保温材料热阻蜕变对墙体传热系数的影响

外保温结构中保温材料的有效热阻随温度、湿度和使用年限的增加而下降,造成墙体传热系数大于设计值。为此,本文以耦合传热传湿模型为基础,结合保温材料的有效导热系数模型,以分别采用发泡聚苯乙烯,挤塑聚苯乙烯和聚氨酯泡沫作为保温材料的外墙保温结构为例,数值预测了长期实际气候下外墙保温结构的传热系数。计算结果表明,采用上述三种保温材料的墙体传热系数在20年后因保温材料热阻蜕变有不同程度的增加,这意味着需要增加保温层设计厚度才能保证墙体热损失达到设计值。     

Numerical and experimental study of conjugate heat transfer in a horizontal air cavity

The demand for general reduction of the energy consumption in civil engineering leads to more frequent use of insulating materials with air gaps or cavities. Heat transfer through a constructional part can be decreased by adding an air gap and low emissivity reflective foils to the structure. In the first part of this paper, the impacts of cavity thickness and inner surface emissivity on combined conduction, convection and radiation heat transfer was experimentally explored in the case of constructional part with a horizontal cavity subjected to constant downward heat flux. The heat flow meter Netzsch HFM 436 Lambda was used for steady-state measurements. Results suggest that the studied parameters seriously affect the combined heat transfer in the composed structure. In the second part the paper reports the numerical study of two-dimensional conjugate heat transfer in closed horizontal cavity having air as the intervening medium. Numerical models validated by related experimental results were performed to further investigate the effect of radiation heat transfer. It was found that in general, the total heat flux through the composed structure decreases with increasing air cavity thickness, which is significant especially when low emissivity inner surfaces are taking into account. The direction of heat flow (downward or upward heat flow) has a significant impact on the convection heat transfer. An important contribution from the present work is the analysis of the optimal thickness of the cavity at different boundary conditions. The optimal thickness of the enclosure with low emissivity surfaces is 16 mm when subjected to upward heat flux.     

Experimental analysis of heat transfer through multiple reflecting bodies in a reactor-based cooling vault

In a nuclear reactor power plant, massive amount of heat generated in the reactor core causes the Reactor vault (RV) temperature to rise, which should be maintained at a permissible temperature range of 65°C–80°C. In order to address this issue we propose to incorporate thermal insulation, which consists of thin stainless steel (SS) sheets of 0.1?mm-thick stacked with uniform gap between them, introduced between the core and RV. The SS sheets (emissivity?=?0.05) are of highly polished, reflective type (mirror finish conforming to No. 8 as per ASTM A480/480M), which are made in the form of panels of suitable size and shape covering the entire outer contour. These types of insulations work on the principle of thermal radiation shielding Heat radiations from the reactor core falling on these reflective plates are reflected back thereby restricting the heat flux into the RV concrete. In order to estimate the effectiveness, 10 such plates are arranged over a length of 150?mm inside a casing made of an insulating material. A plate heater attached at the one end produces the desired heat transfer to analyse the thermal behaviour of the reflective plates. Thermocouples are attached to each plate to understand the temperature distribution in the system. Solidworks simulation and numerical calculations will be carried out.     

Multi-layer heat insulation system for frame construction buildings

One of the most important research areas today is in energy-efficient technologies such as heat insulation in buildings. In this research, insulation panels with multilayer, low-emissivity aluminum–polyethylene sheets were prepared and investigated. The results of the study showed that surface emissivity and convection currents have important influence on heat flow. The aluminum–polyethylene sheets were effective in reflecting heat and reducing heat transfer by radiation. They also divided the air space in the insulation system, resulting in the reduction of convection currents and convection heat transfer. The heat insulation system was built without micro heat bridges. Consequently, heat conduction was not increased by micro heat bridges resulting in lower effective thermal conductivity than the commonly used insulation materials. The connection between heat resistance and the number of sheets was not linear. The first inserted sheet had the highest effect and each additional sheet had less influence on heat resistance.     

关于地暖系统中主要材料选择问题的探析

重点针对低温热水地板辐射采暖系统的材料选择问题展开讨论,通过对地暖的采暖原理及热传导分析,主要探讨了隔热层,热水管,装饰地板等主要材料对系统的影响,以及这些材料的比较和选择.     

