多孔建筑材料内部湿分布及湿传递对导热系数影响研究

发布时间：2018-04-05 18:57

本文选题：建筑材料　切入点：导热系数　出处：《西安建筑科技大学》2017年博士论文

【摘要】：建筑材料导热系数是建筑冷热负荷、能耗计算的重要热工参数之一。实际使用的多孔建筑材料大多有一定的含湿率,材料含湿后传热性能将发生改变。但目前的建筑冷热负荷计算方法及软件中,往往近似认为材料物性恒定,忽略其中湿分变化对材料导热过程的影响,造成传热量计算误差。因此,研究多孔建筑材料湿分与内部传热的定量变化关系,修正导热系数,对于建筑节能分析和暖通空调负荷计算均有重要意义。建筑材料内部湿分含量、分布特征、传递特性是影响材料传热的重要因素,其不仅与材料孔隙结构特征有关,还受热湿传递环境影响。我国各地气候差异大,且材料结构各异。揭示材料中湿分含量、湿传递对传热过程的影响程度,定量计算后,方可准确掌握材料的传热性能。而从湿分的存在状态分析,建筑材料中湿分包括静态湿分布和湿迁移,两者对材料内部传热性能的影响机理并不相同。针对以上问题,以揭示湿分与材料导热系数定量关系,提供含湿多孔建筑材料导热系数的修正计算方法为目标。利用理论分析、实验测试与数值计算的方法,研究多孔材料内部结构特征,探索湿分在其中的静态和迁移分布,揭示湿分布过程对材料内部传热作用,获得导热系数随湿分的因变特征。针对静态湿分对多孔材料导热过程的影响作用,当前固、气共存导热模型仅考虑材料固体骨架和湿空气的传热作用,而忽视了材料内部吸附湿分和凝结态湿分的问题;依据材料内部的湿分状态变化形成的液、气空间替换,提出了固、液、气体共存导热物理模型。根据多孔建筑材料孔隙结构曲折连通特点,利用分形理论建立了基于毛细管结构的三相共存多孔材料导热系数计算模型。利用该计算模型,分析获得孔隙率、孔径分布、含湿量、分形结构等静态湿分布参数对材料导热系数的定量影响关系。研究发现:随孔隙率的增加,导热系数变化率先减小后增加,孔隙率小于0.1时,变化率尤为明显;且饱和度越低,此变化特性越明显。最小与最大孔径比降低一个数量级,导热系数降低约10%。随迂曲度、迂曲度分形维数和面积分形维数的增加,导热系数逐渐减小,尤其是面积分形维数高于1.8时,此变化趋势更为明显。针对湿分传递对导热过程的影响作用,通过热湿耦合机理分析,以水蒸气分压力梯度和温度梯度为驱动势,建立了无湿相变的热湿耦合传递控制方程。对当前材料内部湿凝结采用的宏观湿分扩散理论,无法反映湿分凝结机理的问题,在无湿相变条件的热湿耦合传递方程的基础上,引入相变湿源和热源两项,结合经典蒸发冷凝理论,建立了有湿相变的热湿耦合传递控制方程。根据多孔材料内部湿相变与温湿度场的相互影响作用,提出了附加导热量循环迭代计算方法,获得了湿迁移和湿相变引起的附加导热系数。基于上述有无相变热湿耦合传递数学模型,通过大量分析计算发现,材料导热系数低于0.2W/(m·K)时,湿迁移引起的材料导热系数附加修正率变化较为敏感,而附加修正率随水蒸气渗透系数变化较为稳定。随两侧传热温差减小,湿迁移引起的附加导热系数修正幅度增大。在传热温差为10℃,当两侧水蒸气分压力从1000Pa增至4000Pa时,普通混凝土、粘土砖和加气混凝土导热系数可分别增大约2%、10%和35%。针对居住建筑,在采暖期、空调期工况下,分析了外保温围护结构各层材料中静态湿分布和湿迁移对导热系数的定量影响。研究发现在上述两种工况下,静态湿分布存在导致普通混凝土主体材料导热系数约增大8%~10%。湿迁移对材料导热系数的影响,严寒及寒冷地区采暖工况下可忽略;在夏热冬冷及夏热冬暖地区空调工况下,普通混凝土附加修正率约为-10%~15%。综上,普通混凝土材料导热系数在采暖期综合修正约为8%~9%,空调期综合修正约为-5%~25%。
[Abstract]:The thermal conductivity of building materials is one of the important building cooling load, energy consumption calculation. The thermal parameters of porous building materials used mostly have a certain moisture content, material moisture heat transfer performance will change. But the building load current calculation methods and software, to approximate the material constant, which is ignored the moisture change influence on heat conduction process, resulting in heat transfer calculation error. Therefore, study on the relationship between the wet porous building materials and internal heat transfer, correction coefficient of thermal conductivity, the calculation has important significance for building energy analysis and HVAC load. Building materials internal moisture content, distribution, transfer characteristics is an important influence the material factors of heat transfer, which is not only related to the pore structure characteristics of materials, the heat and moisture transfer environmental impact. Our climate is different, and different material structure. Jie In the material moisture content, moisture transfer effect on the heat transfer process, quantitative calculation, we can accurately grasp the heat transfer performance of materials. From the existing state of moisture in building materials, including static wet moisture distribution and moisture migration mechanism, their influence on internal heat transfer properties of materials are not the same for. The above problems, in order to reveal the relationship between moisture and thermal conductivity of materials to provide quantitative calculating method of porous building materials thermal conductivity as the goal. By theoretical analysis, experimental test and numerical calculation method, study on the characteristics of structure in porous materials, explore the moisture in static migration and distribution of the wet process to reveal the distribution of the internal heat transfer material, to obtain thermal conductivity and moisture dependent characteristics. Based on the static moisture effect of porous materials in the heat conduction current solid, gas heat conduction model considering material coexistence The material of solid skeleton and wet air heat transfer effect, while ignoring the internal moisture adsorption and moisture condensation state; according to the material internal