固体燃料燃烧的分形模型研究

在固体燃料燃烧的分形模型中,把固体燃料（煤）的燃烧用一个整体的分形动力－扩散方程来描述。在燃烧过程中,内部孔洞体积与反应表面积存在分维指数关系,而且反应面积的增长是两种分形增长模式的叠加。同时考虑孔洞合并的因素,得到了描述煤燃烧反应速率的分形模型。燃烧反应速率先增加后减少的规律不仅在理论上得到解释,而且与实验结果十分相符。     

多孔介质导热的分形模型

多孔介质中热量传递与多孔介质内部的几何结构有密切的关系，讨论了多孔介质的分形结构和相关的分形维数，利用能量方程，导出了分形维数为D的有限尺度多孔介质中的广义热传导方程，在此基础上，假定热量在多孔介质中的传导路线也是一种分形结构，提出了一个筒化的多孔介质并联通道分形导热模型，求出了基于分形理论的多孔介质有效导热系数表达式。     

回转水泥窑采用硅酸铝纤维隔热保温试验与节能效益分析

回转水泥窑采用硅酸铝纤维隔热保温试验与节能效益分析上海医药设计院孙恕心，施瑾上海市节能技术服务中心郭梦骅１概述硅酸铝纤维作为一种高效隔热保温材料，除具有容重轻、比热小、导热系数低等优点外，在耐高温、抗热振性、化学稳定性及回弹性方面也很突出，正在逐步取...     

建立汽油机燃烧模型的新方法——分形几何

本文介绍了分形几何应用于汽油机燃烧模型的发展概况。国内外研究表明：汽油机燃烧火焰具有分形特性,因此用分形来描述火焰是恰当的。汽油机燃烧火焰的分形尺度大约在０．１３ ｍ ｍ 到几个ｍ ｍ 之间。利用分形建立的汽油机准维燃烧模型对发动机性能进行模拟可以很好地反映燃烧过程。     

建立汽油机燃烧模型的新方法——分形几何

本文介绍了分形几何应用于汽油机燃烧模型的发展概况。国内外研究表明：汽油机燃烧火焰具有分形特性,因此用分形来描述火焰是恰当的,汽油机燃烧火焰的分形尺度大约在０．１３ｍｍ到几个ｍｍ之间,利用分形建立的汽油机准燃烧模型对发动机性能进行模拟可以很好地反映燃烧过程。     

土壤样品有效导热系数的分形计算模型

选择上海地区具有代表性的两种土壤样品,根据其微结构的特点,在粒径分布、剖面颗粒分布自相似规律的基础上建立了土壤结构分形模型;结合并联传热,得到土壤样品有效导热系数的分形表达式.通过与土壤样品有效导热系数的实测数据相比较,确立了符合样品有效导热系数的分形表述的最佳面积度量尺度.     

基于三维DTRT传热模型的地源热泵土壤热导率测试方法研究

为进一步提升土壤热导率测试精度和效率,基于建立的三维分布式热响应测试（DTRT）理论模型,结合粒子群算法提出一种新型土壤热导率的DTRT测试方法,并基于该方法对土壤热导率的时空变化规律进行研究。首先针对U型埋管换热器建立DTRT三维传热分析模型,然后通过建立DTRT实验平台获得流体温度数据并验证建立的传热模型,同时利用建立的模型计算分析土壤热导率、管间距和加热功率对流体温度的影响,最后基于不同时间和深度的温度数据并采用粒子群算法对土壤热导率随时间变化和空间分布规律进行研究。研究结果表明,建立的DTRT理论模型具有较高精度,流体进出口温度的平均分析误差约为0.5℃;利用不同深度的温度数据预测出的土壤热导率稳定性较好,最大偏差仅为1.49%;不同时间土壤热导率的预测结果收敛性也较好,在5和20 h下的测试结果偏差仅为4.67%。此外,土壤热导率预测结果与参考值吻合度较高,说明该文提出的方法可在短时间内对不同深度的土壤热导率进行有效预测。     

分形在汽油机准维燃烧模型中的应用研究

针对汽油机的准维湍流燃烧模型进行了发展研究,将分形表示的湍流燃烧速度并入燃烧模型.研究表明：分形是一种模拟燃烧过程的有效手段,通过对分形中低端转捩尺度的选取分析,得出Gibson尺度比Kolmogorov尺度更适合作为低端转捩尺度.     

