Calculation of wind-driven cross ventilation in buildings with large openings

A simplified macroscopic method is commonly used for wind-driven ventilation analysis of buildings with small openings. Consequently, it is reasonable to question if and under what conditions will this method provide accurate results in predicting ventilation flow rates in buildings with large openings. We investigate a single-zone cubic building with two equal large openings using a computational fluid dynamics approach. We analyzed the driving forces and the ventilation flow rates due to wind as a function of the geometry, size and relative location of the two openings. The ventilation flow rates are found to be affected by both wind flows around and through the building when the two openings are relatively large. The simplified macroscopic method can provide reasonable engineering accuracy (i.e., less than 10% error) when the porosity of the building envelope does not exceed a critical value. This critical value is not a constant; instead it depends significantly on the degree of alignment between the wind direction and the character of the dominant stream tube associated with the flow through the room. We found that the simplified macroscopic method fails to provide acceptable accuracy when this stream tube is truly dominant and parallel to the wind direction. The effects of wall thickness and aspect ratio of openings are also investigated.     

An experimental study of wind-driven cross ventilation in partitioned buildings

This study uses wind tunnel experiments to investigate wind-driven cross ventilation in partitioned buildings. The discharge coefficients of internal opening under various flow conditions were determined by a fan technique. It was found that the internal discharge coefficient is dependent on the internal porosity, but independent of external porosity, and opening location. Based on the experimental results, a predictive model for internal pressure and ventilation rate of multi-room buildings was developed and verified. This model also revealed that the ventilation rate increases as the internal porosity increases, and that the maximum ventilation rate occurs when the windward and leeward opening areas are equal. Because internal partitions reduce the difference between external and internal pressure, the ventilation rate of partitioned buildings is always smaller than that of buildings without partition. Furthermore, this study provides a quantitative scheme to regulate the ventilation rate of partitioned buildings by controlling the opening areas of windward, leeward and internal openings.     

Prediction of performance of a wind-driven ventilation device

The paper presents the aerodynamic design of a ventilation device suitable for remote open areas, such as desert terrains. The device is attached to the top of an enclosure roof and employs wind energy to create a partial vacuum in a specially designed duct to draw out the stale air from within the enclosure. The design is developed with the aid of fluid flow simulation software that is employed to predict the flow details corresponding to different geometric shapes so that an efficient shape may be found. The software is also employed to predict the performance of the adopted shape for different operating conditions and the results are presented in the form of characteristics curves and a correlation equation. Reliability of predictions is backed by results of experimental validation presented elsewhere, and by performing a detailed grid-dependence study, as well as a domain extent sensitivity analysis.     

The effect of ceiling configurations on indoor air motion and ventilation flow rates

The purpose of this paper is to evaluate the effects of a building parameter, namely ceiling configuration, on indoor natural ventilation. The computational fluid dynamics (CFD) code Phoenics was used with the RNG k–? turbulence model to study wind motion and ventilation flow rates inside the building. All the CFD boundary conditions were described. The simulation results were first validated by wind tunnel experiment results in detail, and then used to compare rooms with various ceiling configurations in different cases. The simulation results generated matched the experimental results confirming the accuracy of the RNG k–? turbulence model to successfully predict indoor wind motion for this study. Our main results reveal that ceiling configurations have certain effects on indoor airflow and ventilation flow rates although these effects are fairly minor.     

旋流通风方式下室内二氧化碳分布的实验研究

对8种旋流通风方式进行了实验测试,分别得到了各种通风方式下的速度场和污染物浓度场.结果表明,旋流通风方式用于改善气流组织和室内空气品质是经济可行的.     

