Infiltration of Liquid Metals in Porous Compacts: Modeling of Permeabilities During Reactive Melt Infiltration

Reactive infiltration is a fast and cost-effective technique for manufacturing ceramic-matrix composites (CMCs). CMCs are used in elevated temperature applications like rocket engine casings, jet nozzles, gas turbine blades and nuclear cladding. There is an urgent need for minimizing experimental costs as well as optimizing process parameters during manufacture, so that we have minimized manufacturing costs and reduced infiltration times. Towards this end, the objective of this research was to develop an integrated micro-macro model of reactive flow of molten silicon in a porous preform consisting of carbon-coated silicon carbide fibers and then optimize process parameters computationally. The overall objective of the research was to arrive at a modified equation of Darcy's law for flow through a porous medium with the help of numerical/computational modeling. This paper deals with the flow of silicon through porous carbon at the macro level. The macro flow of silicon was integrated with an available micro model by determining the transient porosity from the micro model and using it in Darcy's law written for the macro flow of silicon. From the results of this study, we recommend suitable process parameters such as initial temperature of the solid reactant and the specific kind of reactants to be used for achieving complete infiltration. These conclusions are drawn after observation of the rate of decrease of permeability with more reaction.     

Gas Injection in a Liquid Saturated Porous Medium. Influence of Gas Pressurization and Liquid Films

We study numerically and experimentally the displacement of a liquid by a gas in a two-dimensional model porous medium. In contrast with previous pore network studies on drainage in porous media, the gas pressurization is fully taken into account. The influence of the gas injection rate on the displacement pattern, breakthrough time and the evolution of the pressure in the gas phase due in part to gas compressibility are investigated. A good agreement is found between the simulations and the experiments as regards the invasion patterns. The agreement is also good on the drainage kinetics when the dynamic liquid films are taken into account.     

一维流体饱和粘弹性多孔介质层的动力响应

本文研究了不可压流体饱和粘弹性多孔介质层的一维动力响应问题。基于粘弹性理论和多孔介质理论,在流相和固相微观不可压、固相骨架服从粘弹性积分型本构关系和小变形的假定下,建立了不可压流体饱和粘弹性多孔介质层一维动力响应的数学模型,利用Laplace变换,求得了原初边值问题在变换空间中的解析解,并利用Laplace逆变换的Crump数值反演方法,得到原动力响应问题的数值解。数值研究了饱和标准线性粘弹性多孔介质层的动力响应,分析了固相位移、渗流速度、孔隙压力及固相有效应力等的响应特征。结果表明,与不可压流体饱和弹性多孔介质相同,不可压流体饱和粘弹性多孔介质中亦只存在一个纵波,并且固相骨架的粘性对动力行为有显著的影响。     

液态铝在多孔介质中低压渗流过程的模拟实验研究

本文根据相似原理对液态铝在多孔介质中的低压向上渗流过程进行了模拟试验研究，试验表明，渗流液面以平面沿垂方向推进，位移－时间关系符合四次方根规律；某一时刻Ｔ的雷诺数，阻力系数和沿程损失可用相应的公式计算外加渗流气压的最佳值为０．４０ａｔ．。实验还将模拟充填时间与原型充填时间进行了对比，两者得到了良好的一致性。     

铝液在多孔介质中渗流过程的模拟试验研究

选择低压渗流法制备泡沫铝的工艺过程为模拟对象,根据相似原理,在有机玻璃模型中用液体作为流体,对铝液在多孔介质中的低压向上渗流过程进行了模拟试验研究。试验表明,在低压渗流制备泡沫铝的过程中,原型的渗流液面是以平面沿铅垂方向向上推进的,位移和时间的关系满足四次方根规律;渗流过程中某一时间t的雷诺数、阻力系数和沿程损失可用相应的公式计算;渗流过程中,外加渗流气压的最佳值为0.04kPa,高于该值的外加渗流气压对于渗流时间的影响不大。实验还将模拟实验得到的结果与原型进行了比较,模型的充填时间与原型的充填时间一致性良好,证实了本实验结果的正确性。     

