Large eddy simulation of wind-induced interunit dispersion around multistory buildings

Emission and spatial variation characteristics of odorous pollutants in the aerobic tank of an underground wastewater treatment plant (UWWTP) in southern China

In this study, the spatial concentration of odorous pollutants in the aerobic tank of an underground wastewater treatment plant (UWWTP) in southern China is monitored. The odour activity value, odour contribution rate, and chemical concentration contribution rate are used to evaluate the degree of contribution of odorous substances. Computational fluid dynamics (CFD) simulations of odorous pollutant diffusion are also established. The study shows that the odorous substances detected in the aerobic tank mainly included ammonia (NH3), hydrogen sulfide (H2S), trimethylamine (C3H9N), and methanethiol (CH3SH), and their concentrations are 1.160, 0.778, 0.022, and 0.0006 mg/m3, respectively. The total odour activity value of the aerobic tank is 450.72 (dimensionless), of which the odour activity value of H2S is 432.22, and the contribution rate reaches 95.9%. H2S is the main contributor to odour and a key controlled substance. The air inlets and exhaust outlets in the aerobic tank are cross-arranged at the top of the space, and the CFD model of odorous pollutant diffusion shows that the gas flow organization determines the odorous pollutant diffusion. The spatial distribution of gas flow and odorous substances in the aerobic tank is relatively uniform, and the odour collection efficiency is higher. The production flux and production coefficient of H2S in the aerobic tank are calculated as 25.831 mg/(m2·h) and 14.149 mg/t, respectively. This study determines the reasonable air supply and exhaust design of the aerobic tank, the number of odour pollutants, and the key controlled substances. These findings offer guidance and serve as useful references for the prevention and control of odour pollution in aerobic tanks of the same type of UWWTPs.

A CFD-based framework to assess airborne infection risk in buildings

The COVID-19 pandemic has prompted huge efforts to further the scientific knowledge of indoor ventilation and its relationship to airborne infection risk. Exhaled infectious aerosols are spread and inhaled as a result of room airflow characteristics. Many calculation methods and assertions on risk assume 'well-mixed' flow conditions. However, ventilation in buildings is complex and often not showing well-mixed conditions. Ventilation guidance is typically based on the provision of generic minimum ventilation flow rates for a given space, irrespective of the effectiveness in the delivery of the supply air. Furthermore, the airflow might be heavily affected by the season, the HVAC ventilation, or the opening of windows, which would potentially generate draughts and non-uniform conditions. As a result, fresh air concentration would be variable depending upon a susceptible receptor's position in a room and, therefore, associated airborne infection risk. A computational fluid dynamics (CFD) and dynamic thermal modelling (DTM) framework is proposed to assess the influence of internal airflow characteristics on airborne infection risk. A simple metric is proposed, the hourly airborne infection rate (HAI) which can easily help designers to stress-test the ventilation within a building under several conditions. A case study is presented, and the results clearly demonstrate the importance of understanding detailed indoor airflow characteristics and associated concentration patterns in order to provide detailed design guidance, e.g. occupancy, supply air diffusers and furniture layouts, to reduce airborne infection risk.

CFD Simulations of Ventilation and Interunit Dispersion in Dormitory Complex: A Case Study of Epidemic Outbreak in Shanghai

Since the beginning of March 2022, a new round of COVID-19 outbreaks in Shanghai has led to a sharp increase in the number of infected people. It is important to identify possible pollutant transmission routes and predict potential infection risks for infectious diseases. Therefore, this study investigated the cross-diffusion of pollutants caused by natural ventilation, including external windows and indoor ventilation windows, under three wind directions in a densely populated building environment with the CFD method. In this study, CFD building models were developed based on an actual dormitory complex and surrounding buildings under realistic wind conditions to reproduce the airflow fields and transmission paths of pollutants. This paper adopted the Wells-Riley model to assess the risk of cross-infection. The biggest risk of infection was when a source room was located on the windward side, and the risk of infection in other rooms on the same side as the source room was large in the windward direction. When pollutants were released from room 8, north wind resulted in the highest concentration of pollutants in room 28, reaching 37.8%. This paper summarizes the transmission risks related to the indoor and outdoor environments of compact buildings.

