MODELING OF DIELECTRIC FLUID SOLIDIFICATION WITH CHARGED PARTICLES IN ELECTRIC FIELDS AND REDUCED GRAVITY

A mathematical model and an explicit finite-difference iterative integration algorithm for two-dimensional laminar steady flow and solidification of an incompressible, viscous, electrically conducting but neutrally charged melt containing electrically charged panicles and exposed to an externally applied electrostatic field were developed. The system of governing electrohydrodynamic equations was derived from a combination of Maxwell's equations and the Navier-Stokes equations, including thermally induced buoyancy, latent heat release, and Joule heating, while accounting for the mushy region. Physical properties were treated as arbitrarily temperature-dependent. Numerical results demonstrate the existence of strong electrothermoconvective motion in the melt and quantify its influence on the amount of accrued solid, deposition pattern of the electrically charged particles inside the accrued solid, and the melt/solid interface shape.     

NUMERICAL CALCULATIONS FOR COMBINED CONDUCTION AND RADIATION TRANSIENT HEAT TRANSFER IN A SEMITRANSPARENT MEDIUM

A numerical computer code was developed for calculating the combined conduction and radiation transient heat transfer in cylindrical, semitransparent materials that have temperature-dependent thermal properties. The radiative component is combined with the equation of conduction heat transfer by adding it as a heat source. The finite element method (FEM) was used for calculating the radiative component and for solving the temperature field in the medium. Very good agreement was observed between results obtained by using our code and those that exist in the literature for several steady-state cases. The advantage of the code is due to the fact that it incorporates temperature-dependent properties; thus it leads to more realistic and accurate results. The code was applied to calculate the cooling path of a large cylindrical sapphire boule while using varying, transient, temperature-dependent, combined heat transfer coefficients.     

CONVECTIONAL MODE AND ELECTRICAL FIELD IN SILICON MELTS UNDER VERTICAL,HORIZONTAL, AND ROTATING MAGNETIC FIELDS

Numerical computations were carried out to study a convectional mode and an electrical field in silicon melts in a cylindrical container that is sufficiently long in an axial direction under various magnetic fields. Under both no and vertical magnetic fields, the fluid represented a rigid rotation. Under a horizontal magnetic field, flow in the circumferential direction was suppressed strongly. Under a rotating magnetic field, fluid rotated with the rotational rate of a magnetic field when the rotational rate of a magnetic field and that of a container were different. These flow patterns could be successfully explained by considering an electric field.     

NUMERICAL PREDICTIONS OF FLOW AND THERMAL FIELDS IN A RECIPROCATING PISTON-CYLINDER ASSEMBLY

An efficient solution method for predicting unsteady, compressible flow and thermal fields in a reciprocating piston-cylinder assembly is developed in this study. The solution method, based on a two-stage pressure correction scheme, is applied for simultaneously determining the absolute pressure, density, temperature, and velocity components of the fluid inside the cylinder at any instant during the start-up and the periodically stable periods. Discretization equations are derived from the integral mass, momentum, and energy equations on a moving grid, which is deforming to accommodate the movement of the piston. A test problem is solved by means of the proposed method to illustrate the validity of the numerical procedure. Results show that the two-stage pressure correction scheme can be readily incorporated into existing numerical techniques to yield reasonably accurate results. Effects of the influential factors, including gravity (g) and rotation speed of the crank shaft (f), thus can be evaluated.     

A NUMERICAL STUDY OF ANISOTROPY AND CONVECTION DURING SOLIDIFICATION

A fixed-grid enthalpy formulation is developed to model convection and anisotropic conduction during solidification from the side or top wall of a rectangular cavity containing pure gallium. The solid-liquid interface is tracked with the use of an interface energy-balance equation. An improved discretization method is developed to prevent nonphysical oscillations due to discontinuous temperature and velocity gradients in the vicinity of the solid-liquid interface. Numerical results are compared to experimental data from the literature on the morphology and positions of the interface. The effect of anisotropic conduction in the solid interacting with thermal convection in the liquid has been studied.     

建筑室内人体太阳辐射得热特性及数值模拟

太阳辐射对建筑室内人体热舒适和建筑能耗有着显著影响.通过实测验证了daylight coefficient(DC)算法模拟太阳辐射强度的准确性.随后基于假人仿真模型采用DC算法计算室内人体平均辐射温度增量(ΔMRT),与SolarCal(SC)算法结果作对比,并对SC算法进行改进.相比原SC算法,改进SC算法与DC算法...     

NUMERICAL STUDY OF G-JITTER DURING DIRECTIONAL SOLIDIFICATION

A study of directional solidification of a weak binary alloy (specifically, Bi-1% Sn) using a fixed grid single domain approach has been undertaken. The enthalpy method is used to solve for the temperature field over the computational domain, including both the solid and liquid phases. The vorticity-stream function formulation is used to describe thermosolutal convection in the liquid region. Results on the solute field and segregation are presented, showing the effects of the periodic disturbances for a range of amplitudes and frequencies (multicomponent) and for actual acceleration data obtained during a space flight.     

