A NUMERICAL STUDY OF THE HEAT TRANSFER TO FLUID FLOW THROUGH CIRCULAR POROUS PASSAGES

A detailed numerical study of heat transfer to a fluid passing through a saturated porous medium is the subject of this study. The Green's function solution method is selected in order to accomplish this task. The interesting features of this methodology are the focus of this article. As a test case, primary consideration is given to the computation of heat transfer to a fluid flowing through a circular passage with impermeable walls filled with porous materials. The analysis includes the heating/cooling effects due to a temperature change at the wall of the passage. In addition, the contributions of frictional heating are examined.     

管内流体流动管外PCM发生相变的贮能系统热性能研究

建立了分析空调贮能系统中管内流体流动管外PCM发生相变的相变的贮能器热性能的数学模型，并进行了数值计算。其中，把传热流体看作是沿轴向的—维无粘流动，对PCM相变过程的求解用显热容法。计算结果与文献中的计算结果吻合较好。所得结论对该类贮能系统的设计和性能优化有一定指导作用。     

Laminar heat transfer in an asymmetrically heated rectangular duct

This paper presents a numerical study concerning the effects of non-uniform heating on the heat transfer of a thermally undeveloped gas flow in a horizontal rectangular duct; a vertical side wall is uniformly heated, and the other walls are insulated. As an initial step of the study, the duct flow is assumed to be laminar, and buoyancy effects are considered. The heat transfer rate and drag increase with the secondary flow due to buoyancy; the effects of the buoyancy force on the heat transfer and friction coefficient of the thermally undeveloped region are found to depend only upon modified Grashof numbers of the duct entrance.     

Turbulent duct flow with streamwise nonuniform heating at the duct wall

A combined experimental and analytical/numerical investigation has been carried out for turbulent flow in a flat, rectangular duct with streamwise nonuniform heating at one of the principal walls. The investigated heating pattern consisted of adiabatic zones periodically inserted between isothermal heated zones. The relative streamwise lengths of the heated zones and the adiabatic zones were varied parametrically, as was the Reynolds number. Heat transfer coefficients were determined at each of the heated zones, both in the thermal entrance region and in the fully developed region. It was found that the presence of the adiabatic zones can give rise to substantial enhancement of the heat transfer coefficients at the heated zones. For a duct of fixed overall length, the use of the adiabatic zones represents a loss in transfer surface area, which brings about a net decrease in the overall rate of heat transfer. The agreement between the numerical and experimental results was typically in the two percent range, an outcome which recommends the numerical model as a tool for studying other nonuniform heating patterns.     

Numerical study of convective heat transfer to supercritical water in rectangular ducts

Numerical investigation has been performed to analyze forced convective heat transfer to supercritical water in horizontal rectangular ducts. Convective heat transfer near the critical region in the rectangular ducts is strongly influenced by large variations of thermodynamic and transport properties of supercritical fluid with gravity force, especially close to pseudocritical temperature. Fluid flow and heat transfer characteristics such as velocity, temperature, and local heat transfer coefficient with water properties distribution in the ducts are presented. Flow accelerates along the horizontal ducts because of decreased water density from heat transfer at the duct walls. Center of large flow recirculation in the duct section locates near the middle of vertical surface and additional secondary recirculation in clockwise direction appears with the increase of duct height. Local wall temperature severely varies along the inner surface of the duct section and its variation depends on aspect ratio of the duct. The heat transfer coefficient distributions along the ducts for various aspect ratios are compared with the proximity effect to the critical pressure.     

COMPUTATION OF HEAT AND FLUID FLOW IN DUCTS OF ARBITRARY CROSS SECTION

This paper reports on connective heat transfer in irregular ducts maintained under a constant wall temperature. In particular, due to the complexity of the geometry, the paper investigates in detail the fluid flow and convective heat transfer in right-triangular and semicircular ducts. The hydrodynamically fully developed flow and the developing temperature in these geometries are obtained analytically/numerically from the solution of the energy equation employing the method of lines. The energy equation is reformulated by a system of a first-order differential equation controlling the temperature along each line. It was found that reliable closed-form solutions for the temperature distribution in the thermal entrance region can be obtained utilizing 21 lines, or less, displayed in the cross-stream direction of the duct. The grid pattern chosen provides: drastic savings in computing time. Results for the thermal entry region flow heat transfer are presented in tabular and graphical forms. The representative curves illustrating the variation of bulk temperature and Nusselt numbers with pertinent parameters in the entire thermal entry region are plotted. The computed results are compared against some analytical/numerical findings reported in the literature. In all cases, satisfactory comparison is obtained. The asymptotic Nusselt numbers are 1.90, 2.25, and 2.29 for 15°, 30°, and 45° right-triangular ducts, respectively, and 6.030 for semicircular ducts     

