A simple time domain calculation method for transient heat transfer models

This paper presents a simple time domain calculation method to derive thermal response factors and conduction transfer conduction (CTF) coefficients of finite differential models for estimating transient heat transfer through building structures. It is developed on the basis of converting the matrix exponential function, which is a part of the solution of the state equation established from the finite differential equations of building finite differential models, to matrix polynomial. The thermal response factors and CTF coefficients can be easily derived from the matrix polynomial with simple arithmetic and integral in time domain. This method avoids the time-consuming root-finding process of conventional methods and the computation of all the internal temperature of the finite differential model, while utilizing the advantage of the thermal response factors/CTF coefficients which relate the desired outputs at a moment to the previous inputs through a set of coefficients. Various case studies were conducted to validate the performance of this time domain calculation method in calculating the thermal response factors and CTF coefficients of various order finite differential models.     

A criterion for the assessment of the reliability of ASHRAE conduction transfer function coefficients

This paper presents a new mathematical approach, which, when applied to conduction transfer functions (CTFs) of a multi-layered wall, is able to predict the reliability of building simulations. This new procedure can be used to identify the best set of CTF coefficients, which are a critical point at the core of the ASHRAE calculation methodology founded on the transfer function method. To evaluate the performance of different CTF coefficient sets, the authors performed a large number of thermal simulations on the multi-layered walls included in the ASHRAE Handbook, volume fundamentals, and on other walls typical of Mediterranean building heritage. Those data were employed to test an algorithm able to assess the reliability of the simulations. The numerical results show that it is possible to select the optimal number of coefficients on the basis of the size of the poles of a CTF. The proposed criterion, which employs a pole threshold value, is highly accurate, fast and easy to integrate in the most diffuse building simulation tools.     

Fitting conduction transfer function method to low Fourier numbers: application to ground-coupled floors

This paper proposes a practical solution to the problems arising when the conduction transfer function (CTF) method is applied to solve the 1D transient component of heat transfer in the internal zone of ground-coupled floors. The lower boundary condition for these problems must be imposed at a depth of several metres. The proposed method is based on the division of the ground domain into a number of layers that allows calculation of transfer function coefficients while keeping the first cross coefficient negligible. A mathematical expression is proposed for the proper layer thickness that assures the applicability of the CTF method to thick ground domains. A numerical validation for a slab-on-grade configuration has been carried out showing maximum errors of 0.09°C (0.42%) and 0.73?W/m² (2.24%) for temperature and heat flow rates on the floor surface, respectively.     

地源热泵地埋管换热器传热研究(2):传热过程的完全数学描述

建立了管内流体换热和土壤换热的耦合模型,并在此基础上建立了与地面热泵模型耦合的地源热泵系统仿真模型.给出了边界条件和计算参数的确定方法,计算参数包括:流体与管内壁面的表面传热系数、地面的表面传热系数、大气温度、不同埋深土壤温度、多孔介质土壤有效导热系数.     

水冷式表冷器传热研究（4）：表冷器总传热系数的一个新实验公式

认为目前使用的表冷器总体传热系数实验公式中未考虑管壁及助片的导热,故利用接触因数和表冷器外表面换热系数的关系,提出了一个新的实验公式,对该公司进行了实验验证,拟合精度较高。     

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。     

数值模拟技术在多孔砖墙传热性能研究中应用

本文总结了多孔砖的数值设计优化步骤。通过例子介绍了多孔砖墙传热数值模拟过程,以及砖孔可视表面辐射的模拟和墙体表面温度、热流密度的计算方法,并给出了有关结果。     

空气间层对聚苯复合外保温墙热湿耦合传递的影响

聚苯(EPS)复合外保温墙中的空气间层对其热湿耦合传递有着重要的影响.针对不同空气间层厚度的EPS复合外保温多孔砖墙,在以上海为代表的夏热冬冷地区的气候条件下,利用已有的热湿耦合传递数学模型和计算软件CHAMPS-BES,进行热湿耦合传递数值模拟.结果表明:空气间层越厚,多孔砖层的初始水分向空气间层扩散、迁移越快;空气间层越厚,空气间层和EPS板的峰值含湿量越高,达到湿稳定所需时间越长.当空气间层厚度为20mm时,水分对EPS复合外保温多孔砖墙传热系数总的影响最显著,高达34%.tif板施工时,在保证其黏结强度的前提下,宜适当减小空气间层的厚度.     

