the mixer have been obtained using a high speed camera following the experimental procedure as described by Darelius et al. [2007a. Measurement of the velocity field and frictional properties of wet masses in a high shear mixer. Chemical Engineering Science, 62, 2366-2374]. The governing equations for modelling the dense mixer flow have been closed by using closure relations from the kinetic theory of granular flow (KTGF) combined with frictional stress models. The free slip and partial slip boundary conditions for the solid phase velocity at the vessel wall have been utilized. The partial slip model originally developed for dilute flows by Tu and Fletcher [1995. Numerical computation of turbulent gas-solid particle flow in a 90^° bend. A.I.Ch.E. Journal, 41, 2187-2197] has been employed. It was found that the bed height could be well predicted by implementing the partial slip model, whereas the free slip model could not capture the experimentally found bed height satisfactorily. In the simulation, the swirling motion of the rotating torus formed was over-predicted and the tangential wall velocity was under-predicted, probably due to the fact that the frictional stress model needs to be further developed, e.g. to tackle cohesive particles in dense flow. The advantage of using the Eulerian-Eulerian approach compared to discrete element methods is that there is no computational limitation on the number of particles being modelled, and thus manufacturing scale granulators can be modelled as well.     

Effects of surface roughness and the chemical structure of materials of construction on wall slip behavior of linear low density polyethylene in capillary flow

The presence of wall slip during the flow of polymeric melts has significant ramifications on the melts' processability. In this study, the effects of materials of construction and surface roughness on the wall slip behavior of a linear low density polyethylene were investigated, using capillary flow. Capillaries, constructed from copper, stainless steel, aluminum, and glass, were used. The inner surface roughness of the capillaries were characterized by the employment of a profilometer and scanning electron microscopy. The roughness profiles of copper capillaries were also altered by the employment of chemical etching. Using Mooney's analysis, the wall slip velocity values were determined to be in the range of 0.09 to 1.34 mm/s. The wall slip velocity values were the highest for stainless steel and were negligible for aluminum. The relative work of adhesion values of polyethylene were the smallest for stainless steel and copper and the highest for glass. Overall, the wall slip velocity values increased with decreasing surface roughness of the capillaries and with decreasing work of adhesion. © 1993 John Wiley & Sons, Inc.