Effect of porous hedge on cross ventilation of a residential building

Examined here are the effects of cross-ventilation when a porous hedge is placed ahead of a residential building. Parametric studies include the hedge porosity (0?η?1.0)$(0?\eta ?1.0)$, the space between the hedge and the building (0.25?L/2H?3.0)$(0.25?\mathrm{L}/2\mathrm{H}?3.0)$ and the hedge height (0.1?h/2H?1.0)$(0.1?\mathrm{h}/2\mathrm{H}?1.0)$. A computational fluid dynamics (CFD) simulation is carried out to predict the airflow characteristics around both the porous hedge and the building. Results show that the porous hedge could modify the flow structures behind it, and thus can alter the cross ventilation of the building. In addition, the shelter effect of the porous hedge is increased when the hedge porosity is decreased. It is also discovered that there exists a critical hedge porosity below which the cross ventilation in the building becomes inverse.     

The loss factors of building openings for wind-driven ventilation

This study used wind tunnel experiments to determine the loss factors of building openings for wind-driven ventilation. It is found that the loss factor is a function of internal porosity, but independent of Reynolds number. The loss factor of a partially open door increased as the door angle decreased. In addition, the influence of thickness ratio on the loss factor is much smaller than the internal porosity. The present study also derived the relationship between the loss factor and the discharge coefficient used in the orifice equation. The experimental results demonstrate that the ventilation rate of wind-driven cross ventilation can be predicted by a resistance model, once the loss factors of the openings and external pressure coefficients are known. Besides, it was found that the flow resistance of the internal opening dominated the ventilation rate when the external openings are large. But the resistance of external opening is the governing parameter when the external openings are small.     

Effect of ventilation and filtration on submicrometer particles in an indoor environment

The effect of filtration and ventilation on reduction of submicrometer particle concentration indoors was investigated in an office building. The air-handling system consisting of dry media filters and an air-conditioning unit, reduced particle concentration levels by 34%. The characteristics of indoor airborne particles were dominated by, and followed the pattern of, outdoor air, with vehicle combustion aerosols as the main pollutant. The ratio indoor/outdoor particle concentration varied between 14 and 26% for different sub-zones. The presence of significant source of particles indoors was not observed. A simple mathematical model predicting evolution of particles indoors is presented. The model, based on a particle number balance equation, was validated with experimental data and showed very good agreement between predicted and measured parameters.     

Influence of ventilation on indoor radon level

Detailed radon measurements were conducted at different residential units in Hong Kong in winter time when air-conditioners were off and also in summer time when air-conditioners were on. Ventilation rates were measured concurrently to investigate the influence of ventilation on indoor radon level. The ratio of indoor radon level to outdoor radon level was plotted against ventilation rate, and it was found that a critical value existed after which the indoor radon level could be considered identical to the outdoor level. This result is important for use in ventilation design to reduce indoor radon pollution.     

Effect of mechanically induced ventilation on the indoor air quality of building envelopes

Experiments were conducted to study the effect of mechanically induced fresh-air ventilation on the indoor air quality (IAQ) of the Tuskegee Healthy House (THH), selecting the outdoor weather conditions almost identical during the “fan OFF” and “fan ON” periods. Measurements of outdoor and indoor temperature and relative humidity (RH), in addition to the indoor dust particle concentration levels and interior wall moisture content, were systematically carried out during the summer month of August 2008. Results show that the effect of mechanically induced ventilation (“fan ON” period) is to raise the indoor RH, interior wall moisture content, and indoor dust particle concentration values significantly above those measured during the “fan OFF” period. The indoor temperature increases only slightly during the “fan ON” period.     

Studies on buoyancy driven two-directional smoke flow layering length with combination of point extraction and longitudinal ventilation in tunnel fires

This paper investigates the buoyancy-driven smoke flow layering length (both upstream and downstream) beneath the ceiling with combination of point extraction and longitudinal ventilation in tunnel fires. A theoretical model is developed based on previous back-laying model with only longitudinal ventilation, with modified actual heat release rate, as well as modified upstream and downstream opposing longitudinal air flow velocities by the induced flow velocity due to point extraction. Experiments are carried out in a reduced scale model tunnel with dimensionless of 72 m×1.5 m×1.3 m. A LPG porous gas burner is used as fire source. The smoke flow layering length both upstream and downstream are identified based on temperature profiles measured along the ceiling, for different experiment conditions. CFD simulations with FDS are also performed for the same scenarios. Results show that with combination of point extraction and longitudinal ventilation, the smoke flow layering length is not symmetric where it is longer downstream than that upstream. The upstream smoke layering length decreases, while the downstream layering length increases with increase in longitudinal ventilation velocity; and they both decrease with increase in point extraction velocity. The predictions by the proposed theoretical model agree well with the measurements and simulation results.     

