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Indoor environmental control has become primordial to minimize inhalation risk of pollutants through the implementation of next-generation ventilation systems. The performance of these ventilation strategies should be evaluated in terms of risk mitigation using a multi-indicator approach to support the realistic outcome of this evaluation. The present study has evaluated five main ventilation strategies (mixing, displacement, underfloor air, impinging jet and stratum ventilation) widely used in current society and four further cases based on occupant position with respect to supply flow. Their performance was assessed through computational fluid dynamics and two interacting virtual manikins with autonomous respiratory functions to calculate age of air, particle dispersion in a room, particles in the local breathing zone, maximum long-term exposure, deposition rate on human tissue and transfer probability. Results showed that well-mixed air in up-supply strategies increase age of air, particle dispersion in the room, transfer probability and deposition in the human body. Down- and mid-supply methods show high local age of air while improving the other indicators but create asymmetric flow in some cases. Overall, displacement ventilation was the optimal solution because it presented the lowest transfer probability although slightly increased age of air. Furthermore, deposited particles were mainly located in the nasal cavity, which might lower adverse health effects. • Large eddy simulation improves accuracy of indoor particle tracking. • Virtual manikins allow exact inhalation dosage through transfer probability. • Displacement ventilation mitigates particle deposition compared to other types. • Particle tract deposition locale affects human body metabolization. [ABSTRACT FROM AUTHOR]

Cross-ventilation using rooftop wind-catchers is very complex as it is influenced by a wide range of interrelated factors including aerodynamic characteristics of the wind catcher, approach-flow conditions and building geometry. Earlier studies on wind-driven cross-ventilation in buildings have shown the significant impact of the geometry and position of openings on the flow and ventilation performance. However, this has not yet been investigated for cross-ventilation using wind catchers. This paper, therefore, presents a detailed evaluation of the impact of the outlet openings on the ventilation performance of a single-zone isolated building with a wind catcher. The evaluation is based on three ventilation performance indicators: (i) induced airflow rate, (ii) age of air, and (iii) air change efficiency. High-resolution coupled 3D steady RANS CFD simulations of cross-ventilation are performed for different sizes and types of outlet openings. The CFD simulations are validated based on wind-tunnel measurements. The results show that using outlet openings very close to the wind catcher will not increase the induced airflow, while it leads to a considerable reduction in the indoor air quality. A combination of one-sided wind-catcher and window is superior, while the use of two-sided wind-catchers leads to the lowest indoor air quality and air change efficiency. [ABSTRACT FROM AUTHOR]

Chilled beams provide sensible cooling to an occupied space by using chilled water flowing through modular beams mounted to a room ceiling. Previous studies show that active chilled beam systems can achieve larger sensible cooling than all-air systems. However, most studies have focused on active chilled beams and little information is available for performance of passive chilled beam (PCB) under varied ventilation conditions. The objective of the present study is to investigate ventilation and air conditioning performance of combined PCB and displacement ventilation systems. Using a validated and verified computational fluid dynamics (CFD) model, parametric analyses were performed to examine the ventilation and air conditioning performance depending on five major operating parameters: 1) ventilation strategy, 2) PCB cooling output, 3) supply air temperature, 4) supply diffuser type, and 5) internal heat source arrangement. The results show that thermal comfort and ventilation performance of a combined PCB-DV system notably vary with the PCB cooling output and supply air temperature. When the PCB cooling output increases from 33% to 53% of the total cooling load, air change effectiveness decreases from 1.6 to 1.2 and vertical temperature difference decreases from 4.0 °C to 1.5 °C in the breathing zone. However, spatial heat source distribution has a marginal effect on the air mixing and temperature distribution. The results suggest that a relatively high PCB cooling output (>50% of the total cooling) combined with a low supply air temperature ( e.g. , 17 °C) could disrupt thermal stratification of displacement ventilation and increase draft rate in the occupied zone. [ABSTRACT FROM AUTHOR]

