Dispersal of exhaled air and personal exposure in displacement ventilated rooms

The influence of the human exhalation on flow fields, contaminant distributions, and personal exposure in displacement ventilated rooms is studied together with the effects of physical movement. Experiments are conducted in full-scale test rooms with life-sized breathing thermal manikins. Numerical simulations support the experiments. Air exhaled through the mouth can lock in a thermally stratified layer, if the vertical temperature gradient in breathing zone height is sufficiently large. With exhalation through the nose, exhaled air flows to the upper part of the room. The exhalation flow from both nose and mouth is able to penetrate the breathing zone of another person standing nearby. The stratification of exhaled air breaks down if there is physical movement in the room. As movement increases, the concentration distribution in the room will move towards a fully mixed situation. The protective effect of the boundary layer flow around the body of a moving person disappears at low speed, and is reduced for a seated person placed nearby due to horizontal air movements, which can also cause rebreathing of exhaled air for the seated person. The results indicate that the effect of the exhalation flow is no acute problem in most normal ventilation applications. However, exhalation and local effects caused by movement may be worth considering if one wishes to contain contaminants in certain areas, as in the case of tobacco smoking, in hospitals and clinics, or in certain industries.     

置换通风在办公建筑中的应用与分析

利用计算流体力学(CFD)方法，对使用置换通风的办公建筑进行计算机数值模拟对室内空气的速度场、温度场进行分析，认为置换通风在提高空内空气品质和节能方面具有一定的效果。     

双热源置换通风系统的实验研究

通过实验手段细致分析研究了置换通风系统中双热源的情况，结果表明：在相同的总热气流送入量下，单热源和双热源形成的温度场有很大的不同，辅热源的存在降低了温跃层的高度和上部区域的温度，分散的热源对置换通风系统不利。讨论了辅热源气流温度、辅热源气流流量、辅热源焓值等影响因素。     

Theoretical and experimental investigation of impinging jet ventilation and comparison with wall displacement ventilation

This paper focuses on evaluating the performance of a new impinging jet ventilation system and compares its performance with a wall displacement ventilation system. Experimental data for an impinging jet in a room are presented and non-dimensional expressions for the decay of maximum velocity over the floor are derived. In addition, the ventilation efficiency, local mean age of air and other characteristic parameters were experimentally and numerically obtained for a mock-up classroom ventilated with the two systems. The internal heat loads from 25 person-simulators and lighting were used in the measurements and simulations to provide a severe test for the two types of ventilation systems. In addition to a large number of experimental data CFD simulations were used to study certain parameters in more detail. The results presented here are part of a larger research programme to develop alternative and efficient systems for room ventilation.     

Air quality in rooms conditioned by chilled ceiling and mixed displacement ventilation for energy saving

A transient-contaminant-transport model is developed for assessing IAQ in the breathing zone when introducing return air into rooms conditioned by CC/DV system to save energy. The steady state transport model of [1] is extended to transient conditions while accounting for significant wall plumes associated with external loads.Experiments are performed to validate the extended model predictions of IAQ expressed in the level of CO₂ concentration. Experiments are conducted in a chamber with two external walls in Kuwait Climate. Measurements are recorded in time of the air temperature and CO₂ concentration at selected locations in the room and compared with values predicted by the model. Experimental results agreed well with model predictions. The maximum errors in predicted CO₂ concentrations are less than ±25 ppm in presence of external load. 60% fresh air fraction resulted in 37% less energy consumption compared with 100% fresh air CC/DV system.The validated model is applied to a case study in Kuwait to evaluate energy saving over the cooling season for a typical office space while using mixed DV air. Energy savings of up to 20.6% can be realized using mixed supply air while maintaining IAQ compared with energy used for the 100% fresh air.     

置换通风系统室内温度分布的实验研究

实验研究了置换通风系统送风参数和环境参数对室内温度场分布特性的影响.测量了不同送风量、送风温度和夹层空气温度下室内不同测点及人体不同部位上的竖直温度分布,并将实验值与CFD模拟结果进行比较,结果表明二者吻合较好.不同参数条件下的实验结果表明,随着送风量的增大,热分层高度相应提高,但送风量达到一定值后,其对竖直温度梯度的影响明显减小;送风温度的变化只对室内整体温度产生影响,而几乎不影响室内温度梯度;夹层空气温度的升高对室内下部温度影响不大,而上部区域温度升高明显.     

