Temperature prediction for CO2 laser heating of moving glass rods

This paper presents a heat transfer model to calculate the temperature field in moving glass rods heated by a CO₂ laser. Conduction and radiation heat transfer in radial and axial directions are taken into account in the current model. The Rosseland diffusion approximation is incorporated to analyze the radiation heat transfer in the glass rod. A two-band model is used to simulate the spectral property of the glass. Results of the simulation show that glass rods of sufficiently large optical thickness should be treated as a semitransparent medium for radiative transfer, and it is reasonably accurate to assume it to be opaque to CO₂ laser irradiation. It has been shown that the resulting temperature profile is strongly dependent on the laser parameters, i.e., the size of laser beam and the power of the laser. The diameter and speed of the moving glass rod are also important in determining the temperature field although the convective heat transfer coefficient between the glass rod and the environment has little effect.     

激光二极管阵列端面抽运复合棒状激光器热效应的有限元法分析

建立了激光二极管阵列(LDA)端面抽运棒状激光介质的数值模型.考虑到介质与空气的对流换热和介质的热力学参数随温度的变化,根据经典热传导方程和热弹性方程,运用有限元法得出了复合棒状介质和未复合棒状介质内瞬态温度、热应力和应变的时空分布,分析了温度、热应力和应变随抽运功率、换热系数和时间的变化规律.结果表明,复合棒的最高温度、最大张应力和最大轴向应变的位置与未复合棒不同,并且数值分别为未复合棒的73%,60%和33%.由此可知,利用复合棒可极大地减小热效应的影响.理论分析结果可为LDA抽运固体激光器的结构优化设计和实验研究提供理论参考.     

切向气流对激光辐照树脂基复合材料的影响

 为了考察切向强迫气流对激光辐照下树脂基复合材料热响应的影响,基于边界层换热理论,研究了切向气流与靶面的对流换热系数和热分解气体对表面热交换的覆盖效应,并用有限差分法对激光辐照下树脂基复合材料的1维热响应模型进行数值求解。数值计算表明：高速切向气流的存在会加速靶材表面与外部环境的热交换,从而明显降低激光对靶材的加热效率;边界层换热理论给出的对流换热系数和覆盖因子是合理有效的,适用于数值模拟切向气流对激光辐照下树脂基复合材料热响应的影响;向靶材表面溢出的热分解气体对靶材表面与外部环境的热交换有一定的抑制作用,但影响较小,基本可以忽略不计。     

地热换热器的传热分析

本文综述了作者近年来在地热换热器传热模型方面的研究:提出了分析竖直埋管地热换热器钻孔内的传热过程的准三维模型,考虑流体工质在深度方向上的温度分布,给出钻孔内热阻的解析表达式;求得有限长线热源在半无限大介质中的瞬态温度响应解析解;导得了有渗流时无限大介质中线热源温度响应的解析解。以上工作改进和深化了国际上现有的地热换热器传热模型,并已应用于工程设计和模拟。     

薄层物料受激光脉冲热作用的数值模拟

有别于传统的将加热光源作为第二类边界条件的传热计算方法,考虑到激光与薄层多孔材料热相互作用,将激光加热处理为在被加热物料内沿光程按指数规律衰减的内加热源.考虑到薄层多孔材料在大功率微秒脉冲激光作用下可能会出现极为明显的非傅立叶效应,采用非傅立叶导热的双曲型模型描述多孔材料内的传热过程,并用有限差分方法对其进行数值求解,重点分析并讨论了薄层多孔材料的非傅立叶导热现象与物料的光学性质之间的相关关系.     

