Thermoelastic Deformation of Three-Layer Combined Die with Non-Uniform Temperature Distribution

During precision forging processes, the elastic and thermal deformations always take place simultaneously on the forging dies, which will affect the dimension accuracy of the final forging components. According to the classic Lamé formulas and thermoelastic mechanics theories, the thermoelastic deformation of a combined die was investigated. Directly related to geometric parameters, material properties, external stresses, and temperature distribution, the expressions of die deformation and contact normal stresses were derived. A three-layer combined die with three different temperature distributions was studied as a specific example. The thermoelastic deformations of each layer, as well as the contact normal stresses between them, were both calculated by the derived expressions and by finite element simulation. There was good agreement between the calculated values and simulated results, which demonstrated the effectiveness and accuracy of the theoretical derivation. Based on this, the total deformation on the inner surface of the combined die and the contact stress between contact layers under non-uniform temperature distribution that would influence the practical die initial design were discussed further.     

TWO-DIMENSIONAL AND UNSTEADY NATURAL CONVECTION IN A HORIZONTAL ENCLOSURE WITH A SQUARE BODY

A two-dimensional solution for unsteady natural convection in an enclosure with a square body is obtained using an accurate and efficient Chevyshev spectral collocation method. A spectral multidomain methodology is used to handle a square body located at the center of the computational domain. The physical model considered here is that a square body is located at the center between the bottom hot and top cold walls. To see the effects of the presence of a body on natural convection between the hot and cold walls, we considered the cases that the body maintains the adiabatic and isothermal thermal boundary conditions for different Rayleigh numbers varying in the range of 103 to 106. When the Rayleigh number is small, the flow and temperature distribution between the hot and cold walls shows a symmetrical and steady pattern. At the intermediate Rayleigh number, the fluid flow and temperature fields maintain the steady state but change their shape to the nonsymmetrical pattern. When the Rayleigh number is high, the flow and temperature fields become time dependent, and their time-averaged shapes approach the symmetric pattern again. The Rayleigh number for the fluid flow and temperature fields to become nonsymmetrical and time dependent depends on the thermal boundary conditions of a body. The variation of time- and surface-averaged Nusselt numbers on the hot and cold walls and at the body surfaces for different Rayleigh numbers and thermal boundary conditions are also presented to show the overall heat transfer characteristics in the system.     

Analytical solution for transient temperature and thermal stresses within convective multilayer disks with time-dependent internal heat generation,Part I: Methodology

This work deals with the exact solution for asymmetric transient problem of heat conduction and accordingly thermal stresses within multilayer hollow or solid disks which lose heat by convection to the surrounding ambient. The combination of the separation of variables method (SVM) and Duhamel's theorem is applied to the heat conduction problem which provides a versatile technique. The temperature distribution is obtained by the SVM which concerns the heat conduction problem with time-independent internal heat generation. Applying Duhamel's theorem on the previous solution, temperature distribution with time-dependent internal heat generation can be achieved. Accordingly, assuming plane stress condition, radial and tangential stresses are obtained which are incorporated into the equivalent tensile stress formulation to calculate von Mises stress. The comprehensive methodology described here can be useful addition for many new emerging fields in which both transient and steady-state temperature distributions and thermal stresses for composite disks are important.     

A CFD study of hygro–thermal stresses distribution in PEM fuel cell during regular cell operation

A three-dimensional, multi-phase, non-isothermal computational fluid dynamics model of a proton exchange membrane fuel cell has been developed and used to investigate the displacement, deformation, and stresses inside the whole cell, which developed during the cell operation due to the changes of temperature and relative humidity. The behaviour of the fuel cell during operation has been studied and investigated under real cell operating conditions. A unique feature of the present model is to incorporate the effect of hygro and thermal stresses into actual three-dimensional fuel cell model for a complete cell with both the membrane-electrode-assembly and the gas distribution flow channels. The results show that the non-uniform distribution of stresses, caused by the temperature gradient in the cell, induces localized bending stresses, which can contribute to delaminating between the membrane and the gas diffusion layers. The non-uniform distribution of stresses can also contribute to delaminating between the gas diffusion layers and the channels, especially in the cathode side. These stresses may explain the occurrence of cracks and pinholes in the fuel cells components under steady-state loading during regular cell operation, especially in the high loading conditions.     

