Effects of Wall's thermophysical properties on time lag and decrement factor

In this study, the effects of thermophysical properties and thickness of a wall of a building on time lag and decrement factor have been investigated. For this purpose, one dimensional transient heat conduction equation was solved using Crank-Nicolson scheme under convection boundary conditions. To the outer surface of the wall, periodic boundary conditions were applied. A very general code which can take care of composite walls under any kind of boundary condition was developed. Single and combined effects of the thickness and thermophysical properties on the time lag and decrement factor were investigated. It was found that thermophysical properties have a very profound effect on the time lag and decrement factor. The computations were repeated for different building materials and the results are discussed.     

Periodic solution of transient heat flow through multilayer walls and flat roofs by complex finite Fourier transform technique

An analytical solution method for the estimation of space heat gain through multilayer walls and flat roofs submitted to periodic boundary conditions is developed. Formulation of the transient heat transfer problem consisting of differential equation and the initial and boundary conditions is converted into dimensionless form and it is solved by the application of complex finite Fourier transform (CFFT) technique. This analytical solution is used to determine hourly variation of heat flow into the space through the walls and the temperature at the inner surface of the walls. A computer program is developed based on the analytical model to perform numerical calculations considering six different wall and two different roof constructions, characteristics to the buildings in Turkey. The CFFT technique provides itself as an alternative to other analytical and numerical methods and it appears to be useful in that it allows the calculations to be performed for various multilayer wall and roof constructions and for various climatological locations with varying ambient air temperatures and solar heat inputs.     

复合保温墙体传热研究

针对复合保温墙体传热问题 ,在分析其组成及传热特点的基础上 ,建立数学模型 ,并采用有限差分隐式格式求解。求解中结合实测数据进行数值计算 ,得到复合保温墙体内各点温度在外部温度扰动情况下随时间的波动值。计算结果与实测结果比较表明 :使用数值方法研究复合保温墙体的传热问题具有效率高、计算精度高的特点 ,能方便地用于解决优化建筑结构组成及分析热负荷或冷负荷方面的实际计算中     

Correlating thermal transmittance limits of walls and roofs to orientation and solar absorption

The presented research addresses the association of maximum thermal transmittance (U value) of walls and roofs with orientation and solar absorption. The study is performed on walls and a roof typically used in Kuwait when subjected to local hot climate conditions. A computer program employing the total equivalent temperature difference (TETD) method is developed to estimate the U values corresponding to solar absorption coefficients ranging from 0 to 1. At zero solar absorption coefficient, the maximum U values stipulated in the existing national energy code are used to induce the constant total daily heat flux for the developed correlations. Furthermore, the thermal inertia parameters affected by the established U values are also predicted. The results show that the orientation and absorption coefficient have a profound effect on the U value particularly for the roof and west and east wall orientations. Moreover, the decrease in the thermal transmittance significantly enhances the thermal inertia parameters of the aerated autoclaved concrete wall type.The developed correlations can be added to the Kuwait building energy code to extend the application scope of the U value limits based on surface orientation and external solar properties.     

Numerical computation of time lags and decrement factors for different building materials

In this study, time lags and decrement factors for different building materials have been investigated numerically. For this purpose, one dimensional transient heat conduction equation was solved using the Crank–Nicolson scheme under convection boundary conditions. To the outer surface of the wall, periodic boundary conditions were applied. Twenty-six different building materials were selected for analysis. The computations were repeated for eight different thickness of each material and the effects of thickness and the type of material on time lag and decrement factor were investigated. It was found that thickness of material and the type of the material have a very profound effect on the time lag and decrement factor. The results of present study are useful for designing more effective passive solar buildings and other related areas.     

