Effects of Attaching Baffles onto the Inclined Walls of Attic Frames for Purposes of Energy Conservation

This article addresses the buoyant air circulation inside attic spaces of houses and buildings with sloped roofs and horizontally suspended ceilings. In order to counteract the excessive heat transmission through the roof that takes place during the wintertime, when the attic is heated at the base, this study attached an array of baffles onto the top inclined walls to alter the flow characteristics of the buoyant air. To perform the computational analysis, the finite volume method is the vehicle for the discretization of the conservation equations. The Boussinesq approximation is not invoked, and all thermophysical properties are considered as temperature-dependent. Simulations are performed for several values of baffles length and the Rayleigh number. The influence of these parameters upon the flow and temperature patterns is analyzed and discussed. A comparison of the thermal performance between baffled attics and non-baffled attics is presented. The results show that the presence of baffles provides an important energy savings for heating and helps keep the attic at a desired temperature.     

A combined approach for determining the thermal performance of radiant barriers under field conditions

This paper deals with a combined approach for assessing the thermal performance of radiant barriers under field conditions, based both on dynamic simulations and field measurements. The methodology involves the combination of model predictions and experimental results of a complex roof including a radiant barrier installed on a dedicated test cell. During the empirical validation of the building thermal model and more particularly thanks to the results of sensitivity analysis, simplifications of the model were made. These considerations lead to successive simplified versions of the model and finally a very simplified one, which is used to determine the thermal resistance of the complex roof. We first present the detailed thermal model, elaborated with a prototype of building simulation code. We then describe the experimental test cell and put the emphasis on the details of the roof. The simplification of the detailed model is then explained and the results presented. A value of the thermal resistance is finally obtained and confirms the potential of radiant barriers for a tropical climate.     

Field investigation on hygrothermal performance of full-vent perforated soffit and ceiling

This paper reports on field hygrothermal performance of white full-vent perforated soffit and ceiling compared to common wood soffit and gypsum ceiling. Testing was conducted using a two-story residential house located in Bangkok of 100 m² floor surface area. Data collection included ambient conditions, attic temperatures at various positions, and relative humidity of air in the bathroom and in the attic measured at 30 cm above the ceiling.Experimental results pointed out that perforated ceiling is very effective for reducing attic air temperature and moisture in the bathroom. Due to the solar radiation during daytime, the induced stack ventilation decreased heat and moisture accumulations in the bathroom and the attic. Such performance is extremely significant when compared to the performance of the original ceiling (gypsum board) where moisture removal by a small window took much longer time to achieve the same level of performance. After sunset, the temperatures at all positions in the attic were very close to ambient, which is an additional advantage for energy saving and comfort. Therefore, it is expected that perforated soffit and ceiling will be quickly integrated in the new design of modern houses as a new interesting option that promotes passive moisture ventilation and attic heat gain reduction.     

Experimental evaluation of insulation material in roofing system under tropical climate

The objective of this study is to determine the influence of radiant barriers on conductive and radiative heat transfers when they are integrated to a building envelope and to compare their efficiency to traditional insulation material (mineral wools, polystyrene). It is also about determining which insulation material and process can lead to a better heat flux reduction through a building roof. For this study four identical small-scale test cells were used. Their respective roof was equipped with the insulation material to be tested: One with polystyrene, the second with a radiant barrier the third one with fibber glass and the last one with no insulation material was considered as the reference cell. Different test were performed with a view to evaluate the influence of parameters such as roof absorptivity and roof air layer ventilation on the heat flux reduction through the roof. With the measured temperature, the conductive and radiative heat fluxes were calculated. With a white corrugated iron roof top the heat flux reduction provided by the radiant barrier is 37%. With a black one this material allows a reduction of 33%. It is shown that whatever the roof absorptivity value, the radiative heat flux is predominant over the conductive one. With no ventilation, the radiant barrier is comparable to polystyrene and fiber glass; when the airspace is ventilated the radiant barrier provides a better insulation.     

地板辐射采暖地面散热量的简化计算方法

对加装了地板辐射采暖设施的空间采用三维稳态湍流模型,进行空气流动和传热特性的数值研究,并在与实验数据相比较的基础上,提出了一个便于实施计算的地板表面散热量的简化计算方法.提出的简化计算方法的思路即是对原计算公式中难以确定的房间内非加热表面的面积权重平均温度值Tp,用室温的修正量来代替.研究结果表明,数值结果和实验数据吻...     

