Development of new and validation of existing convection correlations for rooms with displacement ventilation systems

Building airflow, thermal, and contaminant simulation programs need accurate models for the surface convective boundary conditions. This is, especially, the case for displacement ventilation (DV) systems, where convective buoyancy forces at room surfaces significantly affect the airflow pattern and temperature and contaminant distributions. Nevertheless, for DV, as a relatively new ventilation system, the convective correlations are adopted from more traditional mixing ventilation correlations, or non-existent. In this study, the existing recommended correlations are validated in a full-scale experimental facility representing an office space. In addition, new correlations are developed for floor surfaces because the current literature does not provide necessary correlations, even though, the floor surface is responsible for >50% of the total convective heat transfer at the envelope. The convective correlations are typically functions of a surface-air temperature difference, airflow parameters, and characteristic room dimensions. Validation results show that the floor convection correlations expressed as a function of volume flow rate are much stronger than the correlations expressed as a function of a temperature difference between the surface and local air. Consequently, the new convection correlation for floor surfaces is a function of the number of hourly room air changes (ACH). This correlation also takes into account buoyant effects from local floor heat patches. Experimental data show that the existing correlation can be successfully applied to vertical and ceiling surfaces in spaces with DV diffuser(s). Overall, the new and the existing convection correlations are tabulated for use in building simulation programs, such as annual energy analyses or computational fluid dynamics.     

下送上回通风方式供冷与供暖运行性能比较

下送上回通风方式目前得到了广泛的研究应用,其供冷运行时就是置换通风,但同样一套通风系统在一些地区的寒冷季节则有可能需要作供暖运行.为了获得下送上回通风系统在分别作供冷与供暖运行时的具体性能参数,本文应用实验测试与计算流体力学(CFD)模拟的方法研究了置于环境实验室内的某办公环境.研究中分析比较了该办公环境内的空气速度、温度以及追踪气体污染物的浓度分布.研究结果表明,下送上回通风方式作供冷运行时空气温度及污染物浓度分层现象明显,空气处于半混合状态,置换效果较好;作供暖运行时,温度及污染物浓度趋于均匀,通风系统性能接近于混合送风系统,不具备良好的抑制交叉污染的能力.     

An investigation of air inlet types in mixing ventilation

Previous studies have shown that dispersion of contaminant concentrations strongly depends on air inlet types. However, these studies were performed computationally, not experimentally. Thus, the purpose of the current research is to obtain contaminant concentrations in a room, to perform qualitative and quantitative comparison for a wall jet (WJ) air inlet and a ceiling diffuser (CD) air inlet, and to determine more efficient inlet and outlet configuration.     

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.     

Influence of air supply parameters on indoor air diffusion

This paper presents the field distributions of air velocity, temperature, contaminant concentration, and thermal comfort in an office with displacement ventilation for different air supply parameters such as the effective area, shape, and dimension of the diffuser and the turbulence intensity, flow rate, and temperature of the air supplied. The research is conducted numerically by using an airflow computer program based on a low-Reynolds-number k-ε model of turbulence. It can be concluded that the effective area, shape, and dimension of the diffuser and the turbulence intensity of the air supplied have little effect on the room air diffusion except at floor level. The influence of the flow rate and temperature of the air supplied is very significant on the air diffusion as well as on the thermal comfort and indoor air quality.     

Removal of contaminants released from room surfaces by displacement and mixing ventilation: modeling and validation

This paper presents the experimental and numerical modeling of contaminant dispersion in a full-scale environmental chamber with different room air distribution systems. For the experimental modeling, an area source with uniform emissions of a hypothetical contaminant (SF6) from the entire floor surface is designed and constructed. Two different types of ventilation are studied: displacement and mixing ventilation. A computer model for predicting the contaminant dispersion in indoor spaces was validated with experimental data. The validated model is used to study the effects of airflow and the area-source location on contaminant dispersion. Results show that the global airflow pattern has a strong impact on the distribution of the contaminants. In general, the personal exposure could be estimated by analyzing the relative source positions in the airflow pattern. Accordingly, the location of an exhaust diffuser may not greatly affect the airflow pattern, but can significantly affect the exposure level in the room. PRACTICAL IMPLICATIONS: When designing ventilation in addition to bringing fresh air to occupants, it is important to consider the removal of contaminants released in the off-gassing of building materials. Typical indoor off-gassing examples are emissions of volatile organic compounds from building enclosure surfaces such as flooring and painted walls. In this study, we conducted experimental and numerical modeling of different area sources in a mock-up office setup, with displacement or mixing ventilation. Displacement ventilation was as successful as mixing ventilation in removing the contaminant source from the floor area. Actually, the most important consideration in the removal of these contaminants is the relative position of the area source to the main airflow pattern and the occupied zone.     

