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《Journal of Building Performance Simulation》2013,6(2):125-140
A CFD model was developed to study thermal performance of hollow cement wall constructions of buildings under hot summer conditions. The approach employed couples conjugate, laminar natural convective flow of a viscous fluid in hollow building blocks with long-wave radiation between the cavity sides. Realistic boundary conditions were employed at the outdoor and indoor surfaces of the wall. A state-of-art building energy simulation program, ESP-r, 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 wall domain. A detailed parametric study showed that using a wider cavity within a building block does not necessarily reduce heat flux through the block. Radiation heat transfer between cavity sides may account for a significant fraction of heat flux through the block and neglecting its effect can lead to errors that could be as large as 46%. The geometry of the hollow blocks was demonstrated to affect the heat flux by as much as 30%. 相似文献
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《Journal of Building Performance Simulation》2013,6(4):248-262
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. 相似文献
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This paper presents an analysis of the impact of selected design alternatives on the thermal and energy performance of atriums based on the methodology outlined in the accompanying paper. Computer simulation programs were used to predict the impact of the selected design alternatives on the design performance outputs of atriums. Design alternatives focused on fenestration glazing types, fenestration surface area, skylight shape, atrium type, and interaction of the atrium with its adjacent spaces. Design performance outputs, evaluated with respect to a basecase design, included seasonal solar heat gain ratio, cooling and heating peak load ratios and annual cooling, heating and total energy ratios. Design tools were developed to quantify the impact of the design alternatives on the performance outputs. The design tools were cast into two-dimensional linear relationships with the glazing U-value and SHGC ratios as independent parameters. The results for enclosed atriums showed that the annual cooling energy ratio increased at a rate of 1.196 per unit of SHGC ratio and decreased at a rate of 0.382 per unit of U-value ratio. However, the annual heating energy ratio increased at a rate of 1.954 per unit of U-value ratio and decreased at a rate of 1.081 per unit of SHGC ratio. Similar trends were also found for the three-sided and linear atriums. Pyramidal/pitched skylights increased the solar heat gain ratio by up to 25% in the heating season compared to flat skylights. The effect of the skylight shape on the annual cooling and heating energy may be positive or negative, depending on the glazing U-value and SHGC ratios and the atrium type. Atriums open to their adjacent spaces reduced the annual cooling energy ratio by up to 76% compared to closed atrium spaces. However, open atrium spaces increased the annual heating energy ratio by up to 19%. 相似文献
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ABSTRACT Light-emitting diode (LED) has several advantages and applications due to energy efficiency, versatility, performance, and long life. LED converts only 20–30% of the power input into light, converting the remaining 70–80% of the energy into heat that must be conducted from the LED die to the underlying circuit board, heat sinks, and housings. Improper thermal management will lead to an increase in the junction temperature above the safe limit, thus leading to premature failure of the LED. The present investigation aims to study the variation of case and junction temperature for three types of heat sinks, namely, diagonal, cylindrical, and spiral using 16?W LED. Light output in terms of lux is measured. Experiments were conducted by varying currents from 100 to 300?mA. It was observed that the spiral heat sink has the least junction temperature and thermal resistance among the three heat sinks at all current ratings. 相似文献
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Solar heated tall ventilation cavities including solar chimneys are used to enhance natural ventilation of buildings. A validated CFD model was used to predict the buoyancy-driven air flow and heat transfer rates in vertical ventilation cavities with various combinations of heat distribution on two vertical walls ranging from symmetrical to fully asymmetrical heating. The natural ventilation rate and heat transfer rate have been found to vary with the total heat input, heat distribution on the cavity walls, cavity width and height and inlet opening position. General expressions for these variables have been obtained and presented in non-dimensional terms, Nusselt number, Reynolds number, Rayleigh number and aspect ratio (H/b), as Nu = f(Ra, H/b) and Nu = f(Ra, Re) or Re = f(Ra, Nu), for natural ventilation design. 相似文献
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Heat exchangers are widely used in almost all industrial activities. Turbulent promoters used in heat exchanger tubes are an effective way of enhancing the performance. This paper summarises various investigations using twisted tapes, wire coil ribs, baffles, and swirl flow generators. The main objective of this paper is to review various studies, in which different obstacle roughness elements are used to enhance the rate of heat transfer in the heat exchanger tubes. It has been found that a lot of experimental and analytical studies reported in the literature. On the basis of correlations developed by various investigators an attempt has been made to compare the thermal hydraulic performance of obstacles in heat exchanger tubes. In this work, a comparative study is also carried out to select the best obstacle roughness shapes for higher heat transfer rate and low pressure drop losses. 相似文献