基于分形理论的柴油引燃天然气发动机燃烧模型

以柴油引燃天然气发动机为研究对象,针对柴油引燃油的扩散燃烧和天然气气体燃料的均质预混燃烧的特点,分别建立了引燃油多区燃烧模型和基于分形理论的预混天然气气体燃料燃烧模型.在试验验证所建模型的基础上,分析了引燃油量、喷油提前角和发动机转速对柴油引燃天然气发动机性能的影响.研究表明,适当增加引燃油量和减小喷油提前角可以降低柴油引燃天然气发动机的最大缸内压力升高率,从而有利于遏制柴油引燃天然气发动机高负荷时的爆震倾向.     

含甲烷水合物的石英砂渗透率实验和分形模型对比研究

含水合物的多孔介质渗透率是影响水合物开采的关键参数,多孔介质渗透率与水合物的饱和度密切相关。定量研究多孔介质渗透率随水合物饱和度的变化,对自然界中天然气水合物藏内渗流场的研究具有重要的理论价值。本文以平均粒径为139.612 μm的石英砂为多孔介质,采用稳态注水法测量在不同甲烷水合物饱和度（0 ~ 28.56%）下的石英砂渗透率,将实验数据与两种不同水合物赋存形式（颗粒包裹、孔隙填充）下的石英砂渗透率二维分形模型进行了对比。结果表明,石英砂渗透率比K_r随甲烷水合物饱和度S_h的增大呈现指数减小的趋势。当水合物饱和度低于11.83%时,渗透率比下降缓慢。而当水合物饱和度高于11.83%时,渗透率比下降迅速;当饱和度指数n = 12时,渗透率分形模型与实验数据吻合良好。通过分形模型与实验数据对比,发现当水合物饱和度低于11.83%时,甲烷水合物的赋存形式为颗粒包裹型。在11.83% ~ 28.56%水合物饱和度范围内,甲烷水合物的赋存形式为孔隙填充型。本研究成果量化了石英砂渗透率与甲烷水合物饱和度的关系,确定了含甲烷水合物的石英砂的渗透率分形模型的参数取值。     

Effective thermal conductivity of various filling materials for vacuum insulation panels

Three thermal transport mechanisms of various filling materials for Vacuum Insulation Panels (VIPs) are theoretically investigated with special emphasis on the solid conduction. As the first, the solid conductivities of porous materials such as powder, foam, fiber and staggered beam subject to external atmospheric compression are derived using simplified elementary cell models. The results show that the solid conductivities of the fiber and staggered beam insulation are lower than those of the powder and foam due to the relatively long thermal path. The second mechanism, i.e., gaseous conductivity shows the lowest for the fine powder among the considered materials due to its smallest pore size. The radiative conductivity as the last is calculated using the diffusion approximation. If radiation shields are installed for the staggered beam, the radiation effect can be lowered to a negligible order of magnitude. The predicted total effective conductivities suggest that the fiber and staggered beam structures are promisingly proper filling materials for VIPs.     

Lattice Boltzmann modeling of the effective thermal conductivity for fibrous materials

This paper presents a full set of numerical methods for predicting the effective thermal conductivity of natural fibrous materials accurately, which includes a random generation-growth method for generating micro morphology of natural fibrous materials based on existing statistical macroscopic geometrical characteristics and a highly efficient lattice Boltzmann algorithm for solving the energy transport equations through the fibrous material with the multiphase conjugate heat transfer effect considered. Using the present method, the effective thermal conductivity of random fibrous materials is analyzed for different parameters. The simulation results indicate that the fiber orientation angle limit will cause the material effective thermal conductivity to be anisotropic and a smaller orientation angle leads to a stronger anisotropy. The effective thermal conductivity of fibrous material increases with the fiber length and approach a stable value when the fiber tends to be infinite long. The effective thermal conductivity increases with the porosity of material at a super-linear rate and differs for different fiber location distribution functions.     