Comparison between PCM filled glass windows and absorbing gas filled windows

From the thermal point of view, windows represent the weak link between the internal and external ambients of a room. In cold climates, they are responsible for 10–25% of the heat lost from the heated ambient to the external atmosphere. In hot climates, the excessive solar radiation entering the internal ambient through the windows leads to increasing the cooling load of the refrigeration system. The use of absorbing gases filling the gap between glass sheets appears to be an alternative solution for thermally insulated glass windows. The other options one may incorporate filling materials such as silica aerogel or a PCM. In this work, a comparison between the thermal efficiency of two glass windows one filled with an absorbing gas and the other with a PCM and exposed to solar radiation in a hot climate is done. To model double glass window filled with infrared absorbing gases, a CW real gas model is used. A radiative convective conductive model and a radiative conductive model were investigated. Three mixtures of gases were used; a strongly absorbing gas mixture, an intermediate absorbing gas mixture and a transparent to infrared radiation mixture. To model the double glass window filled with a PCM, a relatively simple and effective radiation conduction one dimensional formulation is used. Heat transfer through the window is calculated and the total heat gain coefficients are compared and discussed.     

Combined heat transfer of radiation and conduction in stacked radiation shields for vacuum insulation panels

Combined heat transfer of radiation and conduction considering depthwise conduction phenomena in stacked radiation shields for vacuum insulation panels (VIPs) having an artificial core structure is investigated. The purpose of this study is to establish a reliable analysis method for actual shield design. A one-dimensional problem is first analyzed. Actual two-dimensional problem is simplified for theoretical modeling and the depthwise conduction around contact spot is further considered. Test radiation shields are manufactured using press molding process with SUS 304 (AISI grade 304) plates, since this sample depicts the parametric effects well. Measurement of the effective thermal conductivity is performed with a VGHP apparatus. While various analytical and numerical models predict the effective thermal conductivity in the case of this specific specimen within about 40% error, the combined heat transfer analysis considering depthewise conduction phenomena is accurate within 10% error and it is thus recommended in estimating the effective thermal conductivity of stacked radiation shields.     

Heat transfer in a cylinder sheathed by flame-resistant fabrics exposed to convective and radiant heat flux

A numerical model is developed which investigates heat transfer in a cylinder sheathed by flame-resistant fabrics when suddenly exposed to convective and radiant heat flux from simulated pre-flashover fire radiation. The column inside the cylinder system simulating the human body is assumed to keep at a constant temperature. This model incorporates characteristics of the heat-induced changes in flame-resistant fabrics and dry air thermo-physical properties. Temperature distribution was calculated with the help of a one-dimensional radial heat transfer model. A skin burn equation is quoted to predict second-degree skin burn injuries based on the numerical model. The effects of air gap thickness on mean incident heat flux to the skin simulant surface are also discussed. Results from the numerical model contribute to a better understanding of the heat transfer process within flame-resistant fibrous materials and fabrics in intensively high-temperature environment. At the same time, the method in the paper also helps to establish a systematic method for analyzing heat transfer in other cylindrical applications.     

Effective thermal conductivity of a staggered double layer of vacuum insulation panels

The use of vacuum insulation panels in buildings has gradually increased in the past few years and developments in their production have occurred in parallel. This has mainly lead to an optimization of different hygro-thermal properties of both the core material as well as the envelope. The issue of thermal bridges caused by the 300 nm thin metallic layers of the barrier envelope and by the joints between two adjacent panels remains. The present study investigates the influence of these thermal bridges on the effective thermal conductivity of a staggered double-layer of vacuum insulation panels. For this purpose a series of guarded hot plate measurements on single- and double-layers of vacuum insulation panels was carried out. The variety of the existing thermal bridge situations was accounted for by different panel size compositions. The results hereof were compared to those of a simplified numerical model. The target is to determine the effective thermal conductivity by a minimum number of required tests and the simplest numerical model. This will enable the determination of additional heat loss induced by the mentioned thermal bridges for a specific envelope laminate type without knowing the details of its composition especially the metallic layers.     