Street networks serve as the main open corridors for wind flow within urban areas, so understanding their ventilation performance and capacity of removing pollutants helps improve urban air quality. However, conventional modelling methods using an exact building model (EBM) are hard-pressed to simulate urban-scale areas, due to their extremely high computing load. To reduce time and cost of numerical simulations, and accurately predict the flow field and ventilation within the street network, a simplified modelling approach (i.e., EBM&PMM) is proposed, in which only the first row of buildings flanking the street is modelled by EBM and the rest of the urban buildings are modelled by a porous media model (PMM). The performance of EBM&PMM is evaluated by three common modelling approaches, i.e., EBM, PMM, and sealed concrete model (SCM) in which the building complexes with gaps are treated as an enclosed entity. A typical crossing street network flanked by four communities (building arrays) is used for this study. The age of air (τ P) and ventilation flux (Q) obtained from the four modelling methods are compared. The results show that EBM&PMM predicts τ P and Q well, and the flow pattern by this model agrees well with EBM. PMM significantly underestimates τ P but exaggerates Q in street networks, and it fails to give the detailed flow patterns. SCM over-predicts τ P , and Q between the community and street is zero. Furthermore, the first row of buildings facing the street plays a decisive role in flow pattern and ventilation effectiveness of the street network. [Display omitted] • Airflow in streets is simulated by exact building model and porous media model (EBM&PMM). • EBM&PMM predicts the flow pattern and ventilation processes well in street networks. • PMM and sealed concrete models fail to simulate flows and pollution in the network. • First building rows beside a street play decisive roles in simulations by the EBM. [ABSTRACT FROM AUTHOR]

Most people spend approximately 90% of their lives indoors. Thus, designing effective ventilation systems is essential to mitigating problems with indoor air quality. The measures of mechanical ventilation design performance considered in this study are age of air, air residual life time, air residence time, and ventilation effectiveness. This paper presents two different methods to help quantify these measures. The first method is based on transport equations, where a continuous representation of these quantities are calculated. The second method is based on Markov matrices, where a discrete representation of these quantities are calculated. We show 1) how both the continuous and discrete methods are related and 2) that the age of air and residual life time are adjoints. A new transport equation for residual life time along with methods for these quantities using Markov matrices are established. The two approaches are validated and compared using previously established experimental data. The results show that both approaches provide similar results. Using these techniques allows for the quantities of residual life time and residence time to be integrated into the design processes. This paper provides a simple framework that enables designers to get a comprehensive picture of the ventilation systems they design. [ABSTRACT FROM AUTHOR]

Knowledge of temperature stratification in indoor environments is important for occupant thermal comfort and indoor air quality and for the design and evaluation of displacement ventilation systems. This paper presents a detailed and systematic evaluation of the performance of 3D steady Reynolds-Averaged Navier–Stokes (RANS) CFD simulations to predict the temperature stratification within a room with a heat source and two ventilation openings. The indoor air quality within the room is also investigated by assessing the distribution of the age of air. The evaluation is based on validation with full-scale measurements of air temperature. A sensitivity analysis is performed to investigate the impact of computational grid resolution, turbulence model, discretization schemes and iterative convergence on the predicted temperatures and age of air. The results show that steady RANS with the SST k-ω model can accurately predict the temperature stratification in this particular indoor environment. However, of the five commonly used turbulence models, only the SST k-ω model and the standard k- ω model succeed in reproducing the thermal plume structure and the associated thermal stratification, while the three k-ε models clearly fail in doing so. In addition, the iterative convergence criteria have a major impact on the predicted age of air, where it is shown that very stringent criteria in terms of scaled residuals are required to obtain accurate results. These required criteria are much more stringent than typical default settings. This paper is intended to contribute to improved accuracy and reliability of CFD simulations for displacement ventilation assessment. [ABSTRACT FROM AUTHOR]