Effect of cross section and ventilation on heat release rates in tunnel fires

Model scale fire tests were performed in tunnels with varying tunnel widths and heights in order to study the effect of tunnel cross-section and ventilation velocity on the heat release rate (HRR) for both liquid pool fires and solid fuel fires. The results showed that for well ventilated heptane pool fires, the tunnel width nearly has no influence on the HRR whilst a lower tunnel height clearly increases the HRR. For well ventilated solid fuel fires, the HRR increases by approximately 25% relative to a free burn test but the HRR is not sensitive to either tunnel width, tunnel height or ventilation velocity. For solid fuel fires that were not well ventilated, the HRRs could be less than those in free burn laboratory tests. In the case of ventilation controlled fires the HRRs approximately lie at the same level as for cases with natural ventilation.     

强化室内通风研究进展

首先解释了自然通风和强化室内通风。接着介绍了强化室内通风的主要结构形式,包括太阳能强化室内通风(太阳能通风墙、太阳能烟囱、太阳墙、太阳能通风屋顶、太阳能蓄能通风)、多元通风和通过改良设计强化室内通风,并对国内外专家和学者的主要研究进展进行阐述。最后指出目前强化室内通风在研究应用的一些不足之处和未来的研究方向。     

An experimental study on effects of increased ventilation flow on students' perception of indoor environment in computer classrooms

The effects of ventilation in computer classrooms were studied with university students (n = 355) in a blinded study, 31% were women and 3.8% had asthma. Two classrooms had a higher air exchange (4.1-5.2 ac/h); two others had a lower air exchange (2.3-2.6 ac/h). After 1 week, ventilation conditions were shifted. The students reported environmental perceptions during the last hour. Room temperature, RH, CO2, PM10 and ultra-fine particles were measured simultaneously. Mean CO2 was 1185 ppm at lower and 922 ppm at higher air exchange. Mean temperature was 23.2 degrees C at lower and 22.1 degrees C at higher air exchange. After mutual adjustment (temperature, RH, CO2, air exchange), measured temperature was associated with a perception of higher temperature (P < 0.001), lower air movement (P < 0.001), and poorer air quality (P < 0.001). Higher air exchange was associated with a perception of lower temperature (P < 0.001), higher air movement (P = 0.001), and better air quality (P < 0.001). In the longitudinal analysis (n = 83), increased air exchange caused a perception of lower temperature (P = 0.002), higher air movement (P < 0.001), better air quality (P = 0.001), and less odor (P = 0.02). In conclusion, computer classrooms have CO2 levels above 1000 ppm and temperatures above 22 degrees C. Increased ventilation from 7 l/s per person to 10-13 l/s per person can improve thermal comfort and air quality. PRACTICAL IMPLICATIONS: Computer classrooms are crowded indoor environments with a high thermal load from both students and computer equipment. It is important to control room temperature either by air conditioning, sun shields, or sufficiently high ventilation flow. A high ventilation flow is also crucial to achieving good perceived air quality. Personal ventilation flow should be at least 10 l/s. Possible loss of learning ability due to poor indoor air quality in university buildings deserves more attention.     

