Numerical study on temperature stratification in a room with underfloor air distribution system

In this study, numerical prediction using computational fluid dynamics (CFD) was utilized to investigate air temperature stratification in a room with an underfloor air distribution (UFAD) system. The numerical modeling using CFD computation was validated with physical test in a full size experimental room with an UFAD system. The different supply air conditions and heat loads were discussed. The results show that the effect of three parameters, heat load, supply volume flux and supply air velocity, on room air temperature would be expressed by the length scale of the floor supply jet. When the length scale increased from 0.8 to 1.56 m, the ratio of vertical temperature difference between 2.5 and 0.1 m at the occupied zone to the difference between return and supply air temperature decreased from 0.62 to 0.25. When there was only one local heat source in the room, there was a thermal stratified interface at the occupied zone. The interface height was about 1.42 times the length scale. The results may suggest ways to optimize UFAD design and operation.     

Indoor thermal stratification and its statistical distribution

Thermal stratification is established when warmer air rises and cooler air descends under thermal buoyancy. It occurs in indoor environment situations including large warehouse‐type buildings, buoyancy‐driven ventilated spaces with displacement, underfloor ventilation, and/or natural ventilation, and enclosure fires with hot smoke layer on top of cold air layer. This paper reports a recent study that thermal stratification of indoor environment follows the statistical Beta distribution so the vertical temperature distribution is the Cumulative Distribution Function of the Beta distribution defined by two shape parameters, Alpha (α) and Beta (β), despite ventilation types, heat source and other details. It is then possible to estimate a complete vertical temperature profile under thermal stratification by four temperature points (ie, 4‐point Beta distribution), or as few as two points (ie, 2‐point Beta distribution) with a slight loss of accuracy. The study was confirmed by the field measurement data of five warehouse‐type buildings, and eleven thermal stratification studies from the literature. A few applications were demonstrated including quantitative characterization of thermal stratification; estimation of mean and spatial temperature uniformities and other key parameters. The dimensionless nature of the methodology may also be potentially applied to other indoor stratification phenomena.     

地板送风不同送风温差对冷负荷减小系数的影响

利用数值模拟方法对不同送风温差下地板送风系统房间的总热源和单热源进行流场和温度场的模拟,分析了各单一热源冷负荷减小系数的变化规律,模拟结果表明在相同的送风速度下,除了放置于地面上的热源(主机)冷负荷减小系数基本维持在0.81,其它热源的冷负荷减小系数随着送风温差的增大而增大,且分层高度附近的热源冷负荷减小系数受送风温差的影响最大。     

Airflow and air temperature distribution in the occupied region of an underfloor ventilation system

Measurements were carried out in an office-type experimental room ventilated by a floor return-type underfloor ventilation system to investigate the distributions of airflow velocity and air temperature. A fan-powered floor air unit (FAU) with rectangular supply and return air outlets covered by straight-profile linear bar-type air diffusers was installed to deliver the conditioned air in the experimental room. Turbulence intensity and draught rate distributions inside the room were also calculated by using the measured data. From the experimental results, it is found that undesirable high air velocities and high draught rates were created within a small region near the supply outlet of the FAU. Temperature differences between different height levels were maintained within an acceptable comfort level under the tested supply air conditions and heat loads. The results indicated that the temperature stratification could be maintained at an acceptable comfort level by designing the supply air conditions properly. A clearance zone is suggested as a design consideration for locating the FAUs and occupants to avoid undesirable draught discomfort to the occupants.     

地板送风系统中无量纲数对室内CO2浓度的影响

本文对地板送风系统进行数值模拟,通过分析室内温度场、速度场以及不同工况下CO2浓度随空间高度的变化,结果表明不同工况下的CO2排除效率有所不同,通过比较热长度尺度Lm/H和阿基米德数Ar与CO2排除效率的关系曲线,指出热长度尺度Lm/H与阿基米德数Ar都与分层高度有关,当阿基米德数Ar越小,CO2的排除效率变化越明显,当阿基米德数Ar很大的时候,CO2的排除效率几乎不变;而热长度尺度Lm/H不论变大还是变小,其CO2的排除效率都变化很明显。     

地板送风技术条件与舒适条件的研究

本文阐明了下部送风房间热力分层和垂直温度分布规律，给出了确定垂直温度分布曲线的方法及确定最佳送风量的原理。为了计算冷负荷，作者初步推荐了不同位置热源的负荷系数，并且说明了如何决策送风口的型式与数量和相关数据。最后在实验基础上分析了地板空调的热感觉，其结果说明这种送风系统在适宜的风量与温度下能够提供舒适条件，并使风感百分数低于１５％。     

