室内自然对流数值模拟分析

研究建筑围护结构传热与流体流动综合作用下室内自然对流数值模拟,建立了一套同时在固体-流体区域整体求解连续性方程、动量方程和能量方程的数值模拟方法。具体分析了瑞利数变化范围为104到106时建筑围护结构传热对室内自然对流的影响。数值预测结果表明:该方法能够真实反映室内自然对流问题。为室内自然对流问题数值模拟找到了一种实用有效的方法。     

居住建筑能耗预测分析方法的研究

本文通过正交实验设计、DeST建筑能耗软件模拟及多元线性回归方法,研究了影响居住建筑能耗的主要建筑设计因素,得到了重庆市居住建筑冷、热负荷及全年负荷的预测方程,并进行了误差分析。通过预测方程和模拟软件分别计算所得结果的对比分析,验证了该方程的可靠性,为重庆市居住建筑能耗预测分析提供了一种简便适用的方法。利用可靠的预测方程,可以快速而简单地评价分析不同设计方法和围护结构方式对建筑能耗的影响。     

沈阳周边农村地区住宅建筑节能改造实测分析

为了解决沈阳周边农村地区住宅建筑能耗较大的问题,研究了传统非节能建筑的特征,以及新建节能建筑的节能措施。选取该地区节能住宅和非节能住宅各一例,对建筑围护结构的热工性能和建筑环境参数进行检测分析,确定出采暖供回水温度、室内环境温度、相对湿度的数值,通过现场调研和仪器检测确定出PMV-PPD室内环境评价指标。结果表明,节能建筑与非节能建筑相比,采用围护结构节能等措施的建筑的节能率提升了17.7%,同时,室内环境舒适度也有大幅度提高。     

建筑室内环境数值模拟中的耦合方法

针对建筑室内环境模拟中的耦合特点,同时考虑建筑围护结构中的导热与室内空气的对流问题,详细分析了建筑室内空气流动及建筑围护结构传热过程的物理数学描述方法,对热流耦合特性的数值计算方法做了探讨;建立了宏观描述和微观描述的耦合模拟模型,同时对这两种耦合模型的优缺点做了定性讨论;最后利用CFD软件把建筑围护结构传热与室内流动作为整体进行了微观耦合的数值模拟。结果表明,微观耦合模型并不适合室内环境的通风与热耦合问题的计算,有待于宏观耦合模型的研究及实用高效热耦合分析软件的进一步开发。     

严寒寒冷地区居住建筑围护结构内表面结露预测方法

随着生活水平的提高,人们对室内环境要求也越来越高。对于严寒寒冷地区,居住建筑围护结构内表面结露是影响室内环境的重要因素。本文分析居住建筑围护结构结露的原因,并结合寒冷地区某实际工程运用内表面温度法计算,运用Ptemt软件对该工程围护结构内表面温度进行模拟分析,从而有效预测建筑内表面温度,进而判断是否结露,可为建筑热工参数的设计提供技术指导。     

基于机器学习的外窗百叶外遮阳性能预测与敏感性研究

在建筑围护结构中越来越多地使用透明围护结构,会导致高能耗和光热环境不适等问题。为了解决这一问题,遮阳作为一种减少建筑的能源消耗和改善室内环境较为有效的手段被越来越多地使用。为探究百叶外遮阳参数对遮阳性能的影响程度,采用机器学习算法预测遮阳性能,利用基于机器学习的改进敏感性分析法探讨了影响遮阳性能的2类参数(建筑和遮阳)的局部和全局敏感性,确定影响最大的参数。研究结果表明：XGBoost预测热环境、能耗指标精度最高,而随机森林算法预测光环境指标效果最好。同时发现遮阳参数是影响室内热环境和建筑能耗的最重要因素,整体权重均在0.5以上;建筑参数显著影响室内采光,其权重高达0.9左右。     

