非饱和土孔隙气、水、汽、热耦合运动之模拟(英文)

旨在介绍一个描述非饱和土孔隙气、水、汽、热耦合运动的理论模型。该模型假定孔隙气和孔隙水运动分别遵循达西定律 ,而影响水蒸汽运动的两种主要因素分别是分子扩散和孔隙气运动 ,其中受分子扩散影响的孔隙水蒸汽运动可用Fick定律描述。热转移则主要包括了三种形式 ,即传导、对流和汽化潜热。根据有限单元法 ,编制了一个三维的计算机程序用以模拟非饱和土孔隙气、水、汽、热的耦合运动。通过数值分析与干沙试验结果之比较 ,验证了文中之理论模型和计算机程序的可靠性     

A second order turbulence model for the prediction of air movement and heat transfer in a ventilated room

A full-scale test room is used to investigate experimentally and numerically the velocity and temperature fields in the case of a mechanical ventilation. Detailed fields are measured for three cases of ventilation air temperature: an isothermal case, a hot case, and a cold case. The experimental data are used to test two turbulence models: a first order k-ε realizable turbulence model and a second order quadratic RSM (Reynolds Stress Model) turbulence model. The RSM model predicts the temperature and velocity fields better than the k-ε turbulence model. In particular, global values of velocity and temperature coming from experiments are in good agreement with the RSM turbulence model. This conclusion is confirmed using a turbulence analysis based on Lumley triangles.     

大空间分层空调辐射转移热特性规律研究

大空间建筑室内垂直温度差异显著,非空调区壁面对空调区壁面存在辐射转移热。依据典型大空间结构形式和辐射转移热计算方法,对大空间建筑宽长比、相对分层高度、室外干球温度等因素下辐射转移热的特性规律进行了系统研究。     

架空通风地板传热系数研究

采用稳态传热方法和等量传热原则得出了以架空层通风量为主要影响参数的地板传热系数计算公式,应用CFD软件模拟了通风口面积比与通风量的关系,给出了常用通风口设置情况下地板传热系数建议值.     

Data-based mechanistic modelling approach to determine the age of air in a ventilated space

In literature, local mean age of air is used as an important index to evaluate indoor air quality in ventilated rooms. In this research, a data-based mechanistic approach is used to model the spatial–temporal mass distribution in an imperfectly mixed forced ventilated installation. A first-order transfer function model has proved to be sufficiently good in describing the mass transfer dynamics $(R_t^2=0.987)$ of the system. Furthermore, it was possible to fully understand the physical meaning of the model parameter. The parameter is found to be an inverse of the age of air. This Data-Based Modelling approach proved to be more robust when dealing with measurement noise. Finally, the modelled age of air was validated with a classical step up determination of the age of air for experimental data. Good correlation $(R_t^2=0.77)$ was found between both results, which proved the physical background of the model parameter.     

Modelling mass transfer phenomena and quantification of ventilation performance in a full scale installation

This paper outlines development of a low order model that can be used for control purposes and quantification of ventilation performance in ventilated systems. First informative pollutant transport data is generated using numerical simulations. Later on, identification procedures are followed to build a low order transfer function model from the CFD generated input–output data. The obtained results demonstrate that first order model can sufficiently describe the dominant mass transfer dynamics in the ventilated air space. Afterwards classical mass balance equation is used to explain the objectively formulated model in a meaningful manner. The developed model is compact in structure and accurate in nature making it an ideal input for model based controller algorithm development. Furthermore its model parameter is found to be an inverse of the local mean age of air. Therefore the model can also be used to assess ventilation performance.     

