A 3D modeling of static and forward smoldering combustion in a packed bed of materials

A three-dimensional (3D) model based on the first principles of mass, momentum and energy was developed that numerically simulates the processes of static and forward smoldering in a porous packed bed of plant materials. The packed bed contains cellulose material or tobacco (cigarette) wrapped in a porous paper and surrounded by an ambient air. Other major characteristics of the model are including the effects of buoyancy forces in the flow field, separate treatment of solid and gas in a thermally non-equilibrium environment, and use of multi-precursor kinetic models for the pyrolysis of staring material and oxidation of char. The changes in porosity due to pyrolysis and char oxidation and the effect of porosity on the bed permeability and gas diffusivity are included. The mass, momentum, energy, and species transport equations are solved in a discretized computational domain using a commercially available computational fluid dynamics (CFD) code. The simulation results show that the model reasonably reproduces the major features of a burning cigarette during smoldering and puffing and are in a good agreement with the existing experimental results for cigarettes. Results include the velocity profiles, gas and solid temperatures, coal shape, burn rates, profile and transport of gas and vapor species throughout the packed bed, dilution through the wrapper paper and ventilation in the filter section, and the mass fraction of some pyrolysis and oxidation products in the mainstream and sidestream flows.     

Notes on modelling of environmental transport in wetland

This paper presents an effort towards a basic model for environmental transport of momentum, heat and mass transfer in the wetland. To smear out the discontinuity between the two phases of water and solid in the wetland, the continuum models distinctively applying for the water body and solid frame are transformed via the technique of phase average to give equations for a virtual single-phase flow in the entire domain of the wetland. Then to filter out the vortex and fluctuation common in the wetland flow, the operation of large eddy simulation (LES) is applied to yield a basic model for practical simulation. With reference to the modelling of flows in porous media and turbulent flows, closure relations are presented for the correlation terms due to the phase average and large eddy simulation.     

Modelling and solving fixed-bed adsorption

In this paper we develop a mathematical model for packed-bed adsorption of a single component, directed toward the purification of β-lactamic antibiotics. The model describes the removal of sorbates from an aqueous stream entering a bed packed with spherical sorbent particles. Then, assuming that the adsorption isotherm is linear, an exact solution for this model is found.     

A 3D non-linear k–ε turbulent model for prediction of flow and mass transport in channel with vegetation

The results from a 3D non-linear k–ε turbulence model with vegetation are presented to investigate the flow structure, the velocity distribution and mass transport process in a straight compound open channel and a curved open channel. The 3D numerical model for calculating flow is set up in non-orthogonal curvilinear coordinates in order to calculate the complex boundary channel. The finite volume method is used to disperse the governing equations and the SIMPLEC algorithm is applied to acquire the coupling of velocity and pressure. The non-linear k–ε turbulent model has good useful value because of taking into account the anisotropy and not increasing the computational time. The water level of this model is determined from 2D Poisson equation derived from 2D depth-averaged momentum equations. For concentration simulation, an expression for dispersion through vegetation is derived in the present work for the mixing due to flow over vegetation. The simulated results are in good agreement with available experimental data, which indicates that the developed 3D model can predict the flow structure and mass transport in the open channel with vegetation.     

计算有弥散和吸附的径向渗流问题的局部化间断Galerkin方法

在Corkburn和Shu新近发展的求解对流扩散方程的局部化间断Galerkin方法的基础上,针对有弥散和吸附的径向渗流问题中出现的推广的对流扩散方程的形式,构造了一种计算有弥散和吸附的径向渗流问题的局部化间断Galerkin有限元方法,为径向渗流问题的求解提供了一个高阶的新方法．对对流-弥散和对流-弥散-吸附两种情况进行了数值实验, 所得结果的相应部分与已知的一些精确解结果和数值结果是一致的,表明方法是可靠的．从计算速度上看,方法也是可行的．     

Two dimension-curvilinear grid for open channel flow simulation

A mathematical model for two-dimensional flow simulation in an open channel is developed. The model is obtained through the use of a stretched curvilinear grid which defines points where velocity and surface elevation are simulated. Simulation is achieved by numerically integrating the Navier-Stokes mass and momentum equations using centred finite difference approximations of derivatives. The model uses an implicit discretization scheme, the Newton-Raphson iterative technique and a customized Gauss elimination solving algorithm. The computer program developed for this model was tested for uniform, non-uniform and unsteady flow conditions. The results have been found consistent with theoretical solutions and/or field measurements. Limitation and verification of the model is also outlined.     