碰撞射流通风供暖房间热环境特征的研究

采用数值模拟方法研究了碰撞射流通风系统的供热特性,对供热房间内热环境特征以及热舒适性的分析结果表明：在典型送风参数工况下,碰撞射流供热房间内靠近送风口附近位置处存在垂直温度分层,空气温度随房间高度增加而降低,工作区热舒适情况良好。     

Comparison of underfloor and overhead air distribution systems in an office environment

This study compares thermal environment of two air distribution systems in an office setting. Airflow, heat and mass (water vapor and contaminant gas) transfer in steady-state condition are modeled for an underfloor air distribution system and an overhead air distribution system. The models include a typical cubicle in a large office floor with a chair, a desk with a personal computer on top, and heat sources such as seated person, desktop computer, and lights. For underfloor air distribution system, cool air enters the occupied zone through an inlet located at the floor level supplying a vertical upward inflow. Three different locations of the inlet diffuser are considered. For overhead air distribution, the inlet is located on the ceiling with slower and cooler inflow. Three inlet angles are considered. For both systems, the air return location is on the ceiling at the same place. Distributions of velocity, temperature, relative humidity, and contaminant concentration in various cases for both systems are computed. Thermal comfort factors are assessed for the two systems. The results are compared among cases of each system, as well as between two typical cases of the two systems and to experimental data for an actual office building given in literature. The results provide a detailed understanding of air transport and its consequence on thermal comfort and indoor air quality that are useful for office building air conditioner design. It is found that underfloor system gives better performance than overhead system in contaminant removal and significantly in energy saving while maintaining the same thermal comfort condition.     

Determination of particle concentration in the breathing zone for four different types of office ventilation systems

Many factors affect the airflow patterns, thermal comfort, contaminant removal efficiency and indoor air quality at individual workstations in office buildings. In this study, four ventilation systems were used in a test chamber designed to represent an area of a typical office building floor and reproduce the real characteristics of a modern office space. Measurements of particle concentration and thermal parameters (temperature and velocity) were carried out for each of the following types of ventilation systems: (a) conventional air distribution system with ceiling supply and return; (b) conventional air distribution system with ceiling supply and return near the floor; (c) underfloor air distribution system; and (d) split system. The measurements aimed to analyse the particle removal efficiency in the breathing zone and the impact of particle concentration on an individual at the workstation. The efficiency of the ventilation system was analysed by measuring particle size and concentration, ventilation effectiveness and the indoor/outdoor ratio. Each ventilation system showed different airflow patterns and the efficiency of each ventilation system in the removal of the particles in the breathing zone showed no correlation with particle size and the various methods of analyses used.     

A note on energy saving in heated enclosures

When a closed space is heated, thermal stratification can be established such that the temperature at floor level is lower than that near the roof. There are two methods by which, during heating, the temperature at floor level can be raised to provide a convenient microclimate at the lower region. The first is by providing a surplus of heat to the space without destroying the thermal stratification, and the second is by mixing mechanically the air (while heating) to establish a uniform temperature distribution. A simple one-dimensional model is developed to calculate the energy required by each method. The results show that air mixing requires much less energy than surplus heating and is therefore a more economical method. In both methods, surplus heating and air mixing, the energy needed increases with the initial vertical temperature gradient existing in the heated enclosure and with the enclosure height.     

架空地板送风系统技术探讨

探讨了架空地板送风系统的工作原理和特点，分析了该系统的送风静压层和送风特性，并从这种送风特性出发，综述了架空地板送风系统在室内空气品质、热舒适性和节能等方面的特点。通过分析，得出架空地板送风系统能够减小空气龄，提高换气效率和通风效率，显著改善室内空气品质；在热舒适方面，架空地板送风系统可以通过使用者调节出风方向和风量达到热舒适：在能耗方面，架空地板送风系统由于仅需维持工作区环境参数，使得送风量和风机压头减小，制冷机效率提高，在风机能耗和冷热源能耗上有着显著的经济效益。最后得出了架空地板送风系统将被广泛应用的结论。     

某机加工车间空调设计的优化

论文针对无锡某项目机加工车间空调系统设计的落地式射流柜机无法满足现场安装尺寸要求的情况,提出将空调由落地式射流柜机改为吊装射流机组,由外墙向厂房中间射流。同时,为保证靠外墙处人员的空调要求及制热时热风上升的问题,在吊装射流机组后端连接竖向风管并在低位设置回风口。为确定合适的吊装高度以满足人员区域温度及风速要求,利用CFD数值模拟方法分别对2种吊装高度下的设计方案进行气流组织分析。通过分析对比,得到满足设计要求的空调方案。     