空心砖瞬态传热数值研究

利用自然对流和热传导的物理耦合模型,控制方程采用有限容积法,求解算法使用SIMPLE算法,研究了空心砖在外界环境变化条件下的瞬态传热规律。通过计算得到空心砖内外壁温的变化情况及延迟特性,同时也研究了通过空心砖的热流变化规律,并与实心砖的传热过程比较,发现实心砖和空心砖的内壁面温度的延迟相差不大。而空心砖与实心砖热流量相差很大,实心砖的平均热流约比空心砖平均热流大41.5%,说明空心砖节能效果显著。     

A two-port envelope model for building heat transfer

A zone envelope thermal model is derived from first principles, based on the exact two-port solution to the heat conduction equation. A single set of matrices, one for each frequency of interest, is used to describe the thermal behaviour of the total envelope. The set of matrices must be computed once for a particular zone and cooling loads for various conditions, or the passive response, can then be obtained. The method is based on the same principles underlying the CTF method. However, the transfer function is calculated for the whole envelope by combining the two-port matrices of the individual walls, after simplification of the room interior heat transfer network. The conduction transfer coefficients of the individual walls forming the zone are not required. Except for the simplification of interior radiation, the method is completely exact and easy to implement. It is also numerically efficient. For load prediction, an implementation in the frequency domain, based on the prime factor FFT algorithm is convenient. For time domain simulation with active control, the envelope conduction coefficients are easily obtained by transforming to the Z-domain. This represents a modern solution of the envelope conduction problem, which is convenient to implement in modern object oriented computer programming languages.     

Verification for transient heat conduction calculation of multilayer building constructions

Validation and verification of building simulation programs and load calculation programs is of continuing interest. Dynamic thermal behavior data, including conduction transfer function (CTF) coefficients, thermal response factors and periodic response factors, are used to calculate transient heat conduction through building constructions. Computational inaccuracy sometimes occurs in calculating CTF coefficients and response factors. In this paper, a method for verification of the CTF coefficients and response factors over the whole frequency range is introduced. This method is based on the equivalence of dynamic models for a linear system and the frequency characteristics of building transient heat transfer models. Bode diagrams and error criteria are proposed to verify the CTF coefficients and response factors. Some examples are given to demonstrate the methodology.     

Study on transient heat transfer through multilayer thermal insulation: Numerical analysis and experimental investigation

This paper reports on a numerical and experimental study of heat transfer phenomena through two different multilayer fibrous insulations for building applications. The investigated samples were composed of different layers of fibrous materials and aluminium foils, placed between one or two air gaps in the vertical dimension. An experimental apparatus (a guarded hot box) has been used to measure heat transfer through the samples, while a finite volume numerical model combined radiation/conduction heat transfer was developed to predict the temperature distribution and heat transfer in such insulation systems comprised of the materials separated by multiple reflective foils. The model takes into account the coupling between the solid conduction of the fibrous system and the gaseous conduction and radiation. The radiation heat transfer through the insulation system has been modelled via the two flux approximation. The numerical results were compared with the experimental data from the guarded hot box for model validation, as well as to assess the effectiveness of the reflective foils in changing the resistance of the insulations. The comparative verification of the model showed that the numerical results were consistent with the experimental data through the environmental conditions under examination.     

Thermal analysis of a building brick containing phase change material

This paper presents the thermal analysis of a building brick containing phase change material (PCM) to be used in hot climates. The objective of using the PCM is to utilize its high latent heat of fusion to reduce the heat gain by absorbing the heat in the bricks through the melting process before it reaches the indoor space. The considered model consists of bricks with cylindrical holes filled with PCM. The problem is solved in a two-dimensional space using the finite element method. The thermal effectiveness of the proposed brick-PCM system is evaluated by comparing the heat flux at the indoor surface to a wall without the PCM during typical working hours. A paramedic study is conducted to assess the effect of different design parameters, such as the PCM's quantity, type, and location in the brick. The results indicate that the heat gain is significantly reduced when the PCM is incorporated into the brick, and increasing the quantity of the PCM has a positive effect. PCM cylinders located at the centerline of the bricks shows the best performance.     

湿传递对建筑墙体传热性能的影响

建筑物的耗能与建筑围护结构的传热传湿密切相关,了解建筑墙体内部的热湿传递对建筑节能有重要影响。以相对湿度和温度梯度为驱动势建立墙体一维非稳态热、湿和空气耦合传递模型(HAM模型),并利用有限元法进行了数值求解,重点关注了湿传递对传热的影响。数值结果表明:考虑传湿时墙体内部温度波动小,墙体进行热湿传递会产生湿积累,降低墙体使用年限;考虑传湿时通过墙体总传热量比不考虑传湿时多7.5%;考虑传湿时内壁面最大平均数比不考虑传湿时大0.78。     

Determining the thermal capacitance,conductivity and the convective heat transfer coefficient of a brick wall by annually monitored temperatures and total heat fluxes

The finite volume scheme and complex Fourier analysis methods are proposed to determine the thermal capacitance (defined as the product of density and specific capacity) and thermal conductivity for a building construction layer using the monitored inner/outer surface temperatures and heat fluxes. The overall heat transfer coefficient for the air gap, and the convective heat transfer coefficient for air gap surfaces and room surfaces are determined by the linear relationship between the surface convective heat flux and the temperature difference. Convective heat flux is obtained by removing the thermal radiation flux from the total surface heat flux. Finally, the predicted surface heat fluxes using the calculated thermal properties and ASHRAE values were compared with the measurements.     

