Conjugate conduction convection and radiation heat transfer through hollow autoclaved aerated concrete blocks

A computational fluid dynamics (CFD) model is developed to study thermal performance of hollow autoclaved aerated concrete (AAC) blocks in wall constructions of buildings under hot summer conditions. The goal is to determine size and distribution of cavities (within building blocks) that reduce heat flow through the walls and thereby lead to energy savings in air conditioning. The model couples conjugate, laminar natural convective flow of a viscous fluid (air) in the cavities with long-wave radiation between the cavity sides. Realistic boundary conditions were employed at the outdoor and indoor surfaces of the block. A state-of-the-art building energy simulation programme was used to determine the outdoor thermal environment that included solar radiation, equivalent temperature of the surroundings, and convective heat transfer coefficient. The CFD problem is put into dimensionless formulation and solved numerically by means of the control-volume approach. The study yielded comprehensive, detailed quantitative estimates of temperature, stream function and heat flux throughout the AAC block domain. The results show a complex dependence of heat flux through the blocks on cavity and block sizes. In general, introducing large cavities in AAC blocks, being a construction material of low thermal conductivity, leads to greater heat transfer than the corresponding solid blocks. Several small cavities in a block may lead to small reductions in heat flux, but the best configuration found is a large cavity with a fine divider mesh in which case heat flux reductions of 50% are achievable.     

多孔砖和空心砖墙体传热过程和冷负荷谐波性能的数值计算与分析

运用有限差分法计算出多种节能多孔砖、空心砖墙体在夏季标准设计日的温度、传热量、空调冷负荷,应用傅里叶级数分析了墙体内部温度、传热量、空调冷负荷的谐波构成,得出了墙体放热衰减度和相位延迟等参数.     

节能空心墙体传热引起的夏季空调负荷的数值计算

介绍了夏季空调室外气象参数和太阳辐射照度的计算及气象参数的数值离散,采用有限容积法,计算了夏季标准设计日多种空心墙体由传热引起的空调冷负荷及冷负荷温度,并与标准Ⅱ~Ⅳ型墙体的相应数值进行了比较。     

Transfer function coefficients for time varying coupled heat transfers in vertically heated hollow concrete bricks

This paper describes a two step numerical procedure to determinate empirical transfer function coefficients (TFCs) for vertically heated hollow concrete bricks. For such systems TFCs cannot be generated using the analytical techniques available in the literature such as the z-transfer function method or the space state representation method because of the nonlinear local character of the heat transfer by natural convection and radiation in the air cells of the hollow concrete bricks. The first step of the procedure consists in predicting coupled heat transfer by conduction, convection, and radiation in realistic time varying conditions using a detailed numerical simulation. In the second step, the results of the simulation (the time-varying heat fluxes at the hollow brick surfaces) are used to obtain empirical transfer function coefficients using an identification technique. Transfer function coefficients are generated for three different types of hollow concrete bricks mostly used in practice. It is shown that the empirical transfer function coefficients permit fast and accurate prediction of heat transfer for thermal excitations that differ markedly from those used to generate these coefficients without solving the complex system of equations governing the coupled heat transfer mechanisms.     

建筑围护结构传热得热量计算方法的比较

比较了围护结构传热得热量的两种计算方法:周期性反应系数法和传递函数法。与传递函数法相比,周期性反应系数法简化了计算,而且周期性反应系数能反映围护结构对周期性扰量的热响应,可作为评价围护结构热稳定性的指标。推导了传递函数系数与周期性反应系数间的关系式,可根据ASHRAE的数据库RP 626建立典型围护结构周期性反应系数的数据库,从而减少周期性反应系数法的计算量。     

建筑围护结构中热桥稳态传热计算研究

在实验室进行了稳态传热条件下热桥表面温度和热流强度的测试实验,将实验结果分别与二维稳态传热模拟软件PTDA的模拟计算结果及我国现行标准中规定的一维简化方法的计算结果进行了对比。结果表明,PTDA的模拟结果较精确,而现行标准中的简化方法的误差较大,应及时修订。     

上海地区建筑能耗计算用典型年气象数据的研究

以同济大学某教学楼为研究对象,对比了用EnergyPlus模拟计算得到的全年耗电量和实测耗电量,研究了用于建筑能耗动态计算的5套典型年气象数据的可靠性,得出了CTYW气象数据最接近现实情况的结论。对其中影响较大的参数如室外气温、湿度和太阳辐射量进行分析和比较,指出所用5套典型年气象数据间的差异来自观测值的缺损和统计处理模型的不完善,特别是日射模型存在一定缺陷。     

