基于分形理论的建筑保温材料导热系数估计模型构建

建筑保温材料微尺度空间结构所呈现的不规律性,导致其导热系数估计精度下降。为解决该问题,以纳米颗粒多孔介质为例,构建基于分形理论的建筑保温材料导热系数估计模型。根据纳米颗粒多孔介质的微尺度空间结构及相关参数获取分形直径以及分形维数。以材料分形特征为基础,计算纳米颗粒多孔介质孔隙率与热阻,构建导热系数估计模型,估计其有效导热系数。实验结果表明,该模型的导热系数估计结果与实际测量结果具有高度一致性,最大误差仅为3.24%;热辐射等效导热系数对材料导热性能影响较小,热传导导热系数对材料导热性能影响较大。     

相变微胶囊/加气混凝土复合材料的热工性能

通过免蒸压法制备加气混凝土(AC),并在制备过程中加入以RT25石蜡为相变材料的相变微胶囊(MPCMs),得到相变微胶囊/加气混凝土复合材料(以下简称复合材料).在17、40℃下测试复合材料的导热系数和比定压热容,研究了这些参数与相变微胶囊掺量的关系,同时计算出复合材料的蓄热系数,据此评价了其蓄热性能.结果表明:随着相变微胶囊掺量的增加,复合材料的导热系数和比定压热容均呈现先增加后降低的趋势;当相变微胶囊掺量为1.0%时,复合材料具有最好的蓄热性能,较不掺相变微胶囊的对照组增强74%,蓄热系数可达3.35W/(m^2·K).     

聚乙二醇/二氧化硅/膨胀石墨相变储能材料的制备与性能研究

本文以聚乙二醇(PEG)为有机组分,膨胀石墨(EG)的多孔结构为材料载体,以二氧化硅(SiO_2)的网状结构完成封装,通过溶胶凝胶法和真空浸渗法相结合,制备具有良好导热能力的聚乙二醇/二氧化硅/膨胀石墨定形相变材料。通过DSC、TG、XDR和导热系数测试分析复合材料的结构和性能,测试结果表明:聚乙二醇最大质量分数为84%,膨胀石墨质量分数为6%,材料没有液相泄露,相变温度为59.9℃,潜热值为129J/g,导热系数为1.867W/(m·k),且热稳定性良好;另外,导热系数与复合体系中膨胀石墨的质量分数为指数关系,此材料为中温潜热蓄热提供了一种蓄热介质选择,在热泵供热、太阳能利用、余热回收等领域有广阔的应用前景。     

使用泡沫铜增强相变材料换热性能的实验研究

针对有机相变材料(PCM)导热系数较低的缺点,通过实验研究了添加通孔泡沫铜金属材料增强相变材料导热系数的方法。选择脂肪酸二元低共熔混合物相变材料作为蓄热介质,通过对其进行DSC测试分析,得到其相变温度和相变潜热。对壳管式潜热蓄热系统填充介质为纯PCM与PCM/泡沫铜复合相变材料两种工况下的熔化过程进行对比实验研究。实验数据表明,与纯PCM蓄热系统相比,添加泡沫铜的蓄热系统换热性能得到增强,整个蓄热器内PCM达到相变温度的时间仅为纯PCM系统的22.5%。     

相变微胶囊硅藻土蓄热板材制备与性能研究

以正十八烷相变微胶囊为蓄热材料,制备相变微胶囊/硅藻土复合材料,并通过压模法制备相变微胶囊/硅藻土蓄热板材。采用差示扫描量热仪、电子扫描显微镜、傅里叶红外光谱仪测试分析复合材料性质,测试蓄热板材的导热系数,对蓄热板材进行蓄放热性能实验,分析其蓄热性能。结果表明:相变微胶囊/硅藻土复合材料熔化、凝固温度分别为25.64、24.04℃,熔化潜热、凝固潜热分别为59.51、56.97 J/g;蓄热板材在温度变化中有明显滞后性,有助于调节室内温度平衡。     

含纳米颗粒石蜡物性参数对玻璃窗蓄热的影响研究

运用FLUENT模拟软件,建立了相变玻璃窗的动态传热数理模型,采用控制变量法,在不同相变温度、相变潜热、导热系数物性参数条件下,研究了夏冬两季双层玻璃相变窗内壁面热流随时间的变化情况,得到了相变材料物性参数对双层玻璃相变窗蓄热的影响。研究结果表明,相变材料的相变潜热和相变温度的升高在夏季可以明显增强隔热效果,改善玻璃窗的蓄热能力,在冬季影响较小;导热系数变化在冬夏两季均对传热性能影响较小。     