NUMERICAL ANALYSIS OF HEAT TRANSFER BY NATURAL CONVECTION AND RADIATION IN PARTICIPATING FLUIDS ENCLOSED IN SQUARE CAVITIES

The influence of thermal radiation on natural convection in a participating fluid contained in a square cavity is studied numerically. The radiative transfer process is solved from the PI approximation. The Navier-Stokes equations are solved by a finite difference scheme integrated over control volumes. A numerical study of the so-called window problem (thermally driven cavity) shows the influence of thermal radiation on this reference problem for Rayleigh numbers in the range of 10³-10⁷ and Planck numbers varying from 1 to 0.05. The isotherms, streamlines, and heat lines show an increase of the dynamical effects in the central part of the cavity and a significant modification of the boundary layers. Results obtained from the simulation of an isotropically scattering medium are given.     

NUMERICAL MODELING OF INTERIOR BOUNDARIES

The numerical modeling of interior boundaries of finite and infinitesimal volume (area) is described for finite volume numerical methods. The treatment of passive structures such as solid obstacles and infinitesimally thin porous and nonporous walls, as well as hydro-dynamically active structures such as pumps and fans, are discussed. Properties peculiar to the pressure-velocity coupling are stressed, while more generally applicable techniques for other dependencies are shown. Heal transfer and turbulence effects complementary to the hydrodynamics are discussed. An example is presented showing application of the techniques to the flow of air about a very large directly air-cooled heat exchanger.     

地板辐射采暖空间温度场的数值模拟

根据地板采暖的特点，建立数学模型，采用有限差分法，模拟地板辐射采暖中地板温度场的变化关系。通过与实测结果进行比较和调整，可有效地控制供暖温度，取得了满意的结果，为工程设计提供比较准确的方法。     

卫星遥感地理信息与数值模拟应用于风能资源综合评估新尝试

该文提出一套综合的应用于风能资源评估的分析系统。应用卫星遥感反演出地形、地貌特征，融合地理信息数字高程数据，获得三维TM图像，在三维地形上进行风场的数值模拟和分析，得到该区域风能分布图，用以进行风电场宏观选址；根据宏观选址结果，通过在备选风电场进行风实测，应用研制的风能资源分析软件对该实测资料进行进一步地可行性分析，为风电场的开发作好前期的准备工作。     

IMPORTANCE OF GRID REFINEMENT IN NUMERICAL MODELING OF CHEMICAL VAPOR DEPOSITION PROCESSES

The importance of grid resolution near the substrate surface for accurate prediction of the deposition rates in chemical vapor deposition modeling has been demonstrated. The exercise is conducted through numerical modeling of the chemical vapor deposition of silicon in an atmospheric-pressure, circular, impinging-jet reactor. Silicon is deposited from gaseous silane (SiH 4 ) supplied in a dilute condition premixed in a hydrogen carrier gas. The substrate temperature is kept fixed at 1,333 K. The model includes variable fluid properties and buoyancy forces in the hydrodynamic model. The Bousinesq approximation is not used because the temperature gradient is large. In addition to the hydrodynamic and thermal solution, both gas-phase reactions in the bulk gas and surface reactions on the susceptor are included in the model. The mesh-independent solution and the deposition rate of silicon on the wafer surface are presented. It is observed that a very fine mesh near the substrate surface, within the concentration boundary layer for the intermediate species such as silylene (SiH 2 ), is required to establish grid independency and accurate prediction of the deposition rate. For the specific deposition process modeled in this study, about 7 control volumes had to be placed within the SiH 2 concentration boundary layer at the substrate surface.     

NUMERICAL MODEL FOR DENDRITIC SOLIDIFICATION OF BINARY ALLOYS

Abstract

A finite element model capable of simulating solidification of binary alloys and the formation of freckles is presented. It uses a single system of equations to deal with the all-liquid region, the dendritic region, and the all-solid region. The dendritic region is treated as an anisotropic porous medium. The algorithm uses the bilinear isoparametric element, with a penalty function approximation and a Petrov-Galerldn formulation. Numerical simulations are shown in which an NH₄Cl-H₂O mixture and a Pb-Sn alloy melt are cooled. The solidification process is followed in time. Instabilities in the process can be clearly observed and the final compositions obtained.     

THREE-DIMENSIONAL NUMERICAL MODELING OF HEAT AND MASS TRANSFER IN A CERAMIC OXYGEN GENERATOR (COG)

A three-dimensional numerical model has been developed to simulate the conjugate heat transfer and the separation of oxygen from air in a single-cell ceramic oxygen generator (COG). Attention is focussed on the temperature distribution in the brittle ceramic electrolyte as a result of heat generated from electrochemical reactions and the resistance to current flow. Investigations conducted to determine the influence on the thermal behaviour of the electrolyte with the presence of metallic current collectors and gas distributors in the model will also be summarised.     

