Solution of an Inverse Problem of Heat Conduction of 45 Steel with Martensite Phase Transformation in High Pressure during Gas Quenching

In order to simulate thermal strains,thermal stresses,residual stresses and microstructure of the steel during gas quenching by means of the numerical method,it is necessary to obtain an accurate boundary condition of temperature field.The surface heat transfer coefficient is a key parameter.The explicit finite difference method,nonlinear estimation method and the experimental relation between temperature and time during gas quenching have been used to solve the inverse problem of heat conduction.The relationship between surface temperature and surface heat transfer coefficient of a cylinder has been given.The nonlinear surface heat transfer coefficients include the coupled effects between martensitic phase transformation and temperature.     

Effect of magnetic fields on heat convection inside a concentric annulus filled with Al2O3–water nanofluid

In the current study, forced convective heat transfer of an MHD fully developed laminar nanofluid between two concentric horizontal cylinders is investigated in the presence of a radial magnetic field. In contrast to a conventional no-slip condition at the surfaces, the Navier’s slip condition is considered at the surface to represent the non-equilibrium region near the surfaces. Employing the modified Buongiorno model, the conservative partial differential equations have been collapsed to two-point ordinary boundary value differential equations before being numerically solved. To consider the effects of thermal boundary condition on nanoparticle migration, two distinctive cases including constant heat flux at the outer wall and adiabatic inner wall (Case A) and constant heat flux at the inner wall with adiabatic outer wall (Case B) have been considered. Our results indicate that due to thermophoresis force, the distribution of nanoparticles was denser at the adiabatic wall for the case A which affects the local and the universal fluid flow and heat transfer characteristics. Moreover, inducing a radial magnetic field on the system, heat transfer rate was increased for the case A which had a decreasing effect on the case B. Finally, slip velocity at the walls enhances heat transfer rate for both cases.     

Application of the decomposition method to thermal stresses in isotropic circular fins with temperature-dependent thermal conductivity

Summary A stress-field of a perfect elastic isotropic circular fin with variable thermal conductivity is obtained. The thermal conductivity is considered temperature dependent. The nonlinear conduction-convection-radiation heat transfer equation of the circular fins subjected to the nonlinear boundary conditions is solved by Adomian's double decomposition method. The thermal stress distribution is obtained by direct integration of the temperature distribution. Fow low temperature difference between the fin base and the ambiance, the effect of thermal conductivity on pure convection and convection-radiation is important and can be negligible in pure radiation.     

钢筋混凝土双向板火灾行为的数值分析

根据傅里叶热传导理论,提出蒸发阶段水分修正函数,建立改进温度场模型;结合非线性板壳理论和热弹塑性本构关系,建立了火灾下钢筋混凝土双向板的数值模型,发展现有计算程序,通过计算结果与试验结果对比,验证了模型及程序的可靠性。在此基础上,分析了火灾工况、约束作用、骨料类型、板厚和保护层厚度等参数对混凝土双向板变形行为和耐火极限的影响规律。结果表明:火灾工况、骨料类型、板厚和保护层厚度对双向板火灾行为影响较大;双向板变形行为和破坏模式对约束作用非常敏感,力学机理较为复杂。     

岩土的非线性冻结模型试验和相似准则

由于冰的比热和导热系数与水的比热和导热系数具有显著的差异性,因此土的比热和导热系数在冻结过程中是不断变化的。依据常热参数建立起来的温度场相似准则,难以准确反映土体在冻结过程中由热参数非线性引起的温度场演变。该文基于考虑热参数非线性变化的热传导微分方程,采用相似变换法导出了冻土模型试验中温度场的相似准则,并给出了采用原土进行模型试验时原型与模型应满足的相似关系。同时,在考虑第三类边界条件对相似准则影响的基础上,导出了模型土应满足的热物性相似条件。在此基础上,分别利用ABAQUS有限元软件对线性、非线性以及考虑第三类边界条件的原型和模型温度场进行了数值模拟,并对冻结过程中特征点的温度演变过程进行了分析。计算结果表明:建立的非线性导热相似准则能够正确反映原型冻土温度场的演变过程,说明模型土热参数的计算方法是合理的。同时表明,满足第三类边界条件的土体非线性冻结相似准则具有明确的物理意义和更高的实用价值。该研究成果为冻土模型试验中模型土的配制提供了切实合理的参数计算方法,为冻土水-热-力三场耦合模型试验的设计和实施提供了理论基础和技术支持。     

The thermal state of a porous wall under conditions of transpiration cooling

Results of numerical experiment are used for analysis of fields of temperature in a laminar boundary layer, in a porous wall, and in a cooling gas delivery chamber, as well as for analysis of heat transfer and of distribution of the temperature difference between the cooling gas and the porous wall frame and cooling efficiency. It is demonstrated that heat transfer between a porous wall of finite thickness and a high-temperature gas flow differs significantly from heat transfer with preassignment of the same intensity of injection and of the homogeneous thermal boundary condition directly on the surface subjected to flow. One of the reasons for this is the formation of wall temperature variable along the boundary layer.     

