On the combined heat transfer of natural convection, radiation and conduction in the non-gray semitransparent thermal fibrous insulation

Non-gray semitransparent fibrous insulation hasbroad application in high-temperature industry, such asthe TPS ( thermal protection systems) of reusablelaunch vehicles. A metallic thermal protection systemgenerally consists of a metallic shell panel partit…     

堆积型含湿多孔介质导热系数测试实验研究

以砂子模拟堆积型多孔介质,采用Hot Disk热常数分析仪（TPS 2500）对不同含湿率砂的有效导热系数进行测量和实验研究。研究发现,非饱和含湿砂有效导热系数随含湿率的增加而增大。低含湿率（体积含湿率低于25％）情况下,砂子的有效导热系数难以准确测得,其主要原因是测试过程中,由于加热影响,贴近探头附近的水分因蒸发扩散而逐渐减少,探头周围试样构成不能保持恒定,导致测试环境持续发生变化。     

大孔隙对多孔介质导热性能影响的数值分析

采用有限体积方法求解Laplace方程,对非均匀多孔介质中大孔隙对导热性能的影响规律进行数值分析。研究结果表明,有效导热系数随着大孔隙的形状、排列方式、连通性和方向的不同呈现出很大的波动性,其中大孔隙的连通性和方向对多孔介质导热性能的影响尤其严重。研究结果可为非均匀多孔介质导热性能预测模型的改进提供参考依据。     

Finite element analysis for radiative heat transfer in multidimensional participating media

A finite element model is developed to simulate the radiative transfer in 2D and 3D complex-geome-tric enclosure filled with absorbing and scattering media. This model is based on the discrete ordinates method and finite element theory. The finite element formulations and detailed steps of numerical calculation are given. The discrepancy of the results produced by different space and solid angle discretization is also investigated and compared. The effect of the six-node quadric element on the accuracy is analyzed by a 2D rectangular enclosure. These results indicate that the present model can simulate radiative transfer in multidimensional complex-geometric enclosure with participating media effectively and accurately.     

考虑炉气散射特性的加热炉内辐射换热分析

考虑加热炉内燃烧产物的散射作用,借助辐射换热网络分析法,给出了吸收、发射和散射介质中炉膛系统黑度及辐射换热的基本公式,模拟计算了散射介质中系统黑度的变化.数值模拟结果表明,与忽略介质散射作用的简化描述相比,散射使得系统黑度值降低,炉膛总换热强度减弱.介质的散射作用越强,忽略介质散射导致的误差越大.     

基于多孔介质理论的冻土水热迁移耦合模型推导

在多孔介质理论的基础上,基于非线性达西定律并假设水分迁移过程为单向、可逆及水分迁移过程中无溶质迁移,推导得出了无相变以及考虑相变的水分迁移方程;引入土体传热方程和土骨架质量密度变化方程,过程中考虑了冻土中温度变化,扩散和对流,以及水分相变和温度、质量变化之间的相互影响。联立各方程得到了非饱和冻土水热分布控制方程,建立了非饱和冻土水热迁移耦合模型。文中亦对模型中具体参数的确定方法提出了建议。方程属于非线性偏微分方程,无法得出解析解,须采用数值解法。     

A novel method to determine effective thermal conductivity of porous materials

1 Introduction Due to some unique structure and characteristics, the porous material behaves as afunctional material and is finding more and more applications in a variety of fields suchas aviation, chemistry and energy[1]. In such cases, the effective thermal conductivity ofthe porous materials is a vital parameter that dominates application performances. Gen-erally speaking, the effective thermal conductivity can be measured experimentally, butin some cases (for example the stuffing is l…     

瞬态法测量木材热物性的理论与实验研究

为了有效测量不同含水率木材的热物性，提出了一种新的瞬态平面热源测量法。在一定的简化条件下求解一维有限厚度平板的非稳态导热微分方程，得到了平板木材试样的温度随时间和平板厚度变化的短时间公式，并由此建立了在恒定热流加热条件下木材热物性的瞬态测量实验台。利用此实验台测量了不同含水率的落叶松和红松试样的径向导热系数、热扩散系数和比定压热容等热物性参数并与文献数据进行了比较。结果表明，该瞬态测量方法具有较高的精度，比准稳态法更好地满足了一维导热条件，利用此方法能够有效而快速地对不同含水率的木材进行热物性测量。     

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.     