Passive cooling in hot,arid regions in developing countries by employing domed roofs and reducing the temperature of internal surfaces

Natural ventilation due to wind effects through buildings employing domed roofs was estimated by a flow network analysis. The dome was assumed to have an opening at its crown. When compared with flat roofs, the domed roofs always increase the air flow rate through the building. The increase in natural ventilation becomes significant in buildings with doors and windows all in one wall, or whenever the wind effects on the building envelope do not produce large pressure differences at the openings.The large air flow rate in the buildings with domed roofs may be utilized to store night air coolness in the structure more effectively and keep the mean radiant temperature of the interior surfaces low for thermal comfort in summer. The lowest internal surface temperatures can be obtained when the surfaces are kept moist and evaporatively cooled.Through a one-dimensional energy analysis the inside surface temperature of a horizontal slab was estimated for various slab materials and thicknesses and external and internal conditions. The inside surface temperature was compared with the case of employing a roof pond. It was found that lower temperatures can be obtained by evaporatively-cooled moist internal surfaces than that which can be obtained by unshaded roof ponds: For a building whose internal surfaces (walls and ceiling) are kept moist a large ventilation rate is needed to prevent water vapor build-up in the space. A domed roof with a hole in its crown can produce the necessary ventilation for such a building.     

建筑墙体的延时与削弱作用

本文采用数值方法分析了建筑墙体的材料与厚度对墙体延时与削弱作用的影响。基于这个目的,考虑了一维墙体带第三类周期性边界条件的非稳态导热。通过分析墙体材料的导热系数七、热容C和墙体厚度对墙体延时与削弱作用的影响,发现热容C和导热系数k对墙体延时与削弱作用影响明显。墙体达到一定厚度则具有比较好的延时与削弱效果。     

住宅节能计算分析

节能计算是节能设计的必要环节,节能计算分析有助于合理地进行节能设计。通过分析体型系数、墙身耗热量指标、屋面耗热量指标等,提出了体型系数设计取值以及较为合理的墙身、屋面保温层厚度。比较分析了住宅建筑节能计算中各部分所占耗热量指标,提出了外墙、门窗、楼梯间、地面等的节能设计要求和应采取的措施。     

住宅节能计算分析

节能计算是节能设计的必要环节,节能计算分析有助于合理地进行节能设计。通过分析体型系数、墙身耗热量指标、屋面耗热量指标等,提出了体型系数设计取值以及较为合理的墙身、屋面保温层厚度。比较分析了住宅建筑节能计算中各部分所占耗热量指标,提出了外墙、门窗、楼梯间、地面等的节能设计要求和应采取的措施。     

Thermal inertia of newspaper sandwiched aerated lightweight concrete wall panels: Experimental study

A short term experimental investigation on the thermal inertia: time lag and decrement factor of aerated lightweight concrete (ALC) wall panels is the main purpose of this study. ALC wall panels of different density and different aerial intensity of newspaper sandwiched were produced and erected on a prototype house. All wall panels were installed facing east direction and subjected to natural environment or tested under transient condition. The inner and outer surface temperatures of the wall panels were recorded at 3 min interval, 24 h a day. Time lag and decrement factor were computed from the surface temperatures recorded. The results showed that newspaper sandwiched ALC panel has greater time lag and lower decrement factor compared to ordinary ALC panel. It is found that time lag and decrement factor were influenced by the thermal diffusivity value of the wall panel. The thermal inertia property of the panels was negatively related to its thermal diffusivity.     

建筑索结构的类型及其应用

近年来索结构在建筑结构中得到日益广泛应用,建造技术迅速发展,将其归纳为:悬索结构、管内预应力结构、张弦结构、拉索结构、斜拉结构、索拱结构、吊挂结构7种类型,结合我国最新的工程实践逐一讨论各种类型建筑索结构的组成与受力特点,介绍其在建筑钢结构与幕墙(采光顶)中的应用及发展.     

Harmonic analysis of building thermal response applied to the optimal location of insulation within the walls

The analysis of heat transfer through building walls using Fourier transforms and the matric method are briefly reviewed. The formalism is applied to a simple one-room building. By making a few simplifying assumptions and by considering only one- or two-layer walls and roofs, the equations are kept sufficiently short to preserve the insight of the reader into the effects of a few construction features upon the building's thermal response. Such construction features, mainly the placement of insulation inside or outside the main wall mass, are extensively discussed, with an eye on their potential energy savings.The results are: (1) the placement of insulation outside the wall masonry reduces the amplitude of the internal temperature swing caused by weather conditions and by internal heat gains. If the inside temperature is left free to oscillate within a few degrees, the amplitude of the heating or cooling load is greatly reduced, allowing for substantial energy savings. However, the building is thermally sluggish and inefficient durign thermostat setbacks because of its large wall heat storage. (2) Inside placement of insulation increases the room temperature response to weather conditions and to internal heat gains. Thus, heating or cooling is needed for temperature peak-shaving. In return the building's response to a thermostat setting change is quick and the heat stored in the walls, lost during a setback, is relatively small.     