The Role of Adsorption and Phase Change Phenomena in the Thermophysical Characterization of Moist Porous Materials

Phase change phenomena in moist porous media with low liquid content, the typical condition of a porous body at ambient conditions and far from the contact of liquid water, are controlled by the shape of the adsorption isotherms and by the effective liquid-vapor thermodynamic condition within the pores. Usually, heat and mass transfer models are developed under the assumption of thermal and hygrometric equilibrium. This gives rise to an expression of the evaporation source that is too complex in view of the dynamic identification of thermophysical and transport properties of a porous material. In this study, the hypothesis of hygrometric equilibrium is dropped. The phase change rate is considered proportional to the amount of local nonequilibrium through an appropriate delay coefficient. This approach leads to a simple representation of the process and makes manageable the formulation of a coupled heat and mass transfer inverse problem. A comparison with a first group of experiments performed with an open-pore light insulating material (expanded perlite board) confirms the suitability of the proposed approach. However, the analysis shows that, for this material, phase change occurs not far from the hygrometric equilibrium.     

塔式太阳能热发电吸热器模拟计算

本文建立了塔式太阳能热发电高温吸热器传热性能数值计算模型,采用有限元分析方法进行模拟计算,编制了预测塔式太阳能热发电吸热器热工模拟程序。本文在综合考虑多场耦合特性的情况下,研究了吸热器单模块周向非均匀热流边界条件下吸热介质对流换热系数、吸热管外壁温度、光热转化效率的数值及分布。数值模拟为工程设计提供可靠的科学依据,为塔式太阳能热发电整体控制策略的制定提供参考。     

40英尺液化天然气铁路及其联运罐式集装箱静态蒸发率计算及测试研究

针对铁路及其多式联运液化天然气罐式集装箱,对其静态蒸发率进行了理论计算,在理论计算中引入了温度修正系数kt来考虑装载LNG介质时对液氮温区下绝热层比热流的影响,同时引入了结构修正系数ks来考虑结构和深冷下罐体收缩对支撑部位热传导的影响,采用液化天然气作为介质开展了静态蒸发率试验测试,综合测试结果对理论计算模型进行了验证,在此基础上开展了相关关键参数对LNG罐式集装箱静态蒸发率的影响分析研究。     

Modeling of Environmental Effects on Thermal Detection of Subsurface Damage in Concrete

A significant form of deterioration in concrete is corrosion of embedded reinforcing steel that can cause subsurface delaminations and spalling. Infrared thermography can be used to detect delaminations based on variations in surface temperature that are caused by the disruption of the heat flow through the delaminated area. The surrounding environmental conditions such as sunlight, ambient temperature variation, and wind speed are critical for heat transfer, and as such the technology depends on these environmental conditions. This paper describes a numerical model developed to predict thermal contrasts for subsurface delaminations based on a given set of environmental conditions surrounding the concrete. The finite element method (FEM) was used to perform 3-D nonlinear transient heat-transfer analysis of a large concrete block with embedded Styrofoam targets intended to provide an idealized model of subsurface delaminations. The effectiveness of the modeling was evaluated by comparing the thermal contrasts predicted by the model and those obtained from experimental testing of an actual concrete block of the same dimensions. The correlation and error between the experimental testing and the model results indicated that the model could be an effective tool for the prediction of anticipated thermal contrasts based on given weather conditions.     

Boundary layer in shape optimization of convective fins using a meshfree approach

We show how shape optimization of fin arrays for increased heat flux through the base under area constraint leads to non‐existence of optimal solutions. An additional constraint in terms of the boundary layer eliminates the apparent paradox. We consider a variable heat transfer coefficient and we use a fixed‐point iteration scheme to solve the resulting non‐linear boundary value problem for the steady‐state heat operator with temperature, flux, and convection boundary conditions. We propose a simple yet effective algorithm for evaluating the boundary layer constraint and eliminating the constraint violation. There are large shape changes between the initial and final design but no remeshing is required because we use a meshfree method that is not sensitive to shape distortion of integration cells as long as they remain convex. The resulting optimal unit cell is repeated by periodicity to produce the optimal fin array. The obtained shapes display similarities to shapes seen in natural systems governed by diffusion/convection and conduction processes. A length‐scale for the unit cell is naturally introduced by the non‐overlap condition imposed on the thermal boundary layer in the cooling ambient fluid. Copyright © 2004 John Wiley & Sons, Ltd.     

