基于瞬态传热的含裂缝环形锻件传热模型

针对目前热态锻件存在的裂缝问题,提出了一种基于传热理论的含裂缝锻件传热模型。首先,基于微元体的传热特性,推导满足微元体热平衡关系的导热微分方程,并以瞬时热流量为中介变量,对传热系数、热导率进行了修正。其次,设定环形锻件内外环境边界条件、裂缝有限空间自然对流边界条件,进而建立了多边界条件下含裂缝的热态环形锻件传热模型,利用分离变量法求解该传热模型。最后,应用有限元软件对含不同尺寸裂缝的环形锻件传热模型进行模拟,通过试验验证了仿真分析及传热模型的可行性。     

Laminar forced convective heat transfer to near-critical water in a tube

Numerical modeling is carried out to investigate forced convective heat transfer to nearcritical water in developing laminar flow through a circular tube. Due to large variations of thermo-physical properties such as density, specific heat, viscosity, and thermal conductivity near thermodynamic critical point, heat transfer characteristics show quite different behavior compared with pure forced convection. With flow acceleration along the tube unusual behavior of heat transfer coefficient and friction factor occurs when the fluid enthalpy passes through pseudocritical point of pressure in the tube. There is also a transition behavior from liquid-like phase to gas-like phase in the developing region. Numerical results with constant heat flux boundary conditions are obtained for reduced pressures from 1.09 to 1.99. Graphical results for velocity, temperature, and heat transfer coefficient with Stanton number are presented and analyzed.     

Thermal analysis of heat pipe shell-fin structures with selective coatings under radiation

The purpose of this study is to investigate the thermal response of a heat pipe shell-fin structure that is selectively coated and is subject to uneven radiation from upper and lower sides. Such a structure would arise when fins are attached to opposing sides of the condenser or evaporator section of heat pipes to enhance the heat transfer performance. Typical examples include radiators for space applications and solar energy collectors for water heating. The temperature distribution in the circumferential direction of the heat pipe shell as well as that in the fin is examined via theoretical modeling and numerical analysis. The model accounts for the effects of selective coatings. Both steady-state and transient solution procedures are presented. Examination of the steady-state results justifies the use of a thermally-lumped formulation for the heat pipe shell region. The effect of fin width is investigated as a typical design variable. The fin efficiency and the collector efficiency are also presented as functions of heat pipe operating temperature and fin widith. The influence of the surface properties on the thermal performance of the heat pipe shell-fin structure are also examined.     

高效热扩展技术在干式T/R组件散热中的对比研究

T/R组件散热是有源相控阵雷达领域的重要课题,大功耗、高热流密度器件在冷板上会形成局部热障,产生扩展热阻。文中以干式风冷T/R组件散热为研究对象,采用三维数值模拟方法对铝冷板、金刚石/铜、热管及蒸汽腔等高效热扩展技术进行了对比研究,探讨了导热系数、冷板厚度、对流换热系数对扩展热阻的影响规律。结果表明,提高冷板的等效换热系数是减小扩展热阻、强化传热最有效的途径之一。同时合理优化冷板厚度及散热器对流换热系数能有效降低高热流密度器件的工作温度。     

Comparison of performances of the various shapes of asymmetric fins

A comparison between the heat loss of the asymmetric triangular fin and the asymmetric trapezoidal fins which have various slopes of the fin’s upper lateral side is performed. The relation between the slope factor of the fin and the non-dimensional fin length for equal amount of heat loss is shown. Further, the relation between the Biot number and the non-dimensional fin length for equal amount of heat loss is given. For these analyses, a forced analytic method is used. In particular, the same equation is used for both the asymmetric triangular fin and the asymmetric trapezoidal fins just by adjusting the value of the slope factor. It is shown that this equation can also be applied to a rectangular fin with very good accuracy. The base temperature, thermal conductivity of fin’s material and the heat transfer coefficient are assumed constant.     