Research on the cross-contamination in the confined conference room with the radiant floor heating system integrated with the down-supply ventilation

The performance of a confined conference room under radiant heat floor combined with down-supply ventilation (RFDS) is investigated. The purpose of the study is to reveal the mechanism of indoor contaminants transmission from infected person and to protect the occupants. The exhaled contaminant dispersion was simulated using a CFD model and experimentally validated in an office with the down-supply ventilation (DS). The effects of radiant floor (RF) combined with down-supply ventilation on airflow and contaminant distribution in the room was evaluated with regard to different RF temperatures and inlet velocities. The influence of downdraft from the envelope on the airflow pattern of the room is also discussed. It is proved that the exposure risk of pollutants can be reduced by strengthening the insulation of the envelope. The simulation results showed that the proposed RFDS system could significantly reduce the level of exposure to contaminants in the breathing zone (BZ) of occupants. In this research case, the RFDS reduced the average exposure rates by more than 50% relative to the case with the only down-supply warm air heating. Furthermore, With improved envelope insulation, the probability of infection can be reduced by more than 80% only by avoiding a simultaneous inlet velocity of 0.3 m/s at RF temperatures no less than 27 °C. The results suggested an improved fresh air heating mode for passive ultra-low energy consumption buildings with good thermal insulation and air tightness.

Airborne transmission of biological agents within the indoor built environment: a multidisciplinary review

The nature and airborne dispersion of the underestimated biological agents, monitoring, analysis and transmission among the human occupants into building environment is a major challenge of today. Those agents play a crucial role in ensuring comfortable, healthy and risk-free conditions into indoor working and leaving spaces. It is known that ventilation systems influence strongly the transmission of indoor air pollutants, with scarce information although to have been reported for biological agents until 2019. The biological agents' source release and the trajectory of airborne transmission are both important in terms of optimising the design of the heating, ventilation and air conditioning systems of the future. In addition, modelling via computational fluid dynamics (CFD) will become a more valuable tool in foreseeing risks and tackle hazards when pollutants and biological agents released into closed spaces. Promising results on the prediction of their dispersion routes and concentration levels, as well as the selection of the appropriate ventilation strategy, provide crucial information on risk minimisation of the airborne transmission among humans. Under this context, the present multidisciplinary review considers four interrelated aspects of the dispersion of biological agents in closed spaces, (a) the nature and airborne transmission route of the examined agents, (b) the biological origin and health effects of the major microbial pathogens on the human respiratory system, (c) the role of heating, ventilation and air-conditioning systems in the airborne transmission and (d) the associated computer modelling approaches. This adopted methodology allows the discussion of the existing findings, on-going research, identification of the main research gaps and future directions from a multidisciplinary point of view which will be helpful for substantial innovations in the field.

Patterns of SARS-CoV-2 aerosol spread in typical classrooms

Although current industry guidelines to control the spread of aerosols such as SARS-CoV-2 (COVID-19) have adopted a six-foot (~1.8 m) spacing between individuals indoors, recent evidence suggests that longer range spread is also responsible for infections in public spaces. The vehicle for long-range spread is smaller (<5 μm) droplets or particles, termed bio-aerosols, or aerosols for short, which have a large surface area to volume ratio such that aerodynamic drag is much larger than gravity forces. The aerosols remain suspended in air for extended time periods, and they essentially move with air currents. Prediction of the danger to occupants in a closed room when exposed to an infected individual requires knowledge of the period of exposure and the concentration level of aerosols in the breathing zone of an occupant. To obtain an estimate of the concentration level, a common assumption is well-mixed conditions within an interior space. This is obtained from a mass balance between the level of aerosol produced by an infected individual along with the airflow rate into and out of the entire space. In this work, we use computational fluid dynamics, compared with experimental results in several cases, to explore the aerosol concentration distribution in a typical classroom for several common conditions and compare these results to the well-mixed assumption. We use a tracer gas to approximately simulate the flow and dispersion of the aerosol-air mixture. The two ventilation systems examined, ceiling diffusers and open windows, yield average concentrations at occupant breathing level 50 % greater than the well mixed case, and some scenarios yield concentrations that are 150 % greater than the well mixed concentration at specific breathing-level locations. Of particular concern are two conditions: horizontal air flow from an open window in line with a row of seating and, second, an infected individual seated near a sealed cold window. For the former, conditions are improved if a baffle is placed inside the open window to direct the air toward the floor, creating a condition similar to displacement ventilation. In the latter, the cold air flowing down along the cold window recirculates aerosols back into the breathing zone. Adding window covers or a portable heater below the window surface will moderate this condition.