The moisture content distribution of a room with radiant ceiling cooling and wall-attached jet system

Radiant ceiling cooling is a comfortable and energy saving air conditioning system, but condensation on the panels limits its application. The moisture distribution of a radiant cooling room is one of the key factors affecting the condensation of radiant panels. This paper uses a simulation model validated by experimental data to investigate the influence of wall-attached jets on the moisture content distribution of a room with radiant cooling. The influence of diffuser types and air velocities on the moisture content distribution is analyzed in the pre-dehumidification stage. This paper also studies the moisture content distribution and dew point temperature variation of the attached air layer with the cooling system operating with and without pre-dehumidification. The results show that a linear diffuser has a better attach effect, and the moisture content distribution is more uniform. The time it takes to reduce the moisture content to the design value of different diffusers is similar, approximately 25 min. With increasing air velocity and decreasing linear diffuser length, the non-uniformity coefficient (NUC) of the moisture content distribution increases and the pre-dehumidification time becomes longer. When the radiant cooling system operates without pre-dehumidification, the attached layer moisture content is high at the beginning, which may cause condensation in the first 7 min. Pre-dehumidification can decrease the moisture content and dew point temperature and effectively prevent condensation.     

Performance of three air distribution systems in VOC removal from an area source

Building-related health complaints and symptoms represent a significant occupational health problem. Elevated concentrations of various types of indoor pollutants, frequently associated with inadequate ventilation, have been implicated as a potential cause. The objective of this research is to model and evaluate the performance of several ventilation methods in pollutant removal from indoor environments. Pollutant sources are assumed to be at the floor level, one with a constant emission rate and the other a fast decaying source (volatile organic compound emissions from a wood stain). Three ventilation methods, namely displacement ventilation and two mixing systems using a side grille and ceiling square diffuser respectively are studied. A computer model has been applied to simulate the distributions and the time history of the pollutant concentrations in a mockup office. Experimental data of velocity, temperature, and tracer gas concentration distributions in the chamber with the displacement diffuser are obtained to validate the airflow model. Simulation results show that different ventilation methods affect the pollutant distributions within the room. When the pollutant sources are distributed on the floor and not associated with a heat source or initial momentum, displacement ventilation behaves no worse than perfect mixing ventilation at the breathing zone. Conventional “mixing” diffusers, on the other hand, could perform better or worse than a perfect mixing system. The computer model could be used for selecting appropriate ventilation systems to maximize indoor air quality for occupants.     

Respiratory symptoms, perceived air quality and physiological signs in elementary school pupils in relation to displacement and mixing ventilation system: an intervention study

Schools may be poorly ventilated and may contain furry pet allergens, particles and microorganisms. We studied health effects when changing from mixing ceiling ventilation to two types of displacement ventilation, front ventilation system (FVS) and floor master system (FMS). The study included pupils in three elementary school classes (N = 61), all with floor heating. One class received blinded interventions; the two others were unchanged (controls). Ventilation flow and supply air temperature was kept constant. The medical investigation included tear film stability (BUT), nasal patency and a questionnaire containing rating scales. When changing from mixing ventilation to FVS, the pupils (N = 26) perceived better air quality (P = 0.006) and less dyspnoea (P = 0.007) as compared to controls (N = 35), and BUT was improved (P = 0.03). At desk level, mean CO(2) was reduced from 867 to 655 ppm. Formaldehyde and viable bacteria were numerically lower, while total bacteria and molds were higher with displacement ventilation. There was no difference in symptoms or signs when changing from FVS to FMS. Cat (Der p1), dog (Can f1) and horse allergen (Equ cx) were common in air at all conditions. In conclusion, displacement ventilation may have certain positive health effects among pupils, as compared to conventional mixing ceiling systems. PRACTICAL IMPLICATIONS: Displacement ventilation may be a suitable ventilation principle for achieving good indoor environment in classrooms. The type of supply air diffuser does not seem to be of major importance. The combination of floor heating and displacement ventilation can be a useful way of avoiding the previously described problem of thermal discomfort.     

大空间建筑室内气流组织数值模拟与舒适性分析

分别对采用百叶侧送侧回、喷口侧送侧回、散流器顶送下回、分层空调、置换通风方式的大空间建筑空调室内气流的速度场和温度场进行了数值模拟,并对其结果进行了实验验证。根据ADPI指标对这几种送回风方式进行了热舒适性评价.结果表明,分层空调和置换通风是大空间建筑中较好的气流组织方式。     

Comparison of gaseous contaminant diffusion under stratum ventilation and under displacement ventilation

The gaseous contaminant diffusion under stratum ventilation is investigated by numerical method which is validated by experiments carried out. The concentration of gaseous contaminants along the supply air jet is found to be lower than the other parts of the room. Compared with displacement ventilation, the formaldehyde concentration in breathing zone is lower when a contaminant source locates close to the occupant. The concentration is at the same level when the contaminant source locates up-steam to the occupant. The concentration in the occupied zone (<1.9 m from the floor) is also lower when the contaminant source locates on the floor. At supply air temperature optimized for displacement ventilation, the toluene concentration in breathing zone for stratum ventilation is higher than that for displacement ventilation when the area source locates on the four surrounding walls of the room.     