建筑墙体的动态响应特性研究

建立了墙体在外界交变温度作用下的动态响应的理论模型并进行了数值求解,研究了建筑墙体的动态响应特性。该模型中考虑太阳辐射和室内通风量的影响。本文以南京市的天气数据为例,给出了建筑墙体的室内温度响应以及墙体材料对动态响应的影响。研究结果表明,相对室外温度变化,室内温度响应存在时间滞后和振幅衰减。夏季炎热,可采用通风和敷设反光材料的方法降低室内温度。     

Five-year data of measured weather,energy consumption,and time-dependent temperature variations within different exterior wall structures

This paper presents coherent 5-year measured data that have been gathered for analyses of building energy consumption and thermal performance of exterior walls. The data is also very suitable for calculations and simulations of heating and cooling energy need of buildings. The data was collected from six identical test buildings, having exterior walls that are constructed of different building materials. The data include the following: indoor–outdoors temperatures; temperatures at various depths within the northern, southern, eastern, and western exterior wall facades; indoor–outdoors relative humidity, heating energy, wind speed and direction; air tightness, infiltration, and horizontal global solar radiation. A computer system (data logger) was used to monitor, check, calculate, integrate, and save the data acquired from approximately 520 sensors in each test building. Measurements were taken with a time interval of 20 s. The 20 s values were then integrated over a time interval of 30 min and the minimum, maximum, and mean values were subsequently stored to a computer database. Analyses of the results indicated that temperatures within the buildings’ exterior walls are constantly changing and, that occasionally the flow of conduction heat is reversed (i.e. outside–inside) due to solar radiation. For accurate results of temperature distribution and the actual heat losses through building envelopes, none steady-state calculations are essential. Depending on the intensity of solar radiation and the material characteristics of the walls, temperature gradient at the inner surfaces of exterior walls may become milder compared to that of the outer surfaces.     

Thermal performance investigation of a PCM-enhanced wall/roof in northern Morocco

Energy efficiency improvement in building sector has become a real challenge in Morocco, especially in the northern region evolving a rapid urban growth. In this context, using the phase change materials (PCM) in the construction is presented as one of the promising solutions to enhance the thermal behaviour of building envelope. This work aims to investigate the thermal performance of a PCM integrated external wall and roof under the summer climate conditions of northern Morocco. Dynamic thermal characterization methodology is adopted through the calculation of decrement factor (DF) and time lag (TL) parameters. For that, a one-dimensional numerical model based on resistance–capacitance (RC) approach is developed and implicitly solved to simulate the heat transfer process through a wall/roof structure. The model is validated through a new small-scale experimental device. Different qualities of PCM, regarding its peak phase change temperature, have been evaluated. Moreover, the PCM layer emplacement within the wall/roof structure was evaluated considering two possible configurations. The obtained results show a significative enhancement of the thermal performance of different wall facades and roof through the use of the suitable PCM quality. An appropriate selection of PCM layer emplacement yields an evidenced enhancement in the case of the roof. As for the walls, the integration of PCM from the interior side increase the TL period but has a negative impact on the DF parameter.     

Shear lag analysis by the adaptive finite element method: 1. Analysis of simple plated structures

Due to the presence of sharp stress gradients, traditional finite element analysis using uniform meshes for the solution of shear lag problems for thin-walled structures is inefficient and will result in inaccurate values of effective breadth ratio. By using the adaptive finite element analysis, it is possible to obtain results with predetermined accuracy with a minimum amount of computational cost. An adaptive finite element analysis procedure is used to deal with the problem of shear lag effects of plated structures with arbitrary dimensions and geometry. This study consists of two parts. In Part 1 of the study, shear lag effects in simple plated structures, such as straight rectangular, single-cell box girders are studied. In Part 2 of the study, the adaptive refinement procedure will be extended to the shear lag analysis of complex plated structures such as core walls with openings, multi-cell box girders and box girders with curve flanges.     