侧向失热率对新型墙体热阻现场测试方法的影响分析

在常规测试建筑围护结构热阻的热流计法的基础上提出一种新型墙体热阻现场测试方法。依据现场测试试验以及数值模拟验证新型测试方法的可行性中对包括新型测试方法中加热面积大小的选取、所需加热面边长与墙体厚度的关系的基础上进一步探究墙体侧向失热率影响对测试偏差的影响并得到侧向失热率对测试的影响偏差与无量纲参数D/d(加热边长D与墙体厚度d的比值)的回归拟合方程。     

康定传统民居围护结构优化研究

围护结构作为住宅建筑的重要组成部分,其保温隔热性能对于建筑能耗有着重大影响。通过对康定藏族传统民居建筑的广泛调研和分析,对不同围护结构进行了改造,提出了12种围护结构优化方案,比较分析了不同方案的节能效果,并探究了围护结构的最优方案。结果表明:康定传统民居围护结构传热系数较小,建筑采暖耗热量较大;建筑采暖耗热量与传热系数呈线性关系且外墙对其影响较大;外墙、屋面聚苯板的厚度分别为80 mm和50 mm,外窗空气层厚度为8 mm时,建筑围护结构达到最优。     

住宅太阳墙的原理与使用

创造舒适的室内环境是建筑围护结构的基本目标之一。然而，在舒适与节能之间往往存在矛盾。在建筑围护结构设计中引入生态概念，将太阳能的利用与建筑围护结构的设计巧妙结合，太阳墙这种复合式多功能围护结构为绿色节能建筑提供了广阔的前景。     

基于能耗模拟的商场建筑围护结构热工性能优化研究

以某大型商场建筑为研究对象,利用清华大学自主研发的全年动态能耗模拟软件DeST-c,模拟研究该建筑在三种不同气候分区下(严寒气候区、寒冷气候区及夏热冬冷气候区)围护结构热工性能与建筑能耗的关系。模拟结果表明,保温层厚度改变对建筑累计热负荷影响较大,对建筑累计冷负荷影响较小。利用全寿命期费用评价方法,得出该商场建筑在哈尔滨、北京、上海的最佳保温层厚度区间分别为90mm至100mm、70mm至80mm、30mm至40mm。研究成果对制定公共建筑围护结构热工性能节能方案具有一定的借鉴性。     

伴随自然对流叠置圆管外蓄冰特性的试验研究

针对伴随自然对流效应的管道蓄冰问题，将立方体空间内水平叠置三圆管外的蓄冰过程作为研究对象，研究了恒壁温和水温变化引发的自然对流及密度反转条件下的圆管外蓄冰情况。调查了在高于或低于密度逆转温度点的不同初始温度下自然对流形成的情况，及其对水平叠置恒壁温三圆管外冰层形成过程的影响。研究了密度反转和等间距三管间相互作用下，围绕上，中，下三管外蓄冰过程蓄冰量的变化规律。     

高温围岩封闭空气夹层隔热的理论与数值分析

随着矿井开采深度的增加以及深埋隧道的发展,高地温地下空间热害问题日益突出,研究如何控制围岩散热在其热害治理中具有重要意义。结合空气夹层在建筑围护结构中保温隔热的作用,提出应用封闭空气夹层控制围岩散热量的隔热方法。利用FLUNET建模分析了巷道为不同截面形状时相应封闭空气夹层结构内的自然对流换热特性,拟合了不同形状巷道的空气夹层内平均自然对流强化系数的计算式K_ave=f(Ra,2δ₂/d₃)。得到以下结论:(1)不同形状巷道的自然对流强化情况不同,差异产生的原因是空气夹层顶部对流换热强度不同;(2)空气夹层隔热方法可有效降低巷道壁面的总放热量;(3)空气层厚度对岩壁散热量影响不显著,但巷道底部宽度对总放热量有明显影响,最佳空气层厚度为8 cm左右。     

Experimental and numerical investigation on bare tube cross flow heat exchanger-using COMSOL

ABSTRACT

In this study, COMSOL multi-physics modelling software was used to make a computational model of a bare helical tube cross flow heat exchanger in order to simulate the temperature changes in the heat exchanger. The computational results of heat transfer are validated by using the analytical models. A conjugate convection/conduction heat transfer model was developed, which exhibited good agreement to the experiments. A different velocity of air taken into the consideration to find out the temperature distribution through the pipe and air temperature inside the duct. The temperature profile, and the overall heat transfer rate from the wall of the tube were calculated and plotted for theoretical, experimental and Numerical method using the k- conjugate heat transfer model. The model is validated through comparison with theoretical relations for single-pass cross-flow arrangements and with experimental results also. Simulation results showed good agreement with experimental values with respect to different mass flow rates.     

