Field study on occupant comfort and the office thermal environment in rooms with displacement ventilation

A field survey of occupants' response to the indoor environment in 10 office buildings with displacement ventilation was performed. The response of 227 occupants was analyzed. About 24% of the occupants in the survey complained that they were daily bothered by draught, mainly at the lower leg. Vertical air temperature difference measured between head and feet levels was less than 3 degrees C at all workplaces visited. Combined local discomfort because of draught and vertical temperature difference does not seem to be a serious problem in rooms with displacement ventilation. Almost one half (49%) of the occupants reported that they were daily bothered by an uncomfortable room temperature. Forty-eight per cent of the occupants were not satisfied with the air quality. PRACTICAL IMPLICATIONS: The PMV and the Draught Rating indices as well as the specifications for local discomfort because of the separate impact of draught and vertical temperature difference, as defined in the present standards, are relevant for the design of a thermal environment in rooms with displacement ventilation and for its assessment in practice. Increasing the supply air temperature in order to counteract draught discomfort is a measure that should be considered carefully; even if the desired stratification of pollution in the occupied zone is preserved, an increase of the inhaled air temperature may have a negative effect on perceived air quality.     

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.     

Modelling of cowl performance in building simulation tools using experimental data and computational fluid dynamics

Exhaust cowls are used in conjunction with hybrid ventilation systems to efficiently convert wind energy into negative pressure and thus minimize the electrical energy required by the extract fan. Yet the fact that cowl performance is largely dictated by operating conditions imposes particularly stringent demands on modelling. This paper demonstrates, by way of a concrete example, the need for and potential benefits of a new methodological approach to the modelling of cowls. The study focuses on a specific modelling strategy, applied within a building simulation program, for a cowl used in a hybrid ventilation system. The method is progressively simplified to produce four variants, which chiefly vary according to their level of detail and, hence, the associated modelling effort. Wind pressure coefficients at facade, above roof and in the cowl are needed for all model variants. Some of the investigated variants rely on CFD computations of airflow around the building to determine these values. This study uses the example of a single-family house (SFH) to identify those criteria requiring particular attention in the performance of CFD numerical flow analyses. All four variants are examined on the basis of this example to determine which simplifications to the model are appropriate and permissible without unduly compromising the accuracy of the results.     

Evaluation of buoyancy-driven ventilation in atrium buildings using computational fluid dynamics and reduced-scale air model

This research focuses on developing a reliable methodology for predicting the performance of buoyancy-driven ventilation in atrium buildings during the design stage using both computational fluid dynamics (CFD) and scale model tests. The results show several features. First, the agreement between CFD simulation and measurement results in the heated zone is better with rng k–? and zero-equation turbulent schemes; whereas, in the atrium space, the laminar and zero-equation CFD models provide better results. Second, the external ambient temperature has a larger effect on the temperature distribution in the atrium space than the thermal load inside the building. Third, the position of the stack openings that create a direct ventilation path can improve the internal thermal environment. The size of the stack openings also affects the temperature distribution in the atrium space. Lastly, due to the small temperature difference in hot and humid climates, a buoyancy-only ventilation strategy is not very effective in such a situation. That is, when a low-rise atrium building is situated in a hot and humid environment, additional efforts such as wind-driven ventilation, wind-buoyancy ventilation or mechanically driven ventilation will be necessary to achieve the thermal comfort desired.     

A new model on simulating smoke transport with computational fluid dynamics

A new model is proposed for simulating smoke movement induced by a fire. Smoke is taken as a collection of particles with size described by a certain distribution function. Movement of the particles will be studied by dividing the physical problem into two parts: solid phase and air phase. The Lagrangian approach is used for studying motion of the solid phase, with the air flow simulated by computational fluid dynamics (CFD). Interaction between the air phase and the solid phase will be described by the particle-source-in-cell method. k–ε types of turbulence models are used in the simulation of air flow. Some of the results are also compared with the fire dynamics simulator model based on large eddy simulation developed at the Building and Fire Research Laboratory, National Institute of Standards and Technology, USA. Application of the model for designing smoke management system is also illustrated. This model should be useful for designing smoke management system as it describes an intermediate step while using CFD.     