Theoretical Investigation on Rapid Evaporation of a Saline Droplet During Depressurization

This paper reports a theoretical investigation on rapid evaporation of a saline droplet during depressurization. A mathematical model was developed to simulate the droplet temperature variation by considering the ambient pressure change, the heat transfers due to evaporation and convection at the droplet surface, accompanying the heat and mass transfer inside the droplet. The component diffusion and the temperature gradient inside the droplet were mainly discussed by comparing the numerical droplet temperature with the experimental data. The result shows that, the variation of internal concentration is small, while the temperature gradient within the droplet is significant during the evaporation process. In addition, the influencing factors of the droplet temperature variation were analyzed, such as: the final ambient pressure, theinitial salt concentration and the initial droplet temperature. The present model calculations help to understand the thermodynamic process of rapid evaporation of a saline droplet during depressurization.     

不同压力下具有内热源的包装箱内传热特性

目的研究具有内热源的某产品包装箱在不同压力下的传热特性。方法搭建具有内热源的某产品包装箱传热特性实验系统,对不同压力下内部具有热源的某产品包装箱内的温度场进行测量,讨论产品各部件的传热特性,并基于计算结果,拟合各部件的传热关联式。结果常压状态下各测点温度较低压状态更易达到平衡,箱体内温度场的分布符合被加热物四周热边界层厚度分布规律。包装箱内对流传热能力随着压强的增大而增强,且箱内压强处于较小范围时,压强的增加对于箱内各部的冷却效果更加显著。结论正常工作的温度范围内,该产品包装箱内传热能力的强弱主要受对流传热影响,对流传热能力随着压强的增大而增强;得到了各部件壁面传热关联式,误差在5%以内。     

多孔泡沫金属中指数规律变热流平板表面的层流对流传热分析

采用Brinkman-Forchheimer-extended Darcy流动模型和局部非热平衡传热模型(双温度模型),对指数规律变热流密度条件下的多孔泡沫金属中平板表面的层流对流传热进行了分析,并得出了平板表面的热边界层的厚度和局部的对流传热系数的表达式。结果发现:平板表面的热边界层的厚度发展沿流动方向逐渐增大,但是增大的趋势由迅速趋向平缓;局部对流传热系数沿流动方向逐渐减小,而后趋于稳定。最后推导出了局部的对流传热Nusselt数的准则方程。     

Thermal analysis of a constructal T-shaped porous fin with radiation effects

This paper presents an analytical technique based on the decomposition method to determine the temperature distribution and thermal performance parameters of a constructal T-shape porous fin. The effect of radiation on natural convective heat transfer is considered in the analysis. The governing energy equations of the stem and flange part of this T-shaped porous fin for the aforementioned conditions are highly nonlinear. The adopted decomposition solution gives an explicit expression of temperature distribution in the fin as a function of a coordinate expressed by infinite power series from which fin performance parameters and heat transfer rates can easily be calculated without the need of linearization. The effects of different geometric and thermophysical parameters on the dimensionless temperature distribution and fin performances are studied. Finally, the increase in heat transfer is noticed by selecting porous medium condition in the fin.     

A calorimeter for multilayer insulation (MLI) performance measurements at variable temperature

Here we describe a concentric cylindrical calorimeter with radiation guards developed to measure the thermal performance of multilayer insulation (MLI) for low temperature applications. One unique feature of this calorimeter is its ability to independently control the boundary temperatures between room temperature and about 15 K using two single-stage Gifford–McMahon cryocoolers. Also, unlike the existing calorimeters that use the evaporation rate of a liquid cryogen to measure the heat load, in the present system the total heat transfer through the MLI is measured by recording the temperature difference across a calibrated heat load support rod that connects the cold inner cylinder to the lower temperature cryocooler. This design allows the continuous mapping of MLI performance over a much wider temperature range with independently controlled boundary conditions. The calorimeter is also suitable for performing a variety of radiation heat transfer experiments including the determination of the temperature dependence of the total emissivity.     

