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.     

NUMERICAL PREDICTION OF BUOYANCY-INDUCED PERIODIC FLOW PATTERN AND HEAT TRANSFER IN A LID-DRIVEN ARC-SHAPE CAVITY

Numerical simulation has been carried out to study the unsteady phenomenon of a buoyancy-induced periodic flow and convection heat transfer in a lid-driven arc-shape cavity. The governing equations in terms of the stream function-vorticity formulation are solved by the finite-volume method coupled with a body-fitted coordinate transformation scheme. In the range of the Reynolds number (Re) from 100 to 2,000 and Grashof number (Gr) up to 5 2 10 7 , the heat transfer characteristics and flow pattern have been predicted. Attention has been focused on the combined effects of the inertial and buoyant forces exerted on the fluid. Results show that only when the inertial and buoyant forces are of approximately equal strength that can the periodic flow pattern be observed. For an inertia-dominant or buoyancy-dominant situation, the periodic flow pattern is not visible.     

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.     

INCOMPRESSIBLE FLOW AND HEAT TRANSFER COMPUTATIONS USING A CONTINUOUS PRESSURE EQUATION AND NONSTAGGERED GRIDS

A pressure-based algorithm for incompressible flows is presented. The algorithm employs a finite-volume discretization in general curvilinear coordinates on a nonstaggered mesh. This approach is derived from a finite-element algorithm, and is here extended to the finite-volume/finite-difference context. The algorithm can be classified as a SIMPLE-like sequential method, and is validated in two classical test cases: the lid-driven cavity and the differentially heated cavity problems. Good results, with no pressure checkerboarding, are achieved up to Reynolds numbers Re = 10⁴ and Rayleigh numbers Ra = 10⁸ , respectively.     

NUMERICAL SIMULATION OF HEAT TRANSFER IN MIST FLOW

A numerical simulation of heat transfer over a row of tubes, in the presence of mist flow, is described. Computations include the solution of the flow field around the tubes, the prediction of the motion of water droplets, and the evaluation of the cooling effect of the water film on the tube surface. The entire analysis is carried out using FENSAP-ICE (Finite Element Navier-Stokes Analysis Package for In-flight icing), a simulation system developed by Newmerical Technologies for icing applications. The numerical model is described, including the Navier-Stokes solution, the water thin film computation, the droplet impingement prediction, and the conjugate heat transfer procedure. The predictions are verified against experimental data for different droplet mass flow rates, showing satisfactory agreement and allowing a useful insight in the physical characteristics of the problem.     

NUMERICAL INVESTIGATION OF FLOW AND HEAT TRANSFER IN TWISTED CIRCULAR-SECTOR DUCTS

Flow and heat transfer in a twisted circular-sector duct are analyzed numerically for steady, fully developed, and incompressible laminar flow with a uniform-wall-temperature boundary condition. A rotating coordinate system is employed to account for the duct twist. The friction factors and Nusselt numbers are predicted for duct sector angles ranging from 15° to 90°, Reynolds numbers ranging from 1 to 1,000, and Prandtl numbers ranging from 1 to 100. Results show significant influence of duct twist on both friction factors and Nusselt numbers, particularly at large values of Reynolds and Prandtl numbers. Accurate correlations are developed to predict the friction factors and Nusselt numbers for the entire range of geometric and operating conditions studied.     

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.     

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.     

NUMERICAL INVESTIGATION OF CONVECTIVE HEAT TRANSFER AND PRESSURE DROP IN WAVY DUCTS

Flow field and heat transfer in corrugated ducts have been investigated from the numerical solutions of the Navier-Stokes equations in the laminar to transitional range of Reynolds numbers (600 - 2000). First, the geometric parameters for the best ratio of heat transfer to pressure drop have been determined for two-dimensional channels. Then for these parameters, flow structure and heat transfer have been investigated for three-dimensional (3-D) channels consisting of corrugated plates. The angles 45^o and 90^o between the corrugations and the main stream direction have been considered. A performance evaluation criterion identifies the 3-D corrugated plate at 45^o with the best ratio between heat transfer and pressure drop.     

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.     

多孔介质太阳能集热组合墙的耦合传热与流动分析

针对接触型和分隔型多孔介质太阳能集热组合墙系统,分析了太阳辐射及环境温度变化时,组合墙内传热与流动变化.多孔介质太阳能集热组合墙中,多孔介质起半透明隔热体和蓄热体的作用.多孔介质集热层的孔隙率、粒径、材料热导率和多孔介质集热层在组合墙中的位置对系统的采暖效果影响较大.     

PREDICTION OF SURFACE RADIATIVE HEAT TRANSFER USING THE MODIFIED DISCRETE TRANSFER METHOD

An ideal surface radiation model applied in many manufacturing and materials processing systems must be able to take into account specular, spectral, and shadowing effects with complex geometries, and must be computationally efficient to permit its inclusion in the fluid flow and heat transfer models. In this study, a novel surface radiative heat transfer method is developed to meet all these practical needs. The present model is based on the discrete transfer method (DTM). A direct application of the DTM to modeling surface radiative heat transfer may result in a large error due to strong ray effects. In order to eliminate these ray effects, the DTM is modified by considering radiation contribution from all surface cells intercepted by a control angle. Calculation of these surface cell areas represents one of the most important tasks in the modified DTM (MDTM), and it is described in detail in this study. To investigate the accuracy and efficiency of the MDTM, three benchmark problems covering different geometric and boundary conditions are considered, and the present solution is compared with the solutions from the exact approach, the DTM, and discrete ordinates method (DOM). For each problem, the accuracy of the MDTM, DTM, and DOM appears to be affected by the angular discretization. For a reasonable fine angular discretization, the solutions from the MDTM and DOM match the exact solution very well, while the solution from the DTM usually shows strong ray effects. The CPU times spent on the MDTM and DTM are very similar, but they are usually orders of magnitude less than that for the DOM. The present study indicates that the MDTM is not only accurate but also very efficient for modeling complicated surface radiation problems. Such a model will greatly benefit the simulation of many manufacturing and materials processing systems.     

