A new approach on zonal modeling of indoor environment with mechanical ventilation

This paper reports the development of a new zoning approach based on room air age, a parameter that indicates the mixing condition of the air. Zoning criteria are developed based on the deviation ratio of air age as well as location of the key source that is of concern (e.g., temperature, air pollutant). By integrating the zonal model with other models such as dynamic models for heat and moisture transfer, and source/sink models for air pollutant, the dynamic characteristics of indoor parameters such as air temperature, humidity, and pollutant concentrations can be simulated. A case study was presented for a displacement ventilated room, and simulation results using the new zonal model were compared with those using a computational fluid dynamics (CFD) model and a conventional zonal model. Results demonstrated that the new zonal model is more accurate in calculating the zonal temperature distributions than the conventional zonal model. The model is suitable for dynamic simulations (e.g., whole year) of indoor environmental parameters.     

CFD modelling of natural displacement ventilation in an enclosure connected to an atrium

Computational fluid dynamics (CFD) is used to investigate buoyancy-driven natural ventilation flows in a single-storey space connected to an atrium. The atrium is taller than the ventilated space and is warmed by heat gains inside the single-storey space which produce a column of warm air in the atrium and drive a ventilation flow. CFD simulations were carried out with and without ventilation openings at the bottom of the atrium, and results were compared with predictions of analytical models and small-scale experiments. The influence of key CFD modelling issues, such as boundary conditions, solution controls, and mesh dependency were investigated. The airflow patterns, temperature distribution and ventilation flow rates predicted by the CFD model agreed favourably with the analytical models and the experiments. The work demonstrates the capability of CFD for predicting buoyancy-driven displacement natural ventilation flows in simple connected spaces.     

Application of integrating multi-zone model with CFD simulation to natural ventilation prediction

Predicting the performance of natural ventilation is difficult, especially for large scale naturally ventilated buildings, due to the lack of accurate and efficient prediction tools. This paper presents a strategy, integrating a multi-zone model and computational fluid dynamics (CFD), to improve natural ventilation prediction and design methods. Large openings and atrium configurations are broadly used in naturally ventilated buildings to promote buoyancy force and optimize air movement. How to properly deal with this typical configuration for a multi-zone model and integrated simulation is discussed and compared in this paper. In order to validate a newly developed multi-zone model program, MMPN, this paper investigated both buoyancy ventilation and wind-buoyancy combined ventilation. Integration strategies, transferring data (velocity or pressure) from a multi-zone model program to CFD as boundary conditions, are also studied.     

Investigation of operation performance of air carrying energy radiant air-conditioning system based on CFD and thermodynamic model

This study aims to investigate the operation performance of a new terminal form of radiant air-conditioning system called the air carrying energy radiant air-conditioning system (ACERS). Three summer operation conditions, namely steady condition (without opening door and window), open-door condition and open-window condition, are researched in a residential apartment using experimental, computational fluid dynamics (CFD) simulation and thermodynamic methods. The concept of dynamic synergistic operation of mechanical ventilation driven by the air-conditioning system and natural ventilation driven by the open door or window is proposed. A thermodynamic model formulated by the dynamic enthalpy equation, dynamic temperature equation and dynamic moisture equation is developed to analyze the heat and mass transfer process of the test room under the synergistic operation of mixing ventilation. Moreover, the CFD simulation results are used to analyze the synergistic operation and thermodynamic energy transfer of the test room under the mixing ventilation of ACERS and open door/window. It is indicated that ACERS is an important technology with a low temperature gradient of less than 0.1 °C between the head (1.5 m) and ankle level (0.1 m) and low velocity of approximately 0.1 m/s in the occupied zone under the steady condition. The thickness of the boundary zone under the orifice plate of ACERS under the steady, open-door and open-window conditions is 12, 6, and 8 cm, respectively, which can effectively prevent condensation. This study proves that ACERS is a promising technology for air conditioning in residential buildings in regions with hot and humid summers.     

进风口对变压器室通风效果的影响

通风设计是户内变电站变压器室设计的难点问题。运用CFD方法,对某变压器室温度场和速度场进行模拟,并将模拟值与实测值对比,验证数学模型的有效性。在此基础上,以某变电站变压器室为模拟对象,通过改变进风口位置和面积,设计并模拟6种通风工况,通过对比各工况温度场、速度场和特征温度值的变化规律,重点研究进风口对变压器室通风效果的影响。模拟结果表明：进风口面积不变时,进风口应布置在散热器一侧且其中心高度宜控制在散热器中心高度或稍偏下位置,不宜高于散热器;进风口位置固定时,增大进风口面积改善通风效果时,宜选择沿高度方向增大进风口面积。所得结论可为变压器室通风设计提供技术参考。     

Numerical simulation of transient flow development in a naturally ventilated room

Numerical simulations were conducted to model the transient flow development in a naturally ventilated space containing a centrally located localized source of heat. The simulations were compared with a series of small-scale laboratory experiments and existing theoretical models. The aim of the work was to benchmark CFD models for time-dependent buoyancy-driven natural ventilation against previously published experimental results and theoretical models. The simulations agree well with experimental results during the initial development of the room stratification. The CFD results accurately predict the maximum depth of the hot buoyant layer at the top of the room as well as the steady-state interface height which separates the warm upper buoyant layer from the cooler air below. The simulations also predict well the time taken for the buoyant upper layer to reach its maximum depth. However, at longer times the results diverge. This may be due to thermal diffusion and mixing at the interface between the upper and lower layers due to the inflow from the floor level vents.     

