多孔微晶玻璃导热理论模型的验证

为了研究多孔微晶玻璃的孔隙率与导热系数之间的关系,以伟晶岩为原料,采用粉末烧结法和添加造孔剂方法,烧制出SiO2-Al2O3-CaO-ZnO-R2O乳白色多孔微晶玻璃.根据多孔微晶玻璃的内部结构和孔隙率情况提出3种热传导的理想等效物理模型,推导出数学公式,验证了多孔微晶玻璃的导热系数.实验结果表明,实验测得的多孔微晶玻璃导热系数值与理论导热系数之间具有较好的一致性.如果已知多孔微晶玻璃的孔隙率,那么可以利用理想等效物理模型来推算多孔微晶玻璃的导热系数.

煤矸石层降温的数值模拟方法研究与验证

为了研究煤矸石组成的多孔介质内部热传导规律与热棒工程降温效果,并对其降温进行评价。以OpenFoam中Volume of Fluid(VOF)两相模型为基础,通过植入热传导与相变方程的方法来实现2种不同状态物质之间(热棒材料与煤矸石)的能量传递过程。与实验进行比较,结果表明:该模型与实验测试的误差较小,能够全方位反映稳定工况时地层区内部温度。可通过数值模拟调节求解器中热棒的冷源温度来进一步实现调控煤层温度内部的变化。有助于进一步应用到更大范围或复杂工况的煤矸石层降温工程,以快速实现矸石工程降温的工程效果的数值分析评价。

Determination of Effective Thermal Conductivity ForReal Porous Media Using Fractal Theory

In this paper, using fractal theory3 the geometric structure of real soil was described with its sectionview and section particle area fractal dimension d of porous media was counted. The volumetric solidcontent and the relation between volumetric solid content and porous media particle arrangementsas well as measure scale were obtainted. A heat conduction model was established and the effectivethermal conductivity of real soil based on the volumetric solid content was calculated.

Experimental Study of Melting Heat Transfer in anEnclosure with Three Discrete Protruding HeatSources

To explore the possibility of using solid-liquid phase change cooling to those periodically or intermit-tently operated electronic equipment, a detail of experimeatal study of time-dependent melting heattransfer in a rectangular encforure with three discrete protruding heat sources is presented. A seriesof experimntal measurement in an enclosure with phase change materials (PCM) of n-octadecaneheated by three discrete Protruding heat sources at a constant rate on one of its vertical wall has beendescribed. The opposite wall of the enclosure is cooled at a constant temperature and all other wallsof the enclosure are insulated. The time-dependent solid-liquid shapes of the interface during meltingprocess, the variation of surface temperature and the effects of subcooling are investigated. An empirical correlation predicting the relationship between tbe liquid phase fraction and subcooling is given.

A novel method to determine effective thermal conductivity of porous materials

A 2D Lattice-Boltzmann (LB) model is proposed for analyzing the heat conduction process in the porous media. The effective thermal conductivities of several porous materials are calculated by means of this model. The calculated results are found to be in excellent agreement with the experimental data of the existing references. The factors affecting the effective thermal conductivity of porous materials are discussed. The results show that the effective thermal conductivity is strongly dependent upon the porosity and the pore structure and only has imperceptible dependence on the pore density. Then the correlation for estimating the effective thermal conductivity of the porous material is established. This LB model can be used conveniently to calculate and analyze the heat conduction problems of porous media or other materials with complex geometry boundary in pore scale. Keywords Lattice-Boltzann - porous media - effective thermal conductivity

Forced Convective Heat Transfer in a Porous Plate Channel

Forced convective heat transfer in a plate channel filled with metallic spherical particles was investigated experimentally and numerically. The test section, 58 mm×80mm×5mm in size, was heated by a 0.4 mm thick plate electrical heater. The coolant water now rate ranged from 0.015 to 0.833 kg/s.The local wall temperature distribution was measured along with the inlet and outlet fluid temperatures and pressures. The results illustrate the heat transfer augmentation and increased pressure drop caused by the porous medium. The heat transfer coefficient was increased 5-12 times by the porous media although the hydraulic resistance was increased even more. The Nusselt number and the heat transfer coefficient increased with decreasing particIe diameter, while the pressure drop decreased as the particle diameter increased. It was found that, for the conditions studied (metallic packed bed),the effect of thermal dispersion did not need to be considered in the physical model, as opposed to a non-metallic packed bed, where thermal dispersion is important.

