Wind tunnel investigation on influence of fluctuating wind direction on cross natural ventilation

The performance of wind driven natural ventilation is influenced significantly by the boundary conditions set for the wind. In real conditions the wind direction is fluctuating constantly so it is important to consider this fluctuation in experiments and simulations. This paper investigates the influence of fluctuating wind direction on cross-ventilation using wind tunnel experiments with the aim of improving the evaluation accuracy for natural ventilation. A periodically fluctuating wind direction was designed and reproduced in the experiment. Rapid Response FIDs (Flame Ionization Detector) were used to monitor the concentration of tracer gas. An index named diluting flow rate (DFR) is introduced to evaluate the ventilation performance of this kind of experiment. The results indicate that the DFRs of fluctuating cases are approximately 65–100% of the maximum airflow rate and DFR is influenced by the wind speed, the opening size and the wind direction fluctuation. Informed by the experimental data the mechanism of this combined influence is discussed in this paper.     

Investigating a safe ventilation rate for the prevention of indoor SARS transmission: An attempt based on a simulation approach

This paper identifies the “safe ventilation rate” for eliminating airborne viral infection and preventing cross-infection of severe acute respiratory syndrome (SARS) in a hospital-based setting. We used simulation approaches to reproduce three actual cases where groups of hospital occupants reported to be either infected or not infected when SARS patients were hospitalized in nearby rooms. Simulations using both computational fluid dynamics (CFD) and multi-zone models were carried out to understand the dilution level of SARS virus-laden aerosols during these scenarios. We also conducted a series of measurements to validate the simulations. The ventilation rates (dilution level) for infection and non-infection were determined based on these scenarios. The safe ventilation rate for eliminating airborne viral infection is to dilute the air emitted from a SARS patient by 10000 times with clean air. Dilution at lower volumes, specifically 1000 times, is insufficient for protecting non-infected people from SARS exposure and the risk of infection is very high. This study provides a methodology for investigating the necessary ventilation rate from an engineering viewpoint.     

Natural ventilation for reducing airborne infection in hospitals

High ventilation rate is shown to be effective for reducing cross-infection risk of airborne diseases in hospitals and isolation rooms. Natural ventilation can deliver much higher ventilation rate than mechanical ventilation in an energy-efficient manner. This paper reports a field measurement of naturally ventilated hospital wards in Hong Kong and presents a possibility of using natural ventilation for infection control in hospital wards. Our measurements showed that natural ventilation could achieve high ventilation rates especially when both the windows and the doors were open in a ward. The highest ventilation rate recorded in our study was 69.0 ACH. The airflow pattern and the airflow direction were found to be unstable in some measurements with large openings. Mechanical fans were installed in a ward window to create a negative pressure difference. Measurements showed that the negative pressure difference was negligible with large openings but the overall airflow was controlled in the expected direction. When all the openings were closed and the exhaust fans were turned on, a reasonable negative pressure was created although the air temperature was uncontrolled.     

Numerical study of the influences of different patterns of the building and green space on micro-scale outdoor thermal comfort and indoor natural ventilation

Citizens could enjoy a healthy and comfortable living environment if outdoor thermal comfort and sufficient natural ventilation are available in their dwellings. In this paper, numerical studies were performed with the Simulation Platform for Outdoor Thermal Environment (SPOTE) to investigate: (1) the thermal environment and pedestrian thermal comfort of the occupants in the open space with different patterns of the building and green space; (2) the wind pressures on the building facades and the natural ventilation rate of these buildings. The conclusions are summarized as follows: (1) it has been observed that the long facades of building and green space, which are parallel to the prevailing wind direction, can accelerate horizontal vortex airflow at the edges where such airflow could strengthen the convective exchange efficiency of hot air in low altitude and cold air in high altitude, and can obtain thermal comfort and sufficient natural ventilation at the pedestrian level; (2) after a series of simulations and comparisons, the configuration in which buildings are grouped in staggered layout with a centralized green space can provide better ventilation conditions and suitable air movement as a result of attenuated revised standard effective temperature (SET*). This configuration is regarded as the optimum pattern of the building and green space.     

Effects of double skin envelopes on natural ventilation and heating loads in office buildings

The influence of natural ventilation on heating load and energy savings in a building with a double skinned envelope (DSE) was examined in this study. Field measurements and computer simulations were performed under various weather conditions. The DSE was effective for saving energy and creating natural ventilation rates under clear and partly cloudy skies. Due to insufficient irradiance, the DSE was not effective for reducing the heating load under overcast sky conditions. When natural airflow rates from the cavity space between internal and external skin to the indoor space were controlled, the southwest-facing DSE effectively reduced heating loads due to the accumulation of solar irradiance.Regression models showed that outdoor air temperature was the most significant factor governing variations in cavity temperature under all sky conditions. Computer simulations indicated that natural ventilation was practical at an appropriate supply temperature only when the sky ratio was less than 0.7. The airspace in cavity of the DSE provided additional natural ventilation rates to the indoor space and effectively reduced heating loads. Natural ventilation was available for 135 h during three winter months without consuming additional energy to heat the outdoor air. The heating load was reduced by the DSE ranged from 17.98% to 18.7% depending on the airflow control options for the cavity space.     