ROUGH SIZE ESTIMATION OF A THERMALLY FRACTURED ZONE IN AN INFINITE HOT ROCK MASS

Abstract

Concerning the extraction of geothermal energy from a deep thermal reservoir by the downhole coaxial heat exchanger with a thermally insulated inner pipe proposed by Morita et al, we obtained rough estimates of a size of the fractured zone induced by thermal stresses due to injecting cold water into the hot rock mass through the pipe. We assumed complete spherical symmetry of the temperature and stress fields. At the rough estimation, we considered three typical or extreme cases. (1) The fracturing affects neither the loading capacity of a fractured rock mass nor the temperature distribution within the formation. (2) The fractured zone completely loses its loading capacity and is fully invaded by the borehole water. No disturbance of the fracturing makes any difference in the temperature. (3) The rock formation is assumed to have an appropriately increased fictitious conduction substituted for the heat transfer enhanced by the expected convection within the fractured zone in order to discuss the effects of an occurrence of heat convection within the fractured zone on the temperature and stress distributions and the fractured zone size. As a result, the size of the zone has been estimated to be about ten or more times the borehole radius.     

Torsional strength comparison between two assembling techniques for aluminium drill pipe to steel tool joint connection

Two techniques for assembling an aluminium drill pipe to a steel tool joint were investigated: ‘hot’ assembling and ‘cold’ assembling. During hot assembling a difference of temperature between the two parts is exploited to impose contact interferences and then to obtain a friction bond that generates torsional strength. Hot assembling is ordinarily used. The cold assembling technique is introduced here as an alternative. During cold assembling connection make up, the contact interferences are obtained by imposing an external torque. This technique requires a proper lubricating compound between the mating surfaces, in order to reduce the torque required to make up the connection. After lubricating compound curing, the coefficient of static friction rises and a high torsional strength of the connection is obtained. Full scale experimental tests were performed both on hot assembled connections and on cold assembled connections. Cold assembling showed higher torsional strength than hot assembling; moreover cold assembling does not expose the aluminium alloy to high temperature, that can be a risk for the aluminium alloy microstructure. However, the use of the compound can be a reason of cold assembling torsional strength scatter.     

Enhancing heat transfer rates from closed-sided,open-topped heat exchangers,each having vertical rectangular fins extending upwards from a horizontal base

The almost two-dimensional steady-state rates of heat loss from arrays of uniformly-spaced vertical rectangular fins, extending upwards—in otherwise stagnant air—from horizontal heated bases, have been measured. (The vertical air gaps between the fins were closed at their sides, by insulated vertical end-barriers.) The effects of various combinations of height, thickness and spacing of the fins, for different base temperatures (in the range 40 to 100°C), have been studied.For the configuration considered, in a normal ambient environment (～ 20°C), there is an optimal fin spacing (? 16 mm) corresponding to the greatest steady-state rate of free convective/conductive heat loss through the air from the finned system, and this is almost independent of the temperature of the heat exchanger base (in the range 40–100°C). At this optimal spacing for base temperatures not greater than 50°C, the convective/conductive heat transfer rate from the array increases with the fin height up to about 60 mm, so that it would be uneconomic to employ taller fins if convection/conduction is dominant compared with radiation.If the radiation contribution is also considered, then the optimal spacing corresponding to the maximum total steady-state rate of heat loss through the air is somewhat less than the optimal spacing for which, under the same temperature conditions, the maximum steady-state rate of convective/conductive heat leak occurs. The greater the emissivity of the heat exchanger surfaces, the narrower the optimal uniform gaps between the fins.A two-dimensional finite-difference computer program has been composed to predict the temperature distribution throughout the heat exchanger for a stipulated ambient environmental temperature and experimentally-determined distribution of the heat transfer coefficient over the surfaces of the exchanger. This enables, for instance, any hot spots to be located prior to a proposed design being built.     

数控机床导轨的热变形研究

机床导轨的热变形是引起导轨导向误差的一个主要原因,本文根据导轨热变形的有限元理论,利用UG软件建立数控机床导轨的热变形的有限元模型并对其进行了热-结构耦合分析,找出其热变形最大区域,发现了对流系数和进给速度对导轨热变形的影响规律,提出了选择合理的对流系数和进给速度能有效的降低导轨的热变形,减少其导向误差,提高其的导向精度的热变形补偿方法。     

Modeling, experimental study on the heat transfer characteristics of thermoelectric generator

This paper investigates the heat transfer characteristics of a thermoelectric generator. The influence of heat dissipation intensity to the sub-thermal resistances distribution is experimentally studied. Based on the thermal network analysis and finite time thermodynamics, an analytical model including all thermal resistances (in both thermocouples and external heat exchangers) is developed to predict the performance of the generator. The results show that the computed values of output power agree well with the experimental values. The heat transfer enhancement on the generator cold side greatly reduces the cold side temperature and thermal resistance, and obviously improves the output power. Compare with air natural convection cooling, the main thermal resistance changes from the resistance between the fins and the ambient to the thermal contact resistances between the generator and the heat sink at the conditions of forced convection and water cooling. This study may be guide the optimization of generator structure.     