冷却顶板对置换通风系统的影响:CFD研究

以计算流体力(CFD)的模型为基础,采用有限容积法对带有冷却顶板的置换通风系统和不带冷却顶板的普通置换通风系统的温度场、气流分布及人体的热舒适性进行了模拟分析,模拟结果表明,冷却顶板-置换通风系统可以减小室内温度梯度,提高人体热舒适性。     

Chilled ceiling and displacement ventilation aided with personalized evaporative cooler

This paper aims at studying the energy impact of a chilled ceiling displacement ventilation CC/DV system aided with a personalized evaporative cooler (PEC) directed towards the occupant trunk and face. A simulation model is developed for integrating the personalized cooler with the ascending thermal plume. The thermal model of the conditioned room air around the person is integrated with a segmental bioheat and thermal comfort model to predict the human thermal comfort.The model is validated with experimental data on the vertical temperature distribution in the room, and the recorded overall comfort perceived by surveyed subjects. Experimental results agreed well with predicted values of temperature and comfort level. When using personalized cooling, the DV supply air temperature can be as high as 24 °C while the PEC at flow rates of 3–10 l/s achieved similar comfort with a DV system at supply temperature of 21 °C. At equal thermal comfort level, the integrated CC/DV system, PEC model resulted in up to 17.5% energy savings compared to the CC/DV system without a PEC. When mixed air is used in the CC/DV system additional 25% savings in energy is realized when compared with energy used for the 100% fresh air without the PEC.     

透过玻璃窗的太阳辐射对置换通风系统的影响的实验研究

采用电热膜或电加热器等模拟太阳辐射透过玻璃窗照射到室内的地板、家具或其它物品上形成的水平、倾斜或垂直的局部热表面，定性和定量地研究这些局部热表面对置换通风系统的影响。研究表明：置换通风系统的热力分层现象总是存在的；热地板降低了温跃层的高度，对置换通风系统有不利影响；倾斜和垂直的局部热表面表现出相同的热力行为，热流量的大小对温跃层高度无影响，只是改变上部区域空气的温度。     

置换式通风房间室温分布的分类表达

置换通风及其衍生形式的发展,提供了改善室内环境及系统性能的可能性。这些性能的评价指标多基于实际的室温竖直分布。然而现有预测室温变化的经验模式又往往存在一些功能上的局限性,难以避免出现较大误差。提出了以特定的近似分析解作为基本分析架构的分类表达法,它有助于辨析室内存在不同类型热源分布及流率条件的室温分布问题。     

单、双热源置换通风系统流场及温度场的数值模拟

利用计算流体力学(CFD)方法,分别对单、双热源置换通风系统进行了数值模拟。分析和比较了两种情况下室内空气的速度和温度分布,指出室内温度场在垂直方向上存在三个温升层;单、双热源形成的垂直温度分布有较大差别;双热源送风参数的计算不能简单按单热源的计算参数进行,应考虑热源分散的影响。     

Optimization of ventilation system design and operation in office environment,Part I: Methodology

Ventilation principles that integrate flexible and responsive elements have grown in popularity in office buildings due to increasing concerns about the impact of indoor environment quality on office workers' well-being and productivity, as well as concerns over the rising energy costs for space heating and cooling in the office building sector. Such advanced elements as underfloor air distribution (UFAD), passive swirl diffusers, and demand controlled ventilation have posed challenges to system design and operation. This paper is concerned with the development and implementation of a practical and robust optimization scheme, aiming to assist office building designers and operators to enhance thermal comfort and indoor air quality (IAQ) without sacrificing energy costs of ventilation. The objective function was constructed in a way attempting to aggregate and weight indices (for thermal comfort, IAQ, and ventilation energy usage assessment) into one indicator. The path taken was a simulation-based optimization approach by using computational fluid dynamics (CFD) techniques in conjunction with genetic algorithm (GA), with the integration of an artificial neural network (ANN) for response surface approximation (RSA) and for speeding up fitness evaluations inside GA loop.     