孔隙深度对多孔储液介质摩擦学性能的影响研究

多孔储液介质凭借其独特的孔隙结构可以储存并释放润滑介质,具备良好的自润滑性能. 利用计算流体力学(CFD)方法研究了孔隙深度对多孔储液介质摩擦界面流体压力分布的影响;考虑气-液界面的弯月面力作用,研究了不同孔隙深度的多孔储液介质气-液承载模型以及气-液二相的最小压差分布规律. 基于模拟计算结果,采用3D打印技术制备了不同孔隙深度的多孔储液介质,进一步考察了孔隙深度对其摩擦学性能的影响. CFD模拟结果表明合理设计孔隙深度能够增强多孔储液介质的流体动压润滑效应,孔隙深度较低会使得润滑升力不足,孔隙深度过高又会使得孔隙中流体产生回流循环,削弱楔形效应. 气体进入多孔储液介质摩擦副表面后,在孔隙中形成气-液二相受压承载,其最大承载力随着孔隙深度的增加先升高后趋于平稳,但孔隙深度越小,对润滑作用的积极效果越显著. 摩擦试验表明多孔储液介质的摩擦系数随着孔隙深度的增加呈先降低后增加的趋势,与模拟计算结果一致. 因此合理设计多孔储液介质的孔隙深度,能优化多孔储液介质的润滑性能.      

Pore Network Simulation of Evaporation of a Binary Liquid from a Capillary Porous Medium

A pore-network model of evaporation of a binary liquid mixture into a ternary gas phase is developed. The model is applied to study the influence of surface tension gradients induced by composition variations of the liquid on the phase distribution within a capillary porous medium. Numerical simulations based on the proposed model show that the surface tension gradients lead to the accumulation of liquid near the open edge of the network. This surface tension gradient effect is only significant for weakly disordered porous media.     

Effect of Wetting on Gravity Drainage in Porous Media

A series of benchmark experiments on the effect of the wetting state on the flow properties in porous media were performed, allowing us to relate the wetting properties at the pore scale to the macroscale hydrodynamics. Drainage of n-alkanes (oils) displaced by air in a model porous medium consisting of water-wet sand was studied using gamma-ray densitometry and weight measurements. The enormous advantage of our system is that we know and control the wetting properties perfectly: we can tune the wetting properties by changing the salinity of the water. This allows us to perform porous medium flow experiments for the different wetting states without changing the transport properties (viscosity and density) of the oil. Drainage is found to be more efficient, and consequently oil recovery more important for partial wetting.     

Heat Transport in an Anisotropic Porous Medium Saturated with Variable Viscosity Liquid Under Temperature Modulation

Thermal instability in a horizontal porous medium saturated with temperature-dependant viscous fluid has been considered, and the effect of time-periodic temperature modulation has been investigated. The amplitudes of temperature modulation at the lower and upper surfaces are considered to be very small and the disturbances are expanded in terms of power series of amplitude of convection. A weak non-linear stability analysis has been performed for the stationary mode of convection, and heat transport in terms of the Nusselt number, which is governed by the non-autonomous Ginzburg–Landau equation, is calculated. The effects of thermo-rheological parameter, amplitude and frequency of modulation, thermo-mechanical anisotropies, and Vadasz number on heat transport have been analyzed and depicted graphically. It is found that an increment in the value of thermo-rheological parameter results in the enhancement of heat transport in the system. Further, the study establishes that the heat transport can be controlled effectively by a mechanism that is external to the system.     