Diagnostic test cases for verifying surface heat transfer algorithms and boundary conditions in building energy simulation programs

Verification and validation are crucial in developing and implementing models. Although there are standards to test energy simulation software, this article describes an additional set of eight test cases that are a combination of analytical cases and numerical cases for solid conduction heat transfer. These tests focus on diagnosing and verifying conductive heat transfer algorithms and boundary conditions in building envelopes or fabrics. As an example, EnergyPlus versions 5, 6 and 7 are tested using these eight test cases. The test cases were useful for detecting several bugs in the code. The authors recommend these test cases as useful complements to existing verification test cases for building envelopes.     

A second degree approximation for the calculation of the transfer function coefficients for heat conduction through walls

Calculation of the conduction transfer function coefficients using a state space representation requires the transient governing differential equation to be discretized in space by the use of finite difference or finite element methods, in order to obtain a set of first order differential equations. The use of FEM to discretize the media gives an additional advantage due to it is possible to use a higher order approximation of the dependent variable, which gives us a better accuracy with less elements. In this paper, the transient heat flow problem is tackled using a quadratic finite element. The variational formulation for the governing differential equation is developed, the Ritz approximation to construct the finite element formulation is used and the approximation functions are presented using a normalized local coordinate system for elements with three equally spaced nodes for the one-dimensional problem. The 2D transient problem is presented using a rectangular 8 node element. Results with 1, 2 and 3 three-node elements are compared with the ASHRAE conduction transfer functions for the 3, 5, 6, 8 and 32 wall groups and a 2D-example is given.     

区域建筑冷热负荷预测方法及其研究进展

区域建筑冷热负荷预测是建筑区域用能规划和区域供冷供热系统设计的基础和关键。对已有区域建筑冷热负荷预测方法进行了回顾,比较分析了各种预测方法的优缺点,根据其各自特点探讨了区域用能规划阶段和区域能源系统设计阶段负荷预测方法的适用性。     

Assessment of building cooling energy need through a quasi-steady state model: Simplified correlation for gain-loss mismatch

The objective of this work is to implement a simplified calculation procedure for building net energy need, based on a quasi-steady state model and on a monthly data set. In particular, it is intended to supply a formulation of the dynamic parameters and to adapt them to Italian climatic, typological, constructive and user data. The method was validated by determining the numerical correlations of the gain/loss utilization factor, through a comparison with a detailed building energy simulation software (EnergyPlus). The simulation was run on some test rooms defined by CEN (European Committee for Standardization) and on some real buildings that are representative of the Italian building stock, assuming weather data from different Italian locations (Torino, Roma, Palermo). The work shows that the accuracy of results is greatly affected by nonlinearities in the determination of the heat transfer and that the dynamic parameters are sensitive to some building features which are not taken into account in the CEN correlations.     

Simplified calculation of effective flange width for shear walls with flange

The shear lag effect of shear walls with flange is normally analyzed by introducing the concept of effective flange width. Therefore, it is crucial to accurately evaluate the effective flange width for tall buildings design. In this paper, based on the energy variational method, the normal stress distribution in the flange of shear walls is deduced, and the analytical solution of effective flange width in elastic stage is obtained according to the stress equivalent principle. For simplification, a parametric study on a serial T‐shaped cross‐section shear walls was conducted using finite element method in order to clarify the characteristic of effective flange width along with the loading history and the variations of shear lag effect with different parameters accordingly. Based on the numerical results, simplified formulas of the effective flange width in different loading stages are deduced for practical structure design. Moreover, the rationality of the values derived from different design codes is evaluated by comparing with simplified formulas. Copyright © 2015 John Wiley & Sons, Ltd.     

本科生科技创新能力培养的实践与思考 ——以建环专业传热学课程为例

传热学是建筑环境与能源应用工程专业的专业基础课之一。随着新工科建设的推进,传热学教学将承担创新人才培养的任务,课程目标与现行的本科生科创能力培养体系一致。南京工业大学建环专业传热学课程教学引入“五问反思报告”体系,鼓励学生将所学知识及时应用至本科生科技创新训练项目中,并在授课过程中多次引入挑战杯、“互联网+”创新创业大赛等案例讲解传热学基本原理和开展科创项目讨论。此类实践为建环专业传热学教学质量的提升和学生创新能力的培育提供了参考。