EXPERIMENTAL RESEARCH AND DESIGN ON HEAT TRANSFER OF EVAPORATOR USED IN THE LARGE QUICK FREEZE PLANT

The evaporator is the main part of a quick-freeze equipment. There are many factors influencing the heat transfer coefficient of an evaporator. The most important factors among them are the fin shape, tube diameter, distance of fin space, frost, and velocity of air flow etc. They mainly influence the thermal efficiency of an evaporator, and therefore its thermal efficiency has direct relationship with the whole efficiency of the quick freeze plant. Evaporators with different structural types have different heat transfer efficiency. In order to obtain high efficiency structure of evaporator, 8 evaporator models with different fin shape, tube diameter and tube arrangement are analyzed and compared. The calculation results show that the integral waved fins, equilateral-triangle arranged small diameter tubes and varying fin-spacing has the highest heat transfer coefficient. The experimental result also shows that the evaporator with this type of structure has better thermal efficiency. The experimental result is in good agreement with the calculation result. It can instruct engineering design for usual designer. A real quick-freeze equipment is designed and put into production. The result shows that, compared with traditional domestic quick-freeze equipments, this equipment decreases by 40% in size and by 20% in energy consumption.     

水冷盘式制动器散热结构优化

为提高水冷盘式制动器散热性能,基于强化对流传热原理,通过添加扰流柱对制动器散热结构进行优化,设计了4种扰流柱散热结构,运用CFD方法模拟制动盘流固耦合传热过程,采用Fluent软件进行热流固耦合仿真计算,获得制动盘温度特性和换热特性以及流动阻力特性,并使用综合性能评价因子对不同扰流柱散热结构进行评价。结果表明:通过在安装盘水槽内添加扰流柱可以有效地提高水冷盘式制动器的散热效果;在相同的工作条件下,正三角形扰流柱散热结构的盘面温度最低,平均努塞尔数与流动阻力最高,其综合散热性能较圆形、椭圆形以及水滴形扰流柱散热结构分别提高了3.4%,2.4%和4.4%,较无扰流柱散热结构提高了6.7%,正三角形扰流柱散热结构具有更好的综合散热性能。研究结果为水冷盘式制动器散热结构的优化设计提供了参考。     

An experimental apparatus measuring convective heat transfer coefficient from a heated fine wire traversing in nanofluids

Most of the previous convection experiments for nanofluids have been performed for internal tube flow with constant heat flux boundary condition. In contrast, a simple experimental apparatus measuring convective heat transfer coefficient from a heated wire to external nanofluids is proposed and its working principles are explained in detail. The convective heat transfer coefficient provided by the present system might be used as a useful indication justifying the adoption of prepared nanofluids as new efficient heat transfer fluids. Validation experiments by comparing convective heat transfer coefficients between the conventional correlation and measured values are carried out for base fluids. Also the effect of increased thermal conductivity of nano lubrication oil on the enhancement of convective heat transfer coefficient is investigated.     

Measuring heat transfer coefficient in convection reflow ovens

In this paper, the evaluation of a measurement method is discussed which can determine the heat transfer coefficient in convection reflow ovens. Nowadays the reflow ovens apply forced convection heating with nozzle-matrix blower system. In these ovens the heat transfer coefficients of the heater gas streams determine mainly the efficiency of heating. A method is presented which has two steps: in the first step, the heat transfer coefficient of the heater gas streams is studied above the assembly in function of height; in the second step, the heating efficiency of the nozzle-lines is compared as a distribution of the heat transfer coefficient in the oven. The heat transfer coefficients are calculated from the heat equation of the reflow oven. It is also presented with the distributions of the heat transfer coefficient that how the contamination of the nozzles affects the heating efficiency of the reflow oven.     