Enlightenment of re-entry airflow: The path of the airflow and the airborne pollutants transmission in buildings

Viable aerosols in the airflow may increase the risk of occupants contracting diseases. Natural ventilation is common in buildings and is accompanied by re-entry airflow during the ventilation process. If the re-entry airflow contains toxic or infectious species, it may cause potential harm to residents. One of the Covid-19 outbreaks occurred in a public residential building at Luk Chuen House (LC-House) in Hong Kong. It is highly suspected that the outbreak of the disease is related to the re-entry airflow. The study attempts to explain and discuss possible causes of the outbreak. In order to understand the impact of airflow on the outbreak, a public residential building similar to LC-House was used in the study. Two measurements M - I and M - II with the same settings were conducted for a sampling unit in the corridor under low and strong wind conditions respectively. The sampling unit and the tracer gas carbon dioxide (CO₂) were used to simulate the index unit and infectious contaminated airflow respectively. The CO₂ concentrations of the unit and corridor were measured simultaneously. Two models of Traditional Single-zone model (TSZ-model) and New Dual-zone model (NDZ-model) were used in the analysis. By comparing the ACH values obtained from the two models, it is indicated that the re-entry airflow of the unit is related to the corridor wind speeds and this provides a reasonable explanation for the outbreak in LC-House, and believes that the results can help understand the recent frequent cluster outbreaks in other residential buildings.

Numerical investigation of gaseous pollutant cross-transmission for single-sided natural ventilation driven by buoyancy and wind

Single-sided natural ventilation was numerically investigated to determine the impact of buoyancy and wind on the cross-transmission of pollution by considering six window types commonly found in multistory buildings. The goal of this study was to predict the gaseous pollutant transmission using computational fluid dynamics based on the Reynolds-averaged Navier-Stokes equations and baseline k-ω turbulence equations. The results indicated that ventilation rates generally increased with increasing wind speeds if the effects of buoyancy and wind were not suppressed; however, the re-entry ratio representing the proportion of expelled air re-entering other floors and the corresponding risk of infection decreased. If the source of the virus was on a central floor, the risk of infection was the highest on the floors closest to the source. Different window types were also considered for determining their effectiveness in controlling cross-transmission and infection risk, depending on the source location and driving force (e.g., buoyancy and wind).

Numerical investigation of gaseous pollutant cross-transmission for single-sided natural ventilation driven by buoyancy and wind

Single-sided natural ventilation was numerically investigated to determine the impact of buoyancy and wind on the cross-transmission of pollution by considering six window types commonly found in multistory buildings. The goal of this study was to predict the gaseous pollutant transmission using computational fluid dynamics based on the Reynolds-averaged Navier-Stokes equations and baseline k-ω turbulence equations. The results indicated that ventilation rates generally increased with increasing wind speeds if the effects of buoyancy and wind were not suppressed; however, the re-entry ratio representing the proportion of expelled air re-entering other floors and the corresponding risk of infection decreased. If the source of the virus was on a central floor, the risk of infection was the highest on the floors closest to the source. Different window types were also considered for determining their effectiveness in controlling cross-transmission and infection risk, depending on the source location and driving force (e.g., buoyancy and wind).