Simulation of thermal stresses in anode-supported solid oxide fuel cell stacks. Part II: Loss of gas-tightness, electrical contact and thermal buckling

Structural stability issues in planar solid oxide fuel cells arise from the mismatch between the coefficients of thermal expansion of the components. The stress state at operating temperature is the superposition of several contributions, which differ depending on the component. First, the cells accumulate residual stresses due to the sintering phase during the manufacturing process. Further, the load applied during assembly of the stack to ensure electric contact and flatten the cells prevents a completely stress-free expansion of each component during the heat-up. Finally, thermal gradients cause additional stresses in operation.The temperature profile generated by a thermo-electrochemical model implemented in an equation-oriented process modelling tool (gPROMS) was imported into finite-element software (ABAQUS) to calculate the distribution of stress and contact pressure on all components of a standard solid oxide fuel cell repeat unit.The different layers of the cell in exception of the cathode, i.e. anode, electrolyte and compensating layer were considered in the analysis to account for the cell curvature. Both steady-state and dynamic simulations were performed, with an emphasis on the cycling of the electrical load. The study includes two different types of cell, operation under both thermal partial oxidation and internal steam-methane reforming and two different initial thicknesses of the air and fuel compressive sealing gaskets.The results generated by the models are presented in two papers: Part I focuses on cell cracking. In the present paper, Part II, the occurrences of loss of gas-tightness in the compressive gaskets and/or electrical contact in the gas diffusion layer were identified. In addition, the dependence on temperature of both coefficients of thermal expansion and Young's modulus of the metallic interconnect (MIC) were implemented in the finite-element model to compute the plastic deformation, while the possibilities of thermal buckling were analysed in a dedicated and separate model.The value of the minimum stable thickness of the MIC is large, even though significantly affected by the operating conditions. This phenomenon prevents any unconsidered decrease of the thickness to reduce the thermal inertia of the stack. Thermal gradients and the shape of the temperature profile during operation induce significant decreases of the contact pressure on the gaskets near the fuel manifold, at the inlet or outlet, depending on the flow configuration. On the contrary, the electrical contact was ensured independently of the operating point and history, even though plastic strain developed in the gas diffusion layer.     

Natural convection heat transfer in a square enclosure containing water near its density maximum

The steady-state flow structure, temperature and heat transfer in a square enclosure heated and cooled on opposite vertical walls and containing cold water near its density maximum are investigated numerically. Interpretation of the results hinges upon a dimensionless density distribution parameter which fixes the orientation of the hot and cold wall temperatures with respect to the extremum temperature and also serves to characterize the distribution of the buoyancy force in the enclosure. Multicellular flow structures are observed for certain ranges of the density distribution parameter independent of the value of Rayleigh number (10³ ⩽ Ra ⩽10⁶). The effect of the density distribution parameter on cross-cavity heat transfer is found to be substantial and is discussed in the context of the changing flow structure. Comparisons with previous studies in the literature are made.     

Natural convection heat transfer from a finite vertical thick plate at low Grashof numbers

The purpose of this investigation was to discover the natural convection heat transfer characteristics of a finite vertical thick plate on an adiabatic floor, enclosed by a solid wall. One vertical side of the plate was kept cold and the other side hot. Numerical results were obtained for a Grashof number in the range 0.1 to 10⁵, ratio of the temperatures of the hot and cold walls ranging from 0.05 to 1.0, and a Prandtl number of 0.71. The effect of the mutual interferences of the natural convection fields on both walls was investigated. It was found that in the case of a small temperature ratio, when Gr becomes large, convection fields on the hot- and cold-wall sides become fully independent convection fields that do not interfere with each other since a strong natural convection field forms on both wall sides. On the other hand, the fluid flow from the hot-wall side invades the cold-wall side with decreasing Gr. The heat transfer coefficients on the cold-wall side have a negative value at the upper part of the thick plate, resulting in an endothermic phenomenon. © 2000 Scripta Technica, Heat Trans Asian Res, 29(8): 609–622, 2000     

含加热圆管方腔内自然对流的数值研究

采用数值计算方法对含不同直径圆管以及相同直径圆管位置不同方腔内的层流自然对流进行了研究。以冷热壁面温度差为基准的瑞利数Rn为10^6,以圆管壁面热流密度为基准的Ra为10^8。计算结果表明,当圆管处于方腔中间位置时,随着圆管直径的增大,圆管表面局部努塞尔数呈减小趋势。当圆管直径不变时,由于在不同位置处浮力作用的强弱不同,随着圆管在方腔内位置的改变,方腔内流场结构和温度场分布也会发生变化。整个计算结果可为工程设计提供参考。     