A boundary element solution approach for the conjugate heat transfer problem in thermally developing region of a thick walled pipe

This paper presents a sole application of boundary element method to the conjugate heat transfer problem of thermally developing laminar flow in a thick walled pipe when the fluid velocities are fully developed. Due to the coupled mechanism of heat conduction in the solid region and heat convection in the fluid region, two separate solutions in the solid and fluid regions are sought to match the solid-fluid interface continuity condition. In this method, the dual reciprocity boundary element method (DRBEM) with the axial direction marching scheme is used to solve the heat convection problem and the conventional boundary element method (BEM) of axisymmetric model is applied to solve the heat conduction problem. An iterative and numerically stable BEM solution algorithm is presented, which uses the coupled interface conditions explicitly instead of uncoupled conditions. Both the local convective heat transfer coefficient at solid-fluid interface and the local mean fluid temperature are initially guessed and updated as the unknown interface thermal conditions in the iterative solution procedure. Two examples imposing uniform temperature and heat flux boundary conditions are tested in thermally developing region and compared with analytic solutions where available. The benchmark test results are shown to be in good agreement with the analytic solutions for both examples with different boundary conditions.     

Thermophoretic transport and deposition of particles in vertical tube flow with variable wall temperature and thermal radiation

A particulate two phase flow with variable wall temperature has been studied for examining the deposition of particles in the thermal radiation and mixed convection flow associated with the manufacture of optical fiber preforms. The two-dimensional governing equations of continuity, momentum and energy have been solved numerically including the effects of thermal radiation and buoyancy (upward or downward flow) in the vertical tube flow. A particle trajectory model has been adopted to predict the particle transport, and P-1 approximation has been used to evaluate the radiation heat transfer. In the upward flow case, a high deposition efficiency is obtained and the deposition zone of the downward flow is broader than that of the upward flow. Thermal radiation makes the deposition zone broader and the deposition efficiency smaller.     

Optimization of a rectangular profile annular fin based on fixed fin height

The optimum performance and fin length of a rectangular profile annular fin are presented using a variations separation method. For fixed fin height, the optimum fin length and efficiency are arbitrarily defined as those for which the heat loss is in the range between 90% and 99% of the maximum heat loss. The maximum heat loss, the maximum effectiveness, the minimum fin resistance, the optimum fin length and the optimum efficiency are presented as a function of the inside fluid convection characteristic number, fin base thickness, fin height and ambient convection characteristic number. One of the results shows that the optimum fin length decreases almost linearly with the increase of the fin base thickness.     

污水换热器性能测试与分析

对污水源热泵系统中污水换热器进行了现场测试,依据测试数据分析污水换热器换热量、传热系数的衰减规律。依据测试数据和理论计算分析了污水换热器内污水侧对流换热热阻和污垢热阻的变化规律,以及各热阻在总热阻中的所占比例。结果表明:洁净的污水换热器投入运行225h后,实际换热量为初始值的43.8%;实际传热系数为440W/(m2.℃),为初始值的46.3%;污水侧对流换热热阻占总热阻的60%,污垢热阻占总热阻的20%。提高污水换热器换热效率,保持污水换热器换热量的稳定应从降低污水侧对流换热热阻着手。     

Effect of thermal radiation on convection heat transfer in cooling channel of photovoltaic thermal system

The effect of thermal radiation on convection heat transfer in flat-box type cooling channel of photovoltaic thermal system with tilt angle of 30 degree was studied by 3D numerical simulation under constant heat flux boundary condition. The temperature contours and velocity fields of fluid near the outlet were obtained. The variations of wall temperature and convection Nusselt number along flow direction for all the separate walls composing the cooling channel were compared and analyzed. The results show that due to thermal radiation, the deflection of the maximum velocity region to heated top wall, together with the asymmetry of temperature field is weakened. For natural convection, radiation promotes the formation of multi-vortices. For mixed convection, heat transfer on all the cooling channel walls is enhanced under the condition of lower heat flux while heat transfer on heated top wall is deteriorated when the heat flux is relative high. Also, pressure re-rising is promoted by thermal radiation.     