Modelling solar effects on the heat and mass transfer in a street canyon, a simplified approach

In urban areas, the climatic loads on buildings in summer conditions are largely affected by solar radiation. In this paper a modified simplified method for radiant interchange determination is used in a solar energy study. The good agreement with the radiosity method allows one to use this simplified method in the street canyon case. In a building pilot study, parametric analysis and building thermal behaviour can be assessed by simplified models which are useful for long-period simulation. Then this radiant interchange model is introduced in a zonal model of a canyon street and performed with a variable climatic conditions show case. The solar radiation is the only driving force in the street air movement. The interest of such approach for complex coupled phenomena studies is highlighted by obtained results and the assessment of variable climatic loads for different building zones can be considered with the model detailed herein. Future developments are planned in order to improve simulation accuracy by the addition of other local phenomena.     

Numerical simulation of airflow in naturally ventilated attics for different attic geometries

Attic geometry and ventilation in a different attic geometry play a very important role in residential building energy performance. To evaluate the effect of ventilation ratio and vent balance on attic performance, a two-dimensional finite volume model is employed to simulate the buoyancy-driven turbulent ventilation and heat transfer in an attic space of different geometries like gable, gambrel, and saltbox roofs of residential buildings under winter conditions. The impact of ventilation ratio on ventilating airflow rate, heating load rate, and rate of heat gained is investigated for all three types of roof geometries. The model under consideration consists of a passive ventilation system with a ridge and soffit vent. Meshing is done by using ANSYS Workbench and numerical simulations are carried out by using ANSYS FLUENT 19 analysis software. The effect of passive ventilation on ridge-vent attic performance is evaluated in this study. Ventilation ratios for soffit vents ranging from 1/400 to 1/25 are investigated. Convection boundary conditions are used for the attic walls to account for thermal resistances of ceilings and roofs. In addition, the performance of these vented attics is compared to the heat transfer in a sealed attic. The results show that the symmetrical airflow pattern exists in a vented attic, in contrast to the asymmetrical airflow patterns found in a sealed attic. From the study, it is evident that an increase in ventilation ratio reduces attic heating load irrespective of the increase in ventilation airflow rate. The rate of airflow increases with an increase in ventilation ratio in all three types of roof geometries that is gable, gambrel, and saltbox roofs. In the case of gable roof configuration, increasing the ventilation ratio from 1/200 to 1/100 results in an increase in ventilating mass flow of 75%, while an increase of ventilation ratio from 1/50 to 1/25 only increases the ventilating mass flow by 40%. In the case of gable roofs increasing ventilation ratio from 1/200 to 1/100 results in a 20% decrease in attic heating, whereas an increase in ventilation ratio from 1/50 to 1/25 results in a 10% decrease in attic heating.     

低温地板辐射采暖构造层传热模拟

本文针对以哈尔滨为代表的寒冷地区的低温热水地板辐射供暖系统的传热特点和铺设方式，建立了构造层内传热过程的数学模型，分析了低温热水地板辐射供暖系统的热工性能，确定了构造层内温度分布、地板表面温度分布、单位面积散热量等设计指标与管间距、水温、表面材料之间的定量关系。     

Numerical Simulation of Heat Transfer in Firefighters' Protective Clothing with Multiple Air Gaps during Flash Fire Exposure

A finite volume model was developed to simulate transient heat transfer in firefighters' protective clothing during flash fire exposure. The model domain consists of three layers of fire-resistant fabrics (outer shell, moisture barrier, and thermal liner) with two air gaps between the clothing layers, the human skin, and the air gap between the clothing and the skin. The model accounts for the combined conduction-radiation heat transfer in the air gaps entrapped between the clothing layers, and between the clothing and the skin. The variation in the air gap properties and energy content during both the exposure and the cool down periods was accounted for. Predictions were obtained for the temperature and heat flux distributions in the fabric layers, skin, and air gaps as a function of time. The influence of each air gap on the clothing performance was investigated as well. This article demonstrates the importance of accurately modeling the contributions of the air gaps in order to predict the protective clothing performance.     

Natural convection and heat transfer in attics subject to periodic thermal forcing

The heat transfer through the attics of buildings under realistic thermal forcing has been considered in this study. A periodic temperature boundary condition is applied on the sloping walls of the attic to show the basic flow features in the attic space over diurnal cycles. The numerical results reveal that, during the daytime heating stage, the flow in the attic space is stratified; whereas at the night-time cooling stage, the flow becomes unstable. A symmetrical solution is seen for relatively low Rayleigh numbers. However, as the Ra gradually increases, a transition occurs at a critical value of Ra. Above this critical value, an asymmetrical solution exhibiting a pitchfork bifurcation arises at the night-time. It is also found that the calculated heat transfer rate at the night-time cooling stage is much higher than that during the daytime heating stage.     