下送风空调技术原理及应用探讨

本文针对下送风空调的两种主要形式——置换通风和地板送风就其各自工作原理、室内空气分布特性、优点及存在问题等进行了分析比较,并对两种空调送风方式的适用性进行了分析,为合理设计和采用下送风技术提供参考。     

Displacement Ventilation - the Influence of the Characteristics of the Supply Air Terminal Device on the Airflow Pattern

Displacement ventilation is acknowledged to be an efficient system for the removal of contaminants and excess heat from occupied zones of rooms. However, airflow rates, temperature and the design of the air supply device strongly influence the parameters which determine thermal comfort. This paper reviews experiments and theoretical models which show the connection between these parameters. The width and shape of the air supply device have been varied, and a porous media has been used on the inlet area of the air supply device. The velocity and temperature profiles have been measured. The results presented show also that the flow can be described with respect to width and form of the profiles for temperature and velocity. The flow does not operate like a turbulent jet due to thermal stratification. It is shown that the Archimedes number of the supply air is the parameter which determines the air velocity in the area close to the floor. (The Archimedes number is here defined as the ratio between buoyancy and inertia forces.) The results show that it is possible to remove considerable amounts of excess heat from a room, typically 40-50 W/m², without exceeding the limits for thermal comfort. However, this requires relatively high airflow rates and supply air terminal units at least along one of the walls.     

下送风空调房间的数值模拟

介绍了一种下送风空调系统的设计方法,利用Airpak软件对该设计方法下的空调房间进行数值模拟分析。通过模拟结果得到,下送风空调房间在垂直方向出现温度分层现象,下部工作区温度较低,这使得下送风空调比传统全室性空调系统节能;下送风空调在下部工作区具有较好的热舒适性;下部工作区空气龄较小,能够改善工作区的空气品质。模拟结果证实了该设计方法可靠有效。     

Floor heating and cooling combined with displacement ventilation: Possibilities and limitations

Design guidelines envisage that floor heating can be used together with displacement ventilation (DV), provided that the supply air is not overly heated before it can reach heat and contaminant sources. If this is not controlled a mixing flow pattern could occur in the room. The use of floor cooling with DV is also considered possible, although draught risk at ankle level and vertical air temperature differences must be controlled carefully, because they could increase.Few studies on these topics were found in the literature.An indoor environmental chamber was set up to obtain measurements aimed at analysing the possibilities and limitations of combining floor heating/cooling with DV. Air temperature profiles, air velocity profiles, surface temperatures and ventilation effectiveness were measured under different environmental conditions that may occur in practice. These values were compared to equivalent temperature measurements obtained using a thermal manikin.The measurements show that floor heating can be used with DV, obtaining high ventilation effectiveness values. A correlation between the floor heating capacity and the air temperature profile in the room was found. Measurements showed that floor cooling does not increase draught risk at ankle level, although it does increase vertical air temperature differences.     

置换通风几个问题的讨论

分析了讨论下部送风与置通风的关系,热力分层高度的计算财面空气层温升与工作区温升的确定,送风量计算等问题。     

置换通风热力分层高度的数值研究

根据置换通风系统的工作原理,介绍了置换通风热力分层高度的定义及数值计算方法,并对送风温度、送风速度等因素对热力分层高度的影响作了定性分析.在此基础上拟合出量纲一热力分层高度的经验公式.     

车站高大空间空调系统气流组织与热舒适性分析

兼顾人体热舒适和建筑节能的要求,对目前车站高大空间空调气流组织的数值模拟研究报道进行对比分析。分析结果显示,人们对高大空间建筑室内热舒适要求高于居住建筑和办公建筑;从满足人体热舒适角度出发,空调送风加地板辐射供冷方案适于夏季满员工况,地板辐射供热加空调加湿方案适用于冬季满员工况;高大空间的空调系统适宜采用上送上回的送风方式,其温度场和速度场均优于上送下回的空调送风方式;在高大空间内设置分层空调系统将在一定程度上降低空调能耗,且分层空调中送风速度对分层界面的位置影响较大,送风温差对高大空间分层空调的温度分布和流场分布有较大影响。     

两种典型地板送风系统形式热舒适模拟分析

通过模拟地板送风系统的两种典型系统形式,从获得的温度场、速度场、PMV场表明,下送上回系统较容易满足垂直空气温差的限值,下送下回系统容易超出垂直空气温差的限值,在低热力长度尺度（lm/H）时下送下回系统比下送上回系统容易获得舒适的环境,在高热力长度尺（lm/H）时,两种系统形式均易产生偏冷的室内环境。