Investigation of interunit dispersion in 2D street canyons: A scaled outdoor experiment

Interunit dispersion problems have been studied previously mainly through on-site measurements, wind tunnel tests, and CFD simulations. In this study, a scaled outdoor experiment was conducted to examine the interunit dispersion characteristics in consecutive two-dimensional street canyons. Tracer gas ( C O 2 ) was continuously released to simulate the pollutant dispersion routes between the rooms in street canyons. The wind velocity, wind direction, air temperature, and tracer gas concentrations were monitored simultaneously. Two important parameters, the air exchange rate and reentry ratio, were analyzed to reveal the ventilation performance and interunit dispersion of the rooms in the street canyons. Based on the real-time weather conditions, it was found that the ventilation performance of the source room varied according to the room location. The air exchange rate distribution of the leeward-side room was more stable than that of the windward side. The tracer gas was mainly transported in the vortex direction inside the street canyon, and the highest reentry ratio was observed at the room nearest to the source room along the transportation route. In addition, under real weather conditions, the rooms in the street canyon have a high probability of experiencing a high reentry ratio based on the maximum reentry ratio of each room. This study provides authentic airflow and pollutant dispersion information in the street canyons in an urban environment. The dataset of this experiment can be used to validate further numerical simulations.

Investigation of interunit dispersion in 2D street canyons: A scaled outdoor experiment

Interunit dispersion problems have been studied previously mainly through on-site measurements, wind tunnel tests, and CFD simulations. In this study, a scaled outdoor experiment was conducted to examine the interunit dispersion characteristics in consecutive two-dimensional street canyons. Tracer gas ( C O 2 ) was continuously released to simulate the pollutant dispersion routes between the rooms in street canyons. The wind velocity, wind direction, air temperature, and tracer gas concentrations were monitored simultaneously. Two important parameters, the air exchange rate and reentry ratio, were analyzed to reveal the ventilation performance and interunit dispersion of the rooms in the street canyons. Based on the real-time weather conditions, it was found that the ventilation performance of the source room varied according to the room location. The air exchange rate distribution of the leeward-side room was more stable than that of the windward side. The tracer gas was mainly transported in the vortex direction inside the street canyon, and the highest reentry ratio was observed at the room nearest to the source room along the transportation route. In addition, under real weather conditions, the rooms in the street canyon have a high probability of experiencing a high reentry ratio based on the maximum reentry ratio of each room. This study provides authentic airflow and pollutant dispersion information in the street canyons in an urban environment. The dataset of this experiment can be used to validate further numerical simulations.

Airborne transmission between room occupants during short-term events: Measurement and evaluation

This study experimentally examines and compares the dynamics and short-term events of airborne cross-infection in a full-scale room ventilated by stratum, mixing and displacement air distributions. Two breathing thermal manikins were employed to simulate a standing infected person and a standing exposed person. Four influential factors were examined, including separation distance between manikins, air change per hour, positioning of the two manikinsand air distribution. Tracer gas technique was used to simulate the exhaled droplet nuclei from the infected person and fast tracer gas concentration meters (FCM41) were used to monitor the concentrations. Real-time and average exposure indices were proposed to evaluate the dynamics of airborne exposure. The time-averaged exposure index depends on the duration of exposure time and can be considerably different during short-term events and under steady-state conditions. The exposure risk during short-term events may not always decrease with increasing separation distance. It changes over time and may not always increase with time. These findings imply that the control measures formulated on the basis of steady-state conditions are not necessarily appropriate for short-term events.

Qualitative and Quantitative Investigation of Multiple Large Eddy Simulation Aspects for Pollutant Dispersion in Street Canyons Using OpenFOAM

Air pollution is probably the single largest environment risk to health and urban streets are the localized, relevant hotspots. Numerous studies reviewed the state-of-the-art models, proposed best-practice guidelines and explored, using various software, how different approaches (e.g., Reynolds-averaged Navier–Stokes (RANS), large eddy simulations (LES)) inter-compare. Open source tools are continuously attracting interest but lack of similar, extensive and comprehensive investigations. At the same time, their configuration varies significantly among the related studies leading to non-reproducible results. Therefore, the typical quasi-2D street canyon geometry was selected to employ the well-known open-source software OpenFOAM and to investigate and validate the main parameters affecting LES transient simulation of a pollutant dispersion. In brief, domain height slightly affected street level concentration but source height had a major impact. All sub-grid scale models predicted the velocity profiles adequately, but the k-equation SGS model best-resolved pollutant dispersion. Finally, an easily reproducible LES configuration is proposed that provided a satisfactory compromise between computational demands and accuracy.