A transient induction motor model including saturation and deep bareffect

Transient cage induction motor models for use in inverter-fed drives and controllers are reviewed. A simple transient model is presented that includes rotor deep bar effect and magnetic saturation of the magnetising and rotor leakage flux paths. The improved model requires motor details in the form of simple impedance versus frequency characteristics which can be obtained from a variety of external sources. These can range typically from detailed steady-state finite-element solutions to simple experimental measurements. The model is verified experimentally using a 75 kW, 4 pole vector controlled AC motor drive     

Off-design performance of a chemical looping combustion （CLC） combined cycle： effects of ambient temperature

The present work investigates the influence of ambient temperature on the steady-state off-design thermodynamic performance of a chemical looping combustion （CLC） combined cycle. A sensitivity analysis of the CLC reactor system was conducted, which shows that the parameters that influence the temperatures of the CLC reactors most are the flow rate and temperature of air entering the air reactor. For the ambient temperature variation, three off-design control strategies have been assumed and compared： 1） without any Inlet Guide Vane （IGV） control, 2） IGV control to maintain air reactor temperature and 3） IGV control to maintain constant fuel reactor temperature, aside from fuel flow rate adjusting. Results indicate that, compared with the conventional combined cycle, due to the requirement of pressure balance at outlet of the two CLC reactors, CLC combined cycle shows completely different off-design thermodynamic characteristics regardless of the control strategy adopted. For the first control strategy, temperatures of the two CLC reactors both rise obviously as ambient temperature increases. IGV control adopted by the second and the third strategy has the effect to maintain one of the two reactors＇ temperatures at de- sign condition when ambient temperature is above design point. Compare with the second strategy, the third would induce more severe decrease of efficiency and output power of the CLC combined cycle.     

Coupled finite-element/state-space modeling of turbogenerators inthe abc frame of reference-the short-circuit and load cases includingsaturated parameters

In this paper, a coupled finite-element/state space modeling technique is applied in the determination of the steady-state parameters of a 733 MVA turbogenerator in the abc frame of reference. In this modeling environment, the forward rotor stepping-finite element procedure described in a companion paper is used to obtain the various machine self and mutual inductances under short-circuit and load conditions. A fourth-order state-space model of the armature and field winding flux linkages in the abc frame of reference is then used to obtain the next set of flux linkages and forcing function currents for the finite-element model. In this process, one iterates between the finite-element and state-space techniques until the terminal conditions converge to specified values. This method is applied to the determination of the short-circuit, and reduced- and rated-voltage load characteristics, and the corresponding machine inductances. The spatial harmonics of these inductances are analyzed via Fourier analysis to reveal the impact of machine geometry and stator-to-rotor relative motion, winding layout, magnetic saturation, and other effects. In the full-load infinite-bus case, it is found that, while the three-phase terminal voltages are pure sinusoidal waveforms, the steady-state armature phase currents are nonsinusoidal and contain a substantial amount of odd harmonics which cannot be obtained using the traditional two-axis analysis     

Assessment of Building External Wall Thermal Performance Based on Temperature Deviation Impact Factor under Discontinuous Radiant Heating

As the variation and timely meeting thermal environment requirement of indoor air temperature has a close relationship with the thermal performance of building external wall under discontinuous radiant heating condition, one appropriate assessment method or index for assessing the building external wall thermal performance is very necessary. In order to reasonably evaluate the thermal performance of external wall under discontinuous radiant heating condition and build the direct connections and interactions among the indoor air temperature, external wall inner surface temperature and outdoor air temperature, the first and second impact factors of temperature deviation were established, based on one mathematical model of room heat transfer. For one experimental room and four types of external walls under discontinuous radiant heating condition, both the influence of the external wall inner surface temperature deviation on the indoor air temperature and that of the outdoor air temperature deviation on the external wall inner surface temperature were determined effectively with the first and second impact factors of temperature deviation. In addition, favourable performance for the self-insulation and inner insulation walls were found, due to their superiority in effectively and timely improving the indoor thermal environment under discontinuous radiant heating condition.     

Effect of radiant spectral characteristics on performance of a near-infra-red heating module

We simulated the heat transfer phenomena of the heating module that is primarily based on the radiant energy in the near-infra-red(NIR) domain. In the module, the power emitted by the lamp filament is distributed to the lamp glass, reflector, and the target medium, which are cooled by an air flow. The radiant heat transfer is simulated by using the ray-tracing scheme, in which the spectral characteristics of the emission and the materials are incorporated. The heat transport from the lamp glass to the cooling air is analyzed by using the finite volume method. As the lamp-filament temperature rises in the range of 3000–3400K, the NIR radiant power on the target medium increases. However, the lamp-glass temperature also rises, and the proportion of the NIR power to the entire radiation has a peak in the temperature range. The spectral distributions of the absorbed energies in all the components in the module are highly non-uniform, and a monochromatic model of the radiant heat transfer may result in a significant discrepancy.     

