NUMERICAL METHOD FOR HEAT TRANSFER,FLUID FLOW,AND STRESS ANALYSIS IN PHASE-CHANGE PROBLEMS

This work presents a finite-volume method for simultaneous prediction of physical phenomena occurring during a solid / liquid phase change, including buoyancy-driven flow in the liquid, deformation and stresses in the solid, and heat transfer in both the liquid and solid parts of the solution domain. The liquid is treated as a Newtonian incompressible fluid and it is assumed that the solid behaves as a thermoelastic body, although other constitutive equations for liquid and / or solid could easily be incorporated. The method solves integral equations of mass, momentum, and energy balance discretized on numerical meshes consisting of cells of arbitrary polyhedral shape. The method is validated by comparing numerical results with analytical solutions and available measurement data.     

NUMERICAL ANALYSIS OF FLUID FLOW AND HEAT TRANSFER CHARACTERISTICS IN THREE-DIMENSIONAL PLATE FIN-AND-TUBE HEAT EXCHANGERS

This numerical study provides three-dimensional (3-D) time-dependent modeling of unsteady laminar flow and heat transfer over single- and multirow plate fin-and-tube heat exchangers. The complex nature of the flow field featuring a horseshoe vortex is investigated for both configurations. The time-dependent evolution of the horseshoe vortex mechanism on the forward part of the tube and its journey to the rear of the tube are studied to provide fundamental information on the local flow structure and the corresponding heat transfer characteristics. The effects of various governing parameters, such as fin spacing, Reynolds number, tube row number, and tube arrangement, on the heat transfer and flow characteristics are also studied for the Reynolds number range investigated. It is found that the local flow structure including formation and evolution of vortex systems and singular-point interactions correlates strongly with the heat transfer characteristics. The numerical results for the integral heat transfer parameters agree well with available experimental measurements.     

ERROR ESTIMATION AND ADAPTIVITY IN FINITE ELEMENT ANALYSIS OF CONVECTIVE HEAT TRANSFER PROBLEMS,PART II: VALIDATION AND APPLICATIONS

Abstract

This is the second of two articles concerning error estimation and adaptive refinement techniques applied to connective heat transfer problems. In the first article ( ) the development of the proposed methodology was presented. This article( pari II) concerns the validation of the formulation. Examples dealing with heat and momentum transfer were used to verify the efficiency and accuracy of this technique. Applications include sterilization of food products and pasteurization of liquids contained in bottles. The desired accuracy level was always attained. Refined meshes agreed with the physical aspects of the problems. Results showed significant improvements when compared with the conventional finite element approach.     

ERROR ESTIMATION AND ADAPTIVITY IN FINITE ELEMENT ANALYSIS OF CONVECTIVE HEAT TRANSFER PROBLEMS,PART I: THEORETICAL CONSIDERATIONS

Abstract

This is the first of two articles concerning error estimation and adaptive refinement techniques applied to convective heat transfer problems. This study presents the detailed development of the proposed error estimator. The error estimator takes into account the coupling effects of the dependent variables ( e.g.. velocity and temperature) on the discretization error and, consequently, on the adaptive meshes. An averaging procedure is also proposed as a substitute for the smoothing /recovery procedure used to evaluate the gradients. The implementation of the averaging technique is simple and cost effective. Numerical experiments demonstrate that the proposed methodology results in a dramatic reduction in computational costs.     

MULTIPLE TRANSIENT POINT HEAT SOURCES IDENTIFICATION IN HEAT DIFFUSION: APPLICATION TO NUMERICAL TWO- AND THREE-DIMENSIONAL PROBLEMS

This article deals with an inverse problem, which consists of the location and strength identification of multiple-point heat sources in transient heat conduction. The identification procedure is based on a boundary integral formulation using space and time Green functions. The discretized problem is nonlinear if the location of the point heat sources is unknown. In order to reduce the sensitivity of the solution to errors, we use the future time step procedure associated to a Tikhonov regularization procedure. The proposed numerical approach is applied to numerical two- and three-dimensional examples.     