Improving pedestrian level low wind velocity environment in high-density cities: A general framework and case study

An acceptable pedestrian level wind environment is essential to maintain an enjoyable outdoor space for city residents. Low wind velocity environment can lead to uncomfortable outdoor thermal experience in hot and humid summer, and it is unable to remove the pollutants out of city canyons. However, the average wind velocity at pedestrian level is significantly lowered by closely spaced tall buildings in modern megacities. To improve the low wind velocity environment at pedestrian level in high-density cities, a general framework and detailed guidelines are needed. This study is the first time to develop such a framework, and provide detailed guidelines for improving pedestrian level low wind velocity environment in high-density cities. Additionally, a detailed review and summarisation of evaluation criteria and improvement measures are presented in this paper, which provide additional options for urban planners. To investigate the performance of the framework, the Hong Kong Polytechnic University campus was utilised as a case study. Results showed that pedestrian level wind comfort was greatly improved with the developed framework. The outcomes of this study can assist city planners to improve the low wind velocity environment, and can help policy makers to establish sustainable urban planning policies.

The influence of envelope features on interunit dispersion around a naturally ventilated multi-story building

This study examines the influence of building envelope features on interunit dispersion around multi-story buildings, when the presence of an upstream interfering building is also considered. Validated CFD methods in the steady-state RANS framework are employed. In general, the reentry ratios of pollutant from a source unit to adjacent units are mostly in the order of 0.1%, but there are still many cases being in the order of 1%. The influence of envelope features is dependent strongly on the interaction between local wind direction and envelope feature. In a downward dominated near-facade flow field, the presence of vertical envelope features forms dispersion channels to intensify the unidirectional spread. Horizontal envelope features help induce the dilution of pollutant to the main stream and weakens largely the vertical interunit dispersion. The large influences caused by the presence of envelope features extend the existing understanding of interunit dispersion based on flat-facade buildings.

Airborne spread of expiratory droplet nuclei between the occupants of indoor environments: A review

This article reviews past studies of airborne transmission between occupants in indoor environments, focusing on the spread of expiratory droplet nuclei from mouth/nose to mouth/nose for non-specific diseases. Special attention is paid to summarizing what is known about the influential factors, the inappropriate simplifications of the thermofluid boundary conditions of thermal manikins, the challenges facing the available experimental techniques, and the limitations of available evaluation methods. Secondary issues are highlighted, and some new ways to improve our understanding of airborne transmission indoors are provided. The characteristics of airborne spread of expiratory droplet nuclei between occupants, which are influenced correlatively by both environmental and personal factors, were widely revealed under steady-state conditions. Owing to the different boundary conditions used, some inconsistent findings on specific influential factors have been published. The available instrumentation was too slow to provide accurate concentration profiles for time-dependent evaluations of events with obvious time characteristics, while computational fluid dynamics (CFD) studies were mainly performed in the framework of inherently steady Reynolds-averaged Navier-Stokes modeling. Future research needs in 3 areas are identified: the importance of the direction of indoor airflow patterns, the dynamics of airborne transmission, and the application of CFD simulations.

CFD investigation on the effects of wind and thermal wall-flow on pollutant transmission in a high-rise building

The solar radiation can heat the building outer surface, and then cause the upward natural convection flows adjacent to the wall. This phenomenon is especially obvious on a windless sunny day. The near wall thermal plume can drive gaseous pollutants released from lower floors to upper floors. Combined with the effect of ambient approaching wind, the transmission routes will be very complicated. The paper aims to investigate the airflow patterns and pollutant transmission within a building under the effects of wind and thermal forces. A hypothetical twenty-storey slab-shape high-rise building in Shanghai with single-sided natural ventilation is set as the research object in the present study. The intensity of solar radiation on a typical day during transition season is theoretically analysed. The temperature difference between the heated building envelope and the ambient air is calculated by a simplified heat balance model. Finally, the tracer gas method is employed in the numerical simulation to analyse the influence of the wind and wall thermal plume flow on the inter-flat pollutant transmission characteristics. The results show that, the temperature difference between sunward and shady side wall is about 10 K at noon on the designate day. When the source is set as a point with steady intensity and the buoyancy is stronger than or approximately equivalent to the wind, the reentry ratio of the flat immediately above the source can be around 25%.