载人航天器舱内通风空调特性和数值模拟

由于载人航天器这一密闭狭小空间及其微重力这一特殊环境,舱内通风空调问题和热舒适环境与地面的HVAC问题不同,微重力效应特别是自然对流大为减弱影响着通风换热效果。本文首先分析了舱内通风空调问题的特殊性,通过无量纲分析和计算微重力的流体力学效应,然后利用FLUENT软件对两种集中通风方式进行数值模拟。模拟结果表明：微重力下几乎不存在“冷风下坠”或者“热羽”现象;集中斜进风在一定的进风角度和Re数下出现分岔解现象;与常重力相同通风条件下,微重力下自然对流的减弱使得舱内温度降低,换热减少,因此满足常重力热舒适要求的通风条件不一定满足微重力下热舒适性的要求。     

Numerical evaluation of louver configuration and ventilation strategies for the windcatcher system

The windcatcher system is a green architectural feature that uses natural ventilation to induce external airflow into residential buildings. This paper presents different configuration and ventilation strategies for the windcatcher to evaluate the performance of the system in relation to ventilation and indoor particle dispersion. A commercial computational fluid dynamic (CFD) code is used to evaluate the windcatcher’s performance using different numbers of louvers and louver lengths. The effects of buoyancy and window positions on the system’s performance are considered. The flow rate of air induced into the windcatcher is found to increase with the number of louver layers and the highest ventilation rate is reached when the louver length equates with the reference length. With respect to the buoyancy effect, the results show that the system performs well in stimulating airflow and removing contaminants when a window is positioned on the leeward side. A uniform and low particle concentration is created when a window is positioned on the leeward side. However, due to the high air velocity below the windcatcher, the general airflow distribution of the system is not uniform. A damper or egg crated grill should be installed at the terminal of the system, especially when the external wind is strong.     

Ventilation rates in the bedrooms of 500 Danish children

The ongoing “Indoor Environment and Children’s Health” (IECH) study investigates the environmental risk factors in homes and their association with asthma and allergy among children aged 1–5 years. As part of the study, the homes of 500 children between 3 and 5 years of age were inspected. The selected children included 200 symptomatic children (cases) and 300 randomly selected children (bases). As part of the inspection, the concentration of carbon dioxide in the bedrooms of the children was continuously measured over an average of 2.5 days. The ventilation rates in the rooms during the nights when the children were sleeping in the room were calculated using a single-zone mass balance for the occupant-generated CO₂. The calculated air change rates were log-normally distributed (R² > 0.98). The geometric mean of the air change rates in both the case and the base group was 0.46 air changes per hour (h⁻¹; geom. SD = 2.08 and 2.13, respectively). Approximately 57% of both cases and bases slept at a lower ventilation rate than the minimum required ventilation rate of 0.5 h⁻¹ in new Danish dwellings. Only 32% of the bedrooms had an average CO₂ concentration below 1000 ppm during the measured nights. Twenty-three percent of the rooms experienced at least a 20-minute period during the night when the CO₂ concentration was above 2000 ppm and 6% of the rooms experienced concentrations above 3000 ppm. The average air change rate was higher with more people sleeping in the room. The air change rate did not change with the increasing outdoor temperature over the 10-week experimental period. The calculation method provides an estimate of the total airflow into the bedroom, including airflows both from outdoors and from adjacent spaces. To study the accuracy of the calculated air change rates and their deviation from the true outside air change rates, we calculated CO₂ concentrations at different given air change rates using an indoor air quality and ventilation model (Contam). Subsequently we applied our calculation procedure to the obtained data. The air change rate calculated from the generated CO₂ concentrations was found to be between 0% and 51% lower than the total air change rate defined in the input variables for the model. It was, however, higher than the true outside air change rate. The relative error depended on the position of the room in relation to the adjacent rooms, occupancy in the adjacent room, the nominal air change rate and room-to-room airflows.     