钢结构建筑复合墙板结构保温性能研究

文章介绍了钢结构建筑墙体对保温性的一些要求,论述了一些节能的墙体材料体系,根据其中一种体系设计出一种符合节能要求的复合墙板结构,在实验室通过热箱法实验测得该复合墙板结构的传热系数,并且与理论值比较,证明其保温性能满足节能要求,在夏热冬冷地区有良好的推广前景。     

CFD and experimental studies of room fire growth on wall lining materials

CFD simulation and experimental tests have been carried out to study the room corner fire growth on combustible wall-lining materials. In the CFD simulation, the turbulent mass and heat transfer, and combustion were considered. The discrete transfer (DT) method was employed to calculate the radiation with an absorptivity and emissivity model employed to predict the radiation property of combustion products including soot, CO₂ and H₂O, which are usually the primary radiating species in the combustion of hydrocarbon fuels. The temperature of the solid boundary was determined by numerical solution of the heat conduction equation. A simple and practical pyrolysis model was developed to describe the response of the solid fuel. This pyrolysis model was first tested against the Cone Calorimeter data for both charring and non-charring materials under different irradiance levels and then coupled to CFD calculations. Both full and one-third scale room corner fire growths on particle board were modelled with CFD. The calculation was tested with various numbers of rays and grid sizes, showing that the present choice gives practically grid- and ray number-independent predictions. The heat release rate, wall surface temperature, char depth, gas temperature and radiation flux are compared with experimental measurements. The results are reasonable and the comparison between prediction and experiment is fairly good and promising.     

添加隔热板对竖直U型地埋管换热器换热能力的影响

本文就如何减轻竖直U型地埋管换热器2个支管间的热短路损失、提高地源热泵系统的单位井深换热量进行了研究。在对竖直U型地埋管钻孔内准三维传热模型进行研究的基础上,运用数值模拟的方法对竖直U型地埋管换热器有、无隔热板2种情况下的换热能力进行了分析。     

茌平体育馆弦支穹顶叠合拱结构的温度场研究

太阳辐射作用下,茌平体育馆弦支穹顶叠合拱结构的温度场分布极为复杂和不均匀.为精确确定其温度场分布,为结构分析提供数据支持,本文基于APDL语言对ANSYS软件进行了二次开发,采用ASHRAE晴空模型编制了温度场数值模拟程序,对夏季太阳辐射、对流换热和热传导等作用下茌平体育馆弦支穹顶叠合拱结构的温度场进行了数值模拟,得到了整个结构的温度场分布规律,并由数值模拟结果分析得到室外钢拱构件的平均温度要比空气温度高很多、构件沿截面的温度梯度很高等重要结论,为随后的结构设计和分析提供了有力支持.     

离心玻璃棉热湿传递特性实验研究

离心玻璃棉具有良好的保温隔热性能,被广泛用于建筑保温技术。为了研究夏热冬冷地区用于墙体内保温的离心玻璃棉热湿传递特性,设计了一套实验装置,通过实验测试了特定温湿度条件下离心玻璃棉的水蒸气渗透性能,研究了温度对离心玻璃棉湿传递过程的影响,并验证了一维稳态传热传质模型对离心玻璃棉保温材料的适用性。实验结果表明,离心玻璃棉内部所传递的湿流密度随着水蒸气分压力差的增加而增大;其水蒸气渗透系数值受水蒸气分压力差的影响较小;而且在特定的温湿度范围内,温度对离心玻璃棉的湿传递过程几乎没有影响。     

内嵌秸秆板复合墙体热工性能测试及性能优化

将秸秆板装配在由肋梁和肋柱构成框格形式的墙体中,可作为一种新型装配式节能生态复合墙体应用于村镇住宅中。通过标定热箱实验方法直接进行墙体传热系数的测试,再基于有限元软件ABAQUS对该墙体进行一维稳态传热数值模拟。对比发现,数值模拟求得的传热系数与实测值相对误差仅为8.8%。最后再针对传热系数对复合墙体的保温隔热性能进行优化分析。     

A numerical simulation study for the human passive thermal system

The objective of this study is to create a dynamic model representing a transient three-dimensional passive thermal model of the human body. The model is a multi-segmental, multi-layered representation of the human body with spatial subdivisions which simulates the heat transfer phenomena within the body and at its surface. In order to represent the mechanisms of heat transfer within the body, energy balance equations including conduction with adjacent tissue, heat storage, metabolic heat generation, and convective heat transfer due to the blood flow in the capillaries are taken into consideration for each tissue. The present model of the passive system accounts for the geometric and anatomic characteristics of the human body and considers the thermo-physical and the basal physiological properties of tissue materials. It is assumed that the body is exposed to combination of the convection, evaporation and radiation which are taken into account as boundary conditions when solving the passive thermal system equation. The model is capable of predicting human body temperature in any given environmental conditions. Finite difference solution scheme is used to find out the temperature distribution of human body. The results are compared with the experimental data of previous studies present in the literature. Consequently, the numerical results of present model show good agreement with the experimental data.     

