Optimization of volume to point conduction problem based on a novel thermal conductivity discretization algorithm

A conduction heat transfer process is enhanced by filling prescribed quantity and optimized-shaped high thermal conductivity materials to the substrate. Numerical simulations and analyses are performed on a volume to point conduction problem based on the principle of minimum entropy generation. In the optimization, the arrange-ment of high thermal conductivity materials is variable, the quantity of high thermal-conductivity material is constrained, and the objective is to obtain the maximum heat conduction rate as the entropy is the minimum. A novel algorithm of thermal conductivity discretization is proposed based on large quantity of calculations. Compared with other algorithms in literature, the average temperature in the substrate by the new algorithm is lower, while the highest temperature in the substrate is in a reasonable range. Thus the new algorithm is fea-sible. The optimization of volume to point heat conduction is carried out in a rectangular model with radiation boundary condition and constant surface temperature boundary condition. The results demonstrate that the al-gorithm of thermal conductivity discretization is applicable for volume to point heat conduction problems.     

The Role of Vibrations of Medium-Range Order Clusters in Thermal Conduction of Materials with a Disordered Structure

A microscopic model that provides a satisfactory explanation of the temperature dependence of the thermal conductivity coefficient for dielectric glasses in the temperature range above the plateau is proposed for the first time. According to this model, the transfer of thermal perturbations occurs through harmonic interaction of oscillators localized at medium-range order clusters with hinged bonds. It is demonstrated that the interaction coefficient of these clusters depends on the overlap integral of the spectral lines attributed to natural vibrations of the clusters, whereas the temperature behavior of the thermal conductivity coefficient is governed by the temperature evolution of the widths of the vibrational lines. With due regard for the results of investigations into the broadening of the vibrational lines, the temperature dependence of the thermal conductivity coefficient is calculated in the range 10–500 K. The calculated curve is in good agreement with the experimental data. The proposed model accounts for the low thermal conductivity of glasses in the temperature range under consideration.     

Effect of isotope content on the cubic boron nitride lattice thermal conductivity

New theoretical data are presented on the lattice thermal conductivity temperature dependence of cubic boron nitride single crystals. Their thermal conductivity increases with increasing isotope purity, with the maximum thermal conductivity occurring with either isotopically pure ¹¹B or ¹⁰B. The thermal conductivity is a symmetric function of isotopic composition with a minimum at 50% ¹⁰B.     

Microthermal Analysis of Heat Conduction in Silicon Nitrides

Local thermal response in silicon nitride materials was investigated with a microthermal analyzer. Two gas-pressure-sintered silicon nitride materials having basically different ultimate firing temperatures were examined. In sintered materials with coarse grains grown at higher temperatures, the bulk thermal conductivity was high while that of individual particles was rather lower due to the presence of impurities. The higher bulk thermal conductivity may be attributed to heat conduction through coarser grains. In sintered materials with finer grains, the bulk thermal conductivity was lower owing to the effects of grain boundaries, despite higher conductivity within individual grains.     

Thermal Conductivity and Heat Capacity of the Monophosphide and Monosulfide of Plutonium

The thermal properties of PUP and PUS were measured from room temperature to 650° C by a heat-pulse technique. Disk-shaped specimens of 90% theoretical density were made by isostatic cold-pressing and sintering. The thermal conductivity of PUP corrected to theoretical density ranged from 0.014 cal sec^-cm^-1°C^-1 at room temperature to 0.019 at 650° C. Over this temperature range the heat capacity increased from 0.055 to 0.056 cal g^-l°C^-1. The thermal conductivity of PUS decreased approximately 16% from its room temperature value of 0.023 cal sec^-cm^-1°C^-1 and then rose to 0.037 at 650°C. The temperature dependence of the thermal conductivity of PUP is apparently related to the quasimetallic electrical properties of this compound. PUS is a semiconductor and the rise in its thermal conductivity (after passing through a minimum) may be partly attributed to a bipolar thermal diffusion mechanism.     