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Convective heat transfer inside the gap of double glazed windows is studied numerically using a commercial CFD code (Fluent v6.3), for different Rayleigh numbers and aspect ratios. A reference window with empty gap is compared with windows where the gap contains fins arranged in such a way as to reduce heat transfer. The effects of convective air flow inside the cavities were estimated both at the onset of convection and at steady-state in real environmental conditions. The global Nusselt numbers were calculated for different configurations of the fins in the window gap, in order to apply the standard heat loss estimation method to this type of windows. 相似文献
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本文采用数值方法研究了建筑室内同时存在温度梯度、湿度梯度和有机污染物(VOCs)浓度梯度时的自然对流传热传质现象,展示了流场、温度场和浓度场等随浮力比N_C的变化状况,考察了多物理场(空气、水蒸气和VOCs)中交叉耦合扩散效应对流动和传热传质的影响特征,探讨了边壁处的无量纲传热努谢尔特数和传质舍伍德数的变化规律。 相似文献
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混凝土结构对流换热效应的研究 总被引:1,自引:0,他引:1
综合分析了国内外学者对混凝土对流换热系数的研究成果,结合风洞实验,提出了混凝土对流换热系数计算公式,可用于混凝土结构温度效应分析。并利用有限元程序进行预应力混凝土简支箱梁的温度效应分析,计算中采用了标准化的太阳日辐射强度曲线和日气温变化曲线,以梁的跨中挠度和轴向变形的变化规律为例,讨论了对流换热系数取值对结构温度效应的影响。 相似文献
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An Earth-to-Air Heat Exchanger (ETAHE) uses the ground's thermal storage capacity to dampen ambient air temperature oscillations by delivering the outdoor air to the indoors through a horizontally buried duct. With their lower airflow resistance, large cross-sectional area ETAHEs have been found to be more energy efficient than the conventional small ones, especially when integrated in hybrid ventilation systems. However, the lack of available methods for determining the heat convection at the duct surfaces has made accurate energy simulation and proper system design overly difficult. In this study, numerical experiments using computational fluid dynamics (CFD) were conducted to investigate the airflow and thermal behavior in the large ducts. A two-layer turbulence model was used to ensure accuracy in resolving the flow information in the near-wall region, which is critical for predicting accurate heat convection. The modeling method was verified by comparing its results with measurements from literature. Parametric studies were conducted to identify the influential design and operation variables for the heat convection. Thirty numerical experimental setups designed with the Latin Hypercube Sampling method were simulated to prepare a database with six design parameters as the simulation inputs and average Nusselt numbers over the duct ceiling, wall, and floor as the outputs. Based on the database an artificial neural network (ANN) model was trained to build a mathematical relation between the design variables and the Nusselt numbers. The developed ANN model showed very accurate prediction when compared with test data. 相似文献
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Suvash C. Saha 《Energy and Buildings》2011,43(10):2908-2917
Unsteady natural convection inside a triangular cavity subject to a non-instantaneous heating on the inclined walls in the form of an imposed temperature which increases linearly up to a prescribed steady value over a prescribed time is reported. The development of the flow from start-up to a steady-state has been described based on scaling analyses and direct numerical simulations. The ramp temperature has been chosen in such a way that the boundary layer is reached a quasi-steady mode before the growth of the temperature is completed. In this mode the thermal boundary layer at first grows in thickness, then contracts with increasing time. However, if the imposed wall temperature growth period is sufficiently short, the boundary layer develops differently. It is seen that the shape of many houses are isosceles triangular cross-section. The heat transfer process through the roof of the attic-shaped space should be well understood. Because, in the building energy, one of the most important objectives for design and construction of houses is to provide thermal comfort for occupants. Moreover, in the present energy-conscious society it is also a requirement for houses to be energy efficient, i.e. the energy consumption for heating or air-conditioning houses must be minimized. 相似文献
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This paper presents a review of studies on natural convection heat transfer in the triangular enclosure namely, in attic-shaped space. Much research activity has been devoted to this topic over the last three decades with a view to providing thermal comfort to the occupants in attic-shaped buildings and to minimising the energy costs associated with heating and air-conditioning. Two basic thermal boundary conditions of attic are considered to represent hot and cold climates or day and night time. This paper also reports on a significant number of studies which have been performed recently on other topics related to the attic space, for example, attics subject to localized heating and attics filled with porous media. 相似文献
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建立了被动式太阳能温室系统的热平衡方程和土壤热、湿迁移过程的理论模型,针对武汉市11月份的典型气候条件,采用数值模拟和实验研究了温室中土温、气温的变化规律;分析了温室北墙内表面采用不同材料或涂层时,对温室中气流和温度分布的影响。 相似文献
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《Journal of Building Performance Simulation》2013,6(4):226-238
The main aim of this work is to study the influence of a roof monitor skylight on comfort criteria. The roof monitor skylight simultaneously provides lighting and outdoor ventilation air for a living room or a master bedroom in a house with an underfloor heating/cooling system. The air is introduced into the room through an air inlet in the carpentry of the opening in the roof monitor skylight and located – for the purposes of this study – in different positions in the carpentry. To evaluate the comfort variations caused by this change of position, a series of lighting and thermal numerical indicators are applied to a set of simulation models, generating a complete analysis to determine optimal positions for the air inlet. These models are designed for the context of a Mediterranean climate, with extreme calculation conditions (overcast sky in winter and clear sky in summer). The simulation tools Daylight Visualizer 2.5 and Design Builder 2.42 have been selected to generate the models. This work concludes that the optimal position for the air inlet is in the side of the carpentry of the roof monitor skylight opening, ensuring the maximization of both lighting and thermal comfort conditions. 相似文献
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