带有空穴子模型的石墨烯气凝胶复合相变材料导热系数的分形研究

在分形理论的基础上,确定了基于石墨烯气凝胶(graphene aerogel,GA)骨架复合相变材料的分形维数,并基于改进的Sierpinski地毯建立了带有空穴的导热系数预测模型。预测结果表明,对于基于GA骨架的复合相变材料,在所制得的材料孔隙率为0.7的条件下,无论空穴尺寸如何,均可将导热系数从相变材料本身的0.250 W/(m·K)提升至10.900 W/(m·K),增长幅度达40倍以上。结果显示,复合相变材料的导热系数随着孔隙率的减小而增加,且在低孔隙率下,导热系数随空穴尺寸的减小而增加。     

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-aerogel composite insulation material based on its microstructure features. Effective thermal conductivity comparisons between xonotlite-type calcium silicate and aerogel as well as xonotlite-aerogel composite insulation material are presented. It is shown that the density of xonotlite-type calcium silicate is the key factor affecting the effective thermal 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 elevated temperature.     

建筑绝热材料导热系数测试方法探讨

以GB/T 10294—2008为例,对使用防护热板法测试建筑绝热材料导热系数过程中易忽略及不明确的地方进行阐述,主要包括:试件的加工及干燥的处理方法 ;试件厚度的测量方式;环境温、湿度的控制范围;修正系数的标定及参数设置;测量不确定度等方面,并给出了指导性意见和建议。     

Fractal model for prediction of effective thermal conductivity of gas diffusion layer in proton exchange membrane fuel cell

The process of heat transfer within porous media is usually considered as a transport through large numbers of straight channels with uniform pore sizes. For the prediction of effective thermal conductivity of gas diffusion layer (GDL), morphological properties such as the tortuosity of channels and pore-size distribution of this porous layer should be considered. Thus in this article, novel parallel and series-parallel prediction models of effective thermal conductivity for the GDL in proton exchange membrane fuel cell (PEMFC) have been derived by fractal theoretical characterization of the real microstructure of GDL. The prediction of fractal parallel model for carbon paper, a basal material of the GDL, is in good agreement with the reference value supplied by Toray Inc. The prediction results from the proposed models are also reasonable because they are distributed between the upper and lower bounds. Parametric effect has been investigated by using the presented models in dimensionless formalism. It can be concluded that dimensionless effective thermal conductivity (k^′eff${k^{\prime }}_{\text{eff}}$) has a positive correlation with effective porosity (?) or the pore-area fractal dimension (D_p) when k_s/k_g < 1; whereas it has a negative correlation with ? or D_p when k_s/k_g > 1 and with tortuous fractal dimension (D_t) whether k_s/k_g < 1 or not. Furthermore, these fractal models have been modified by considering the effect of polytetrafluoroethylene (PTFE) incorporated into the pore spaces of carbon paper, and the corresponding model prediction shows that there is an increase in the effective thermal conductivity due to the filling of PTFE that has high thermal conductivity.     

Fractal structure reconstruction for alumina-silicate refractory fiber and simulation of the thermal conductivity

In this paper, it is proved that the intemal porous structure of alumina-silicate refractory fiber has fractal characteristics, which is reconstructed by the computer and the reconstructed structure further proved to have fractal characteristics. Based on the reconstructed structure, the network-thermal-resistance model is established to calculate the thermal conductivity of the fiber. It is shown that the calculated results agree well with the previous experimental ones, proving the correctness of the method.     

Simple model for thermal conductivity of nanofluids using resistance model approach

In this work thermal conductivity of the suspension is modeled using resistance model approach. This model considers Brownian motion and interfacial layer as well as a new mechanism where proposed by considering nanoparticles as liquid-like particles. The model can be used for estimation of upper and lower limits of nanofluid thermal conductivity, without any adjustable parameter. Thermal conductivity data of CuO nanofluids are obtained experimentally and the results show that the model is consistent to data. This model has been used for determination of various mechanisms' portions on thermal conductivity of nanofluids. The results are consistent with our knowledge about nanofluids' thermal conductivity mechanism.