Experimental and numerical study of a mechanically ventilated enclosure with thermal effects

Full-scale experimental and computational fluid dynamics (CFD) methods are used to investigate the velocity and temperature fields in a mechanically ventilated enclosure. Detailed airflow characteristics were measured in three cases of ventilation air temperature: an isothermal case, a hot case and a cold case. The experimental data were used to validate two CFD models: a Reynolds averaged Navier–Stokes equation (RANS) modelling and a large Eddy simulation (LES) modelling. The RANS model provides results in better agreement with experimental data, excepted for the cold case. It has been found that the LES model underestimates the expansion of the jet in the three cases, disabling the use of this model for the prediction of the flow field in ventilated rooms.     

Modelling and control of heat transfer phenomena inside a ventilated air space

In this paper a data based mechanistic (DBM) model is proposed using a simplified heat balance formulation for modelling the temperature distribution inside a full scale ventilated room. The model has a number of parameters which are physically meaningful and determined using time temperature data obtained from experiments for several inlet air flow rates. At the inlet a step input in air temperature is applied and temperature responses at 36 sensor locations were recorded. For all ventilation rates used, the parameters of the model are extracted using statistical identification technique. Later, model based predictive control (MBPC) algorithm is developed to control temperature profiles on pre-selected sensor locations. The developed DBM model is compact in structure and found to capture the temperature distribution with high accuracy. The MBPC, which is distinguished by explicit use of process models, is robust for disturbance and noise effects. Besides it has high tracking capability of the reference trajectory.     

Full-scale test and CFD-simulation of radiant panel integrated with exposed chilled beam in heating mode

Thermal conditions in an office room with a chilled beam having integrated radiant panel (CBRP) were analysed in the full-scale laboratory test and using computational fluid dynamic (CFD) simulation. Thermal conditions in the office room were measured in six heating cases under steady state conditions that had different window surface temperatures and internal heat gain conditions. CFD simulations of two measured cases were done in order to investigate indoor climate conditions in more detail and comparing CFD results to measured results. An additional CFD simulation was done with office desk location to near to window to study thermal conditions below the desk. Ventilation efficiency was studied with CFD-simulation. Indoor climate conditions with all measured cases were at good level. Radiant asymmetry and vertical temperature stratification fulfil the requirements of category A in ISO-7730. The highest room air velocities and draught rate readings were 0.16 m/s and 17% respectively. The results indicate that radiant panel heating is applicable solution also in cold climate. Good thermal conditions could be ensured when the temperature of window surface is at least 14–15 °C. At the same time, vertical temperature stratification is acceptable and thus maintains high energy efficiency. Results indicate also that when room is occupied and ventilation is introduced, the temperature gradient is much smaller compared to unoccupied room space.     

A state space model for predicting and controlling the temperature responses of indoor air zones

Indoor temperature distributions and air flows lead to the variation of local thermal comfort from place to place. To have more precise predictions and better control of the thermal conditions in the working zone where the room occupant sits and works, a both detailed and fast model of the dynamic indoor temperature distributions is needed. Unfortunately, very few papers studied such models due to the complexity of fluid (air) flows. This paper discusses a zonal model which is derived from computational fluid dynamics (CFD) and the output of a CFD code. The model is validated with experimental results. In order to design better control systems, the zonal model is transformed into a state space representation form. One example is given on how the state space model can be used for temperature predictions and more precise temperature controls.     

Optimization of bathroom ventilation design for an ISO Class 5 clean ward

Ventilation is a main method to control the contaminant dispersion within clean wards. In this paper, we investigated the effects of various ventilation designs of the bathroom in an ISO Class 5 clean ward. Specifically, the contaminant dispersion and particle concentrations corresponding to three different ventilation design schemes were characterized and compared using computational fluid dynamics (CFD) analysis. For each design, we examined airflow and particle concentrations for contaminant sources located at two places (i.e., at the toilet seat and on the floor), respectively. Field test was conducted to compare the measured and simulated air velocities and particle concentrations in a hospital clean ward. The implemented CFD modeling of ventilation effects of various designs in this study has proven to accurately characterize airflow and contaminant control in the ventilated space, and has led to optimizing ventilation for the bathroom in an ISO Class 5 clean ward.     