Numerical Investigation of Heat Transfer Performance of Rectangularly Coiled Pipes

A numerical simulation was conducted to investigate convective heat transfer from small and compact coiled pipes heat exchangers using computational fluid dynamics (CFD) software Fluent V6. One fluid (air) moves over the coiled pipe while a second fluid (refrigerant R141B) at different temperature flows through the pipe. The studied heat exchanger is composed with bends and straight tubes. Calculations were done for two cases with different outside flow arrangements. The simulation results showed remarkable differences in the flow characteristics and heat transfer rate of different single tubes of the entire heat exchangers. The temperature distribution and heat transfer are mainly influenced by temperature gradient, backflow conditions, exterior flow velocity, and surface area. The results also show the effect of the bends on the flow in straight tubes and vice-versa.     

Numerical and experimental investigations of heat transfer performance of rectangular coil heat exchangers

Both numerical and experimental investigations were conducted to understand convective heat transfer from a single round pipe coiled in rectangular pattern. The studied heat exchangers are composed with inner and outer coils so that the exterior flow is very similar to flow within tube-bundles. The inner and outer coils of the heat exchangers are in turn composed of bends and straight portions. Calculations and experiments were done for two cases with different outside flow arrangements. The results showed the effects of geometric arrangement with better heat transfer for the case 1 of staggered arrangement due mainly to its more tortuous flow characteristics and better mixing of the exterior fluid. The numerical and experimental results qualitatively agree well with each other. The numerical and experimental results showed that coiling a pipe so that an exterior fluid flows over or in tube bundle can help to induce the turbulence without increasing the velocity.     

Boiling heat transfer in vertical minichannels. Liquid crystal experiments and numerical investigations

The paper presents the results of experimental and numerical studies of boiling heat transfer in the flow of refrigerants R123 and R11 through vertical, rectangular minichannels, with one wall heated. An application of liquid crystal thermography has helped detect two-dimensional temperature distribution on the heating surface, allowing determination of boiling heat fluxes and experimental boiling curves. The main objectives of the paper included the development of two-dimensional approach to solve the inverse heat conduction boundary problem for determining local values of internal heating surface temperature, boiling heat flux and heat transfer coefficient, and the improvement of the applied numerical method making use of the equalizing calculus and heating surface temperature measurement errors. A detailed discussion of temperature, heat flux and heat transfer coefficient errors is also provided.     

Three dimensional numerical and experimental study of forced convection heat transfer on solar collector surface

In this study, experimental and three dimensional numerical work was carried out to determine the average heat transfer coefficients for forced convection air flow over a rectangular flat plate. Three dimensional numerical simulations were obtained using a commercial finite volume based fluid dynamics code called Fluent 6.3. The experiments were performed for mass transfer using the naphthalene sublimation technique. The results were presented in terms of heat transfer parameters using the analogy between heat and mass transfer. All the experimental results are correlated within an accuracy of ± 12%.     

EXPERIMENTAL STUDIES ON FLOW IN CONVERGENT AND DIVERGENT DUCTS OF RECTANGULAR CROSS SECTION

This paper describes the experimental examination of the pressure drop and heat transfer of the flow in convergent and divergent ducts of rectangular cross section. The aspect ratio based on the dimensions of the large end of the duct was 0⋅1. It has been found that at a given convergent or divergent angle pressure drop decreases while heat transfer increases with increasing Reynolds number. Along a given duct of small convergent angle, pressure drop increases while heat transfer decreases along the duct. However, heat transfer may increase near the downstream end of ducts of high convergent angle. At a given Reynolds number, both pressure drop and heat transfer increase with increasing convergent angle. As for flow in divergent ducts, the effects of Reynolds number on pressure drop and heat transfer are somewhat similar to those of flow in a convergent duct. © 1997 by John Wiley & Sons, Ltd.     