Experimental Studies on Condensation Heat Transferof the Moist Air outside the Horizontal Circular Pipe witha Porous Layer

This paper presents the structure design of four kinds of circular pipes with porous layer and the experimental resultsof condensation heat transfer of the moist air outSide the horizontal choular pipes. By comparison with theexperiments on bare Pipes, it is concluded that, the designed pipes not only have good condensation heattransfer Performance, but also have the ability to collect and remove condensed liquid under zero gravity. They canbe aPPlied to the thermal control system for future large spacecraft.

Effective Thermal Conductivity Analysis of Xonotlite-aerogel Composite Insulation Material

A 3-dimensional unit cell model is developed for analyzing effective thermal conductivity of xonotlite-aerogelcomposite insulation material based on its microstructure features. Effective thermal conductivity comparisonsbetween xonotlite-type calcium silicate and aerogel as well as xonotlite-aerogel composite insulation material arepresented. It is shown that the density of xonotlite-type calcium silicate is the key factor affecting the effectivethermal conductivity of xonotlite-aerogel composite insulation material, and the density of aerogel has little influence.The effective thermal conductivity can be lowered greatly by composite of the two materials at an elevatedtemperature.

Theoretical and experimental study of the thermal conductivity of nanoporous media

The nanoparticle thermal conductivity and nanoscale thermal contact resistance were investigated by molecular dynamics(MD) simulations to further understand nanoscale porous media thermal conductivity.Macroscale porous media thermal conductivity models were then revised for nanoporous media.The effective thermal conductivities of two packed beds with nanoscale nickel particles and a packed bed with microscale nickel particles were then measured using the Hot Disk.The measured results show that the nano/microscale porous media thermal conductivities were much less than the thermal conductivities of the solid particles.Comparison of the measured and calculated results shows that the revised combined parallel-series model and the revised Hsu-Cheng model can accurately predict the effective thermal conductivities of micro-and nanoparticle packed beds.

Non－Darcian and Anisotropic Anisotropic Effects on Natural Convection in Horizontal Porous Media Enclosure

Natural convection heat transfer in a horizontal enclosure filled with anisotropic porous media,being isothermally heated at bettom and cooled at top while the vertical walls being adiabatic, is numerically studied by applying the Brinkman model-a modified form of Darcy model giving consideration to the viscous effect. The results show that:(1)a larger permeability ratio (K*) causes a lower flow intensity in the enclosure and a smaller Nusselt number, all Nusselt numbers approach unity in the limit of K* → ∞;a larger thermal conductivity ratio (λ*) causes a stranger distortion of isotherms in the enclosure and a higher flow velocity near the walls, all the Nusselt numbers approach unity in the limit of λ*→0; the permeability and thermal conductivity ratios generally have opposing effects on the Nusselt number. (2) an increasing Darcy number decreases the flow intensity and heat transfer rates,which is more significant at a lower permeability ratio. In particular, with K*0.25, the Nusselt number for Da=10-3 would differ from that of Darcy flow up to an amount of 30%, an analysis neglecting the non-Darican effect will inevitably be of considerable error.

Investigation of Enhanced Boiling Heat Transfer from Porous Surfaces

Experimental investigations of boiling heat transfer from porous suffaces at atmospheric pressure were performed. The porous surfaces are plain tubes covered with metal screens, V-shaped groove tubes covered with screens, plain tubes sintered with screens, and V-shaped groove tubes sintered with screens.The experimental results show that siatering metal screens around spiral V-shaped groove tubes can greatly improve the boiling heat transfer. The boiling hysteresis was observed in the experiment. This paper discusses the mechanism of the boiling heat transfer horn those kinds of porous surfaces stated above.

The periodic unfolding method for the heat equation in perforated domains

We study the heat equation with non-periodic coefficients in periodically perforated domains with a homogeneous Neumann condition on the holes. Using the time-dependent unfolding method, we obtain some homogenization and corrector results which generalize those by Donato and Nabil(2001).