Heat‐flux‐specified boundary treatment for gas flow and heat transfer in microchannel using direct simulation Monte Carlo method

Direct simulation Monte Carlo (DSMC) method has been widely used to study gaseous flow and heat transfer in micro‐fluidic devices. For flows associated with microelectromechanical systems (MEMS), the heat‐flux‐specified (HFS) boundary condition broadly exists. However, problems with HFS boundary have not been realized in the simulation of microchannel flows with DSMC method. To overcome this problem, a new technique named as inverse temperature sampling (ITS) is developed. This technique provides an approach to calculate the molecular reflective characteristic temperature from the specified heat flux at the wall boundary. Coupling with DSMC method, the ITS technique can treat the HFS boundary condition in DSMC method for both simple gas and gas mixtures. For validation, heat flux obtained from two‐dimensional Poiseuille flows with wall‐temperature‐specified (WTS) boundary condition is employed as the initial thermal boundary condition of our new method. Sampled wall temperature by the ITS method agrees well with the expected value. Pressure, velocity and temperature distributions under these two thermal boundary conditions (WTS and HFS) are compared. Effects of molecule collision model and gas–surface interaction model are also investigated. Results show that the proposed ITS method could accurately simulate gaseous flow and heat transfer in MEMS. Copyright © 2007 John Wiley & Sons, Ltd.     

电磁微锻机构热效应模拟与实验研究

为研究电磁微锻机构在工作过程中的温度分布情况以及提高其功率密度,首先,在现有电磁微锻机构的基础上,从能量角度对其热效应与输出功率的关系进行分析,明确了热效应会限制机构的最大输出功率。然后,利用COMSOL Multiphysics软件分别对风冷和水冷方式下电磁微锻机构的温度场进行CFD （computational fluid dynamics, 计算流体力学）仿真分析,确定了不同入口边界条件下的等效对流换热系数;同时,根据等效对流换热系数与入口边界条件的关系,建立了该机构的瞬态温度分析模型。最后,搭建了电磁微锻机构温度测量实验平台,并对该机构的温升特性和稳态温度特性进行了实验研究。实验结果表明,所提出的电磁微锻机构热效应仿真分析方法较为合理、准确,可为微锻机构的温度控制和结构优化提供参考。     

Influence of boundary conditions on the cooling effect of crushed-rock embankment in permafrost regions of Qinghai–Tibetan Plateau

The crushed-rock layer is a highly porous medium that has been used to ensure the stability of embankment in permafrost regions. At present, depending on different boundary conditions (impermeable and permeable) of crushed-rock layer in embankment, the crushed-rock embankments are divided into two kinds of structures in the construction of Qinghai–Tibetan railway in China. One is a closed-boundary crushed-rock embankment; the other is an open-boundary crushed-rock embankment. In order to investigate the influence of boundary conditions (impermeable and permeable) on the cooling effect of a crushed-rock embankment, two numerical models of the unsteady two-dimensional hydrokinetic equations for incompressible fluid are presented to analyze the velocity and temperature characteristics of crushed-rock embankment with different embankment heights under impermeable and permeable boundary conditions for a period of 50 years. The results indicate: (1) the boundary conditions (impermeable and permeable) of crushed-rock embankment can have a very large impact on the heat transfer pattern within it in windy permafrost regions of Qinghai–Tibetan Plateau. The cooling effect of the closed crushed-rock embankment mainly relies on natural convection within crushed-rock layer, which is caused by the thermal boundary condition, but the cooling effect of the open crushed-rock embankment is due to the heat transfer enhancement because of internal forced convection induced by the external low temperature air flow (wind); (2) from the temperature distributions of crushed-rock embankments, it can be found that, under the assumption that the air temperature will be warmed up by 2.6 °C in a period of 50 years and in the areas where the mean annual air temperature is − 4.0 °C, when embankment is low, the cooling effects of crushed-rock embankment have no obvious difference under the two boundary conditions, and the cooling effect of closed crushed-rock embankment is only a little better than that of open one; however, when embankment is high, the boundary conditions cause a distinct influence on the temperature distribution of crushed-rock embankment, and the cooling effect under the permeable boundary condition is far better than that under the impermeable boundary condition. However, the asymmetric temperature distribution problem of the high crushed-rock embankment, caused by permeable boundary and external wind, must be considered when it is designed and constructed.     

Coupled conduction-convection problem for a cylinder in an enclosure

The coupled heat conduction/convection problem for a solid cylinder in either a rectangular or a circular enclosure filled with air is solved by an operator-splitting pseudo-time-stepping finite element method, which automatically satisfies the continuity of the interfacial temperature and heat flux. The temperature distribution in the cylinder and in the fluid is obtained showing that the usual practice of prescribing a uniform heat flux boundary condition at the interface may not lead to an accurate solution. From the profile of the local Nusselt number, which is strongly dependent on the Rayleigh number but weakly dependent on the thermal conductivity ratio, it is concluded that most of the heat transfer takes place in the lower half of the cylinder through a convective mode.