A NUMERICAL STUDY OF SOLIDIFICATION IN THE PRESENCE OF A FREE SURFACE UNDER MICROGRAVITY CONDITIONS

A numerical study of the relative importance of Marangoni effects under microgravity conditions is presented. The mathematical formulation adopted is based on the enthalpy porosity method. One of the advantages of the fixed grid method is that a unique set of equations and boundary conditions is used for the whole domain, including both solid and liquid phases. The governing equations written in a vorticity-velocity formulation are discretized using a finite volume technique on a staggered grid. A fully implicit method has been adopted for the mass and momentum equations, while the temperature field is solved separately in order to evaluate the variation in the local liquid mass fraction. The resulting algebraic system of equations is solved using a preconditioned BI-CGStab method. Numerical results modelling the free surface, including the effects on it of Marangoni convection, are presented. The influence of the presence of argon in the gap above the free surface is investigated. During the numerical simulations presented in this paper 161 2 41 and 641 2 161 uniform meshes on the whole computational domain for values of Marangoni number ( Ma ) up to 16,120 and Rayleigh number ( Ra ) of 5 have been used.     

生物质超临界水气化制氢反应建模及数值模拟

建立了管式反应器中生物质超临界水气化制氢反应的数学模型,同时提出了以葡萄糖做为生物质模型化合物的全局气化反应动力学模型。模型计算结果与实验值的比较表明该模型能较好的预测反应器出口温度与气体产物组份分布。利用该模型数值模拟计算得到了反应器中温度场、速度场基本情况以及化学反应速率分布的基本规律。该文通过计算还讨论了反应器入口水温、反应器壁温以及物料和预热水之比对反应器内气化反应的影响,得出一系列重要结论。该模型对生物质超临界水反应器系统的优化设计与化学反应最佳工况的选择有一定的实用价值。     

NUMERICAL MODELING OF HELICAL FLOW OF PSEUDOPLASTIC FLUIDS

The laminar helical flow of non-Newtonian pseudoplastic fluids in concentric and eccentric annuli with a rotating inner cylinder has been investigated numerically. A finite volume algorithm with a nonstaggered grid system is used to analyze the problem. A nonorthogonal curvilinear coordinate system is employed to handle the irregular geometry of aneccentric annulus. The power-law constitutive equation is used to model the shear rate dependent viscosity of a pseudoplastic fluid. The computer code is validated against an available analytical solution for helical flow in a concentric annulus. It is observed that for a certain axial pressure gradient the axial flow rate increases within creasing rotational speed of the inner cylinder. The torque needed to rotate the inner cylinder decreases with increasing axial pressure gradient. These are explained in terms of the shear-thinning effect of a pseudoplastic fluid. The discharge as well as torque are found to increase with increasing eccentricity. The flow field in an eccentric annulus is complex in nature since vigorous secondary flow is produced in addition to the primary axial helical flow. The location and extent of the secondary flow is studied and theresults are presented for various eccentricities. The results will be useful in planning oil and gas well drilling operations.     

NUMERICAL MODELING OF A NUCLEATE BOILING SURFACE

ABSTRACT

A computer program developed to analyze nucleate boiling over a heated surface is described. The model solves the three-dimensional transient conduction equation within the heater. The conduction solution is coupled with closure relationships to mimic the bubble dynamics and the associated heat transfer coefficients. Sample problems are run using a copper surface subject to partial nucleate boiling in saturated water at atmospheric pressure. The results are shown to be in good qualitative agreement with the pertinent experimental observations.     

福建省太阳总辐射和地面辐射平衡的分布

首先利用福州市的辐射观测资料，建立计算太阳总辐射的经验公式，并计算福建省各市、县的太阳总辐射的年、月平均辐照度。然后利用地区代表站的地面反射率，求得福建省各月、年地面所吸收的太阳辐射能的分布。再利用地面温度、气温、水汽压和总云量，计算得出福建省各地的地面有效辐射的月、年平均辐照度。最后得出福建省各月、年地面辐射平衡的分布。     

NUMERICAL SOLUTION OF MELTING PROCESSES FOR FIXED AND UNFIXED PHASE CHANGE MATERIAL IN THE PRESENCE OF MAGNETIC FIELD--SIMULATION OF LOW-GRAVITY ENVIRONMENT

Transport processes associated with melting of an electrically conducting phase change material (PCM), placed inside a rectangular enclosure, under a low-gravity environment, and in the presence of a magnetic field, is simulated numerically. Electromagnetic forces damp the natural convection as well as the flow induced by sedimentation and/or floatation, and thereby simulate the low-gravity environment of outer space. Computational experiments are conducted for both side-wall heating and top-wall heating under a horizontal magnetic field. The governing equations are discretized using a control-volume-based finite difference scheme. Numerical solutions are obtained for a true low-gravity environment as well as for the simulated low-gravity conditions that are a result of the presence of a horizontal magnetic field. The effects of magnetic field on the natural convection, solid phase floatation/sedimentation, liquid/solid interface location, solid melting rate, and the flow patterns are investigated. It is found that the melting under a low-gravity environment reasonably can be simulated on earth via applying a strong horizontal magnetic field. However, the flow patterns obtained for the true low-gravity environment are not similar to the corresponding cases solved for the simulated low gravity.