A naturally ventilated cavity roof as potential benefits for improving thermal environment and cooling load of a factory building

The impact of natural ventilation of a roof cavity on improvement of the thermal environment and reduction of cooling load of a factory building is discussed. A computer program was developed with the logic in a companion paper [1] to observe the effect of cavity ventilation on the operative temperature of the occupied zone in the factory. Comparisons were made between factories with a cavity roof and a single roof in the Japanese climate. Results showed that the cavity roof was superior to the single roof in lowering the operative temperature by about 4.4 °C. When the factory was air conditioned, the cooling load reduction reached approximately 50% during the summer to maintain an operative temperature of 26 °C. Results showed that a naturally ventilated cavity roof has excellent potential for improving the indoor thermal environment and energy savings of factory buildings without complicated cooling installations and life time power consumption.     

Determination of transient two-dimensional heat transfer in ventilated lightweight low sloped roof using Fourier series

In this paper, the analysis of transient two-dimensional (2D) heat transfer in low sloped roof with forced ventilated cavity made from lightweight building elements (LBE) is presented. For the heat transfer analysis the 2D numerical model, which was verified with experiments, was used. Forced ventilated cavity was configured in two different ways. In the first case the cavity was configured with coloured thin metal sheet and in the second case with thin metal sheet with added layer of thermal insulation and radiation barrier. Beside the influence of the ventilated cavity configuration on the transient 2D heat transfer in the LBE and on the cavity outlet air temperature also the influence of the LBE thickness, specific air flow rate through the cavity, inner air temperature and wind velocity was analysed. Multi-parametric equations for determination of Fourier series coefficients were formed. These coefficients were used for evaluation of transient 2D heat transfer on the inner side of the roof and cavity outlet air temperature for a clear day.     

Investigation of the performance of a ventilated wall

The need for environmental friendly and energy efficient building design has stimulated the design of new facade technologies, including various configurations of double skin facades. This paper investigates the thermal performance of a ventilated wall, both for heating and cooling. A thermal analysis was carried out, paying special attention to the characterization of the heat convection resulting from the buoyancy-induced flow in the open air channel which proved to be a critical aspect of the ventilated wall's behaviour. An integrated thermal and air flow model for the entire system was developed. A model of the ventilated wall construction was developed with the ESP-r simulation program and checked against experimental data from a real-scale test cell facility.The thermal benefits of adding a radiant barrier layer were also investigated. The results showed that this layer was beneficial in terms of the energy performance of the construction. Also, the comparison between the experimental and simulation model results showed satisfactory levels of convergence with the exception of the night hours during the summer period. A sensitivity analysis was also undertaken in order to investigate the main factors and the extent of their effect on the temperature variation inside the ventilated facades.     

Effects of roof tile permeability on the thermal performance of ventilated roofs: Analysis of annual performance

This paper shows the results of the second part of an experimental study aimed at analysing the effects of roof tile permeability on the thermal performances of ventilation ducts. Ventilation ducts under the layer of tiles are typically used in south European countries to limit the energy load during the summer period. The results of the first part of the study, carried out by analysing 14 different types of roof, proved that the air permeability of the layer of tiles determines a certain amount of heat to be released, in addition to the release connected with the stack effect, in ventilation ducts which have the same characteristics but are perfectly airtight. However, the study did not completely resolve some issues since it was carried out on a model roof (6 m × 1.5 m) with devices to raise the layer of tiles and to create the ventilation duct but without those building elements which are present in real roofs and are used to stop insects and small animals from entering the ventilation duct. These elements narrow the inlet and outlet and consequently cause important reductions in pressure. Moreover, the measurements were based on data collected for limited periods of time during the summer season.     

Effect of roof solar reflectance on the building heat gain in a hot climate

The effect of the roof solar reflectance on the thermal performance of a building is often ignored. However, there are significant differences in heat gain from light and dark-coloured roof surfaces. In this paper an equation for the average daily downward heat flow of a sunlit roof is derived. Using building simulation, it is first shown that the thermal mass of the roof does not significantly affect the overall daily heat gain (although it causes a time lag and reduction in peak heat flow). As a consequence the daily heat gain from the roof may be estimated by integrating the equation for the steady-state downward heat transfer over the day. For north Australia the derived equation suggests that a light-coloured roof has about 30% lower total (air temperature difference and solar-driven) heat gain than a dark-coloured one. The effect of aging (change in solar reflectance with time) is considered in the calculations and a relationship between the solar absorptance of new and aged material is suggested. A classification of roof colours with respect to their solar absorptance (dark, medium, light and reflective) is proposed to enable a quick and simple assessment of the effect of roof colour on the heat gain and R-value.     