含水量和孔结构对多孔建筑陶瓷砖导热系数影响的试验研究

多孔建筑陶瓷砖是一种多相复合材料,其导热性受温湿度和孔隙率等多个因素的综合影响。通过试验研究了不同孔结构、孔隙率和含水量等因素变化条件下多孔建筑陶瓷砖导热系数的变化规律。结果表明,以淀粉和碳粉为造孔剂制备的建筑陶瓷均以开口气孔为主,整体连通性强。淀粉造孔的孔径尺寸相对比较均匀,显气孔率高于碳粉。随着造孔剂含量的增加,显气孔率和吸水率线性增大,孔径分布趋于分散。多孔建筑陶瓷砖导热性的各影响因素之间具有明显的耦合作用特征,即含水率越高,温度变化对其导热性的影响越显著;显气孔率越高,导热系数对含水量的变化越敏感。     

低熔点石蜡微胶囊保温砂浆的热性能

以低熔点石蜡微胶囊为相变材料,制备石蜡微胶囊保温砂浆.测试了保温砂浆的热焓、相变温度、导热系数和相变蓄热性能.结果表明:石蜡微胶囊保温砂浆具有良好的蓄热、调温功能和较长的热循环寿命,砂浆体系的相变温度为33℃,相变潜热13.42 J/g;随着偶联剂和粘结剂掺量的增加,保温砂浆的导热系数呈下降趋势;随着石蜡微胶囊掺量增加,保温砂浆的导热系数先减后增;与空白试件相比较,相变蓄热砂浆的升降温速率明显要滞后,呈现出较好的蓄热、调温性能.     

石蜡储能颗粒粒度分布与相变储能复合石膏板导热系数的关系研究

以石蜡作为储能介质,不同粒径分布的轻质多孔陶粒作为吸附基体,通过包封制备定形相变储能颗粒,并以石膏为基体材料,制备了相变储能复合石膏板.采用数码相机及图像处理软件和导热系数测试仪分别测试储能颗粒的粒度分布和复合石膏板的导热系数;运用灰色关联原理分析计算储能颗粒的粒度分布与复合石膏板导热系数的灰色关联度,分析了储能颗粒各粒度级配与相变储能材料的导热系数的关系.结果表明,当温度在石蜡相变温度附近时,粒级为2～3 mm的粒子对储能复合石膏板导热系数有显著的增大作用;当温度远高于相变温度时,粒级为4～5mm的粒子对储能复合石膏板导热系数有较强的减小作用.     

脂肪酸/无机纳米颗粒基定形相变材料的制备与热性能

以工业水玻璃为纳米SiO2前驱物,以癸酸(CA)和月桂酸(LA)二元低共熔酸为相变芯材,在表面活性剂的参与下,采用溶胶-凝胶法一步制备出纳米级复合定形相变蓄热材料.利用透射电子显微镜,扫描电子显微镜,傅里叶红外光谱仪,方差扫描量热法和热重分析等测试技术对此定形相变蓄热材料的结构和性能进行分析,并采用瞬态热线法测量了其导热系数.结果表明:相变芯材在吸热熔化后不会产生流动和渗漏;复合相变材料中脂肪酸含量(质量分数)为46％,具有良好的相变蓄热性能(相变温度19.57℃,相变潜热71.28 J/g)和热稳定性;复合相变材料导热系数为0.178 W/(m·K),可作为一种良好的隔热、保温建筑材料.     

Estimation of thermal conductivity of cemented sands using thermal network models

Effective thermal conductivity of soils can be enhanced to achieve higher efficiencies in the operation of shallow geothermal systems.Soil cementation is a ground improvement technique that can increase the interparticle contact area,leading to a high effective thermal conductivity.However,cementation may occur at different locations in the soil matrix,i.e.interparticle contacts,evenly or unevenly around particles,in the pore space or a combination of these.The topology of cementation at the particle scale and its influence on soil response have not been studied in detail to date.Additionally,soils are made of particles with different shapes,but the impact of particle shape on the cementation and the resulting change of effective thermal conductivity require further research.In this work,three kinds of sands with different particle shapes were selected and cementation was formed either evenly around the particles,or along the direction parallel or perpendicular to that of heat transfer.The effective thermal conductivity of each sample was computed using a thermal conductance network model.Results show that dry sand with more irregular particle shape and cemented along the heat transfer direction will lead to a more efficient thermal enhancement of the soil,i.e.a comparatively higher soil effective thermal conductivity.     

