共查询到19条相似文献,搜索用时 218 毫秒
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利用稳态电热拉曼技术测量了碳纳米管纤维对流换热环境下的导热系数. 该方法基于材料拉曼信号与温度之间的关系, 实时探测一维材料在不同电加热(内热源)下的中心点温度, 利用对流环境下的稳态导热模型推导出材料的导热系数, 实现了一维微纳材料热物性的无损化和非接触式测量. 实验发现: 碳纳米管纤维的导热系数远低于单根碳纳米管的导热系数, 但高于碳纳米管堆积床的导热系数. 这表明碳纳米管体材料的热物性主要取决于内部管束的列阵和管束间的接触热阻. 相似文献
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当粒子系中粒子平均间距接近甚至小于热辐射特征波长时,由于倏逝波贡献,其辐射换热热流可远超黑体辐射极限热流。由于存在复杂近场作用,密集粒子系内近场辐射热扩散特性尚不清晰。本文基于涨落电磁理论研究了均匀分布纳米粒子系的近场辐射热扩散特性。研究发现:通过与离散尺度多体辐射换热理论及动理学理论关于金属及电介质粒子链近场辐射换热结果对比,常规热扩散理论可更准确地用于研究粒子系近场辐射热扩散特性,SiC纳米粒子链辐射等效导热系数随温度增大而增大,且温度只影响辐射等效导热系数的大小,而对其光谱峰值频率则无影响。当粒子间距一定时,粒子系辐射等效导热系数随粒子尺寸的增大而增大,更多能量可以在大颗粒构成的粒子链上传递。紧密堆积的粒子系内近场光子隧穿作用较强,其辐射等效导热系数也大于较稀疏粒子系的辐射等效导热系数。 相似文献
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金属蜂窝结构的稳态热性能 总被引:2,自引:0,他引:2
采用MCM、FVM方法数值模拟金属热防护系统蜂窝内辐射导热耦合换热,研究了一正六角形蜂窝的稳态热性能及其影响因素,定量分析了各种热量传递机制的作用.结果表明,金属蜂窝表面辐射换热和侧壁导热是主要热量传递方式,对所研究的蜂窝结构,在750 K以上蜂窝内表面辐射换热成为热量主要传递方式. 相似文献
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Jiaqian Li 《Molecular physics》2018,116(10):1297-1305
The heat conduction and its dependence on fullerene coalescence in carbon nanopeapods (CNPs) have been investigated by equilibrium molecular dynamics simulations. The effects of fullerene coalescence on the thermal conductivity of CNPs were discussed under different temperatures. It is shown that the thermal conductivity of the CNPs decreases with the coalescence of encapsulated fullerene molecules. The thermal transmission mechanism of the effect of fullerene coalescence was analysed by the mass transfer contribution, the relative contributions of phonon oscillation frequencies to total heat current and the phonon vibrational density of states (VDOS). The mass transfer in CNPs is mainly attributed to the motion of encapsulated fullerene molecule and it gets more restricted with the coalescence of the fullerene. It shows that the low-frequency phonon modes below 20 THz contribute mostly to thermal conductivity in CNPs. The analysis of VDOS demonstrates that the dominating contribution to heat transfer is from the inner fullerene chain. With the coalescence of fullerene, the interfacial heat transfer between the CNT and fullerene chain is strengthened; however, the heat conduction of the fullerene chain decreases more rapidly at the same time. 相似文献
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Bo-Ming Zhang Xiao-Dong He 《Journal of Quantitative Spectroscopy & Radiative Transfer》2008,109(7):1309-1324
In the present paper, an experimental apparatus has been developed to measure heat transfer through high-alumina fibrous insulation for thermal protection system. Effective thermal conductivities of the fibrous insulation were measured over a wide range of temperature (300-973 K) and pressure (10−2-105 Pa) using the developed apparatus. The specific heat and the transmittance spectra in the wavelength range of 2.5-25 μm were also measured. The spectral extinction coefficients and Rosseland mean extinction coefficients were obtained from transmittance data at various temperatures to investigate the radiative heat transfer in fibrous insulation. A one-dimensional finite volume numerical model combined radiation and conduction heat transfer was developed to predict the behavior of the effective thermal conductivity of the fibrous insulation at various temperatures and pressures. The two-flux approximation was used to model the radiation heat transfer through the insulation. The experimentally measured specific heat and Rosseland mean extinction coefficients were used in the numerical heat transfer model to calculate the effective thermal conductivity. The average deviation between the numerical results for different values of albedo of scattering and the experimental results was investigated. The numerical results for ω=1 and experimental data were compared. It was found that the calculated values corresponded with the experimental values within an average of 13.5 percent. Numerical results were consistent with experimental results through the environmental conditions under examination. 相似文献
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Parham Sadooghi 《Journal of Quantitative Spectroscopy & Radiative Transfer》2005,93(4):461-472