Gaseous pollutant transmission through windows between vertical floors in a multistory building with natural ventilation

Natural ventilation is an effective strategy to control thermal comfort in buildings, and can be enhanced depending on the window style. The combination of natural ventilation and window can also facilitate the removal or dilution of gaseous pollutants from indoor sources in newly decorated buildings. However, the windows on the same facade may cause gaseous pollutant cross-transmission during single-sided natural ventilation between households on different floors close to the source. Although some research has focused on the pollutant cross-transmission in buildings, the simplification of windows into rectangular openings often affects accurate knowledge of pollutant transmission characteristics. Therefore, this investigation explored gaseous pollutant cross-transmission through real windows during single-sided, buoyancy-driven ventilation in a multistory building. Six types of windows were modeled for the indoor pollutant of gaseous formaldehyde (HCHO). Computational fluid dynamics (CFD) was utilized to solve characteristics of pollutant transmission inside and outside the multistory building. The results indicated that the ventilation rates, thermal profiles and pollutant transmission inside and outside the building varied for each window type, although the open window areas were identical. The re-entry ratio of exhausted air entering upper floors and the infection risk of epidemic viruses caused by airborne cross-transmission was sensitive to ventilation rates and window configurations, while the sensitivities for window configurations varied case by case. The comparisons also revealed that the specification of ambient temperature and pollutant release rate ultimately did not affect the evaluation of pollutant cross-transmission using CFD.

Simulations of the impacts of building height layout on air quality in natural-ventilated rooms around street canyons

Numerical simulations were conducted to investigate the effects of building height ratio (i.e., HR, the height ratio of the upstream building to the downstream building) on the air quality in buildings beside street canyons, and both regular and staggered canyons were considered for the simulations. The results show that the building height ratio affects not only the ventilation fluxes of the rooms in the downstream building but also the pollutant concentrations around the building. The parameter, outdoor effective source intensity of a room, is then proposed to calculate the amount of vehicular pollutants that enters into building rooms. Smaller value of this parameter indicates less pollutant enters the room. The numerical results reveal that HRs from 2/7 to 7/2 are the favorable height ratios for the regular canyons, as they obtain smaller values than the other cases. While HR values of 5/7, 7/7, and 7/5 are appropriate for staggered canyons. In addition, in terms of improving indoor air quality by natural ventilation, the staggered canyons with favorable HR are better than those of the regular canyons.

Air infiltration induced inter-unit dispersion and infectious risk assessment in a high-rise residential building

Identifying possible airborne transmission routes and assessing the associated infectious risks are essential for implementing effective control measures. This study focuses on the infiltration-induced inter-unit pollutant dispersion in a high-rise residential (HRR) building. The outdoor wind pressure distribution on the building facades was obtained from the wind tunnel experiments. And the inter-household infiltration and tracer gas transmission were simulated using multi-zone model. The risk levels along building height and under different wind directions were examined, and influence of component leakage area was analysed. It is found that, the cross-infection risk can be over 20% because of the low air infiltration rate below 0.7 ACH, which is significantly higher than the risk of 9% obtained in our previous on-site measurement with air change rate over 3 ACH. As the air infiltration rate increases along building height, cross-infection risk is generally higher on the lower floors. The effect of wind direction on inter-unit dispersion level is significant, and the presence of a contaminant source in the windward side results in the highest cross-infection risks in other adjacent units on the same floor. Properly improving internal components tightness and increasing air change via external components are beneficial to the control of internal inter-unit transmission induced by infiltration. However, this approach may increase the cross-infection via the external transmission, and effective control measures should be further explored considering multiple transmission routes.