可变形孔隙介质中热、水耦合力学问题的代数多格子分析方法

研究了孔隙介质中包括热和质量传递的全耦合多相流问题的代数多格子分析方法。数学模型包括质量、线性矩、能量平衡方程和本构方程,以位移、毛细压力、汽压和温度为基本变量,模型中采用了考虑毛细压力关系的修正有效应力概念,并考虑相变、热传导、对流和潜热交换(汽化-冷凝),气相是由易混合的干空气和水蒸气组成,视为理想气体。考题显示出较高的计算效率。     

Mixed convection magnetohydrodynamic heat and mass transfer past a stretching surface in a micropolar fluid-saturated porous medium under the influence of Ohmic heating,Soret and Dufour effects

A numerical model is developed to examine the combined effects of Soret and Dufour on mixed convection magnetohydrodynamic heat and mass transfer in micropolar fluid-saturated Darcian porous medium in the presence of thermal radiation, non-uniform heat source/sink and Ohmic dissipation. The governing boundary layer equations for momentum, angular momentum (microrotation), energy and species transfer are transformed to a set of non-linear ordinary differential equations by using similarity solutions which are then solved numerically based on shooting algorithm with Runge–Kutta–Fehlberg integration scheme over the entire range of physical parameters with appropriate boundary conditions. The influence of Darcy number, Prandtl number, Schmidt number, Soret number and Dufour number, magnetic parameter, local thermal Grashof number and local solutal Grashof number on velocity, temperature and concentration fields are studied graphically. Finally, the effects of related physical parameters on local Skin-friction, local Nusselt number and local Sherwood number are also studied. Results showed that the fields were influenced appreciably by the Soret and Dufour effects, thermal radiation and magnetic field, etc.     

不可压饱和多孔弹性杆动力响应的多辛方法

研究了不可压饱和多孔弹性杆的一维动力响应问题.基于多孔介质理论,在流相和固相微观不可压、固相骨架小变形的假定下,建立了不可压流体饱和多孔弹性杆一维轴向动力响应的数学模型.利用Hamilton空间体系的多辛理论,构造了不可压饱和多孔弹性杆轴向振动方程的多辛形式及其多种局部守恒律.采用中点Box离散方法得到轴向振动方程的多辛离散格式和局部能量守恒律以及局部动量守恒律的离散格式;数值模拟了不可压饱和多孔弹性杆的轴向振动过程,记录了每一时间步的局部能量数值误差和局部动量数值误差.结果表明,已构造的多辛离散格式具有很高的精确性和较长时间的数值稳定性,这为解决饱和多孔介质的动力响应问题提供了新的途径.     

One-dimensional, two-fluid modelling of turbulent premixed flames

The “two-fluid” mathematical model for turbulent combustion is applied to a one-dimensional, premixed, stabilized ducted flame. The flame is assumed to consist of two interspersed fluids (“reactants” and “products”), each characterized by its own properties and interacting through the exchange of mass, heat, and momentum. The distributions of pressure, densities, velocities, and volume fractions across the duct were successfully simulated. From a parametric study on the effects of the empirical constants involved in the interfluid relations, the significant dependence of the system on the parameters that characterize the mass transfer rate and the relative effect of mass transfer to momentum transfer was confirmed. The application of the model to transient states proved its ability to predict system oscillations.     

The transport of mass, energy and momentum in physiologic systems

A generic equation is developed to define the flux, Φ, of mass, energy and momentum in physiologic systems. This equation is written in terms of the concentration, ρ, of the quantity to be transported, the force driving it, a corresponding kinematic transport coefficient, , and the path and mobility associated with the transport process. Driving forces are expressed in terms of the gradient of some intensive property, ø, of the system, such that:

, where n=0 for energy, for momentum and n=1 for mass; β is a nondimensional coefficient of proportionality. Values of φ and are tabulated for various scenarios typical of transport processes occuring in physiologic systems.     

A perturbation solution for bacterial growth and bioremediation in a porous medium with bio-clogging

We investigate a flow problem of relevance in bioremediation and develop a mathematical model for transport of contamination by groundwater and the spreading, confinement, and remediation of chemical waste. The model is based on the fluid mass and momentum balance equations and simultaneous transport and consumption of the pollutant (hydrocarbon) and nutrient (oxygen). Particular emphasis is placed on the study of processes involving the full coupling of reaction, transport and mechanical effects. Dimensional analysis and asymptotic reduction are used to simplify the governing equations, which are then solved numerically.     

Analytic solution for heat transfer of a third grade viscoelastic fluid in non-Darcy porous media with thermophysical effects

An analytic approximate solution is presented for the natural convective dissipative heat transfer of an incompressible, third grade, non-Newtonian fluid flowing past an infinite porous plate embedded in a Darcy–Forchheimer porous medium. The mathematical model is developed in an $(x\text{,}y)$ coordinate system. Using a set of transformations, the momentum equation is rendered one-dimensional and a partly linearized heat conservation equation is derived. The viscoelastic formulation presented by Akyildiz [Akyildiz FT. A note on the flow of a third grade between heated parallel plates. Int J Non-Linear Mech 2001;36:349–52] is adopted, which generates lateral mass and viscoelastic terms in the heat conservation equation, as well as in the momentum equation. A number of special cases of the general transformed model are discussed. A homotopy analysis method (HAM) is implemented to solve, with appropriate boundary conditions, the coupled third-order, second degree ordinary differential equation for momentum and the second-order, fourth degree heat conservation equation.     