Residential hybrid ventilation: Airflow and heat transfer optimisation of a convector using computational fluid dynamics

Hybrid ventilation systems suitable for residential applications are being developed to reduce the energy demand of the housing sector. This paper describes the development and validation of a computational fluid dynamics (CFD) model of a convector unit that is a component of an existing residential hybrid system. The system incorporates a wall-mounted convector unit that controls ventilation airflow rate and air temperature. Airflow is provided by natural driving forces; a mechanical exhaust fan is used at times of low natural driving forces. The CFD model was used to study the aerodynamics and heat transfer processes of the convector unit with the aim of optimising system performance. Based on the modelling results, alterations to the geometry of a set of louvre blades inside the convector unit are suggested. The new louvre geometry prevents the formation of an airflow separation zone inside the convector unit. This improvement reduces the energy requirements of the system by reducing the convector air resistance by 20% and by increasing the thermal effectiveness of its heat exchanger.     

焊接厂房置换通风效果试验

在正常工况下,对某焊接厂房进行了通风效果试验,现场测试了厂房温湿度、臭氧浓度、粉尘浓度等参数,发现采用置换通风的方式,厂房的速度流场均匀,但臭氧浓度、粉尘浓度均没有达到设计要求,稍微偏高,这与送风速度有关,试验结果为高大厂房置换通风气流组织的研究提供了可靠的数据资料。     

Using computational fluid dynamics to model sail interaction—the ‘slot effect’ revisited

This paper investigates the effects of interaction between the foresail and the mainsail on a sailing vessel, using Reynolds Averaged Navier Stokes based Computational Fluid Dynamics to provide flow visualisation and quantitative analysis. The effect of the foresail sheeting angle upon a sailing rig's performance is compared and discussed. The phenomena of ‘Upwash’ and mainsail separation are investigated in an attempt to demonstrate both visually and quantitatively the importance of the foresail and its correct orientation in maximising lift. The paper hopes to finally lay to rest the misconceptions surrounding the ‘slot effect’ between sails, showing how the foresail increases upwash and reduces the velocity of the flow in the slot. It aims to clarify the theory behind sail interaction.     

Quality control of computational fluid dynamics in indoor environments

Computational fluid dynamics (CFD) is used routinely to predict air movement and distributions of temperature and concentrations in indoor environments. Modelling and numerical errors are inherent in such studies and must be considered when the results are presented. Here, we discuss modelling aspects of turbulence and boundary conditions, as well as aspects related to numerical errors, with emphasis on choice of differencing scheme and computational grid. Illustrative examples are given to stress the main points related to numerical errors. Finally, recommendations are given for improving the quality of CFD calculations, as well as guidelines for the minimum information that should accompany all CFD-related publications to enable a scientific judgment of the quality of the study.     

Validation of computational fluid dynamics simulations for atria geometries

Atria are becoming an increasingly common feature of new buildings. They are often included for their aesthetic appeal; however, their effect on building indoor environment can be significant. Building simulation tools have the potential to assist designers in enhancing energy efficiency by providing information on the temperature and velocity fields inside the atrium for specified geometries and ambient conditions. The unique nature of the physical phenomena that govern the complex flows in atria, however, are not usually considered in traditional building energy simulation programs. These physical phenomena include turbulent natural convection, radiative heat transfer and conjugate heat transfer. Computational fluid dynamics (CFD) has the potential for modeling fluid flow and heat transfer resulting from the phenomena; however, careful validation is required in order to establish the accuracy of predictions. This paper provides a systematic validation of a commercial CFD code against experimental measurements of the underlying physical phenomena. The validation culminates in the simulation of an existing atrium. This work indicates that CFD can be used to successfully simulate the heat transfer and fluid flow in atria geometries and provides recommendations regarding turbulence and radiative heat transfer modeling.     