An analytic solution is presented for describing combined radiation and conduction heat transfer in a spherical fiber thermal protection exposed to combined radiative and convective heating. The solution includes the equation of radiative transfer within the material, coupled to a transient energy equation that contains both radiative and convective terms. At elevated temperatures radiative transfer becomes important, and if several hot surfaces view each other, the radiation exchange process must be considered carefully. Some thermal protections are partially transparent to thermal radiation. Hence, an exchange process is complicated by radiation penetrating into and coming out of material. The radiation leaving an area depends on the temperature distribution inside that area and that is unknown and is affected by the exchange process to other areas. The analysis has allowed for unlimited spectral detail but assumes that the various material properties do not vary significantly with temperature. Transient temperature distributions are obtained for the boundary conditions of external radiation and convection. The present analysis includes the influence of reflectivity, surface radiative properties and spectral properties on the temperature distributions. 相似文献
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A detailed numerical modeling is performed to investigate coupled heat transfer of natural convection, radiation and conduction in high-temperature multilayer thermal insulation (MTI), which consists of high-porous, non-gray semitransparent fibrous materials and reflective foils. Radiation within fibers, radiation between fibers and the reflective foils, conduction within fibers and convection from the fibers to the surrounding fluid are considered. Macroscopic (porous media) modeling is used to determine velocity, pressure and temperatures fields for fibrous insulation with a random packing geometry under natural convection, whereas the radiative transfer equation (RTE) is used to solve the radiative heat flux for non-gray materials. Key features of the macroscopic model include two-dimensional effects, non-gray radiative exchange, and the relaxation of the local thermodynamic non-equilibrium (LTNE). This model was validated by comparison with experimental data and it was used to investigate natural convection of coupled heat transfer in multilayer insulation, numerical results showed that natural convection is more likely to occur when the heated/cooled rate is low, while natural convection can be ignored in simulating steady-state coupled heat transfer in MTI. 相似文献
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本文针对非能动传热机制下简单立方球床堆有效导热系数进行了数值研究,根据有效导热系数的空间分布特性,对球床堆的近壁面区域和主体区域作了划分;分析了不同非能动传热机制下的有效导热系数的壁面效应;最后分析了导热、辐射和自然对流对近壁面和主体区域有效导热系数的贡献。结果发现,近壁面区域是在壁面附近一个球径范围内的区域;由于辐射和自然对流的影响,相同温度下近壁面有效导热系数比主体区域的有效导热系数小了近15%。当温度分别超过950 K和1080 K时,辐射成为球床堆主体区域和近壁面区域的主导传热机制;自然对流对有效导热系数的贡献并不大,当温度超过600 K时,自然对流可以忽略。研究结果可以为高温球床堆的设计与优化提供理论基础。 相似文献
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Markus Spinnler Edgar R.F. Winter Raymond Viskanta 《Journal of Quantitative Spectroscopy & Radiative Transfer》2004,84(4):477-491
The objective of the paper is to model combined heat transfer in multilayer thermal insulations (MTI) for application to high-temperature fuel cells operating at temperatures higher than 650°C. Therefore, solid and gas conduction as well as radiation had to be considered. For conductive heat transfer, referenced models are subjected to sensitivity and plausibility analyses. For modelling the radiation heat transfer, a radiation scaling model reported in literature was adapted to the present problem and compared with experimental data. Finally, internal heat transfer phenomena and a numerical optimisation of MTI are discussed. 相似文献
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Radiation heat loss is an important type of heat loss in thermal systems. In this work, a numerical study of the transient response of two circular radiation heat shields inserted between two parallel and circular surfaces of emissivities ε1 and ε2 is presented. The same dimensions have been assumed for the two main radiating surfaces and the two radiation shields. The radiation shields are assumed to have different emissivities on their top (ε3 and ε5) and bottom ( ε4 and ε6) surfaces, and both are assumed to be different but linear functions of temperature. A specific configuration is investigated in detail to highlight the transient temperature and heat transfer characteristics of the system. Some new results for the transient temperature and heat transfer characteristics of the system such as the effect of shield location, shield emissivities, the temperature dependence of shield emissivities, system dimensions, temperatures of the hot and cold surfaces and emissivities of the hot and cold surfaces are presented for future references. It has been observed that increasing the temperature of the first radiation shield by changing a parameter such as surface emissivity or distance between the radiation shield or the temperature of the hot surface, will not necessarily decrease the temperature of the second radiation shield. 相似文献
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碳遮光石英气凝胶传热机制与热性能数值模拟 总被引:1,自引:0,他引:1
建立了碳遮光石英气凝胶传热机制及热性能数值模拟方法,在交叉立方阵列导热模型、热辐射传输谱带模型、辐射导热耦合传热模型基础上,采用蒙特卡罗方法与有限体积法数值模拟了气凝胶内的热辐射传输及辐射导热耦合传热,并以表观导热系数描述气凝胶传热性能.以某石英气凝胶为例,定量模拟了热性能、各种传热方式的作用及温度依赖性,分析了应用Rosseland扩散近似引起的误差. 相似文献