内呼吸玻璃幕墙综合传热系数CFD模拟计算

采用CFD模拟分析了内呼吸玻璃幕墙的传热现象,比较了不同通风量以及不同幕墙宽度条件下内呼吸幕墙的综合传热系数以及百叶不同位置的遮阳效果,指出与增大通风量及幕墙宽度的方案相比,采取有效遮阳措施的内呼吸幕墙,减少围护结构热损失的效果更显著。     

Measuring ventilation rate through naturally ventilated air openings by introducing heat flux

Existing ventilation rate measuring techniques for natural ventilation do not provide an online quantification of ventilation rate through the building envelope. Therefore, a need for a practical, robust, and cheap direct measuring technique is obvious. This study investigates the possibility of using a heat source to trace ventilation rate through an air inlet/outlet. A rectangular heat source was introduced in the middle of the air inlet with a known ventilation rate, and afterwards, the 2D temperature field in the flow direction was recorded using an infrared camera.     

Modelling of heat and moisture transfer in buildings: II. Applications to indoor thermal and moisture control

The objective of this work is to evaluate the possibility of ensuring indoor temperature and relative humidity in an acceptable range by controlling heating and ventilation associated with minimising the energy consumption or cost by the use of a numerical simulation program. An acceptable indoor range means that indoor temperature is 20 °C with small fluctuation and relative humidity is in the range of 20–70%. A simple heating and ventilation control regulation is proposed. Simulation examples are conducted for different types of buildings based on energy consumption or cost factor. Simulations demonstrate that the indoor condition is much more improved and significant energy consumption or cost can be reduced by using the control regulation developed in this paper.     

玻璃幕墙热通道内气流组织的模拟与分析

用有限元分析软件ANSYS对上海第一人民医院病房玻璃幕墙热通道分别采用自然通风和机械通风时的流场与温度场进行了模拟分析,发现无法实现自然通风,而机械通风不仅可以使幕墙内通风效果大大增强,同时可以节能。为该医院病房玻璃幕墙热通道实现自然通风提出了改进措施,模拟得出在距外壁0.8m左右处安装一内层玻璃时出风口气流速度能达到2.3m/s左右,可有效地把热量带出。在与该医院玻璃幕墙模型相同的边界条件和相同幕墙高度条件下,对不同宽度和不同进出风口高度的情况进行了模拟,得出最佳设计尺寸为宽度1·4m、进出风口高度0.35m左右。     

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.     

Modelling of natural convective heat transfer at an internal surface

The determination of accurate surface convective heat transfer coefficients (CHTC) to model the heat flow within building interiors is of primary importance due to the effect of convection on the thermal comfort and the overall energy consumption of a building. Numerous theoretical and experimental expressions aiming at modelling the convective heat exchange of buoyancy-driven flows along vertical surfaces have been proposed. However, the case of a uniformly heated surface is not as exhaustively treated as that of the isothermal surface despite its relevance in many situations within building interiors. This paper is concerned with laminar and turbulent heat transfer modelling at internal vertical building surfaces (walls, windows, …) uniformly heated. Expressions of both local and average CHTC are derived from a theoretical analysis based on the integral formalism. A simplified expression of the average CHTC is then given as a function of the convective heat flux density (or the average temperature difference) and tested for building simulation purposes. The theoretical results are shown to be in close agreement with Alamdari and Hammond [F. Alamdari, G.P. Hammond, Improved data correlations for buoyancy-driven convection in rooms, Building Services Engineering Research & Technology 4 (1983) 106–112] improved correlation, as well as with experimental data derived from measurements performed on real-size enclosures. Despite the three-dimensionality and limited size of full-scale rooms, less than 10% difference is observed.     

Design and analysis to analyse the heat transfer of a space heater for a boat

As fossil fuels are depleting gradually with the course of time there is a need for developing innovative products that can extract maximum potential that the system can deliver. One of the key areas that still make use of diesel fuels for experiencing thermal comfort is the boat heating system. Due to its user-friendly characteristics with low maintenance requirements, space heater has still be the benchmark for all other types of heaters. The boat cabin heater is an essential setup for all kinds of boats and ships not just for the warmth but also to eliminate the dampness that can spread throughout the boat in cold weather. The paper presents a detailed analysis of thermal behaviour inside a space heater using computational fluid dynamics technique to study the effect of fin numbers and fin location inside the heat exchanger. A total of 11 cases were simulated to study the effect of a fin on the heat transfer rate and pressure drop. An optimisation is done to have maximum heat transfer rate with minimum pressure drop and results showed that case number 9 has satisfied the above criteria to obtain an effectiveness of 90%.     