热管式吸热器单元热管传热的数值模拟分析

热管式吸热器的热性能分析对吸热器设计有着重要意义，但由于其相变过程与热管传热的耦合作用十分复杂，至今仍是很少有人深入研究的领域。本文基于焓法建立单元热管耦合传热的物理和数学模型，模拟计算了热管壁温、蓄热容器壁温、循环工质出口温度及相变材料熔化率等参数，并与基本型吸热器进行比较，验证了热管吸热器明显改善了温度分布的均匀性和相变材料的熔化率。     

U型管埋地换热器三维传热模型及实验对比分析

针对U型管埋地换热器的传热特点,考虑了两管脚之间的相互影响,建立起非稳态传热模型。采用数值解法对该理论模型进行了求解,并将结果与实验值进行了对比分析,发现两者变化趋势一致,但理论计算结果比实验测试值低,其中的一个重要原因是因为换热器与土壤间的热交换不是纯导热过程,还应该进一步考虑土壤中热湿迁移等因素的影响。     

FLOW AND HEAT TRANSFER IN RECTANGULAR ENCLOSURES USING A NEW BLOCK-IMPLICIT NUMERICAL METHOD

This work reports a numerical investigation on buoyancy-induced flows occurring in enclosures of small aspect ratio and inclined with respect to the horizontal direction. The numerical method used consists of the control-volume approach and a new block-implicit error-smoothing operator. Governing equations are written in terms of primitive variables and are recast into a general form. In the proposed method, all governing equation are relaxed locally, in contrast with commonly used segregated schemes. The effects of Rayleigh number, aspect ratio, and cavity inclination on temperature and velocity patterns are discussed. It is expected that more advanced parallel computer architectures can benefit from the error-smoothing operator described here.     

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.     

SOLUTION OF THE INVERSE PROBLEMS IN HEAT TRANSFER AND THERMAL STRESS ANALYSIS

The stable method of soloing inverse problems in heat transfer and inverse problems concerning thermal stress control in elements of complex shapes is presented. Based on the discrete form of Duhamel's integral and on so-called future time steps, a simple and accurate method of solving one- and multi-dimensional inverse problems is developed. Two examples illustrating the application of the described method are presented.     

ITERATIVE METHOD FOR THE NUMERICAL PREDICTION OF HEAT TRANSFER IN PROBLEMS INVOLVING LARGE DIFFERENCES IN THERMAL CONDUCTIVITIES

Heal exchange that occurs between materials with largely differing thermal conductivities is commonly encountered in engineering practice. Conventional iterative solution methods perform poorly for the numerical solution of such problems. A block correction procedure, designed for enhancing the convergence of iterative solution methods, is used in conjunction with the line-by-line iterative solution method. The overall solution algorithm is a multigrid strategy with two grid levels. Results of computations for test problems indicate that the proposed solution procedure enables efficient solution of heat transfer problems with large conductivity differences for which the conventional line-by-line method proves ineffective.     

A NUMERICAL ANALYSIS OF THE EFFECTS OF CONJUGATE HEAT TRANSFER,VAPOR COMPRESSIBILITY,AND VISCOUS DISSIPATION IN HEAT PIPES

The effects of conjugate heat transfer, vapor compressibility, and viscous dissipation in heat pipes are discussed. The accuracy of the partially parabolic versus the elliptic presentation of the governing equations is also examined. The results show that the axial wall conduction has a tendency to make the temperature distribution more uniform for heat pipes with large ratios of pipe wall to effective liquid-wick thermal conductivity. The compressible and incompressible models show very close agreement for the total pressure drop, white the local pressure variations along the heat pipe are quite different for these two models when the radial Reynolds number at the interface is high.     

NUMERICAL SIMULATION OF THREE-DIMENSIONAL SEPARATED FLOW AND HEAT TRANSFER AROUND STAGGERED SURFACE-MOUNTED RECTANGULAR BLOCKS IN A CHANNEL

ABSTRACT

Numerical results simulating a three-dimensional laminar separated flow and heat transfer around staggered surface-mounted rectangular blocks in a plane channel are presented. Treated in the present study is a case of staggered three-row blocks. The finite-difference method is employed to solve the Navier-Stokes and energy Equations directly, and the resulting finite-difference Equations are solved with the SMAC method for Re = 100–500 and Pr = 0.7. The present numerical results are found to simulate well the visualization results such as horseshoe vortices and recirculating flow. The heat transfer coefficient greatly varies on the different side surfaces of blocks and also with Reynolds number.     