Performance of “ductless” personalized ventilation in conjunction with displacement ventilation: Impact of intake height

The importance of the intake positioning height above the floor level on the performance of “ductless” personalized ventilation (“ductless” PV) in conjunction with displacement ventilation (DV) was examined with regard to the quality of inhaled air and of the thermal comfort provided. A typical office room with two workstations positioned one behind the other was arranged in a full-scale room. Each workstation consisted of a table with an installed “ductless” PV system, PC, desk lamp and seated breathing thermal manikin. The “ductless” PV system sucked the clean and cool displacement air supplied over the floor at four different heights, i.e. 2, 5, 10 and 20 cm and transported it direct to the breathing level. Moreover, two displacement airflow rates were used with a supply temperature adjusted in order to maintain an exhaust air temperature of 26 °C. Two pollution sources, namely air exhaled by one of the manikins and passive pollution on the table in front of the same manikin were simulated by constant dosing of tracer gases. The results show that the positioning of a “ductless” PV intake height up to 0.2 m above the floor will not significantly influence the quality of inhaled air and thermal comfort.     

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.     

末端装置特性对置换通风工作区速度场与温度梯度的影响

阐述了稍低于室温的空气经靠墙散流器低速进入后，下沉于地面及扩放的概念。介绍了贴地冷气流层速度分布的一般特性，分析了风口特性对地面气流速度场及工作区空气温度梯度的影响。     

Auxiliary ventilation in mining roadways driven with roadheaders: Validated CFD modelling of dust behaviour

The production of dust when driving mining roadways can affect workers health. In addition, there is a decrease in productivity since Mine Safety regulations establish a reduction in the working time depending on the quartz content and dust concentration in the atmosphere.One of the gate roadways of the longwall named E4-S, belonging to the underground coal mine Carbonar SA located in Northern Spain, is being driven by an AM50 roadheader machine. The mined coal has a high coal dust content.This paper presents a study of dust behaviour in two auxiliary ventilation systems by Computational Fluid Dynamics (CFD) models, taking into account the influence of time. The accuracy of these CFD models was assessed by airflow velocity and respirable dust concentration measurements taken in six points of six roadway cross-sections of the mentioned operating coal mine.It is concluded that these models predicted the airflow and dust behaviour at the working face, where the dust source is located, and in different roadways cross-sections behind the working face.As a result, CFD models allow optimization of the auxiliary ventilation system used, avoiding the important deficiencies when it is calculated by conventional methods.     

Numerical 3D simulation of a longitudinal ventilation system: Memorial Tunnel case

In this work, a numerical 3D simulation of a longitudinal ventilation system (LVS) is developed to analyze the fire behaviour inside a road tunnel. The numerical modelling reproduces the Memorial Tunnel, a two-lane, 853 m long road tunnel, used for experimental purposes. On this tunnel, 98 full-scale fire ventilation tests with different ventilation systems were conducted, constituting the first significant experimental approach to analyze fire incidents inside road tunnels. A total number of 24 reversible jet fans were installed in groups of three, nearly equally spaced over the length of the tunnel, and cantilevered from the ceiling of the tunnel.

The validation of a numerical model is developed in the present paper. For that purpose, the behaviour of the smoke generated during a fire incident inside a road tunnel is predicted and compared with previous experimental data collected in the Memorial Tunnel Project. The smoke evolution and the performance of the LVS is simulated with a commercial code, FLUENT, which allows 3D unsteady simulations of the Navier–Stokes equations for multispecies mixtures of gases. A sufficient mesh density was introduced for the spatial discretization in order to obtain accurate results in a reasonable CPU time. Hence, typical ratios between total number of cells and the overall tunnel length were employed in the modelling. As a result, good agreement was achieved in all the tested cases, defining an accurate methodology to predict the performance of a LVS in case of fire inside a tunnel.     

Experimental study of airflow characteristics of stratum ventilation in a multi‐occupant room with comparison to mixing ventilation and displacement ventilation

The motivation of this study is stimulated by a lack of knowledge about the difference of airflow characteristics between a novel air distribution method [i.e., stratum ventilation (SV)] and conventional air distribution methods [i.e., mixing ventilation (MV) and displacement ventilation (DV)]. Detailed air velocity and temperature measurements were conducted in the occupied zone of a classroom with dimensions of 8.8 m (L) × 6.1 m (W) × 2.4 m (H). Turbulence intensity and power spectrum of velocity fluctuation were calculated using the measured data. Thermal comfort and cooling efficiency were also compared. The results show that in the occupied zone, the airflow characteristics among MV, DV, and SV are different. The turbulent airflow fluctuation is enhanced in this classroom with multiple thermal manikins due to thermal buoyancy and airflow mixing effect. Thermal comfort evaluations indicate that in comparison with MV and DV, a higher supply air temperature should be adopted for SV to achieve general thermal comfort with low draft risk. Comparison of the mean air temperatures in the occupied zone reveals that SV is of highest cooling efficiency, followed by DV and then MV.