微通道换热器空气侧换热性能影响实验研究

针对微通道蒸发器的换热性能,在结露条件下,分析了冷冻水进口温度、空气进口温度、空气流速大小等参数对空气出口温度、换热器换热系数、空气侧压降等实验结果的影响程度,研究发现:提高冷冻水进口温度,空气侧换热系数和压降有不同程度降低,而空气出口温度有所升高。     

暖通空调水系统中水力失调现象及其解决方法

简单介绍了目前暖通空调中水力系统现状,针对暖通空调水系统中水力失调现象进行了分析,阐述了水力失调的种类及解决途径,并对水力平衡阀的安装、选型及调试作了具体说明,以期指导实践。     

Transient buoyancy-driven ventilation: Part 2. Modelling heat transfer

A new mathematical model for buoyancy-driven ventilation [Sandbach SD, Lane-Serff GF. Transient buoyancy-driven ventilation: Part 1. Modelling advection. Building and Environment, 2011] is modified to include heat transfer at the boundaries. Heat transfers at the ceiling and floor are included, using Newton’s law of cooling to model convective heat transfer between the air and the solid boundaries, Fourier’s law to model conductive heat transfer through the floor and ceiling, and a linear version of the Stefan–Boltzmann law to model radiative heat transfer from the ceiling to the floor. The effectiveness of the model was assessed using experimental results obtained in a full-scale test room. In these experiments, the vertical temperature stratification was measured using an array of T-type thermocouples. Speed measurements were obtained to estimate the ventilation flow rate (for displacement ventilation) and the velocity profile across the doorway (for doorway ventilation). Buoyancy was introduced using a twin-hob (∼2.35 kW) heat source, and in most cases a diffuse two-layer temperature stratification developed. The results from these experiments are compared with the model and existing adiabatic models. Our results indicate that the effect of heat transfer at the boundaries on the final stratification is significant and should not be ignored. Furthermore, direct comparisons between the measured and modelled results are in general very good.     

Sensitivity of air change rates in a naturally ventilated atrium space subject to variations in external wind speed and direction

Design guidelines for natural ventilation (NV) in buildings focus on the potential hourly air change (ACH) rates based on the building space parameters. Critically, external airflow data is often assumed on the basis of a single mean wind speed and an associated prevailing wind direction. This can result in significant variation in ventilation rates and comfort conditions when non-design external wind conditions prevail. This paper describes a CFD study aimed at examining the influence of variations in external wind speed and direction on the air change rate for the atrium space of a two-storey naturally ventilated building. The building atrium is ventilated by a series of entry vents on one wall of the building in conjunction with roof vents. External wind speeds from 25 to 250% of the mean site wind speed (5.7 m/s) were examined and found to result in an almost linear increase in the ACH rate. For a single wind speed, the relationship between wind direction and the ACH rate was also found to be approximately linear for wind directions between 0° and 90° (orthogonal and parallel) to the wall vent openings, but non-linear for other wind directions (90–135°). More generally, the significant variation in the atrium ACH rate with changes in external wind conditions, evident in this particular building model, illustrates the importance of considering non-design wind conditions when designing NV buildings.     

A review of natural convection and heat transfer in attic-shaped space

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.     

采用吸湿剂处理湿空气的流程优化分析

分析了吸湿剂与空气直接接触的热湿传递过程,得到了溶液与空气热湿传递过程中传热、传湿阻力表达式,比较了溶液-空气不同进口状态的传热、传湿不匹配系数.在溶液进口状态等浓度线上进行的热湿传递过程传热、传湿不匹配系数最小,而等焓线上发生的过程的传热、传湿不匹配系数较大.在构建溶液与空气的热湿处理流程中,应尽可能使除湿与再生过程贴近溶液等浓度线进行.固体吸湿剂与溶液之间的性质差异使得固体吸湿剂与空气间的热湿传递过程一般沿着等焓线发生.