反向蒙特卡罗法模拟参与性介质中热辐射传递

为解决传统蒙特卡罗法结果收敛时间长的问题,当仅需要了解某个指定位置和方向的信息时,提出了反向蒙特卡罗法替代方法．该反向运算法则与传统的蒙特卡罗法类似,除了能束射线反向跟踪外．能束的传播是从探测位置到参与性介质中,而不是从像传统的蒙特卡罗法从源到探测位置处．利用反向蒙特卡罗法模拟计算一维和二维的热辐射传递问题,并且与相关文献进行比较,结果吻合较好．     

自由堆积多孔介质内超绝热燃烧的试验研究

为探索多孔介质内超绝热燃烧的特性,搭建了自由堆积多孔介质超绝热燃烧试验台架,测量了不同化学当量比（0.4～0.7）的甲烷/空气预混气体的超绝热燃烧特性.自由堆积多孔介质由直径为3和6 mm的Al2O3小球在陶瓷管（Φ38 mm×500 mm)中堆积而成,孔隙率为0.42.试验结果表明,在多孔介质中只有当燃烧波正向传播时才可能产生超绝热燃烧.在贫燃条件下超绝热燃烧的上限化学当量比为0.7,下限化学当量比为0.4;当化学当量比小于0.4或大于0.7时,在贫燃条件下的超绝热燃烧将不能实现.多孔介质中预混燃烧的火焰锋面速度约为7.82 μm/s,最大燃烧锋面温度超过绝热燃烧温度139 K.     

Frequency-dependent dynamic effective properties of porous materials

The frequency-dependent dynamic effective properties (phase velocity, attenuation and elastic modulus) of porous materials are studied numerically. The coherent plane longitudinal and shear wave equations, which are obtained by averaging on the multiple scattering fields, are used to evaluate the frequency-dependent dynamic effective properties of a porous material. It is found that the prediction of the dynamic effective properties includes the size effects of voids which are not included in most prediction of the traditional static effective properties. The prediction of the dynamic effective elastic modulus at a relatively low frequency range is compared with that of the traditional static effective elastic modulus, and the dynamic effective elastic modulus is found to be very close to the Hashin-Shtrikman upper bound.     

Determination of permeability using fractal method for porous media

A theoretical formulation was developed to express permeability as a function of different fractal dimensions and other scales for porous media . The effective fractal void ratio, the spectral dimension and the fractal dimension of particle mass distribution were introduced. The permeabilities for different soils in China are calculated. The predicted permeability for rice soil was compared with the measured data available in literature.     

多孔介质中有效孔扩散系数的试验测量

提出了通过实时测量扩散过程中多孔介质浮重变化,直接测定有效扩散系数的方法.首先建立了联系有效扩散系数和试件浮重变化的数学模型,并求出了该模型的解析解;利用非线性超定方程组优化求解的方法,结合问题的解析解和实测数据,反求出模型中的参数,有效扩散系数和边界层传质系数.用上述参数预测的试件浮重变化与实测浮重变化相差很小.表明该方法的实用性和可靠性.     

采用GMM方法分析聚集粒子的辐射特性

采用广义多粒子M ie理论(GMM)研究了由球形微粒组成的聚集粒子的衰减因子、散射因子、散射相函数等辐射特性参数,考虑单元粒子之间的相互作用,与M ie理论计算的辐射特性参数(不考虑单元粒子之间的相互作用)进行分析比较.结果表明:考虑相互作用与不考虑相互作用所得的聚集粒子辐射特性存在着差异,这种差异随着粒子间的距离、尺度参数的变化而改变,并且具有一定的规律.同时,分析了聚集粒子与等效(等体积或等面积)单粒子之间的辐射特性差异对辐射传输计算的影响,研究发现:考虑相互作用的聚集粒子组成的粒子系与等效球粒子系的温度场之间有明显的差异,相对偏差的最大值约20%,因此,在计算聚集粒子组成的粒子系的温度场时不能忽略粒子间的相互作用.     