载人航天空间站舱内通风对流换热数值研究进展

CFD模拟是研究载人航天器舱内环境控制和地面模型试验验证的有效方法,介绍了国内外载人航天器舱内通风对流换热数值模拟的研究进展,目前的相关研究涉及舱内不同通风方式的数值模拟、通风参数的优化设计仿真、人体散热对通风环境的影响分析、舱内壁面温度分布和结露控制、微重力下通风换热问题的地面模型试验及其数值模拟验证、传热传质等诸多方面。指出数值模拟模型、舱内通风空调环境评定、通风空调系统整体优化、舱内环境数字仿真演示系统等是载人航天器舱内环境数值模拟领域值得进一步研究的问题。     

不同人体座位模型对大空间空气分布影响

本文对座椅送风系统的气流组织进行模拟研究,首先运用座椅送风的简化模型,模拟了人体座椅实体的简化模型和虚区域热源模型下气流组织的情况,分析了两种情况的温度场,速度场和相对湿度场,发现人体座椅实体模型模拟的剧院气流组织速度、湿度与温度分布分层更加明显,此特点更易于微气候的分区控制,用更少的能量达到人体舒适的满意度。该研究为大空间建筑空调系统优化设计、模拟气流组织提供了方法及理论依据,同时对研究空气分布对空调系统的舒适性和节能方面的影响有一定的帮助作用。     

Interzonal air and moisture transport through large horizontal openings in a full-scale two-story test-hut: Part 2 – CFD study

The aim of this paper is to study the air and moisture transport through a large horizontal opening in a full-scale two-story test-hut with mixed ventilation by means of computational fluid dynamics (CFD) simulations. CFD allows extending the experimental study presented in the companion paper [1] and overcoming some limitations of experimental data. More than 80 cases were simulated for conditions similar to those tested experimentally and for additional ventilation rates and temperature difference between the two rooms. CFD simulations were performed in Airpak and the indoor zero-equation turbulence model was used. The CFD model was extensively validated with the distributions of air speed, temperature and humidity ratio measured across the two rooms, as well as with the measured interzonal mass airflows through the horizontal opening. CFD simulation results show that temperature difference between the two rooms and ventilation rate strongly influence the interzonal mass airflows through the opening when the upper room is colder than the lower room, while warm convective air currents from the baseboard heater and from the moisture source placed in the lower room cause upward mass airflows when the upper room is warmer than the lower room. Finally, empirical relationships between the upward mass airflow and the temperature difference between the two rooms are developed.     

CFD study of the effects of furniture layout on indoor air quality under typical office ventilation schemes

The relative freshness of indoor air in breathing zone can be measured by ventilation effectiveness. Numerous research articles in literature have investigated ventilation effectiveness under different ventilation schemes, different inlet/outlet positions, and different diffusor types. These researches seem to have a goal to find a solution to optimize ventilation effectiveness through manipulating ventilation system. In reality, however, the occupants of a rented office room have no right to manipulate the ventilation system; instead, they have to accept whatever rented to them. An important issue thus arises: how to improve ventilation effectiveness without changing ventilation system? This paper has built a CFD model about a typical office room, validated it by published experimental data in literature, and then applied it to twelve typical office situations/cases of different furniture layouts under different ventilation schemes. The simulation results of twelve cases show that furniture layout is an important factor in indoor airflow and temperature fields, and the quality of air in breathing zone can be significantly improved by adjusting furniture layout without making any change in ventilation system.     

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.     

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.     

侧墙上置送风口置换通风流场及通风效果评价

针对设置侧墙上置送风口置换通风系统的房间,采用CFD软件对室内空气速度场、温度场进行模拟。根据模拟结果,分析贴附射流、冲击射流的发展过程,评价侧墙上置送风口置换通风系统的通风效果。     

Numerical procedure for predicting annual energy consumption of the under-floor air distribution system

As compared with the mixing system, indoor air temperature stratification in the under-floor air distribution (UFAD) system offers an opportunity for cooling load reduction in the occupied zone. This stratification is a major feature that offers the energy saving potential, but it has not been thoroughly taken into account in most energy simulation programs. In this article, a numerical procedure, based on coupling two types of modeling, i.e., CFD (computational fluid dynamic) simulation and dynamic cooling load simulation, is proposed to predict annual energy consumption. The dimensionless temperature coefficient is first defined in the UFAD system and obtained from CFD simulation, based on the boundary conditions from a cooling load program ACCURACY. According to this coefficient, temperature stratification input to ACCURACY is then revised to calculate the updated supply and exhaust air temperatures for final annual energy prediction. To demonstrate the method, a small office room is investigated using Hong Kong weather data. With the constant air volume (CAV) supply in the UFAD system, it is found that the dimensionless temperature coefficient is almost a constant, when the locations of heat sources are fixed. As compared with the mixing system, the UFAD system derives its energy saving potential from the following three factors: an extended free cooling time, a reduced ventilation load, and increased coefficients of performance (COP) for chillers.     

住宅建筑室内热环境的数值模拟研究

以现场调查中测试过的住宅建筑为模型房间，以测试结果为边界条件，建立相应的数学物理模型，利用CFD软件PHOENICS对散热器采暖的室内空气流动进行了数值模拟，给出了空气温度、空气流速和辐射温度的分布，对模拟结果进行了分析并与实测值进行了比较：结果表明空气温度和辐射温度的模拟计算值与实测值吻合较好，而速度的模拟计算值和实测值相差较大。