Wind-induced ventilation performances and airflow characteristics in an areaway-attached basement with a single-sided opening

In the present study, we performed both wind tunnel experiments and numerical simulations on a scale model with the focus on wind-driven natural ventilation in an areaway-attached basement with a single-sided opening. In the experiments, the mean value of the effective ventilation rate, purging flow rate (PFR) was measured for nine wind incidence angels based on the homogeneous emission rate method. The experimental results were used to validate two numerical approaches: Reynolds averaged Navier–Stokes (RANS) modeling and large-eddy simulation (LES). The influences of inflow turbulent fluctuations for LES modeling were also examined. The comparisons between the experiment and the numerical simulation indicate that LES can provide more accurate results than RANS and the inflow turbulent fluctuations should be taken into account for LES modeling. Based on LES with the inflow turbulent fluctuations, the mean airflow patterns within and around the areaway-attached basement were further studied for different wind incidence angles to investigate the influence of wind direction on ventilation performance in the areaway space. Furthermore, the relationships between the effective ventilation rate and the kinetic energy in the basement space were analyzed for three wind directions: 0°, 90° and 180°. A close correlation was found between the mean values, whereas the corresponding time variations showed large discrepancies. Finally, we compared the effective ventilation rate obtained using the homogeneous emission rate method and the airflow rates through the opening using two integration procedures. The effective ventilation rates were found lower than the airflow rates through the opening.     

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.     

Natural ventilation with traditional Korean opening in contemporary house

In this study, a natural ventilation opening was designed based on the traditional Korean opening to improve indoor environment on the contemporary house. The prototype of the opening was composed of three hanji papers and two air layers to improve airflow rate, and also to recovery heat lose. The performance of the heat recovery and airflows of the prototype was measured in laboratory, and the CFD simulation was used to verify its performance in the contemporary house. The airflow rate of the prototype is exponentially increased according to the pressure differences, and it ranges from 12.6 to 39.6 m³/m² h in 3–10 Pa, pressure difference. The total amount of heat recovery ranges from 47.8 to 67.7 W/m² in the prototype, and the heat recovery rate is about 25% at 10 Pa. In the CFD simulation, the prototypes were installed at 25% of the total window areas of the model house. The outside air was totally supplied through the prototypes at 67.2 m³/h in the model house, and it is equivalent to 0.2 h⁻¹ ventilation rate. The results show that the prototype is capable of providing natural ventilation even at low wind pressure, and also that it prevent cold draft in heating period. Further analysis of the ventilation performance including the thermal force is needed to apply the prototype to the contemporary house.     

Impact of computational domain on the prediction of buoyancy-driven ventilation cooling

Traditional solar heated cavity structures such as solar chimneys make use of the stored solar energy in the interior wall to enhance natural ventilation of buildings but integration of photovoltaic devices into the exterior wall of such a structure can result in different proportions of heat distribution on both interior and exterior walls. This paper presents results of CFD simulation of the buoyancy-driven airflow and heat transfer in vertical cavities of different heights and widths with different total heat fluxes and wall heat distributions for ventilation cooling. Two sizes of computational domain were used for simulation – a small domain same as the physical size of a cavity and a large extended domain that is much larger than the cavity. The predicted natural ventilation rate and heat transfer coefficient have been found to depend on not only the cavity size and the quantity and proportion of heat distribution on the cavity walls but also the domain size. The difference in the predicted ventilation rate or heat transfer coefficient using the small and large domains is generally larger for wider cavities where heat distribution on two vertical walls is highly asymmetrical; incoming air would be distorted from symmetrical distribution across the inlet opening; and/or significant reverse flow would occur at the outlet opening. The difference in the heat transfer coefficient is generally less than that in the ventilation rate. In addition, a cavity with symmetrical heating has a higher ventilation rate but lower heat transfer coefficient than does an asymmetrically heated cavity.     

Study on inhaled air quality in a personal air-conditioning environment using new scales of ventilation efficiency

A detailed study using computational fluid dynamics (CFD) was conducted on the influence of the difference in the effective diameters of air supply openings (air velocity, assuming the airflow rate to be constant) when using personal air-conditioning (PAC) with isothermal air currents. A new method to analyze the age of air (SVE3*) for individual supplies and the residual lifetime of air (SVE6*) for individual exhausts was developed and proposed. The study focuses on the individual supply openings and exhaust openings in a room with multiple supply openings and exhaust openings when using PAC. PAC with the larger supply opening resulted in less mixture with the surrounding air and a lower age of air than the smaller diameter, which therefore indicated better ventilation characteristics.     