A molecular dynamics simulation of nanoscale convective heat transfer with the effect of axial heat conduction

Effective heat dissipation from nano-fluidic devices is sometimes necessary to ensure their performance and lifespan. In the molecular dynamics simulation of nanoscale convective heat transfer, thermostats cannot be directly applied to the fluid because of the non-uniform temperature distribution. Periodic boundary is typically utilised, but unrealistic axial heat conduction exists when there is a temperature difference between the outlet and images of inlet atoms. In this paper, the effect of axial conduction caused by periodic boundary is investigated through the Péclet number (Pe). Taking viscous dissipation into consideration, the magnitude of outlet thermal diffusion is observed to decrease with increasing Pe. The local average temperature of fluid changes in an exponential form except in the region close to the outlet. Results show that the contribution of outlet axial conduction to the local average temperature is less than 2.0% when Pe > 10. The main reason is that the magnitude of fluid velocity and viscous heat dissipation in nanochannels is much larger than that in macro-channels at the same Péclet number.     

壁面轴向导热对微细管内对流换热的影响

本文通过数值解析的方法研究了考虑壁面轴向导热时微细管内的对流换热。结果表明,当管外为对流换热边界条件时,管内充分发展对流换热的Nu依然在3.66～4.36之间。但若忽略壁面轴向导热,采用一维热阻模型整理微细管内对流换热的实验数据将会导致错误的结论。     

三维有序排列多孔介质对流换热的数值研究

利用CFD软件数值研究了颗粒三维有序堆积多孔介质的对流换热问题. 采用颗粒直径分别为14 mm,9.4 mm和7 mm的球形颗粒有序排列构成多孔介质骨架,在多孔骨架的上方有一恒热流密度的铜板. 采用流固耦合的方法研究了槽通道内温度分布和局部对流换热系数的分布以及对流换热的影响因素. 研究结果表明:热渗透的厚度和温度边界层的厚度在流动方向上逐渐增大,并且随流量的增加而减小;当骨架的导热系数比较高时,对流换热随颗粒直径的减小而略有增大;对流换热系数随聚丙烯酰胺溶液浓度的增大而减小,黏性耗散减弱了对流换热. 关键词： 多孔介质 温度场 局部对流换热系数 数值模拟     

Laser transmission welding of composites – Part B: Experimental validation of numerical model

In this paper, experimental measurements are performed to confirm the global analytical model (refraction and absorption phenomena) presented in the previous work. Initially, an experimental approach to the estimation of the laser beam spread in a semi-transparent composite and at interface (width of the output beam) subjected to an incident heat flux, was presented. This parameter represents a fundamental input data for the global analytical model (refraction and absorption) during the numerical simulation of transmission infrared welding. Then, an experimental setup for the temperature measurement is performed using infrared camera, during infrared transmission welding of materials joints to validate the welding simulation results (a transient numerical model, based both on conduction and radiation mode heat transfer) with the developed analytical model. The commercial FEM software COMSOL Multiphysics® is used to compute temperature distribution by implementing a radiative source term. Numerical simulations are compared with experimental data. The agreement between simulations and experiments is fair, which gives confidence to use the developed model with acceptable accuracy.     

纵向导热对换热芯传热性能的影响

文中数值模拟了套管的导热系数、纵向长度和肋数对纵向导热的影响,随着纵向长度的增大,纵向导热的影响急剧减小.导热系数和肋数的增加都会使纵向导热的影响增加.对纵向长度小于 100 mm 短管来说,纵向导热越大热短路造成的损失越大,使整个换热芯传热效率相应幅度地降低;对管长大于 100 mm,材料导热系数小于 46 W/(m·K) 的回热器及换热介质为气体的来说,可以忽略纵向导热的影响.     

Thermal Performance of Laser Diode Array under Constant Convective Heat Transfer Boundary Condition

Three-dimensional heat transfer model of laser diode array under constant convective heat transfer coefficient boundary condition is established and analytical temperature profiles within its heat sink are obtained by separation of variables. The influences on thermal resistance and maximum temperature variation among emitters from heat sink structure parameters and convective heat transfer coefficient are brought forward. The derived formula enables the thermal optimization of laser diode array.     