Convection in laminar three-dimensional separated flow

Distributions of the wall temperature, the Nusselt number, and the friction coefficient on all of the bounding walls of laminar three-dimensional forced convection flow adjacent to backward-facing step in a rectangular duct are reported. A uniform heat flux is imposed on the bounding walls (stepped wall, sidewalls, and flat wall) downstream from the step, while the walls of the duct upstream from the step and the step are treated as adiabatic surfaces. The flow upstream of the step is treated as hydrodynamically fully developed and isothermal, and the outlet flow downstream from the step is treated as being hydrodynamically and thermally fully developed. Local and average results are presented for a Reynolds numbers range of 150-450, and some results are compared with their equivalent from the two-dimensional case.     

HYDROMAGNETIC FLOW AND HEAT TRANSFER OF A HEAT-GENERATING FLUID OVER A SURFACE EMBEDDED IN A POROUS MEDIUM

Similarity solutions for the laminar boundary-layer equations describing steady hydromagnetic two-dimensional flow and heat transfer in a stationary electrically-conducting and heat-generating fluid driven by a continuously moving porous surface immersed in a fluid-saturated porous medium are obtained. The flow is exposed to a transverse magnetic field and the surface is moving with a constant velocity. Fluid suction or injection is imposed at the wall. The dimensionless similar equations are solved numerically by an implicit finite-difference method. The numerical flow and heat transfer results are illustrated graphically for various parametric conditions to show special features of the solutions. Favorable comparisons with previously published work confirm the correctness of the numerical results. © 1997 Elsevier Science Ltd     

SIMULATION OF SWIRLING TURBULENT FLOWS AND HEAT TRANSFER IN AN ANNULAR DUCT

The article presents a numerical simulation of swirling turbulent flows and heat transfer in an annular duct. The time-averaged governing equations are solved, which are closed by a new algebraic Reynolds stress model (ASM). The simulation is performed under different flow conditions. The calculated results of gas axial and tangential velocities, turbulent kinetic energy, temperature, and local heat transfer coefficients on the inner and outer walls of the annulus are provided. They illustrate the effect of swirl number, inlet axial velocity, and ratio of inner to outer radius on the mean flow and turbulence properties, as well as on enhancing heat transfer in the annular duct.     

纵流壳程换热器新型抗振折流元件的数值研究

提出了一种新型抗振折流杆元件的结构,并对该结构的单元流道提出了简化模型,用数值方法模拟了夹持式折流杆支撑的单元流道内的流动和传热状况,从流体的流动形态、压力分布和温度分布等方面与直折流圆杆单元流道进行了对比。结果表明,除阻力降外,该结构的传热量和传热系数均有所改善。     

FLOW BOILING OF WATER AND n -HEPTANE IN MICRO CHANNELS

Heat transfer characteristics during flow boiling in micro channels with rectangular cross section were studied using a thermographic measuring method. The characteristic length of the channels investigated was varied in a region of 300 w m up to 700 w m. The channels were designed as Joule heating pipes. Thus, the evaporation was achieved under conditions of nearly constant heat fluxes at the heating wall of the channel. The thermographic measuring method was used to examine the wall temperature. The high spatial and temporal resolution of this thermographic measuring method makes it possible to detect the axial position of the different boiling regions. Furthermore, it allows conclusions to be made on which flow conditions occur in the different sections of the channel. Experimental results are shown with water and n -heptane as the fluid to be vaporized. The results of measurements are discussed and a correlation is given of the location where the dryout starts with the characteristic parameters. The dependence of the pressure drop in such channels on the technological parameters has also been presented.     

Study and optimization of the thermal performances of the offset rectangular plate fin absorber plates, with various glazing

The low thermophysical characteristics of air used as a heat transfer fluid in the solar collectors with thermal conversion require a fully developed turbulent flow. This increases the thermal heat transfer between the absorber plate and the fluid, which clearly improves the thermal performances of the solar collector with obstacles arranged into the air channel duct. In the present work, we introduce, in solar collector, the offset rectangular plate fins, which are used in heat exchangers. An experimental investigation carried out showed the generated enhancement of thermal performance. The offset rectangular plate fins, mounted in staggered pattern, are oriented parallel to the fluid flow and are soldered to the underside of absorber plate. They are characterized by high heat transfer area per unit volume. High thermal performances are obtained with low pressure losses and in consequence a low electrical power consumption by the fan in comparison to the flat plate collector. The experimental results are all so compared by using two types of transparent cover; double and triple.     

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.