剧场空间置换空调系统的应用研究之一:地上侧送风方式

以侧送风方式的剧场空间置换空调系统为考察对象，通过实测，考证了该空调方式室内温度分布的特征及通风换气效率的优越性。     

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.     

带冷地面(顶板)置换通风系统回归公式分析

近年来置换通风技术逐步推广运用到各个领域.目前比较常用的是冷地板或者冷顶板分别与置换通风相结合.本文以CFD模型为基础,针对这种带有冷地面(顶板)的置换通风系统提出了一种数据回归方程的设计准则,同时就三种空调方式中风系统的供冷能力以及所能达到的热舒适程度等问题进行对比分析.     

洁净空间新型气流分布方式的机理和特性研究

在阻漏层概念的指导下,建立了１：１的模型。模型试验结果证实了理论上的预见,在高效过滤器不装在末端（如当层高不够时）而又有一定渗漏的情况下,新的末端将使室内含尘浓度降低５０％￣７０％。计算机模拟计算表明了新型末装置的气流分布方式的优越性。     

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.     

CFD assessment of intake fraction in the indoor environment

In this paper we use a validated CFD model to calculate the inhalation exposure, expressed as an intake fraction (iF), of a seated person in an office with different contaminant sources, a floor diffuser, and a ceiling vent. These sources include the floor, the walls, a desk, and the human body. First, experimental data is used to determine the correct turbulent Schmidt number for the computational model to predict the transport of the species in an indoor environment. It was found at a turbulent Schmidt number of ∼0.9 produced the best fit when compared to experimental data. Then, the iF was calculated for two representations of the computer simulated person (CSP): a CSP with detailed surface geometry, and a simplified CSP with multi-block geometry. It was found that the simplified multi-block geometry is not adequate for predicting iF because it radically changes the flowfield of the thermal plume in the breathing zone (BZ). Next, the effect of personal ventilation systems on iF was investigated. The results show that such systems can reduce the iF by an order of magnitude compared with conventional mixing and displacement ventilation systems. Finally, a comparison of iF results were made for a surface body temperature of 32 °C and 28 °C. It was found that a 4 °C change in body surface temperature influenced the iF by less than 10%.     

北京某别墅土壤源热泵空调系统设计

该别墅选择土壤源热泵为空调冷热源，空调系统采用辐射地板/顶板 置换通风方式，围护结构采取了节能措施。简要介绍了空调系统设计和特点，分析了实测数据，实际运行表明，别墅冬夏室内温湿度达到了设计要求。     

Air Change Effectiveness and Pollutant Removal Efficiency during Adverse Mixing Conditions

Abstract The air change effectiveness (ACE), an indicator of the indoor airflow pattern, was measured in twenty-six laboratory experiments. Ventilation air was supplied through induction-type diffusers located in the ceiling and removed through a ceiling mounted return grille. The tracer-gas step-up measurement procedure was employed. In five of the experiments, pollutant removal efficiencies were also measured for simulated pollutant emissions from the floor covering and for simulated emissions from occupants. In experiments with heated supply air, supply airflow rates typical of the minimum supply flow rates of VAV ventilation systems, and 100% outside air, the ACE ranged from 0.69 to 0.89. These results indicate that significant short-circuiting of ventilation air between the supply air diffuser and return air grille does occur under these adverse conditions. Mechanical recirculation of air, so that the supply air contained approximately 50% outside air, increased the ACE by about 0.05. When the supply air was cooled, the ACE ranged from 0.99 to 1.15, adding to existing evidence that short-circuiting is rarely a problem when the building is being cooled. The pollutant removal efficiency for simulated pollutant emissions from the floor covering (PRE_floor) was strongly correlated with ACE (R²= 0.98) and the values of PRE_floor were within approximately 0.1 of the values of ACE. The pollutant removal efficiency for simulated pollutant emissions by occupants varied between workstations and was not as well correlated with the ACE.     

一种高温离心式压缩机扩压器的改进与优化

分析对比了无叶扩压器和叶片扩压器的特性,探讨了一种高温离心式压缩机中采用低稠度叶片扩压器替代无叶扩压器的设计方案,并通过压缩机的性能测试验证了该方案的可行性。