Experimental investigation on the performance enhancement of heat transfer characteristics of electronic chip cooling using metal matrix composites

ABSTRACT

Over the period of years, numerous investigations were made to enhance the heat transfer characteristics of metal matrix composites by various researchers. In these research, eight different groups of Aluminium and silicon carbide (Al & SiC) combinations were used. Each combination group was examined with three-level thermal conductivity materials used for this investigation. Using metal matrix composites, thermal conductive materials of different composition were experimentally investigated for the enhancement of performance of heat transfer characteristics of electronic chip cooling through thermal conduct resistance.     

阻燃抑烟型杨木单板/秸秆板复合材制备研究

以稻草秸秆和杨木单板为原材料,无机胶黏剂为黏结剂,制备了可用于地板的阻燃抑烟型单板/秸秆板复合材.设计了单板/单秸秆板芯层和单板/双秸秆板芯层2种复合材.通过对比无机杨木胶合板、秸秆板和单板/秸秆板复合材的燃烧性能、质量损失率和导热系数,研究单板/秸秆板复合材的阻燃抑烟性能,并对比了2种复合材的力学性能.结果表明:无机胶黏剂覆盖于板材表面能隔断氧气与热量传递,控制火焰从而发挥阻燃作用;单板和秸秆板复合后能保证板材的结构和强度,降低板材的火灾危险性;相对于单板/单秸秆板芯层复合材,单板/双秸秆板芯层复合材的导热系数降低了27.7%,静曲强度和弹性模量显著提高,且能够防止地板加工时产生的碎边现象.     

Enhancing sound absorption using periodic micro-perforated structure with porous layer

To get desired sound absorption,we proposed a novel periodic composite structure comprised of micro-perforated plates(MPPs),porous materials and air cavities.The composite structure is then solved using an equivalent circuit model,with equivalent fluid porous model and Maa's theory.Distributed four-pole elements are used to handle structures which are not compact compared to the sound wavelength.The model procedures are validated and confirmed as satisfactory by published results and finite-element results.Analysis conducted on a single layer shows that,compared with traditional MPP,the porous addition can increase sound absorption in the low-to-medium frequency range;however,the advantage of porous materials in the high-frequency range is lost.Meanwhile,by arranging the porous materials in parallel and controlling their filling ratios,the absorption curve of the composite structure can be tuned.As to periodic composite structures,it is found that the influence of layer number N is mainly in the low-to-medium frequency range.When N varies,the half-absorption bandwidth increases over 40%(≥380 Hz) compared with a single layer.Compared with multi-layered MPPs,N=2 and N=4 produce an increase of bandwidth by 50%(≥400 Hz) and 30%(≥300 Hz) respectively.As N increases,the sound absorption is better but the enhancement weakens as it tends to the limit of the composite structure.These results show the potential enhancements that can be made to the traditional MPP,which can benefit the research on wideband noise reduction in the low-to-medium frequency range.     

太阳能多孔集热墙内传热与流动特性分析

分析了太阳能多孔集热墙内传热与流动过程，在局部非热力学平衡的条件下，采用双方程多孔介质模型对其进行数值模拟。获得了空气流速，多孔材料的当量导热系数，多孔介质骨架与气体间的对流换热系数以及多孔层厚度变化时的温度分布，分析这些因素对太阳能多孔集热墙性能的影响。结果表明，提高多孔材料的当量导热系数，多孔介质骨架与气体间的对流换热系数和增大多孔层的厚度对提高集热器的性能是有利的。降低空气速度可以提高空气的温升幅度，但是总的吸热量将会降低。     

Experimental investigation on natural fibre composite for thermal insulation performance enhancement

ABSTRACT

This experimental investigations deals with the natural fibre composites such as abaca and kenaf with carbon for thermal insulation performance enhancement. The square matrix is used for this analysis. Volume fractions, ratio of thermal conductivity lumen were the most important considerations for this investigation with respect to the thermal conductivity of the corresponding composites. The experimental and theoretical results were compared for the performance identifications.     