含对流与蒸发表面的有限厚度材料的双曲型热传导分析

本文对于含有对流与蒸发表面的有限厚度材料,在遭受一脉冲表面热流作用时的双曲型热传导进行了分析,采用热势函数描述的双曲型热传导方程来描述该问题,并用有限差分法进行数值求解．同时,本文还就表面对流与蒸发对材料表面与内部瞬态温度变化的影响以及材料双曲型热传导对表面蒸发的影响进行了分析．     

翅片管束式管壳式换热器三维数值模拟研究

本文提出了运用多孔介质模型、分布阻力模型和k-ε湍流模型对壳侧为翅片管束的壳管式换热器壳侧速度场与温度场进行三维数值模拟的方法,并对一相应类型换热器壳侧的流动与换热进行了数值模拟,得出了壳侧流场参数的图示以及壳侧进出口压降,温差,换热量随壳侧Re变化的特性曲线。     

Predictions of Conjugate Heat Transfer in Turbulent Channel Flow Using Advanced Wall-Modeled Large Eddy Simulation Techniques

In this paper, advanced wall-modeled large eddy simulation (LES) techniques are used to predict conjugate heat transfer processes in turbulent channel flow. Thereby, the thermal energy transfer process involves an interaction of conduction within a solid body and convection from the solid surface by fluid motion. The approaches comprise a two-layer RANS–LES approach (zonal LES), a hybrid RANS–LES representative, the so-called improved delayed detached eddy simulation method (IDDES) and a non-equilibrium wall function model (WFLES), respectively. The results obtained are evaluated in comparison with direct numerical simulation (DNS) data and wall-resolved LES including thermal cases of large Reynolds numbers where DNS data are not available in the literature. It turns out that zonal LES, IDDES and WFLES are able to predict heat and fluid flow statistics along with wall shear stresses and Nusselt numbers accurately and that are physically consistent. Furthermore, it is found that IDDES, WFLES and zonal LES exhibit significantly lower computational costs than wall-resolved LES. Since IDDES and especially zonal LES require considerable extra work to generate numerical grids, this study indicates in particular that WFLES offers a promising near-wall modeling strategy for LES of conjugated heat transfer problems. Finally, an entropy generation analysis using the various models showed that the viscous entropy production is zero inside the solid region, peaks at the solid–fluid interface and decreases rapidly with increasing wall distance within the fluid region. Except inside the solid region, where steep temperature gradients lead to high (thermal) entropy generation rates, a similar behavior is monitored for the entropy generation by heat transfer process.     

二维平板通道中流动与传热的格子-Boltzmann模拟

格子-Boltzmann数值模拟方法(LBM),在最近十几年来得到迅速发展。本文发展了LBM的流动与传热模型,并对二维平板通道中的流动与传热进行了模拟,采用密度分布函数得到速度场,用单独的内能分布函数得到温度场,并与传统FVM方法所得到的多个特征量结果进行了比较,模拟结果与FVM解均吻合很好。鉴于LBM边界条件处理简单和易于实施等特点,该方法可望成为求解流动与传热的一种有效数值模拟手段。     

高温吸热管内超临界CO2传热特性的数值模拟

研究超临界CO2在高温吸热管内的传热特性是将其应用于聚光太阳能热发电技术中的基础.本文对此进行了数值模拟研究,分析了流体温度、流动方向、系统压力、质量流率和热流密度对对流传热系数和Nu数的影响.结果表明:高温区(800—1050 K)的对流传热系数和Nu数受流动方向和系统压力的影响均很小,但都随着质量流率的增大以及热流密度的减小而明显增大;而随着流体温度的升高,对流传热系数近似线性增大,Nu数则近似线性减小.另外,本文研究发现在高温区可忽略浮升力对传热的影响,而由高热流密度引起的流动加速效应会明显恶化传热.最后,选取了八种管内超临界流体传热关联式与模拟结果进行对比,发现使用基于热物性修正的关联式对高温区传热数据预测的结果优于使用基于无量纲数修正的关联式得到的结果,且其中预测效果最优的关联式得到的计算结果与模拟结果之间的平均绝对相对偏差为8.1%.     