Wind tunnel tests of inter-flat pollutant transmission characteristics in a rectangular multi-storey residential building, part B: Effect of source location

The pollutant behavior in and around a naturally ventilated building requires to be investigated quantitatively as the growing concern on air quality within the built environment. The objective of the present study is to further investigate the wind induced inter-flat pollutant transmission and cross contamination routes in typical buildings in Shanghai. In this paper, a set of experiments was carried out in a boundary layer wind tunnel using a 1:30 reduced scale model that represented the typical configuration of rectangular multi-storey residential buildings. Sulfur hexafluoride (SF6) was employed as a tracer gas in the wind tunnel tests. Two natural ventilation modes, single-sided ventilation and cross ventilation were considered. The conditions under prevailing wind direction with different source locations on the windward side were compared. The pressure coefficients on all of the building façades and tracer gas concentration distributions were monitored and analysed. The experimental results elucidated that contaminant released from windward units could spread vertically and horizontally to other units on the source façade and downstream units. The source location was a significant influence factor on the pollutant concentration in various units. In the single-sided ventilated building, the infected risks of leeward units were even higher than those in some windward units. In the cross ventilated building, the vertical transmission could be suppressed and the horizontal transmission was reinforced. The study is helpful for further understanding of the inter-flat airborne transmission within an isolated building.

Wind tunnel tests of inter-flat pollutant transmission characteristics in a rectangular multi-storey residential building, part A: Effect of wind direction

The inter-flat dispersion of hazardous air pollutants in residential built environment has become a growing concern, especially in crowed urban areas. The purpose of present study is to investigate the wind induced air pollutant transmission and cross contamination routes in typical buildings. In this paper, a series of experiments was carried out in a boundary layer wind tunnel using a 1:30 scaled model that represented the typical configuration of rectangular multi-storey residential buildings in Shanghai. Sulfur hexafluoride (SF₆) was employed as tracer gas in the wind tunnel tests. The conditions under two ventilation modes, i.e. single-sided natural ventilation and cross natural ventilation, were compared. The tracer gas concentration distributions under four approaching wind angles were monitored and analyzed. Computational Fluid Dynamics (CFD) method was adopted to assist in analyzing airflow patterns. The experiment results elucidated that in the two ventilation scenarios, both of the vertical and horizontal inter-flat airborne transmission could proceed. The wind direction played a key role on the pollutant concentration distribution. Compared with the single-sided ventilation mode, cross ventilation could weaken the air pollutant dispersion along the vertical direction when the contamination source was on the windward or on the leeward unit. When the wind blowing parallelly to the source unit window, namely the source room was on the sideward, cross ventilation would not suppress the vertical transport on one hand, but reinforce the horizontal transmission on the other hand. The study is helpful for the analysis of infection risk of respiratory diseases in the residential buildings.

Wind tunnel tests of inter-flat pollutant transmission characteristics in a rectangular multi-storey residential building, part A: Effect of wind direction

The inter-flat dispersion of hazardous air pollutants in residential built environment has become a growing concern, especially in crowed urban areas. The purpose of present study is to investigate the wind induced air pollutant transmission and cross contamination routes in typical buildings. In this paper, a series of experiments was carried out in a boundary layer wind tunnel using a 1:30 scaled model that represented the typical configuration of rectangular multi-storey residential buildings in Shanghai. Sulfur hexafluoride (SF₆) was employed as tracer gas in the wind tunnel tests. The conditions under two ventilation modes, i.e. single-sided natural ventilation and cross natural ventilation, were compared. The tracer gas concentration distributions under four approaching wind angles were monitored and analyzed. Computational Fluid Dynamics (CFD) method was adopted to assist in analyzing airflow patterns. The experiment results elucidated that in the two ventilation scenarios, both of the vertical and horizontal inter-flat airborne transmission could proceed. The wind direction played a key role on the pollutant concentration distribution. Compared with the single-sided ventilation mode, cross ventilation could weaken the air pollutant dispersion along the vertical direction when the contamination source was on the windward or on the leeward unit. When the wind blowing parallelly to the source unit window, namely the source room was on the sideward, cross ventilation would not suppress the vertical transport on one hand, but reinforce the horizontal transmission on the other hand. The study is helpful for the analysis of infection risk of respiratory diseases in the residential buildings.