Impact of pore structure and morphology on flow and transport characteristics in randomly repacked grains with different angularities

The flow and transport behaviors in porous media are closely linked to the structure and morphology of the pore space. A fundamental objective of most studies of porous media is to link the pore structure to the hydraulic functions, such as the permeability, capillary pressure and diffusivity, which are necessary for engineering applications. In this paper, an attempt is made to build a direct link between the hydraulic functions and the morphological measures of diverse porous media. Porous columns with different structures and morphologies are generated by randomly packing grains with different shapes and sizes. The pore structure of the repacked porous media is visualized through X-ray computed tomography and quantified by a series of parameters, including the set of Minkowski functionals, diverse characteristic pore sizes, geometric tortuosity and fractal dimension. The intrinsic permeability, molecular diffusivity and apparent thermal conductivity of the repacked porous media are simulated numerically. The Minkowski functionals have the capacity to characterize the microscale complex pore domain of the porous media in a macroscale way. A good linear relationship is shown among the effective pore size, nominal opening dimension and critical pore neck size obtained from the morphological analysis regardless of the shapes and sizes of the grains. The three different pores may serve as the characteristic pore correlation to the intrinsic permeability. The Kozeny-Carman equation can be used to mimic the intrinsic permeability and to serve as a quality-control tool for porous media with different grain angularities. A topologically based model can generally provide a single relationship for porous media randomly repacked with grains of different angularities. The molecular diffusivity of angular grains is found to be larger than that of round ones. The molecular diffusivity is linearly related to the porosity and fractal dimension. Porous media repacked with round grains tend to attain denser packing, a higher number of contacts per unit volume and higher thermal conductivity than media packed with angular particles. The apparent thermal conductivity has a negative linear correlation to the porosity and fractal dimension of porous media with different grain morphologies.     

Random checkerboard based homogenization for estimating effective thermal conductivity of fully saturated soils

This paper proposes homogenization scheme for estimating the effective thermal conductivity of fully saturated soils. This approach is based on the random checkerboard-like microstructure. Two modeling scales and two modeling approaches are distinguished and used, i.e. microscale and mesoscale and 1-step and 2-step homogenizations, respectively. The 2-step homogenization involves sequential averaging procedure, i.e. first, at microscale, a mineralogical composition of soil skeleton is considered and averaging process results in estimation of the skeleton effective thermal conductivity, and then, at mesoscale, a random spatial packing of solid skeleton and pores via random checkerboard microstructure is modeled and leads to evaluation of the soil overall thermal conductivity. The 1-step homogenization starts directly at the mesoscale and homogenization procedure yields evaluation of the overall soil thermal conductivity. At the mesoscale, the distinct nature of soil skeleton, as composed of soil separates, is considered and random variability of soil is modeled via enriched random checkerboard-like structure. Both approaches, i.e. 1-step and 2-step homogenizations, interrelate mineralogical composition with the soil texture characterized by the volume fractions of soil separates, i.e. sand, silt and clay. The probability density functions (PDFs) of thermal conductivity are assumed for each of the separates. The soil texture PDF of thermal conductivity is derived taking into consideration the aforementioned functions. Whenever the random checkerboard-like structure is used in averaging process, the Monte Carlo procedure is applied for estimation of homogenized thermal conductivity. Finally, the proposed methodology is tested against the laboratory data from our measurements as well as those available from literature.     

AlN/低粘度环氧树脂复合材料的性能研究

通过浇注工艺制备Al/低粘度环氧树脂复合材料,研究了不同AlN含量复合材料的电和热学性能。结果表明,复合材料的介电常数随AlN体积含量增加而升高,介电损耗下降。当AlN体积分数达到46%时,体系的体积和表面电阻率分别为2.67×1015Ω.m和1.04×1014Ω。复合材料的热导率随AlN含量增加而提高,而热膨胀系数下降。当AlN体积分数达到46%时,热导率达到1.14 W.m-1K-1。     

一种新型多孔砖导热系数算法的对比研究

对这一新型无骨料多孔砖的参数及性能作了简要分析,将多孔砖平均导热系数的两种计算方法进行了探讨并作了对比,结果表明:两种方法计算结果比较接近,反映了ANSYS模拟计算值的合理性,为此类多孔砖的导热性能计算提供指导。