磁处理对水溶液pH值和电导率的影响

碳酸钙、碳酸氢钙及其混合水溶液分别以3．7m/s的流速通过磁场强度为6040高斯的正交磁场进行循环运行,磁处理后碳酸钙水溶液的pH值快速下降直至恒定,碳酸氢钙水溶液的pH值先下降后上升出现一最低值,而混合水溶液的pH值则缓慢下降,三种溶液的电导率均上升;室温放置24h后pH值的变化趋势未变,但电导率变得相近,说明磁处理主要通过促进CO2的溶解而起作用的并具有“记忆效应”,这种作用与溶液的性质有关;溶液加热后的pH值及电导率的变化也证明了这一点。     

Non‐Newtonian Heat Transfer on a Plate Heat Exchanger with Generalized Configurations

For the configuration optimization of plate heat exchangers (PHEs), the mathematical models for heat transfer and pressure drop must be valid for a wide range of operational conditions of all configurations of the exchanger or the design results may be compromised. In this investigation, the thermal model of a PHE is adjusted to fit experimental data obtained from non‐Newtonian heat transfer for eight different configurations, using carboxymethylcellulose solutions (CMC) as test fluid. Although it is possible to successfully adjust the model parameters, Newtonian and non‐Newtonian heat transfer cannot be represented by a single generalized correlation. In addition, the specific heat, thermal conductivity and power‐law rheological parameters of CMC solutions were correlated with temperature, over a range compatible with a continuous pasteurization process.     

Heat transfer in nanoparticle suspensions: Modeling the thermal conductivity of nanofluids

This work reviews experimental data and models for the thermal conductivity of nanoparticle suspensions and examines the effect of the properties of the two phases on the effective thermal conductivity of the heterogeneous system. A model is presented for the effective thermal conductivity of nanofluids that takes into account the temperature dependence of the thermal conductivities of the individual phases, as well as the size dependence of the thermal conductivity of the dispersed phase. We demonstrate that this model can be used to calculate the thermal conductivity of nanofluids over a wide range of particle sizes, particle volume fractions, and temperatures. The model can also be used to validate experimental thermal conductivity data for nanofluids containing semiconductor or insulator particles and confirm the size dependence of the thermal conductivity of nanoparticles. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

A Study of Effective Thermal Conduction in Lightweight Foam Corundum Refractories

Thermal conduction of lightweight foam corundum refractories is studied by a hot-wire method. Empirical relations for the thermal conductivity as a function of temperature (20 – 1100°C) and porosity (79 – 93%) are derived. The thermal conductivity at higher temperature is less dependent on porosity because of the increased role of radiation in the heat conduction process.     

液态金属微液滴脉动热管的传热性能

利用室温液态金属和表面活性剂溶液混合工质的振荡运动,在脉动热管中形成液态金属微纳液滴分散的高热导率混合流体并提高其传热性能。本文将液态金属表面活性剂混合工质引入六弯管板式脉动热管中,在不同液态金属填充量和加热功率下开展可视化和传热性能实验。实验结果表明,液态金属在表面活性剂混合工质中通过振荡自分散成球形液滴且相互之间不易发生合并,并在表面活性剂工质中留下粒径在410~520nm的纳米颗粒。传热性能方面,液态金属填充量在20%~25%时,液态金属球形液滴的黏度高、质量大,会阻碍混合工质的振荡运动从而降低脉动热管的传热性能,填充量在5%~10%时,混合工质耦合了液态金属的高热导率特性,有效提高传热性能,热阻最多降低11.21%。     

Specimen Size Effect of the Thermal Diffusivity/Conductivity of Aluminum Nitride

It has been generally found that the thermal diffusivity/conductivity of AIN measured by the laser-flash technique decreases with decreasing specimen size. The results of this study indicate that much of this effect can be attributed to the relatively large temperature rises for thin specimens, especially for high-energy laser pulses, and results from the combination of the noninearity of the commonly used type of IR detector and the strongly negative temperature dependence of the thermal diffusivity of AIN. The experimental results indicate that for reliable data, the specimen temperature rise at thermal equilirium should be kept to less than 1°C by using specimens with a thickness near 3 mm in combination with keeping the pulse energy to a reasonable minimum by attenuation of the laser beam by passing it through an aqueous solution of CuSO₄.     

基于理论的传热结构拓扑优化

以理论为基础,分析了在含有内热源的二维稳态导热问题中的耗散。并以此为目标函数,通过密度法建立拓扑优化模型并用全局移动渐近线（GCMMA）开展拓扑优化研究。对比分析了以最小耗散以及最小熵产得到的拓扑优化构型在传热性能上的异同。然后,以最小耗散为优化目标,进一步分析了不同高导热材料体积占比下最优传热拓扑结构。结果表明,最小耗散以及最小熵产得到的拓扑优化构型结构相似,均可大幅增加传热性能,系统的平均温度均可降低9℃以上。而对于传热结构来说,综合考虑优化效果以及成本,20%的体积占比是一个较优值,在此占比下,优化后的耗散仅为优化前的8.7%。基于理论的传热结构拓扑研究为肋片的结构设计以及传热强化提供了理论指导。     