Application of computational fluid dynamics in building performance simulation for the outdoor environment: an overview

This article provides an overview of the application of computational fluid dynamics (CFD) in building performance simulation for the outdoor environment, focused on four topics: (1) pedestrian wind environment around buildings, (2) wind-driven rain on building facades, (3) convective heat transfer coefficients at exterior building surfaces and (4) air pollutant dispersion around buildings. For each topic, its background, the need for CFD, an overview of some past CFD studies, a discussion about accuracy and some perspectives for practical application are provided. This article indicates that for all four topics, CFD offers considerable advantages compared with wind tunnel modelling or (semi-)empirical formulae because it can provide detailed whole-flow field data under fully controlled conditions and without similarity constraints. The main limitations are the deficiencies of steady Reynolds-averaged Navier–Stokes modelling, the increased complexity and computational expense of large eddy simulation and the requirement of systematic and time-consuming CFD solution verification and validation studies.     

Evaluating long-duration blast loads on steel columns using computational fluid dynamics

Abstract

Long-duration blasts are typically defined by positive pressure durations exceeding 100?ms. Such blasts can generate dynamic pressures (blast winds) capable of exerting damaging drag loads on comparatively slender structural components such as columns. With limited drag coefficient availability for specific structural geometries, Computational Fluid Dynamics (CFD) can be the only satisfactory approach for analysing blast loading on user-specified, finite geometries. The ability to analyse long-duration blasts with commercially available CFD programmes is still not confidently offered, with no prior studies examining the accuracy of modelling interaction with relatively much smaller, finite geometries. This paper presents a comparative investigation between numerical and experimental results to assess the predictive capacity of inviscid Eulerian CFD as a method for calculating long-duration blast drag loading on finite cross-sectional geometries. Full-scale long-duration blast experiments successfully measured surface pressure–time histories on a steel I-section column aligned at four orientations. Calculated pressure–time histories on exposed geometry surfaces demonstrated good agreement although reduced accuracy and under-prediction occurred on shielded surfaces manifesting as overestimated net loading. This study provides new understanding and awareness of the numerical capability and limitations of using CFD to calculate long-duration blast loads on intricate geometries.     

本科流体力学课程中引入CFD内容的探索与实践

流体力学是大部分工科学生的专业基础必修课,计算流体力学软件是流体力学科学与工程研究的重要工具。首先,论证流体力学教学中适当引入计算流体力学的必要性;其次,从理论基础、教学条件、学时安排、教学内容、学时分配和考核办法等方面论证了在流体力学教学中适当引入计算流体力学的可行性;最后,为了激发学生的学习积极性和主动性,提升教学质量,尝试性地在教学中引入了计算流体力学。结果表明,引入计算流体力学后能激发学生对流体力学的兴趣,提高学生的学习能力,培养学生的创新性思维,拓宽学生的知识面,加深学生对基本理论的理解,从而提高教学质量,学生期末成绩明显提高。文章提出的流体力学课程改革新思路,为今后引入计算流体力学教学提供了必要的准备和参考。     

Modelling of a thermally activated building system (TABS) combined with free-hanging acoustic ceiling units using computational fluid dynamics (CFD)

Thermally Activated Building Systems (TABS) have proven to be an energy-efficient solution to achieve optimal indoor thermal environment in buildings. This solution uses the building mass to store heat and by means of water pipes embedded in the concrete slabs adjust the temperature in the premises. The active surfaces of TABS need to be as exposed as possible, but exposing bare concrete surfaces has a negative impact on the acoustic quality in the premises. Acoustic solutions capable of providing optimal acoustic comfort while allowing the heat exchange between the TABS and the room are desirable. This study focuses on the influence of two types of free-hanging ceiling absorbers (horizontal and vertical) on the cooling performance of the TABS. Different scenarios are investigated for each type of sound absorber. Computational Fluid Dynamics (CFD) simulations are used to illuminate the nature of the heat exchange between the TABS and the room and the occupants. The simulations are validated by comparison with full scale measurements in laboratory conditions. The study shows that for equivalent sound absorption levels, free-hanging vertical sound absorbers have a lower impact on the heat exchange between the room and the TABS compared to free-hanging horizontal sound absorbers. Cold air stagnation between the sound absorber units and the TABS has been identified as the major cause of the cooling performance decrease of the TABS.     