moisture state changes into liquid, gas space replacement, proposed solid, liquid, gas coexistence thermal physical model. According to the material characteristics of pore structure of porous building is tortuous. Established the calculation model of three-phase capillary structure of the coexistence of thermal conductivity of porous materials based on fractal theory. Using the model, analysis of porosity, pore size distribution, moisture content, quantitative relationship fractal structure of static moisture distribution parameters on the thermal conductivity of the material. The study found that: with the increase of the porosity, the thermal conductivity first decreases and then change increase the porosity is less than 0.1, the rate of change is obvious; and the saturation is low, the variation is more obvious. The minimum and maximum aperture ratio decreased one order of magnitude, the coefficient of thermal conductivity reduction About 10%. lower with the increase of tortuosity, tortuosity fractal dimension and area fractal dimension, the thermal conductivity decreased gradually, especially in the area of fractal dimension is higher than 1.8, this trend is more obvious. The moisture transfer effect on the heat conduction process, the heat and moisture coupling mechanism analysis, the water vapor pressure gradient and the temperature gradient as the driving potential, a wet wet phase coupling heat transfer control equation. The materials used inside wet condensation macro moisture diffusion theory, can not reflect the moisture condensation mechanism, based on the wet transition condition of coupled heat and moisture transfer equation on the phase transformation of wet and heat source two, combined with the classical theory of evaporation and condensation, a wet wet phase coupling heat transfer control equation. According to the interaction within a porous material wet phase change and temperature and humidity field, put forward additional heat iteration The calculation method, get the additional conductivity of moisture migration and moisture caused by the phase transition. The non phase coupled heat and moisture transfer mathematical model based on the analysis, through a large number of calculations show that the thermal conductivity is lower than 0.2W/ (M - K), the coefficient of thermal conductivity and moisture transfer caused by the additional correction rate of change is more sensitive, and additional correction the rate of the water vapor permeability coefficient change is relatively stable. With both sides of heat transfer temperature decreases, additional thermal conductivity and moisture migration caused by the correction increases. In the heat transfer temperature is 10 DEG C, when the two sides of the water vapor pressure increased from 1000Pa to 4000Pa, the ordinary concrete, the thermal conductivity of clay brick and aerated concrete can be increased by about 2% and 10% respectively. 35%. for residential buildings in the heating period, air conditioning period conditions, analysis of external thermal insulation structure of each layer in the static moisture distribution and moisture migration quantitative effect on thermal conductivity. Now the study found Two under the condition of static moisture distribution due to the thermal conductivity of ordinary concrete material increases about 8%~10%. the main influence of moisture migration on the thermal conductivity of the material, the cold and cold regions under heating condition can be neglected; in the condition of hot summer and cold winter and hot summer and warm winter area, ordinary concrete additive correction rate is about -10%~15%. in material ordinary concrete thermal conductivity on the synthetic correction during the heating period is about 8%~9%, the comprehensive correction air conditioning period is about -5%~25%.

【学位授予单位】：西安建筑科技大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TU50

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