Porous Medium Modeling of Air-Cooled Condensers

This article presents a porous media transport approach to model the performance of an air-cooled condenser. The finned tube bundles in the condenser are represented by a porous matrix, which is defined by its porosity, permeability, and the form drag coefficient. The porosity is equal to the tube bundle volumetric void fraction and the permeability is calculated by using the Karman–Cozney correlation. The drag coefficient is found to be a function of the porosity, with little sensitivity to the way this porosity is achieved, i.e., with different fin size or spacing. The functional form was established by analyzing a relatively wide range of tube bundle size and topologies. For each individual tube bundle configuration, the drag coefficient was selected by trial and error so as to make the pressure drop from the porous medium approach match the pressure drop calculated by the heat exchanger design software ASPEN B-JAC. The latter is a well-established commercial heat exchanger design program that calculates the pressure drop by using empirical formulae based on the tube bundle properties. A close correlation is found between the form drag coefficient and the porosity with the drag coefficient decreasing with increasing porosity. A second order polynomial is found to be adequate to represent this relationship. Heat transfer and second law (of thermodynamics) performance of the system has also been investigated. The volume-averaged thermal energy equation is able to accurately predict the hot spots. It has also been observed that the average dimensionless wall temperature is a parabolic function of the form drag coefficient. The results are found to be in good agreement with those available in the open literature.     

Brinkman Flow of a Viscous Fluid Through a Spherical Porous Medium Embedded in Another Porous Medium

An analytical investigation for a two-dimensional steady, viscous, and incompressible flow past a permeable sphere embedded in another porous medium is presented using the Brinkman model, assuming a uniform shear flow far away from the sphere. Semi-analytical solutions of the problem are derived and relevant quantities such as velocities and shearing stresses on the surface of the sphere are obtained. The streamlines inside and outside the sphere and the radial velocity are shown in several graphs for different values of the porous parameters \({\sigma _1 =(\mu /\tilde {\mu }) (a/\sqrt{K_1 })}\) and \({\sigma _2 =(\mu /\tilde {\mu }) (a/\sqrt{K_2 })}\) , where a is the radius of the sphere, μ is the dynamic viscosity of the fluid, \({\tilde {\mu }}\) is an effective or Brinkman viscosity, while K ₁ and K ₂ are the permeabilities of the two porous media. It is shown that the dimensionless shearing stress on the sphere is periodic in nature and its absolute value increases with an increase of both porous parameters σ ₁ and σ ₂.     

Effect of Wetting on the Dynamics of Drainage in Porous Media

We examine the consequences of the wettability properties on the dynamics of gravity drainage in porous media. The relation between the wetting properties at the pore scale and the macroscale hydrodynamics is studied. Model porous media consisting of hydrophilic and hydrophobic glass beads or sand with well defined wetting properties, are prepared for this study. Gravity drainage experiments with air displacing water (two-phase flow), are performed for different Bond numbers, and using different techniques such as gamma-ray densitometry, magnetic resonance imaging (MRI) and weight measurements. The dynamics of drainage is found to be different for hydrophilic and hydrophobic porous media in the transition zone (funicular regime). Moreover, for hydrophilic (water-wet) porous media, MRI experiments reveal the importance of drainage through the continuous water film, which leads to an increase of the residual quantity of water in the transition zone with time.     

Heat Transport in an Anisotropic Porous Medium Saturated with Variable Viscosity Liquid Under G-jitter and Internal Heating Effects

Thermo-rheological effect of temperature-dependent viscous fluid saturating a porous medium has been studied in the presence of imposed time periodic gravity field and internal heat source. Weak nonlinear stability analysis has been performed by using the power series expansion in terms of the amplitude of gravity modulation, which is considered to be small. Nusselt number is calculated numerically using Ginzburg–Landau equation. The nonlinear effects of thermo-mechanical anisotropies, internal heat source parameter, Vadász number, thermo-rheological parameter and amplitude of gravity modulation have been obtained and depicted graphically. Streamlines and isotherms have been drawn for different times. Comparisons have been made between various physical systems.     