A building corner model for hygrothermal performance and mould growth risk analyses

Combined multidimensional analysis of heat, air and moisture transport through porous building elements is barely explored in the literature due to many difficulties such as modeling complexity, computer run time, numerical convergence and highly moisture-dependent properties. In this way, a mathematical model considering a combined two-dimensional heat, air and moisture transport through unsaturated building upper corners is presented. In order to improve the discretized model numerical stability, the algebraic equations are simultaneously solved for the three driving potentials: temperature, vapor pressure and moist air pressure gradients. In the results section, the convective effects caused by air stagnation are analyzed in terms of heat flux and mould growth risk for different boundary conditions, showing the importance of a detailed hygrothermal analysis – which is normally disregarded by simulation tools – for accurately predicting building energy consumption, indoor air quality, thermal comfort or mould growth risk.     

Heat transfer prediction for radiant floor heating/cooling systems using artificial neural network (ANN)

Radiant floor cooling and heating systems (RHC) are gaining popularity as compared with conventional space conditioning systems. An understanding of the heat transfer capacity of the radiant system is desirable to design a space conditioning system using RHC technology. In the present work, a simplified heat flux model for RHC is developed for both cooling and heating modes of operation. The Artificial Neural Network (ANN) technique is used for the development of the simplified model. Experimental data from literature covering a wide operating range of the RHC is considered for model development and validation. Operating parameters such as mass flow rate (m_f), heat resistance (R_s), mean temperature of water flowing through the pipe (T_m), and operative temperature (T_op) are considered independent variables influencing the heat flux (q_t). The neural network consists of four input layers, one output layer, and one hidden layer with a feed-forward-back-propagation algorithm. A study on the selection of the optimum number of neurons in the range of 1–9 for the hidden layer is also performed. On the basis of the performance parameters, namely, average-absolute-relative-deviation (AARD = 0.11283) percentage, mean-square-error (MSE = 0.00055), and the coefficient of determination (R² = 0.9984), a hidden layer is modeled with five neurons.     

Thermally induced hygroscopic mass transfer in a fibrous medium

A fiberglass sample was used to study the hygroscopic mass transfer produced by a temperature difference across a moist, fibrous medium. Humidity probes and thermocouples were implanted in the sample and used to continuously monitor changes in the relative humidity and temperature as a result of the moisture migration. The effects of average temperature, thermal gradient magnitude and average moisture content were some of the parameters studied. The data was analyzed using a mechanistic analogy to the irreversible thermodynamic model. Vapor and liquid fluxes were evaluated along with vapor and liquid conductivities. The phenomenological coefficients associated with the liquid and vapor fluxes were calculated, and the flux contributions due to the thermal and concentration gradients were determined for steady-state conditions. Transient data for the humidity, temperature and moisture content were also either measured or calculated.     

Automatic generation of skeletal mechanisms for ignition combustion based on level of importance analysis

A level of importance (LOI) selection parameter is employed in order to identify species with general low importance to the overall accuracy of a chemical model. This enables elimination of the minor reaction paths in which these species are involved. The generation of such skeletal mechanisms is performed automatically in a pre-processing step ranking species according to their level of importance. This selection criterion is a combined parameter based on a time scale and sensitivity analysis, identifying both short lived species and species with respect to which the observable of interest has low sensitivity. In this work a careful element flux analysis demonstrates that such species do not interact in major reaction paths. Employing the LOI procedure replaces the previous method of identifying redundant species through a two step procedure involving a reaction flow analysis followed by a sensitivity analysis. The flux analysis is performed using DARS^©, a digital analysis tool modelling reactive systems.Simplified chemical models are generated based on a detailed ethylene mechanism involving 111 species and 784 reactions (1566 forward and backward reactions) proposed by Wang et al. Eliminating species from detailed mechanisms introduces errors in the predicted combustion parameters. In the present work these errors are systematically studied for a wide range of conditions, including temperature, pressure and mixtures. Results show that the accuracy of simplified models is particularly lowered when the initial temperatures are close to the transition between low- and high-temperature chemistry. A speed-up factor of 5 is observed when using a simplified model containing only 27% of the original species and 19% of the original reactions.