A numerical simulation of wing walls using computational fluid dynamics

Larger window openings in the walls of a building may provide better natural ventilation. However, it also increases the penetration of direct solar radiation into indoor environment. The use of wing wall, one of the green features, is an alternative to create effective natural ventilation. Givoni conducted experiments in a wind tunnel to study the ventilation performance of wing walls. This paper presents a numerical study of the ventilation performance of wing walls using computational fluid dynamics (CFD). Two-dimensional and three-dimensional simulations are compared with the results of the experiments of Givoni. The results indicate that wing wall can promote natural ventilation by increasing the air change per hour and the mean indoor air speed relative to wind speed at various wind speeds and wind directions. The best performance of wing wall is at the wind angle of around 45°. The study also shows that 3D CFD simulation produces similar trend to the experimental results though there are some discrepancies.     

计算流体力学在现代建筑消防设计中的应用

随着现代建筑的发展 ,以往为保证建筑安全而制订的处方式消防规范对建筑设计的自由发挥带来许多有争议的限制 ,从 2 0世纪 80年代末开始 ,各发达国家开始重新审视消防设计管理体制 ,推出了“性能化”设计体系。“性能化”设计体系的原意为“表现性”设计 ,它不简单照搬以前消防设计的技术要求 ,而是依据每一栋新建筑的设计和火灾原理及疏散的具体情况来模拟火灾时建筑的防灾能力 ,消防设计也从各个系统分支向总体集成发展。在模拟火灾的过程中 ,火灾的烟气扩散模拟是最主要的一个分支 ,而烟气扩散模拟依赖计算流体力学来实现 ,因此 ,计算流体力学也成为“性能化设计”的主要工具之一 ,随着现代计算机技术的发展 ,计算流体力学在建筑消防设计中也将发挥越来越重要的角色。结合实际的工程实例论述了计算流体力学在现代建筑消防设计中的应用。     

The assessment of the performance of a windcatcher system using computational fluid dynamics

Natural ventilation is increasingly being used in modern residential buildings to minimize the consumption of non-renewable energy and the reliance on active means for environmental control. Innovative green features such as the windcatcher has made use of natural ventilation in residential buildings for increasing ventilation rate. This paper presents a numerical study of assessment of the performance of windcatcher using computational fluid dynamics. A 500 mm square windcatcher system connected to the room has been modeled for different wind speeds in the range of 0.5–6 m/s and four different wind directions. The numerical results generally agree with the published experimental results of a wind tunnel experiment. The numerical results demonstrate that the windcatcher performance is greatly influenced by the external wind speed and direction with respect to the windcatcher quadrants. In all cases studied, the maximum velocity of air entering the room is close to the external wind speed and the windcatcher system is found to be an efficient way to channel fresh air into the room. The study also shows that the airflow rate of the air entering the room increases with the wind speed and slightly decreases with the wind incidence angle when the wind speed is lower than 3 m/s. In addition, the results show that the uniformity of air inlet decreases with increasing the wind speed and the incidence angle.     

CFD study on effect of the air supply location on the performance of the displacement ventilation system

The purpose of this paper is to investigate using a numerical simulation (computational fluid dynamics or CFD) the effect of the air supply location on the design and performance of the displacement ventilation (DV) system. The results are reported in terms of thermal comfort and indoor air quality. The study focuses on the typical Hong Kong office under local thermal and boundary conditions. This includes the high cooling load used in Hong Kong. Several pollutants typically found in the office such as carbon dioxide and volatile organic compounds (VOCs) were investigated. The results indicate that the supply should be located near the center of the room rather than to one side of the room. This will provide a more uniform thermal condition in the office. The DV system was found to be effective in dispersing VOCs within an office environment for all cases studied. The exhaust was found to have minimal effect on the thermal comfort. For a DV system in Hong Kong, it is possible to use 100% fresh air without extra energy consumption.