A perturbation solution for forced convection in a porous-saturated duct

Fully developed forced convection through a porous medium bounded by two isoflux parallel plates is investigated analytically on the basis of a Brinkman–Forchheimer model. The matched asymptotic expansion method is applied for small values of the Darcy number. For the case of large Darcy number the solution for the Brinkman–Forchheimer momentum equation is found in terms of an asymptotic expansion. With the velocity distribution determined, the energy equation is solved using the same asymptotic technique. The results for limiting cases are found to be in good agreement with those available in the literature and the numerical results obtained here.     

On the well-posed coupling between free fluid and porous viscous flows

We present a well-posed model for the Stokes/Brinkman problem with a family of jump embedded boundary conditions (J.E.B.C.) on an immersed interface with weak regularity assumptions. It arises from a general framework recently proposed for fictitious domain problems. Our model is based on algebraic transmission conditions combining the stress and velocity jumps on the interface $\Sigma$ separating the fluid and porous domains. These conditions are well chosen to get the coercivity of the operator. Then, the general framework allows us to prove new results on the global solvability of some models with physically relevant stress or velocity jump boundary conditions for the momentum transport at a fluid–porous interface. The Stokes/Brinkman problem with Ochoa-Tapia and Whitaker (1995) [9], [10] interface conditions and the Stokes/Darcy problem with Beavers and Joseph (1967) [13] conditions are both proved to be well-posed, by an asymptotic analysis. Up to now, only the Stokes/Darcy problem with Saffman (1971) [15] approximate interface conditions with negligible tangential porous velocity was known to be well-posed.     

Solution of nonlinear convection-diffusion problems by a conservative Galerkin-characteristics method

We consider a scheme for nonlinear (degenerate) convection dominant diffusion problems that arise in contaminant transport in porous media with equilibrium adsorption isotherm. This scheme is based on a regularization relaxation scheme that has been introduced by Jäger and Ka?ur (Numer Math 60:407–427, 1991; M²AN Math Model Numer Anal 29(N5):605–627, 1995) with a type of numerical integration by Bermejo (SIAM J Numer Anal 32:425–455, 1995) to the modified method of characteristics with adjusted advection MMOCAA that was recently developed by Douglas et al. (Numer Math 83(3):353–369, 1999; Comput Geosci 1:155–190, 1997). We present another variant of adjusting advection method. The convergence of the scheme is proved. An error estimate of the approximated scheme is derived. Computational experiments are carried out to illustrate the capability of the scheme to conserve the mass.     

重建极性连续统理论的基本定律和原理(Ⅶ)——增率型

目的是建立微极连续统增率型的较为完整的运动方程,边界条件和能率方程.为此,先给出较为完整的变形梯度及其逆的定义.接着推导出各种应力率和偶应力率间的新关系式.最后,作为一种特殊情形得到连续统力学的耦合的增率型运动方程、边界条件和能率方程.     

重建极性连续统理论的基本定律和原理(Ⅵ)——质量和惯性守恒定律

重建极性连续统理论的耦合型质量和惯性的守恒定律和局部守恒方程以及跳变条件.为此推导出新的变形梯度、线元、面元和体元的物质导数,并给出广义Reynolds输运定理.把这些结果和作者以前推导出的耦合型动量、动量矩和能量的基本定律和有关原理结合在一起就大体上构成极性连续统理论相当完整的耦合型基本定律、局部守恒和均衡方程及原理体系.从此体系可以根据常用的局部化方法给出耦合型的非局部质量和惯性守恒方程以及动量、动量矩和能量均衡方程.     

Network modeling of chromatography by stochastic phenomena of adsorption,diffusion and convection

The limitation of macroscopic models to represent structural parameters, such as topology and morphology, as well as population effects, i.e. multi-molecules movement, in the modeling of chromatography systems has implications on the understanding of the phenomenological aspects that contribute to the separation mechanisms in porous media. The representation of the porous structure of chromatographic columns by a three-dimensional cubic network of interconnected sites allows a better analysis of the structural characteristics of the porous column and its connection with the phenomena of adsorption, diffusion and convection. In the present work the application of an interconnected cubic network model associated with a stochastic modeling of the adsorption, diffusion and convection phenomena leads to the proper representation of the dynamic aspects of the breakthrough curves related to separation processes in chromatographic columns. Therefore, it is possible to study the dynamics of solute retention from the molecules distribution in the separation processes throughout the column. Among the mass transfer mechanisms investigated, the convection showed to be closely related to the separation dynamics of chromatography, with the diffusion having little effects. The adsorption influenced both the separation dynamics and solute retention.