An integrated empirical and modeling methodology for analyzing solar reflective roof technologies on commercial buildings

Buildings impact the environment in many ways as a result of both their energy use and material consumption. In urban areas, the emission of greenhouse gases and the creation of microclimates are among their most prominent impacts so the adoption of building design strategies and materials that address both these issues will lead to significant reductions in a building's overall environmental impact. This report documents the energy savings and surface temperature reduction achieved by replacing an existing commercial building's flat roof with a more reflective ‘cool roof’ surface material. The research methodology gathered data on-site (surface temperatures and reflectivity) and used this in conjunction with the as-built drawings to construct a building energy simulation model. A 20-year cost benefit analysis (CBA) was conducted to determine the return on investment (ROI) for the new cool roof construction based on the energy simulation results. The results of the EnergyPlus™ simulation modeling revealed that reductions of 1.3–1.9% and 2.6–3.8% of the total monthly electricity consumption can be achieved from the 50% cool roof replacement already implemented and a future 100% roof replacement, respectively.This corresponds to a saving of approximately $22,000 per year in energy costs at current prices and a consequent 9-year payback period for the added cost of installing the 100% cool roof. The environmental benefits associated with these electricity savings, particularly the reductions in environmental damage and peak-time electricity demand, represent the indirect benefits of the cool roof system.     

Heat transfers in a double-skin roof ventilated by natural convection in summer time

The double-skin roofs investigated in this paper are formed by adding a metallic screen on an existing sheet metal roof. The system enhances passive cooling of dwellings and can help diminishing power costs for air conditioning in summer or in tropical and arid countries. In this work, radiation, convection and conduction heat transfers are investigated. Depending on its surface properties, the screen reflects a large amount of oncoming solar radiation. Natural convection in the channel underneath drives off the residual heat. The bi-dimensional numerical simulation of the heat transfers through the double skin reveals the most important parameters for the system's efficiency. They are, by order of importance, the sheet metal surface emissivity, the screen internal and external surface emissivity, the insulation thickness and the inclination angle for a channel width over 6 cm. The influence of those parameters on Rayleigh and Nusselt numbers is also investigated. Temperature and air velocity profiles on several channel cross-sections are plotted and discussed.     

直立锁边金属屋面系统技术研究

简述了霍高文直立锁边金属屋面系统的性能,阐述了新型屋面设计技术,并利用工程实例分析了直立锁边金属屋面系统的工程应用情况,经跟踪监测,该屋面系统运行良好,值得推广应用。     

A laboratory experiment on natural ventilation through a roof cavity for reduction of solar heat gain

The study targets the reduction of roof solar heat gain through the use of natural ventilation in a roof cavity. This study is mainly concerned with factory buildings. Experimental outcomes were obtained from an inclined cavity model which was heated on the upper surface to mimic solar radiation on a roof. The dimensions of the cavity were 4882 mm× 400 mm × 78 mm. The two opposing smallest sides were allotted as the inlet and outlet, and narrowed to simulate resistance of the air flow in practical applications. Temperature and velocity measuring facilities were prepared in the experimental model. A number of measurements were carried out by varying the combinations of different heat production, inclination angles, and opening ratios. It was found that resistance to heat and air flow in the cavity was strongly influenced by the opening size. When the Reynolds number was examined, it showed that the flow belonged to the laminar region. The average velocity reached to 0.25 m/s at the highest in the examined cases. In other words, the cavity air was turned over 184 times in an hour. Natural ventilation in the roof cavity seemed to be effectively applicable to solar incidence discharges in factory buildings.     

Development and experimental validation of a simulation model for open joint ventilated façades

The investigation of the thermal and fluid dynamical behaviour of open joint ventilated façades is a challenging task due to the complex airflows generated inside of the naturally ventilated cavity by the existence of open joints. For this reason, the use of advanced fluid measurement and simulation techniques is highly recommended. This paper focuses in the development and experimental validation of a simulation model for these façade systems. More specifically, different turbulence and radiation models available in the commercial computational fluid dynamic codes have been tested on a three-dimensional model and the results have been compared to particle image velocimetry measurements. The correlation between experimental and numerical data has been used in order to select the simulation procedure for this type of façades. Best fittings have been found when using the RNG k-epsilon turbulence model and the Discrete Ordinate radiation model. Using the selected scheme, parametrical simulations have been performed to investigate the effect of increasing the cavity height, and correspondingly, the number of slabs. Results show that ventilation air flow inside the cavity is enhanced by incident radiation as well as by the height of the façade.     

坡屋顶在屋顶保温中的研究与应用

根据热传导理论,对坡屋顶的阁楼在不进行通风换气状态下给出了传热系数计算公式,研究了坡屋顶坡度对保温效果的影响,研究表明坡屋顶坡度在30°左右较为合理,最后介绍了坡屋顶施工工艺。     

桂林市平屋面、坡屋面与种植屋面的渗漏情况调查分析及预防措施

屋面渗漏一直是困扰顶层住户的一个重要问题,一直以来大家都认为施工是导致屋面渗漏的重要原因。不过,根据我们的调研,得知后期住户的不正确使用屋面也是造成屋面渗漏的一个相当重要的原因。本文基于我们调研结果,对不同类型的屋面渗漏情况做出了分析,并提出了相应的防治措施。     