Numerical analysis of the flow and heat transfer characteristics for forced convection-radiation in entrance region of an internally finned tubes

The flow and heat transfer characteristics of combined forced convection and radiation in the entrance region of internally finned tubes are investigated numerically in this paper. The uniform flow is considered for an inlet flow condition. A three dimensional parabolic problem is solved by a marching-type procedure envolving a series of two dimensional elliptic problems in the cross-stream plane. The SIMPLER-algorithm and Raithby's pressure-velocity coupling method are employed to analyze the flow and heat transfer characteristics. For the calculation of radiative heat transfer, the P₁-approximation and the weighted sum of gray gases method (WSGGM) are used. The effects of fin height, number of fins, optical thickness, reference temperature, and Planck number on the flow and heat transfer characteristics are examined. It was found that the effect of fin-height on the heat transfer characteristic is more dominant than that of number of fins. The present results show that the optimal non-dimensional fin height and number of fins are 0.4 and 16, respectively.     

基于储热释放的方形锂电池内部热参数测试

方形锂电池是卷芯和外壳组成的非均质结构,卷芯导热系数及其与外壳换热系数是影响散热的关键参数,目前缺乏直接测试方法。提出利用电池储热构造热源,通过冷却面温度变化触发非稳态传热,使用热成像记录外壳温度沿传热方向空间分布及其时间演变,代入三维非稳态传热反演模型,同时计算卷芯面向、纵向导热系数,以及卷芯与外壳底部、最大面换热系数。搭建测试装置,对两种方形锂电池进行多次实验检验重复性,卷芯导热系数测试相对标准差在5%～10%之间,对3种非均质标准样品测试检验准确性,相对偏差小于5%。该方法提供一种适用于非均质样品传热特性实验评估的手段,修改样品模型后也可用于圆柱形锂电池等其他非均质样品。     

ICEPAK在三相PWM变流器模块散热分析中的应用

散热器的设计和散热分析是三相PWM变流器设计的关键技术.本文利用CFD软件ICEPAK分析了三相PWM变流器模块的散热性能,探讨了影响散热性能的各个因素.结果表明:散热器的换热系数随着风速的增大而增大,压力系数则越来越小.散热器的肋片数以及肋片厚度的增加导致流通面积变小,压力系数增大,换热系数逐渐增大.     

Combined convection and radiation in a tube with circumferential fins and circular disks

Combined convection and radiation heat transfer in a circular tube with circumferential fins and circular disks is investigated for various operating conditions. Using a finite volume technique for steady laminar flow, the governing equations are solved in order to study the flow and temperature fields. TheP- 1 approximation and the weighted sum of gray gases model (WSGGM) are used for solving the radiation transport equation. The results show that the total Nusselt number of combined convection and radiation is higher than that of pure convection. If the temperatures of the combustion gas and the wall in a tube are high, radiation becomes dominant. Therefore, it is necessary to evaluate the effect of radiation on the total heat transfer. Key Words: Convection, Radiation, Nongray Radiation,P- 1 Approximation, Weighted Sum of Gray Gases Model     

Constructal design of finned tubes used in air-cooled heat exchangers

The present study documents the constructal design and optimization of finned tubes used in air-cooled heat exchangers. The considered tubes are equipped with annular fins. The aim is to minimize the overall thermal resistance by morphing the geometry. The geometrical and thermo-physical parameters considered are the number of fins, ratio of fin height to tube diameter, Stanton number, ratio of fin conductivity to air conductivity, ratio of in-tube fluid conductivity to air conductivity and dimensionless pressure drop. Two constraints are applied in the optimization process: fixed overall volume of heat exchanger and fixed volume fraction of fin material. It is found that there exist optimal values for the number and the height of fins. Moreover, the optimal heat transfer has an extremum in a special volume fraction of fin material.     

法兰密封中螺栓与螺栓孔的传热学模型建立

建立法兰密封中螺栓与螺栓孔的传热学模型,将辐射换热、对流换热、导热转化为螺栓与螺栓孔间空气层的当量导热,计算了不同规格螺栓在不同温度下的当量导热系数,结果表明:对所有规格的螺栓,空气层的对流换热可以完全忽略;当量导热系数随温度变化是近似线性变化的;导热是主要热传递方式,辐射换热总体比例不超过35%。