Thermal Response of Composite Materials to Elevated Temperatures

Different decomposition models of varying complexity were developed to predict the heat and mass transfer through charring/reinforced materials that are undergoing decomposition. Models included a heat conduction based model, decomposition model neglecting internal pyrolysis gas convection, and decomposition model with internal convection. Experimental methods were developed to measure the decomposition kinetic parameters and thermal properties required for input into the different models. Model results compared well with experimental data. Agreement between the heat conduction model was further improved by modifying the heat of decomposition to account for the internal convection effects.     

Effects of coefficients of solar reflectivity and infrared emissivity on the temperature and heat flux of horizontal flat roofs of artificially conditioned nonresidential buildings

Nonresidential buildings such as shopping centers, supermarkets and factories are constructions characterized as having large roof surfaces in comparison to their external wall surfaces. When conditioned artificially, these buildings become great consumers of electricity. To reduce this consumption, insulation materials are usually used. Notwithstanding the effectiveness of such a procedure, both the cost and environmental issues (the heat-island effect) have been considered as limiting factors for its usage. In this study, we analyze the effect of the application of selective coatings (with very high reflectance for solar radiation and high emittance for infrared radiation) on the surface temperature of the roof and the heat flux that crosses it. Two roof concepts were considered, namely metal roofs both with and without conventional mass-insulation, i.e. an insulating board on the internal side or between two metal foils. The methodology adopted to solve the proposed problem was based on the heat transfer analysis of a roof composed of multiple layers. External air temperature and solar radiation are considered time-varying. The results obtained show the benefits of selective coatings in terms of reductions in both roof temperature and heat flux and, consequently, alleviation of the urban heat-island effect and reduction of electricity consumption.     

Impact of sun patch and three-dimensional heat transfer descriptions on the accuracy of a building’s thermal behavior prediction

A thermal transient numerical model (M_3D) which considers the three-dimensional heat transfer through the envelope of a room and the sun patch through a window has been developed and validated in a recent paper. The use of a refined spatial and temporal discretization allows considering more precise interactions between the sun patch projection with the structure and quick time perturbations in the stresses. This is particularly necessary for highly insulated and low energy consumption buildings. In this new paper, M_3D is subsequently transformed to simpler configurations, close to classical modelling thermal building simulation software that neglects the sun patch and the 3D heat transfer, in order to quantify the main contributions of this model. A first configuration is to consider one-dimensional heat conduction for the envelope and the transmitted solar radiation is only projected onto the floor (M_1D). A second configuration considers also one-dimensional heat conduction but the transmitted beam radiation falls on each wall or floor that is impacted (M_1D,sp). Comparison between experimental data and numerical results of these three models shows, as expected, that M_1D and M_1D,sp are less accurate than M_3D. This is particularly true when wanting to evaluate surface temperature distributions or heating power evolution in winter.     

玻璃幕墙传热系数计算方法及工程应用

在研究玻璃幕墙热传递特点的基础上,基于一维稳态热传导理论,以中空玻璃为例建立了玻璃系统传热系数计算模型;基于二维稳态热传导理论和有限单元法,采用三节点三角形单元对二维温度场进行了离散,推导了单元热传导矩阵和温度载荷列阵,并推导了热对流、热流密度、辐射以及各种边界条件耦合作用下对单元热传导矩阵和温度载荷列阵的修正公式,建立了玻璃幕墙框及附加线传热系数计算模型。利用Visual C++和ObjectARX对AutoCAD进行了二次开发,研发了玻璃幕墙传热系数计算软件TJCW,并通过算例与LBNL系列软件计算结果进行比较,验证了所编软件的正确性和有效性。最后对某工程实例中玻璃幕墙传热系数进行了节能验算。研究结果表明：建立的传热系数计算模型能够正确的计算玻璃幕墙传热系数,基于该计算模型开发出的软件能够应用于实际工程的节能分析和计算中。