New external convective heat transfer coefficient correlations for isolated low-rise buildings

Building energy analyses are very sensitive to external convective heat transfer coefficients so that some researchers have conducted sensitivity calculations and proved that depending on the choice of those coefficients, energy demands estimation values can vary from 20% to 40%. In this context, computational fluid dynamics calculations have been performed to predict convective heat transfer coefficients at the external surfaces of a simple shape low-rise building. Effects of wind velocity and orientation have been analyzed considering four surface-to-air temperature differences. Results show that the convective heat transfer coefficient value strongly depends on the wind velocity, that the wind direction has a notable effect for vertical walls and for roofs and that the surface-to-air temperature difference has a negligible effect for wind velocity higher than 2 m/s. External convective heat transfer coefficient correlations are provided as a function of the wind free stream velocity and wind-to-surface angle.     

Measurement of thermal environment in Kyoto city and its prediction by CFD simulation

In the summer of 2002, measurements were simultaneously performed to investigate the characteristics of heat flow in urban areas at three locations in Kyoto city: (1) a commercial urban area mixed with low-rise traditional residential buildings that represents the urban area of Kyoto; (2) a university campus area with lots of green zones; and (3) a plaza covered with a concrete slab which was used as a reference point of measurement. Heat flux of boundary layer over the three locations and the surface temperatures of building walls and streets were measured to investigate the urban thermal environment. For the analysis, a new simulation code was developed by combining unsteady state heat conduction of building walls and grounds, radiation heat exchange between them, and airflow by computational fluid dynamics (CFD). By using this code, the thermal environment of the urban areas such as air temperature, humidity, wind velocity, and boundary layer heat flux was predicted and compared with the measured results. It was found that this model could predict the real thermal environment of the urban area. Using this code, the effect of additional green on roofs and grounds can be investigated in order to mitigate urban heat island and to improve urban thermal environment at the street level.     

The computation of heat flows through multi-layer slabs

To model the transient thermal behaviour of buildings it is necessary to calculate time-dependent heat flows through multi-layer slabs representing walls, floors and roofs. A numerical method is presented which calculates these from a knowledge of past temperatures on each side of the slab at specified time intervals. Response factors are found using Laplace transforms. The number of response factors is minimized using previously calculated heat fluxes. An estimate of the error introduced by taking only a finite number of response factors is given and a correction term derived. The method is compared with the more standard z-transform method.     

Thermal storage and nonlinear heat-transfer characteristics of PCM wallboard

For the materials with constant thermophysical properties, the thermal performance of wallboards (or floor, ceiling) can be described by decrement factor f and time lag φ. However, the phase change material (PCM) may charge large heat during the melting process and discharge large heat during the freezing process, which takes place at some certain temperature or a narrow temperature range. The behavior deviates a lot from the material with constant thermal physical properties. Therefore, it is not reasonable to analyze the thermal performance of PCM wallboard by using the decrement factor f and time lag φ. How to simply and effectively analyze the thermal performance of a PCM wallboard is an important problem. In order to analyze and evaluate the energy-efficient effects of the PCM wallboard and floor, two new parameters, i.e., modifying factor of the inner surface heat flux ‘α’ and ratio of the thermal storage ‘b’, are put forward. They can describe the thermal performance of PCM external and internal walls, respectively. The analysis and simulation methods are both applied to investigate the effects of different PCM thermophysical properties (heat of fusion H_m, melting temperature T_m and thermal conductivity k) on the thermal performance of PCM wallboard for the residential buildings. The results show that the PCM external wall can save more energy by increasing H_m, decreasing k and selecting proper T_m (α < 1); that the PCM internal wall can save more energy by increasing H_m and selecting appropriate T_m, k. The most energy-efficient approach of applying PCM in a solar house is to apply it in its internal wall.