Experimental unsteady characterization of heat transfer in a multi-layer wall including air layers—Application to vertically perforated bricks

Vertically perforated bricks were developed with a view to building environmentally friendly houses since they make insulating materials unnecessary. By an experimental approach, this study proposes to analyze the propagation of a temperature signal in this kind of brick, in order to characterize the thermal inertia of the brick. The steady-state knowledge is completed by the determination of properties like influence functions or characteristic depth concerned by a surface temperature variation. This allows to validate a simple unsteady surface model to be validated for this heterogeneous material which is classified as an insulating structural material. Furthermore, this study supplements the steady-state knowledge in local heat transfer through the air layers of the brick. Indeed, it is verified that convection heat transfer can be ignored in an unsteady heat transfer in this kind of brick, even in extreme conditions such as a sudden temperature fall.     

Thermal response of brick wall filled with phase change materials (PCM) under fluctuating outdoor temperatures

Based on the enthalpy-porosity technique, a model of thermal conduction accompanied with solidification and melting processes is developed and numerically analyzed to investigate the thermal response of the brick wall filled with phase change materials (PCM). The thermal response, which is represented by indoor wall surface temperature response, of brick wall filled with PCM is evaluated and compared with that of solid brick wall. The effects of PCM filling and its filling amount on thermal response of brick wall are investigated and discussed. It is indicated that, compared to the common solid brick wall, the thermal storage of brick wall filled with PCM is elevated by the alternate process of melting and solidification under fluctuating outdoor temperatures. The use of PCM in the brick walls is beneficial for the thermal insulation, temperature hysteresis and thermal comfort for occupancy. In addition, with the increasing filling amount of PCM, the fluctuation of indoor wall surface temperature is significantly smoothed. Correspondingly, the hysteresis in response to the outdoor temperature fluctuation is enhanced. Moreover, the present model is verified by experimental data available in the literature.     

两种常见墙体的热湿迁移特性分析

大多数建筑墙体均为多孔介质材料,多孔介质墙体中湿迁移与热迁移是互相耦合的过程,湿迁移对热迁移有着重要的作用,例如会使热导率明显增加,同时,热过程对湿过程也有着影响作用。基于Fourier定律、Fick定律、Darcy定律,以温度和空气含湿量为驱动势建立了多层多孔介质墙体热湿耦合迁移数学模型,对两种常见墙体（红砖墙体和加气混凝土墙体）进行计算分析。结果表明,加气混凝土墙体比红砖墙体具有更好的保温隔热性能,但更容易发生湿积累和引发湿破坏。     

Investigation on heat transfer modeling assumptions for radiant panels with serpentine layout

This article aims at determining an optimal modeling method for heat transfer calculation of low thermal mass hydronic radiant cooling or heating panels with serpentine tube layout. It introduces a new semi-analytical procedure based on a well-known analytical modeling method to relax the thermal symmetry condition between tubes that is traditionally used. For the standard reference case presented, the temperature asymmetry was important, reaching 29% of the half inter-tube distance. However, the heat transfer performances were not significantly affected compared to results achieved by assuming thermal symmetry. It is also shown that this result implies that no significant performance gap between parallel layout and serpentine layout panels is expected. The effect of the 1D heat conduction assumption and of straight tube endings in the modeling were also quantified using a 2D finite volume code. None of these assumptions caused significant errors on heat transfer calculations. The same results apply to a large range of panel geometries and operating parameters.     

Detailed numerical simulation of coupled heat transfer by conduction,natural convection and radiation through double honeycomb walls

The aim of the present work is to study numerically 2-D steady state coupled heat transfer by conduction, free convection and infra-red radiation through two honeycomb walls separated by a vertical air layer. Airflow in both holes and separating air layer is laminar. The limiting vertical sides of the double honeycomb wall are assumed to be isothermal but at different temperatures while the upper and lower horizontal surfaces of the structure are insulated. The FVM method and the SIMPLE algorithm are used to solve numerically the equations of conservation of mass, momentum and energy in both air filled cavities and solid partitions. It is found that the global heat flux across the entire wall varies almost linearly with the difference between the outside and the inside temperatures. Based on this linear thermal behaviour, appropriate overall heat exchange coefficients are derived. These coefficients can be used easily in practice to predict the global heat transfer across the studied honeycomb walls without solving the detailed and complex equations that govern the different heat transfer mechanisms. Effect of the thermal conductivity of the construction material on the overall heat transfer through double honeycomb walls is studied.