正交各向异性相变材料的无网格法传热模型及应用

利用无网格迦辽金(EFG)法建立正交各向异性相变材料的传热计算模型,基于该模型编程完成各向异性材料太阳能相变蓄热水箱和管壳式相变蓄热单元的相变传热分析,并探讨热导率因子和材料方向角对复合材料相变传热特性的影响.研究表明:在相同节点布置下EFG法的温度场和相界面计算精度均高于有限元法,EFG法在动态相界面追踪方面具有明显...     

A FAST METHOD OF RADIATIVE HEAT TRANSFER ANALYSIS BETWEEN ARBITRARY THREE-DIMENSIONAL BODIES COMPOSED OF SPECULAR AND DIFFUSE SURFACES

The radiation element method by ray emission model (REM²) has been improved by using the law of reciprocity for the specular view factor and the incomplete Cholesky conjugate gradient (ICCG) method to reduce computational time. This improved method was applied to analyze the radiative heat transfer between arbitrary three-dimensional bodies composed of specular and diffuse surfaces. The accuracy of the improved method was evaluated by comparing analytical solutions. And the method was used to calculate radiative heat transfer between machine elements and heater panels. CPU time to calculate the radiative exchange for a model composed of 3193 elements was reduced to 1/430 of that by the previous numerical method using a decomposition method.     

被动式太阳能温室-采暖房中对流传热的数值分析

针对一种被动式太阳能温室-采暖房，分析采暖房北墙采用或不采用隔热保温措施时，温室-采暖房内的温度和气流分布情况，模拟具有蓄热层的太阳能温室-采暖房中的对流传热，研究温室-采暖房中采用岩石床吸收和贮存太阳能的传输特性。     

NUMERICAL ANALYSIS OF NATURAL CONVECTIVE AND RADIATIVE HEAT TRANSFER IN AN ARBITRARILY SHAPED ENCLOSURE

The flow and thermal characteristics of the interactions of natural convection and radiation in an enclosure containing circular ducts are analyzed numerically. For calculation of flow fields, the SIMPLE algorithm originally developed in Cartesian coordinates is extended and modified to apply to the curvilinear coordinates system. The radiation part of the problem for an arbitrarily shaped domain is solved by using the finite volume method (FVM). Cartesian velocity components are used as the dependent variables in momentum equations, and a nonstaggered grid system is employed. The flow and thermal fields for an irregular geometry are investigated for the variation of such parameters as scattering albedo, optical thickness, and Planck number. The test problem is compared with both the exact solutions and the discrete ordinates method solution. The results show that the FVM is an effective method to predict radiative heat transfer processes in irregular geometries and that the change of optical thickness has more effect than that of scattering albedo on flow and thermal fields.     

太阳能热气流发电系统的传热与流动数值分析

建立了集热棚、烟囱以及多孔蓄热层的太阳能热气流发电系统传热与流动数学模型,分析了太阳辐射对蓄热介质的蓄热特性的影响.计算结果表明,在太阳辐射为200～800W/M2的范围内,随着太阳辐射的增强,蓄热介质的蓄热比例先减小后增大;烟囱底部的最小相对压力显著减小,流动速度增大;系统内空气的温升增大,蓄热介质表面的温度也显著升高.     

CHARACTERISTICS OF THREE-DIMENSIONAL FLOW,HEAT, AND MASS TRANSFER IN A CHEMICAL VAPOR DEPOSITION REACTOR

The three-dimensional flow, heat, and mass transfer characteristics in a horizontal chemical vapor deposition (CVD) reactor with a tilted susceptor are analyzed numerically. As the physical domain for the CVD reactor has an irregular region, the Cartesian coordinates are transformed into a general curvilinear coordinate system. For calculating the governing equations, the finite volume method (FVM) is adopted and the SIMPLE algorithm is extended and modified to employ the curvilinear system. The effects of flow rate and tilted angle of the susceptor on the transport phenomena (i.e., heat transfer rate, uniformity and growth rate of reactant gas, etc.) at the susceptor are investigated. The results show that the existence of return flows leads locally to improvement of the heat transfer, but it is not good for the uniformity of the deposition. As the tilted angle of the susceptor is increased (from 0^o to 9^o), the amount of heat transfer and growth rate in the main flow direction are improved irrespective of the Reynolds number. The growth rate and uniformity are less sensitive to the inclined angle of the susceptor than that of the Reynolds number.