On the basis of the environmental temperature procedure

The procedure advanced by Danter and developed by Loudon at the Building Research Establishment, Watford, represents a major improvement in the means available to the building services engineer and architect to estimate the thermal response of some part of a building. This paper re-examines its basis.The twin roles of radiation and convection as means of transferring heat within an enclosure are first discussed. The concept of a single representative temperature to describe heat loss from the enclosure, the conductances between this central temperature and its several constituents, and the possibility of taking all heat to be input at this temperature are introduced step-by-step. It is then shown that in an enclosure so idealised to entail two surface and one air temperature, these conductances appear simultaneously by a delta to star transformation of the physical conductances, and that the star temperature t_ec so introduced is an overall measure of the enclosure temperature without an internal heat source. The environmental temperature t_ei calculated by equation (A5.18) of the IHVE 1970 Guide is shown to be nearly equal to the duly weighted combination of t_ec and the temperature t_hb of a suitable hot body source of heat contained within the enclosure. t_ei thus serves as the temperature at which this heat can be considered to be input, and from which it is lost by conduction through the fabric or by ventilation. If the heat is input by some other source (such as a convector) it may be convenient to scale it to make it equivalent to an input at t_ei. Account can be taken of periodic variation in an internal source by including the admittance of the wall construction in a manner formally similar to the steady state transmittance of the wall.     

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.     

Development of HAM tool for building envelope analysis

Moisture-related building envelope failures have resulted in costly rehabilitation in various regions of North America. To advance building envelope design towards an engineering approach and reduce the occurrence of future failures, an advanced numerical tool was developed, in conjunction with an extensive full-scale experiment, to investigate hygrothermal performance of various wood-frame wall assemblies. Major features of the tool are multi-dimensional and transient coupling of heat and moisture transport; natural air convection integrated in hygrothermal simulation through Darcy–Boussinesq approximation; heat transfer by conduction and convection of sensible and latent heat; moisture transport by vapor diffusion, capillary suction and convection; material database of common building materials in North America; experimental settings or weather data as boundary conditions; and moisture added in the building envelope to simulate the wetting process. The numerical tool achieves good compliances to the benchmarking cases of the HAMSTAD project, and its predictions have shown good agreement with data from the full-scale wall experiment. The numerical tool employs the commercial finite-element software to solve the governing equations. This approach provides building science researchers the flexibility to modify, maintain and share their modeling work efficiently.     

Unsteady natural convection in a triangular enclosure under isothermal heating

The fluid flow and heat transfer inside a triangular enclosure due to instantaneous heating on the inclined walls are investigated using an improved scaling analysis and direct numerical simulations. The development of the unsteady natural convection boundary layer under the inclined walls may be classified into three distinct stages including a start-up stage, a transitional stage and a steady state stage, which can be clearly identified in the analytical and numerical results. A new triple-layer integral approach of scaling analysis has been considered to obtain major scaling relations of the velocity, thicknesses, Nusselt number and the flow development time of the natural convection boundary layer and verified by direct numerical simulations over a wide range of flow parameters.     

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.     

U型圆管内混合对流换热特性数值研究

本文通过三维数值模拟的方法研究了混合对流作用下U型管管内的换热特性,分析了管内截面自然对流对管内层流换热的影响及主流速度、壁面热流密度和U型管倾角等参数对管内混合对流换热特性的影响.结果表明：与纯强制对流相比混合对流作用下其管内换热系数显著增大;在混合对流作用下,随壁面热流密度增大,管内换热增强,但随进口流速或U型管倾角的增大时,管内换热减弱.