PRESSURE BOUNDARY CONDITIONS FOR A SEGREGATED APPROACH TO SOLVING INCOMPRESSIBLE NAVIER-STOKES EQUATIONS

It has been well accepted that Diricklet and Neumann boundary conditions for Ike pressure Poisson equation give the same solution. The purpose of this article is to reveal that the above statement is computationally acceptable but is not theoretically correct. Analytic proof as well as computational evidences are presented through examples in support of our observation. In this work we address that the mixed finite-element formulation for solving incompressible Navier-Stokes equations in primitive variables is equivalent to the formulation that involves solving the pressure Poisson equation, subject to Neumann boundary conditions, Uerativety with the momentum equations provided the velocity field is classified as having divergence-free and conservative properties.     

除湿条件下波纹通道内对流传热传质的数值研究

通过数值计算对紧凑换热器一种波纹翅片通道内除湿条件下周期性充分发展的对流传热传质情况进行数值研究。计算采用曲线坐标系下压力与速度耦合的SIMPLER算法,湿空气流动Re数的范围为100～1100,Pr数为0.71,Sc数为0.61。讨论了不同波纹高度、波纹间距对阻力与换热的影响,给出了不同Re数下的浓度场,并对动量、传热及传质进行了定量比较分析。计算结果表明,整体Nu数及fRe数随着波纹高度的增加或波纹间距的减小而增加;浓度随着Re数的增加沿着流动方向迅速降低;计算能较好的满足Chilton-Colburn相似,表明传热特性均可类推到传质特性中去。     

NUMERICAL SIMULATION OF THE THERMODYNAMIC,FLUID FLOW,AND HEAT TRANSFER PROCESSES IN A DIESEL ENGINE

A comprehensive model for the complete second-law analysis of a diesel engine has been developed. The model incorporates formulations for zero-dimensional estimation of the convective and radiative heat transfers in the engine cylinder. Models for the valve flow and manifold gas dynamics are also included in order to estimate the second-law losses. The results obtained help identify the engine processes where irreversible losses occur and the magnitudes of these losses relative to the work output. For example, at 2000 rpm and 0.7 overall fuel/air equivalence ratio, the indicated work for the single-cylinder engine simulated in this study is 49% of the fuel exergy. The loss due to irreversibilities is 27.6%, of which 14.8% is due to throttling loss in the exhaust valve, 9.3% is due to combustion irreversibilities, and the remainder is due to throttling toss in the intake valve.     

液膜蒸发强化传热表面的传热与流动特性分析

提出一种新的液膜蒸发强化换热表面－波纹丝网换热表面，在丝网结构尺寸进行６种改变后，通过对其传热和流动表面规律的研究，发现合适的优化结构可广泛应用于蒸发冷却系统中。     

NUMERICAL STUDY ON TURBULENT FLOW AND HEAT TRANSFER IN CIRCULAR COUETTE FLOWS

Abstract

A numerical study is performed to investigate heat transfer and fluid flow in the entrance and fully developed regions of an annulus, consisting of a rotating, insulated inner cylinder and a stationary, heated outer cylinder. Several different k-ε turbulence models are employed to determine the turbulent kinetic energy, its dissipation rate, and the heat transfer performance. The governing boundary layer equations are discretized by means of a control volume finite difference technique and numerically solved using the marching procedure. In the entrance region the axial rotation of the inner cylinder induces a thermal development and causes an increase in both the Nusselt number and the turbulent kinetic energy in the inner cylinder wall region. In the fully developed region, an increase in the Taylor number causes an amplification of the turbulent kinetic energy over the whole cross section, resulting in a substantial enhancement in the Nusselt number. These transport phenomena are also affected by the radius ratio and Reynolds number.     

THE METHOD OF LINES SOLUTION OF THE DISCRETE ORDINATES METHOD FOR RADIATIVE HEAT TRANSFER IN ENCLOSURES

A radiation code based on the method of lines (MOL) solution of the discrete ordinates method (DOM) for transient three-dimensional radiative heat transfer in rectangular enclosures for use in conjunction with a computational fluid dynamics (CFD) code based on the same approach was developed. Assessment of the predictive accuracy of the code by benchmarking its steady-state solutions against exact solutions on one- and three-dimensional test problems shows that the MOL solution of the DOM provides accurate and computationally efficient solutions for radiative heat fluxes and source terms and can be used with confidence in conjunction with CFD codes for transient problems.     

EFFICIENT FINITE ELEMENT METHOD FOR TWO-DIMENSIONAL FLUID FLOW AND HEAT TRANSFER PROBLEMS

Velocity-pressure finite element methods for two-dimensional incompressible viscous flow and natural convection are considered. The governing continuum mechanics equations are summarized, and the finite element formulation is given. Some details of the implementations with the resulting analysis capabilities are presented. Finally, sample results that demonstrate some of the capabilities of the present approach are given.     

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。