Comparison of airflow and particulate matter transport in multi-room buildings for different natural ventilation patterns

This study numerically investigates airflow characteristics and particulate matter (PM) transport in multi-room buildings for different natural ventilation patterns with the same air change rate. Four typical natural ventilation patterns (full-open, pass-through, right short-circuit and left short-circuit), representing the ratios of the outlet-to-inlet opening size ranging from 1.67 to 0.17, are considered to study multi-room airflow characteristics. A measured indoor PM₁₀ profile in Taipei Metropolis is input into the above four ventilation patterns as the initial condition of the PM size distribution. The time variation of indoor PM₁₀/PM_2.5/PM₁ concentrations in each room for various ventilation patterns is next investigated. The effect of ventilation pattern on particle removal mechanism is emphasized. The results show that although the air change rate of the building is the same, airflow characteristics and PM transport behaviors are quite different for various ventilation patterns. The removal efficiencies of PM₁₀ for the four ventilation patterns are all found to be much better than those of PM_2.5 and PM₁. Particle escape is the major mechanism to remove PM for rooms with double-sided ventilation, whereas particle deposition is important for single-sided ventilation rooms.     

Energy-saving strategies with personalized ventilation in cold climates

In this study the influence of the personalized supply air temperature control strategy on energy consumption and the energy-saving potentials of a personalized ventilation system have been investigated by means of simulations with IDA-ICE software. GenOpt software was used to determine the optimal supply air temperature. The simulated office room was located in a cold climate. The results reveal that the supply air temperature control strategy has a marked influence on energy consumption. The energy consumption with personalized ventilation may increase substantially (in the range: 61-268%) compared to mixing ventilation alone if energy-saving strategies are not applied. The results show that the best supply air temperature control strategy is to provide air constantly at 20 °C. The most effective way of saving energy with personalized ventilation is to extend the upper room operative temperature limit (saving up to 60% compared to the reference case). However, this energy-saving strategy can be recommended only in a working environment where the occupants spend most of their time at their workstation. Reducing the airflow rate does not always imply a reduction of energy consumption. Supplying the personalized air only when the occupant is at the desk is not an effective energy-saving strategy.     

Comfort and airflow evaluation in spaces equipped with mixing ventilation and cold radiant floor

In this work the comfort and airflow were evaluated for spaces equipped with mixing ventilation and cold radiant floor. In this study the coupling of an integral multi-nodal human thermal comfort model with a computational fluid dynamics model is developed. The coupling incorporates the predicted mean vote (PMV) index, for the heat exchange between the body and the environment, with the ventilation effectiveness to obtain the air distribution index (ADI) for the occupied spaces with non-uniform environments. The integral multi-nodal human thermal comfort model predicts the external skin and clothing surfaces temperatures and the thermal comfort level, while the computational fluid dynamics model evaluates the airflow around the occupants. The air distribution index, that was developed in the last years for uniform environments, has been extended and implemented for non-uniform thermal environments. The airflow inside a virtual chamber equipped with two occupants seated in a classroom desk, is promoted by a mixing ventilation system with supply air of 28 °C and by a cold radiant floor with a surface temperature of 19 °C. The mechanical mixing ventilation system uses a supply and an exhaust diffusers located above the head level on adjacent walls.     

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.     

Simulation of a building and its HVAC system with an equation solver: Application to benchmarking

The today-availability of powerful engineering equation solvers is opening very new possibilities in technical component modelling and in system simulation. The simulation models, the “user guide” and the “reference guide” are all included in a same file. Reliable “reference” and “simplified” models are currently available for the building zone and for most (heating, ventilation and air-conditioning) HVAC components. Focus is given here on “simplified” models and on a simulation tool, called “Benchmark”. This tool should help an auditor to make the best use of the limited information usually available about actual fuel and electricity consumptions and to get a very first evaluation of the actual performances of a given HVAC system. An example of such use is presented. Another simulation tools and more information about the modelling of HVAC components will be presented in a further paper.     

Ventilation effectiveness as an indicator of occupant exposure to particles from indoor sources

Ventilation effectiveness is an indicator of the quality of supply air distribution in ventilated rooms. It is a representation of how well a considered space is ventilated compared to a perfect air mixing condition. Depending on pollutant properties and source position relative to the airflow, ventilation effectiveness can more or less successfully be used as an indicator of air quality and human exposure. This paper presents an experimentally and numerically based study that examines the relationship between ventilation effectiveness and particle concentration in typical indoor environments. The results show that the relationship varies predominantly with airflow pattern and particle properties. Fine particles (1 μm) follow the airflow pattern more strictly than coarse particles (7 μm), and the high ventilation effectiveness indicates better removal of fine particles than coarse particles. When a ventilation system provides high mixing in the space and ventilation effectiveness is close to one, particle sizes and source location have a relatively small effect on particle concentration in the breathing zone. However, when the supply air is short circuited and large stagnation zones exist within the space, the particle concentration in the breathing zone varies with particle size, source location, and airflow pattern. Generally, the results show that for fine particles (1 μm), increase of ventilation effectiveness reduces occupant exposure; while for coarser particles (7 μm), source location and airflow around the pollutant source are the major variables that affect human exposure.