住宅室内空气品质与通风换气

当今，国际上一些环保专家巳将“室内空气污染”问题列为继“煤烟型污染”、“光化学烟雾型”污染之后的第三代空气污染问题。ASHRAE(美国暖通空调和制冷工程师协会)标准62—1989R中，首次提出了可接受的室内空气品质的概念，定义如下：房间内绝大多数人(80％或更多)没有对室内空气表示不满意，并且空气中没有巳知的污染物达到了可能对人体健康产生严重威胁的浓度。     

蜗舌间隙对贯流风机性能的影响

贯流风机具有动压较高、气流均匀的特点,可形成扁平而高速的气流,是分体空调室内机中不可缺少的一个重要部件.贯流风机主要由一个筒型多叶的贯流叶轮和室内机的蜗壳组成,而蜗舌又是蜗壳中的一个重要组成部分,它既起到改变气流流动方向的作用,又对整个风机的气动性能产生很大的影响.本文通过试验,初步探讨了蜗舌间隙对整个贯流风机性能的影响.     

Experimental investigation of the air flow and indoor carbon dioxide concentration in classrooms with intermittent natural ventilation

Air flow and the associated indoor carbon dioxide concentrations have been extensively monitored in 62 classrooms of 27 naturally ventilated schools in Athens, Greece. The specific ventilation patterns as well as the associated carbon dioxide concentrations, before, during and after the teaching period are analysed in detail. During the teaching period, only 23% of the measured classrooms presented a flow rate higher than the recommended value of 8 l/p/s while the mean daily fluctuation was close to 40%. About, 52% of the classrooms presented a mean indoor CO₂ concentration higher than 1000 ppm. The specific experimental data have been compared against existing ventilation rates and carbon dioxide concentrations using published information from 287 classrooms of 182 naturally ventilated schools and 900 classrooms from 220 mechanically ventilated schools. The relation between the air flow rates and the corresponding indoor carbon dioxide is analysed and then compared to the existing data from naturally and mechanically ventilated schools. It is found that all three data sets present a CO₂ concentration equal to 1000 ppm for air flows around 8 l/p/s. Specific adaptive actions to improve the indoor environmental quality have been recorded and the impact of indoor and ambient temperatures as well as of the carbon dioxide concentration on window opening is analysed in detail. A clear relation is found, between the indoor temperature at which the adapting action takes place and the resulting air flow rate. In parallel, a statistically significant relation between window opening and the indoor–outdoor temperature difference has been established.     

Turbulence effects on the discharge coefficient and mean flow rate of wind-driven cross-ventilation

This experimental study uses a wind tunnel and scale model to investigate turbulence effects on the discharge coefficient and mean flow rate of wind-driven cross-ventilation. The approaching flows include low turbulence smooth flow and grid-generated turbulent flow. Two different cross-ventilation configurations are considered in this study: (1) two opposite walls, each with one opening, and (2) two adjacent walls, each with one opening. The discharge coefficients of different flow conditions are determined by a fan technique. It is found that the discharge coefficient is a function of Reynolds number, wind incidence angle and direction of air flow (inhale or exhale), but independent of external turbulence intensity and wall porosity. However, because the leeward pressure is affected by external turbulence, the internal pressure and inlet velocity are dependent on external turbulence when the openings are on the opposite walls. The experimental results also verify that internal pressure and mean air flow rate can be predicted once the external pressure distribution and opening areas are known, regardless of whether the openings are on opposite walls or on adjacent walls.     

Effects of ventilation strategies and source locations on indoor particle deposition

A computer model for predicting aerosol dispersion in indoor spaces was validated with experimental data found in the literature. The validated model was used to explore the effect of the area or point source locations on aerosol particle transportation in ventilation rooms with rough surfaces. Two different ventilation strategies were studied: mixing ventilation (MV) and underfloor air distribution (UFAD) system. The simulation results show that in MV, the particle concentration and removal effectiveness are little dependent on the position of the pollutant sources. In UFAD, the source location has a strong impact on the distribution and removing of the contaminants. The particle removal performance strongly depends on the ventilation efficiency and the particle deposition loss in the room with rough surfaces. The important consideration in both the relative ventilation efficiency and the deposition rate is the relative position of the sources to the main airflow and the occupied zone in an UFAD room.