金属Ti热学性质第一原理研究

应用基于密度泛函与密度泛函微扰理论的平面波赝势方法计算一组不同晶格常数下六角密堆积（hcp）结构金属Ti的声子谱及相应的静态总能，由此得到不同晶格常数下的自由能，由准谐近似及自由能极小判据得到自由能与温度的关系，进而计算热膨胀系数、定容摩尔热容及定压摩尔热容与温度的关系，对热膨胀系数及定容摩尔热容的第一原理计算值与德拜理论计算值进行比较。结果表明：295K下声子谱理论值与实验值除在[001]方向上的光学纵模有少量偏差外，其余部分符合得很好；hcp结构金属Ti有一定程度的各向异性热膨胀，沿c轴方向与口轴方向的热膨胀系数比值为1．5左右；热膨胀系数、定容摩尔热容及定压摩尔热容第一原理计算值在较宽的温度范围内与已有的实验数据相符；热膨胀系数的德拜理论值仅在室温以下温区与实验结果相符；定容摩尔热容的第一原理计算值与德拜理论值在中温区有少量偏差，在低温及高温区非常接近。     

Experimental and theoretical study on thermal and moisture characteristics of new-type bamboo structure wall

Thermal and moisture characteristics of the bamboo structure wall were tested in natural climate and three representative variation processes of heat and moisture: heating from solar radiation in summer at normal temperature and humidity, heating from solar radiation in summer at normal temperature and high humidity after rain, humidifying from brash in summer at high temperature and normal humidity. The results show that, in summer, the largest temperature difference between external and internal surface of the 28 mm-thick bamboo plywood wall is 11.73 °C (at 15:40) and the largest strain difference is 136 μm/m (at 18:50), both in ambient and indoor conditioned environment. In heating process, lengthways of the wall surface are in contracting strain while transverse ways are in expanding strain at initial stage and in contracting strain during later period. When the high temperature wall is humidified by rain, the surface temperature drops, moisture content increases and the expanding strain is presented on the surface during the whole process. Temperature and moisture content are two important factors which affect thermal and moisture stress (TMS) of the bamboo structure wall. The TMS is not only related to temperature and moisture content, but also greatly affected by temperature gradient, moisture content gradient and rates of changing.     

基于孔隙尺度的方腔内多孔介质融化模拟研究

由于单松弛(LBGK)格子Boltzmann模型在用反弹格式处理无滑移边界时存在缺陷。基于孔隙尺度,采用多松弛(MRT)格子Boltzmann模型研究封闭方腔内多孔介质的自然对流融化过程,其中,通过焓方法考虑相变潜热。分析了Rayleigh数和Prandtl数对融化的影响。结果表明:采用的多松弛模型能很好的预测导热和对流融化过程;多孔介质的导热融化界面不再与垂直壁面平行,自然对流融化界面呈现不规则形状;Rayleigh数和Prandtl数对多孔介质的融化有较大影响。     

采用DDA方法分析非球形粒子辐射特性

利用离散偶极子近似法(DDA)计算了三种几何形状、不同尺度参数、不同复折射率的非球形粒子随机取向时的衰减因子、反照率和散射相函数等辐射特性参数,并与等体积球形粒子进行了比较.分析结果表明:非球形粒子的辐射特性与等效球形粒子的辐射特性存在着差别,这种差别随着粒子的形状、尺度参数和复折射率等参数的变化而变化,并且具有一定的规律,同时存在一个临界尺度参数,此时非球形粒子与等效球形粒子的辐射特性差别可忽略不计.     

微藻生长期辐射特性测量方法

为提高微藻光生物反应器的效率以降低成本及进行规模化培养,提出一种在线测量微藻细胞全生长期辐射特性的方法.采用定制的平板式光生物反应器,给出一种微藻生长期辐射特性在线测量方案,分析传统测量方法应用于生长期微藻辐射特性测量的局限性,基于微藻细胞辐射特性的特征提出改进方案,并以普通小球藻为例进行生长期辐射特性的实验研究.结果表明,提出的辐射特性在线测量方案可以有效地应用于微藻时间相关辐射特性的测量,改进的光谱吸收系数及衰减系数测量方法可以更准确地应用于生长过程中细胞浓度变化时辐射特性的测量,普通小球藻的可见光波段光谱吸收截面和光谱衰减截面随时间变化显著.本研究为微藻生长期辐射特性的测量提供了一种方便可行的实验方案.