LDA侧面抽运棒状激光器热透镜效应的有限单元法分析

建立了激光二极管阵列端面抽运棒状激光介质的数值模型.考虑到介质与空气的对流换热和介质的热力学参量随温度的变化,根据经典热传导方程和热弹性方程,运用有限单元法,得出了棒状介质内瞬态温度、热应力和应变的分布.分析了热透镜焦距随抽运功率的变化规律,所得的规律与有关文献相符合.理论分析结果可为激光二极管阵列抽运固体激光器的结构优化设计和实验提供理论参考.     

Finite Element Analysis of MHD Flow of Micropolar Fluid over a Shrinking Sheet with a Convective Surface Boundary Condition

This paper presents a numerical solution for the steady mixed convection magnetohydrodynamic (MHD) flow of an electrically conducting micropolar fluid over a porous shrinking sheet. The velocity of shrinking sheet and magnetic field are assumed to vary as power functions of the distance from the origin. A convective boundary condition is used rather than the customary conditions for temperature, i.e., constant surface temperature or constant heat flux. With the aid of similarity transformations, the governing partial differential equations are transformed into a system of nonlinear ordinary differential equations, which are solved numerically, using the variational finite element method (FEM). The influence of various emerging thermophysical parameters, namely suction parameter, convective heat transfer parameter, magnetic parameter and power index on velocity, microrotation and temperature functions is studied extensively and is shown graphically. Additionally the skin friction and rate of heat transfer, which provide an estimate of the surface shear stress and the rate of cooling of the surface, respectively, have also been computed for these parameters. Under the limiting case an analytical solution of the flow velocity is compared with the present numerical results. An excellent agreement between the two sets of solutions is observed. Also, in order to check the convergence of numerical solution, the calculations are carried out by reducing the mesh size. The present study finds applications in materials processing and demonstrates excellent stability and convergence characteristics for the variational FEM code.     

Heat conduction from a spherical nano-particle: status of modeling heat conduction in laser-induced incandescence

Laser-induced incandescence (LII) of nano-second pulsed laser heated nano-particles has been developed into a popular technique for characterizing concentration and size of particles suspended in a gas and continues to draw increased research attention. Heat conduction is in general the dominant particle cooling mechanism after the laser pulse. Accurate calculation of the particle cooling rate is essential for accurate analysis of LII experimental data. Modelling of particle conduction heat loss has often been flawed. This paper attempts to provide a comprehensive review of the heat conduction modelling practice in the LII literature and an overview of the physics of heat conduction loss from a single spherical particle in the entire range of Knudsen number with emphasis on the transition regime. Various transition regime models developed in the literature are discussed with their accuracy evaluated against direct simulation Monte Carlo results under different particle-to-gas temperature ratios. The importance of accounting for the variation of the thermal properties of the surrounding gas between the gas temperature and the particle temperature is demonstrated. Effects of using these heat conduction models on the inferred particle diameter or the thermal accommodation coefficient are also evaluated. The popular McCoy and Cha model is extensively discussed and evaluated. Based on its superior accuracy in the entire transition regime and even under large particle-to-gas temperature ratios, the Fuchs boundary-sphere model is recommended for modeling particle heat conduction cooling in LII applications. PACS 44.05.+e; 44.10.+i; 47.45.-n; 61.46.Df; 78.70.-g     

螺旋折流板换热器层流流动与换热的数值模拟

应用多孔介质和分布阻力模型对一螺旋折流板管壳式换热器的壳侧层流流动与换热进行了三维数值模拟,并与该换热器的实验研究结果进行了对比分析,符合程度良好.证明了该方法能有效地模拟螺旋折流板换热器的流动和换热特性.