CFD simulation of the effect of an upstream building on the inter-unit dispersion in a multi-story building in two wind directions

Previous studies on inter-unit dispersion are limited to isolated buildings. The influence of an upstream interfering building may significantly modify the indoor airflow characteristics of the wind-induced natural ventilated downstream interfered building. Motivated by the findings in previous studies, namely that infectious respiratory aerosols exhausted from a unit can re-enter into another unit in the same building through building envelope openings, this study investigates the inter-unit pollutant dispersion around a multi-story building in two wind directions by employing the computational fluid dynamics (CFD) method. The CFD model employed in this study has been validated against previous experimental data. The results show that the presence of an upstream building greatly changes the path lines around the downstream target building and the pollutant transportation routes around it. The presence of a low upstream building also greatly increases the average air exchange rate (ACH) values and the pollutant re-entry ratios (R_k) below the source unit on the windward side of the downstream target building for normal wind incidence. However, the presence of a high upstream building greatly increases the average ACH values on the windward side and increases the R_k on the leeward side of the downstream building for oblique wind incidence.

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Interunit pollutant dispersion in a naturally ventilated building was investigated.

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Effect of an upstream interfering building on the target building was examined.

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A high and a low upstream buildings were considered.

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An upstream building improves ventilation performance of the target building.

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An upstream building enhances Interunit dispersion in the target building.

Associated relationship between ventilation rates and indoor air quality

Ventilation represents a significant portion of building energy consumption.

The airborne transmission of infection between flats in high-rise residential buildings: A review

The inter-flat airborne cross-transmission driven by single-sided natural ventilation has been identified recently in high-rise residential buildings, where most people live now in densely populated areas, and is one of the most complex and least understood transport routes. Given potential risks of infection during the outbreak of severe infectious diseases, the need for a full understanding of its mechanism and protective measures within the field of epidemiology and engineering becomes pressing. This review paper considers progress achieved in existing studies of the concerned issue regarding different research priorities. Considerable progress in observing and modeling the inter-flat transmission and dispersion under either buoyancy- or wind-dominated conditions has been made, while fully understanding the combined buoyancy and wind effects is not yet possible. Many methods, including on-site measurements, wind tunnel tests and numerical simulations, have contributed to the research development, despite some deficiencies of each method. Although the inter-flat transmission and dispersion characteristics can be demonstrated and quantified in a time-averaged sense to some extent, there are still unanswered questions at a fundamental level about transient dispersion process and thermal boundary conditions, calling for further studies with more advanced models for simulations and more sound experiments for validations.

From street canyon microclimate to indoor environmental quality in naturally ventilated urban buildings: Issues and possibilities for improvement

Many buildings in urban areas are more or less naturally ventilated. A good understanding of the current status and issues of indoor environmental quality (IEQ) in naturally ventilated urban buildings and the association with urban microclimate is fundamental for improving their IEQ. This paper reviews past studies on (a) the microclimate in urban street canyons, (b) the potential influence of such microclimate on IEQ of nearby naturally ventilated buildings, and (c) the real-life IEQ status in these buildings. The review focuses mainly on studies conducted by on-site measurements. The microclimate in urban street canyons is characterized by low wind speed, high surface temperature difference, high pollutant concentration, and high noise level. Insufficient ventilation rates and excessive penetration of outdoor pollutants are two key risks involved in naturally ventilated urban buildings. Existing knowledge suggests that reasonable urban planning and careful building envelope design are the primary methods to ensure acceptable IEQ and maximize the utilization of natural ventilation. However, quantitative studies of both microclimate in street canyons and IEQ in buildings are still highly insufficient in many aspects, which make cross comparison and influencing factors analysis currently impossible. Based on the limitations of previous studies and the current issues of naturally ventilated urban buildings, suggestions are made for future studies to better understand and improve IEQ in naturally ventilated urban buildings.