网格屋盖结构柱顶隔震技术的参数分析

采用双线性模型模拟隔震垫,通过非线性有限元程序大量的数值分析,研究了隔震垫刚度、阻尼比对屋盖隔震结构减震效果的影响规律,认为在满足抗风、抗小震的前提下,隔震层刚度应尽可能地小;隔震垫阻尼比存在最优值,一般可取0.1~0.2。     

浅层敞开式蓄水屋面的蒸发换热特性研究

浅层敞开式蓄水屋面由于具有蓄水层浅、屋面荷载小、蒸发性能好、对屋面具有养护功能、夜间散热快等优点,但也存在由于蓄水层浅,其蒸发冷却作用受到一定的限制等缺点,从而影响其工程应用。通过所建立的浅层敞开式屋面蓄水隔热模块测试平台,对其在自然气候条件下的蒸发冷却性能进行了测试研究,分析隔热模块的平均蒸发热流变化特征,获得了其在典型季节月份的蒸发换热系数和换热变化规律,得到了其蒸发热流计算模型,这些实验研究结果为可上人式浅层蓄水屋面的构造模式优化提供基础依据。     

An experimental study of welded splices of reinforcing bars

Manufacturing, fabrication, and transportation limitations make it impossible to provide full length continuous bars in some reinforced concrete structures. In general, reinforcing bars are stocked by suppliers in lengths of 12–18 m. For that reason, and because it is often more convenient to work with shorter bar lengths, it is frequently necessary to splice bars in the field.     

热反射轻质屋面隔热效果对比测试研究

通过对比测试连续晴天条件下,自然通风状态的普通压型钢板屋面厂房和热反射压型钢板屋面厂房的屋面外表面温度、内表面温度、室内空气温度等热环境参数,量化分析热反射屋面的隔热效果和对建筑热环境的改善作用。结果表明,相比普通轻质屋面,太阳辐射强烈的夏季晴日中午和午后,热反射屋面外表面温度、内表面温度、室内空气温度分别降低了12~16℃、5~7℃和1.5~2.5℃,隔热效果显著,有效改善了建筑室外热环境和自然通风状态下室内热环境。     

Lateral restraint forces in Z-section roof systems using the component stiffness method

The component stiffness method is a method to predict lateral restraint forces in roof systems supported by Z-sections. The method approximates a bay of Z-sections as a single degree of freedom system and uses a stiffness formulation to determine the contribution of the different components in the system to the resistance of lateral movement. The forces generated by the roof system requiring restraint are derived from mechanics.     

An experimental study on delamination of FRP plates bonded to concrete

Results of an experimental campaign on FRP–concrete delamination are presented. Specimens with different bonded lengths and plate widths have been tested. Strain gauges along the FRP plate have been used to measure longitudinal strains. For long bonded lengths, progressive debonding along the specimens has been followed. Starting from experimental data, average shear stress–slips data have been computed. By post-processing these data, non-linear interface laws for two different plate widths have been calibrated. Increase of maximum shear stress with decreasing plate width has been observed, whereas no significant plate width effect on fracture energy and delamination force has been found. Experimental tests have been simulated by adopting a numerical bond-slip model and the above mentioned non-linear law for the FRP–concrete interface. Numerical results in good agreement with experimental results have been obtained, both at low and very high loading levels.     

屋顶花园的发展与造景特点初探

从研读屋顶花园的历史渊源出发，探寻屋顶花园的特点演变历程，挖掘屋顶花园在当代存在的意义，通过实地观察与查阅相关文献，了解并总结屋顶花园植物配置的特点，以期作为今后实践的指导。     

Experimental study on cool release process of gas-hydrate with additives

Cool release process of phase change material (PCM) includes the dissolution process of the PCM. Experimental research on the cool release process in a new gas-hydrate cool storage system is performed in this paper. In the system, the inner-heat exchange/outer-crystallization technology and the integrated condenser/evaporator structure design were adopted. The influences of different proportions of calcium hypochlorite or benzenesulfonic acid sodium salt on the dissolution process are studied. The results show that the instantaneous dissolution rate is close to twice as the corresponding instantaneous formation rate of gas-hydrate, and the cold energy release rate, i.e. energy release per unit time, is obviously higher than the corresponding cold energy storage rate at different conditions, which is mainly due to the heat transfer temperature difference of the discharging (release) and charging (store) processes. Both the charging and the discharging processes are simultaneously dominated by the heat transfer and the mass transfer processes. However, as to the dissolution (discharging) process of the gas-hydrate by heating, the effect of the heat transfer process is the main influence factor. The temperature difference between the heating medium and the gas-hydrate in the discharging process, which is larger than that between the cooling medium and the gas-hydrate in the charging process, makes a higher cold energy release rate compared with the cold energy storage rate. The results also indicate that the dissolution rate of gas-hydrate is accelerated, and the cold energy release rate of the cool storage system is increased by adding reasonable proportions of the additives.