基于热传导反问题的各向异性材料热物性预测方法

采用不同方法对基于热传导反问题的固体热导率预测进行了研究。分别采用采用不同方法对基于热传导反问题的固体热导率预测进行了研究。分别采用Bayesian统计方法、Levenberg-Marquardt和遗传算法对二维各向异性材料的热物性进行了预测;并进行了分析比较。研究结果表明;Bayesian方法中热传导反问题的解是其后验概率密度的数学期望;而后验概率密度函数（PPDF）通过测定的温度进行计算获得;用Markov chain Monte Carlo算法计算后验状态空间以得到未知热导率的统计估计;采用Metropolis-Hasting算法进行数据采样构造Markov chain;并截取收敛后的样本进行分析。遗传算法是一种相对较新的用于最优化估计的方法;也可以用于求解反问题。     

导热聚氨酯灌封胶的研制

以聚醚210、聚醚305、1,4-丁二醇、三羟甲基丙烷为原料,氧化铝(Al2O3)为导热填料,与异佛尔酮二异氰酸酯反应合成导热聚氨酯灌封胶。研究Al2O3用量、催化剂和聚醚多元醇的种类对灌封胶性能的影响。结果表明,当Al2O3的质量分数从0增加到80%时,灌封胶的导热系数从0.08 W/(m.K)增加到0.76 W/(m.K),是纯聚氨酯灌封胶的9倍多。SEM测试表明,Al2O3在体系中有着较好的分散性。     

Thermal Conductivity in Nanocrystalline Ceria Thin Films

The thermal conductivity of nanocrystalline ceria films grown by unbalanced magnetron sputtering is determined as a function of temperature using laser‐based modulated thermoreflectance. The films exhibit significantly reduced conductivity compared with stoichiometric bulk CeO₂. A variety of microstructure imaging techniques including X‐ray diffraction, scanning and transmission electron microscopy, X‐ray photoelectron analysis, and electron energy loss spectroscopy indicate that the thermal conductivity is influenced by grain boundaries, dislocations, and oxygen vacancies. The temperature dependence of the thermal conductivity is analyzed using an analytical solution of the Boltzmann transport equation. The conclusion of this study is that oxygen vacancies pose a smaller impediment to thermal transport when they segregate along grain boundaries.     

Utilization of a Radial Temperature Model for Heat Conduction within Spherical Particles to Model Granular Assemblies

A heat transfer (DEM) model for application in the particle based discrete element simulation method is presented. It utilizes an analytical solution of the heat diffusion equation for a solid spherical particle to obtain temporal and radial solutions of the temperature distributions within the particles. This radial temperature model avoids the shortcomings of the usual assumption of spatially uniform temperature profiles in particles. The concept is designed to minimize computing power and memory requirement in order to allow the computation of granular assemblies consisting of a large number of particles. Results obtained for a particle subject to transient convective boundary conditions are compared with a Crank‐Nicholson implicit scheme as numerical reference solution. A first implementation of the radial temperature model in a discrete element code reveals the additional computational cost as negligible compared to the demands of contact identification and force calculation.     

Behaviour of gas-fluidized beds of fine powders part III. Effective thermal conductivity of a homogeneously expanded bed

The effective thermal conductivity of the dense phase in expanded non-bubbling fluidized beds has been studied between minimum fluidization and minimum bubbling for 9 gas—solid systems. Except close to minimum bubbling, the effective thermal conductivity is not a function of bed voidage and is most sensitive to the gas phase thermal conductivity. The experimental results have been compared with 11 packed bed correlations and with suitable modification three of these can be used to accurately predict the effective thermal conductivity of a non-bubbling fluidized bed.     

吸湿性盐溶液振荡热管的传热特性研究

研究了10%(质量分数)LiCl吸湿性盐溶液作为工质的振荡热管传热特性。测试了在45％～90％充液率、10～100 W加热功率下振荡热管蒸发端温度及热阻的变化，并与去离子水工质的实验数据进行了对比。结果表明：在45％、55％的低充液率下，加热功率达到50 W以上时，LiCl溶液振荡热管的热阻明显比去离子水振荡热管低，能有效延迟烧干现象的发生。在62%的中等充液率，35W加热功率以上，LiCl溶液振荡热管的蒸发端温度较离子水振荡热管振荡频率快，幅度小且热阻低。在80%、90%的高充液率下，两种工质振荡热管的蒸发端温度曲线在平均温度、振荡频率、振荡幅度上都较为相似，热阻也比较接近。