液滴二次雾化破碎模式数值模拟

在许多现代工程领域中,二次雾化对于增强液体雾化以及提高混合率都起到了关键的作用．基于SIMPLE方法对这个问题进行了数值模拟,VOF方法与Level-Set方法耦合捕捉界面,自适应网格的运用平衡了对提高计算精度以及降低计算成本两方面的要求．在Oh数较低(Oh < 0.1)的情况下,通过数值模拟得到了四种典型的液滴分裂模式,并且对其中相对复杂的第二第三种分裂模式进行了详细的分析与比较．得出了该模式形成的条件与相关特性．最后,将以上结果与前人的实验结果作了对比,在大多数重要特征方面,两者有非常好的一致性．     

Effects of Precursor Wetting Films in Immiscible Displacement Through Porous Media

A computer-aided simulator of immiscible displacement in strongly water-wet consolidated porous media that takes into account the effects of the wetting films is developed. The porous medium is modeled as a three-dimensional network of randomly sized unit cells of the constricted-tube type. Precursor wetting films are assumed to advance through the microroughness of the pore walls. Two types of pore wall microroughness are considered. In the first type of microroughness, the film advances quickly, driven by capillary pressure. In the second type, the meniscus moves relatively slowly, driven by local bulk pressure differences. In the latter case, the wetting film often forms a collar that squeezes the thread of oil causing oil disconnection. Each pore is assumed to have either one of the aforementioned microroughness types, or both. The type of microroughness in each pore is assigned randomly. The simulator is used to predict the residual oil saturation as a function of the pertinent parameters (capillary number, viscosity ratio, fraction of pores with each type of wall microroughness). These results are compared with those obtained in the absence of wetting films. It is found that wetting films cause substantial increase of the residual oil saturation. Furthermore, the action of the wetting films causes an increase of the mean volume of the residual oil ganglia.     

Oscillating Convection of Nanofluid in Porous Medium

In this paper, oscillatory convection in a horizontal layer of nanofluid in porous medium is studied. For porous medium, Darcy model is applied. A linear stability theory and normal mode analysis method is used to find the solution confined between two free boundaries. The onset criterion for oscillatory convection is derived analytically and graphically. Regimes of oscillatory and non-oscillatory convection for various parameters are derived. The effects of Lewis number, concentration Rayleigh number, Prandtl?CDarcy number (Vadasz Number) and modified diffusivity ratio on the oscillatory convection are investigated graphically. We examine the validity of ??PES?? and concluded that ??PES?? is not valid for the problem.     

An Analysis of the Movement of Wetting and Nonwetting Fluids in Homogeneous Porous Media

The movement of wetting and nonwetting fluid flow in columns packed with glass beads is used to understand the more complicated flows in homogeneous porous media. The motion of two immiscible liquids (oil and water) is observed with different surfactants. Through dimensional analyses, fluid velocity is well correlated with interfacial tension and less dependent on particle size. In water–oil (W/O) experiments, finger pattern flows are observed if water is the displacing fluid that flows in an oil-filled porous media, whereas oil ganglia tend to form if oil is the displacing fluid in the water-wetted porous media. The results are well described by a simple model based on an earlier theory of flow in a tube.     

Hall and Porous Boundaries Effects on Peristaltic Transport Through Porous Medium of a Maxwell Model

Peristaltic motion induced by sinusoidal traveling wave of incompressible, electrically conducting Maxwell fluid in the porous walls of a two-dimensional channel through a porous medium has been investigated in the presence of a constant magnetic field. The Hall effect has been taken into account. Modified Darcy??s law has been used in the flow modeling. The fluid entering the flow region through one plate is considered at the same rate as it is leaving through the other plate. The problem is formulated using a perturbation expansion in terms of small amplitude ratio. We have discussed the problem only for free pumping case. This work can be considered as mathematical modeling to the case of gall bladder with stones. Finally, the effects of various parameters of interest are discussed and shown graphically.     

Peristaltic Flow of a Maxwell Model Through Porous Boundaries in a Porous Medium

In the present investigation we have presented the peristaltic flow of a linear Maxwell model through porous boundaries in a porous medium. The governing non-dimensional partial differential are solved in wave frame by using regular perturbation method and assumed form of solution. We have discussed the problem only for free pumping case. The effects of various physical parameters involved in the problem have been investigated and shown graphically.