Effect of the temperature dependence of fluid properties on the migration of an interface in double-diffusive natural convection

Experimental observation has shown that a slightly tilted sharp interface between two convection layers in double-diffusive natural convection migrates perpetually upward gradually with time. This movement of an interface cannot be explained by a simple mathematical model of constant physical properties. The present paper studies the numerical analyses of two-layer convection with the temperature dependence of the properties of the fluid. The perpetual upward migration of an interface was found to be promoted mainly by the temperature dependence of the volumetric coefficient of thermal expansion and also by that of the kinematic viscosity. However, the diffusion coefficient was independent of the migration. The upward migration of an interface appears to be caused by the difference between the intensity of etching due to the flow along the hot wall in the lower layer and that along the cold wall in the upper layer.     

球床反应堆体心棱柱单元局部高温热点与涡特性分析

球床反应堆燃料球堆积会产生高温热点从而影响燃料球的完整性,高温热点的形成与复杂的氦气流动及产生的涡密切相关。为了确保数值模拟结果的准确性,对比了体心棱柱(BCP)面接触处理方式与人工间隙处理方式的差异,证明面接触结构更为合理。对研究对象构建结构化网格,采用大涡模拟(LES)方法分析不同接触位置处流场与温度的分布。采用Q准则确定局部位置的涡结构,分析涡对球床内流动换热的影响。结果表明,BCP结构的特殊接触面会产生高温热点从而影响整体温度分布,涡的存在能够增强流动换热。     

碳及碳锰锅炉钢板在变形过程中的再结晶与时效硬化

通过对一起高加简节母材裂纹的案例分析．并针对碳及碳锰钢板的形变冷、热加工过程与钢材组织和性能的关系，从理论和实际两方面介绍了碳锰锅炉钢板在冷热变形过程中的再结晶与时效硬化。文章介绍了可以通过对碳锰类钢板的加热，变形和温度的合理控制。使这类钢板的热塑性变形与金属的再结晶相结合，以获得细小的晶粒组织，使钢板具有优异的综合力学性能，从而避免因变形过程中的时效脆性使材料损坏。     

油松林木废弃物热压成型颗粒介质传热、传力特性研究

该研究旨在确定油松热压成型微观机理,并用宏观试验验证微观结果。选取离散元软件EDEM进行微观仿真模拟,用C++语言编写对应的传热应用程序设计（API）,实现热压成型仿真。结果表明：颗粒位移、接触力随应变变化,在形变约为0.5时,逐渐由弹性变形转变为塑性变形阶段,模具内颗粒位移逐渐一致,整体呈现较为杂乱的状态,无上下层颗粒之分,接触力在此处突然激增。生物质成型所受应力越大接触力越杂乱,相互交错程度越明显。模具内颗粒温度上升速率的快慢与微观接触程度有关,宏观压缩程度越大,温度上升速度越快;热量传递的速度与温差相关,然而最终所需传热时间相近。     

Natural Convection Heat Transfer of a Rectangular Block within a Vertical Enclosure

A numerical solution of the natural convection heat transfer between two cold and hot isolated vertical plates is presented for different horizontal and vertical location ratios of an enclosure. Results show that: a) flow configurations of cold and hot plates are different; b) the increase of vertical location ratio, toward that corresponding to the enclosure middle value, is considerably diminishing the temperature differences between heating and cooling conditions; c) the effect of vertical location ratio variation is more prominent on heat transfer for cold and hot plates than that of the horizontal location ratio variation; d) the average Nusselt number, obtained from the hot isolated plate, is 20–39% larger than that of the cold plate under the same conditions when the isolated plate varies horizontally; e) for a narrow distance between the inner plate and the bounding wall, the inner plate Nusselt number is enhanced, but aside from this, the plate average Nusselt number is insensitive to the plate position; and f) different trends are found to affect heat transfer for cold and hot plates when the isolated plate varies vertically. The optimal vertical location can be found at specific Rayleigh number for the hot plate and cold plate.     

Heat-transfer correlations for an immersed finned heat-exchanger coil transferring heat from a hot-water store

Natural-convection heat transfers, to a finned-tube heat-exchanger coil immersed in a hot-water store, have been investigated. Cold water was passed through the pipe of the heat-exchanger in order to extract heat rapidly from the hot water in the store. Natural convection currents in the stored water were created by buoyancy forces, which were induced by the temperature gradients that developed as a result of the heat-extraction process. A heat-transfer correlation in terms of Nusselt and Rayleigh numbers has been deduced in order to predict the natural convection heat-transfer coefficient on the outside surface of the heat exchanger. This correlation, which is valid for heat entering the fins, to within an accuracy of better than 4%, is: Nu=0·280 Ra^0·293 for 100 < Ra < 1500