The age of air, which measures the time elapsed between the emission of a chemical constituent and its arrival at a receptor location, has many applications in urban air quality. Typically it has been estimated for special cases, e.g. the local mean age of air for a spatially homogeneous source. An alternative approach uses the response to a point source to determine the distribution of transit times or tracer ages connecting the source and receptor. The distribution (age spectrum) and first moment (mean tracer age) have proven to be useful diagnostics in stratospheric modelling because they can be related to observations and do not require a priori assumptions. The tracer age and age spectrum are applied to the pollutant ventilation of street canyons in this work. Using large-eddy simulations of flow over a single isolated canyon and an uneven, non-uniform canyon array, it is shown that the structure of the tracer age is dominated by the central canyon “vortex”; small variations in the building height have a significant influence on the structure of the tracer age and the pollutant ventilation. The age spectrum is broad, with a long exponential tail whose slope depends on the canyon geometry. The mean tracer age, which roughly characterises the ventilation strength, is much greater than the local mean age of air. [ABSTRACT FROM AUTHOR]

Air distribution is an effective engineering measure to fight against respiratory infectious diseases like COVID-19. Ventilation indices are widely used to indicate the airborne infection risk of respiratory infectious diseases due to the practical convenience. This study investigates the relationships between the ventilation indices and airborne infection risk to suggest the proper ventilation indices for the evaluation of airborne infection risk control performance of air distribution. Besides the commonly used ventilation indices of the age of air (AoA), air change effectiveness (ACE), and contaminant removal effectiveness (CRE), this study introduces two ventilation indices, i.e., the air utilization effectiveness (AUE) and contaminant dispersion index (CDI). CFD simulations of a hospital ward and a classroom served by different air distributions, including mixing ventilation, displacement ventilation, stratum ventilation and downward ventilation, are validated to calculate the ventilation indices and airborne infection risk. A three-step correlation analysis based on Spearman's rank correlation coefficient, Pearson correlation coefficient, and goodness of fit and a min-max normalization-based error analysis are developed to qualitatively and quantitatively test the validity of ventilation indices respectively. The results recommend the integrated index of AUE and CDI to indicate the overall airborne infection risk , and CDI to indicate the local airborne infection risk respectively regardless of the effects of air distribution, supply airflow rate, infectivity intensity, room configuration and occupant distribution. This study contributes to airborne transmission control of infectious respiratory diseases with air distribution. • Relationships between ventilation indices and airborne infection risk are tested qualitatively and quantitatively. • Different air distributions, ventilation rates, infectivity intensities, room configurations, and occupants are considered. • Integrated index of AUE and CDI robustly indicates overall airborne infection risk control performance of air distribution. • CDI robustly indicates local airborne infection risk control performance of air distribution. [ABSTRACT FROM AUTHOR]

Natural ventilation in residential buildings has a great potential for conserving energy and improving the health of occupants. This paper first presents a design strategy for optimizing natural ventilation in high-rise residential buildings in Chongqing, China, a region with unfavorably low wind speed. Through the use of CFD modeling, building orientation and spacing were adjusted, wind paths were created into internal zones, and two windows were constructed in each bedroom. The optimized design reduced the age of air to less than 6 min in 90% of the rooms, as compared to an age of greater than 30 min in 50% of the rooms in a conventional design. Natural ventilation was found to be effective in the Chongqing area with the optimized design. This investigation also measured the local age of air and air change rate in a building with the optimized design using a tracer-gas method. The measurements confirmed the reliability of the CFD results. [ABSTRACT FROM AUTHOR]

Semi-open street roofs protect pedestrians from intense sunshine and rains. Their effects on natural ventilation of urban canopy layers (UCL) are less understood. This paper investigates two idealized urban models consisting of 4(2 × 2) or 16(4 × 4) buildings under a neutral atmospheric condition with parallel (0°) or non-parallel (15°, 30°, 45°) approaching wind. The aspect ratio (building height (H)/street width (W)) is 1 and building width is B = 3H. Computational fluid dynamic (CFD) simulations were first validated by experimental data, confirming that standard k–ε model predicted airflow velocity better than RNG k–ε model, realizable k–ε model and Reynolds stress model. Three ventilation indices were numerically analyzed for ventilation assessment, including flow rates across street roofs and openings to show the mechanisms of air exchange, age of air to display how long external air reaches a place after entering UCL, and purging flow rate to quantify the net UCL ventilation capacity induced by mean flows and turbulence. Five semi-open roof types are studied: Walls being hung above street roofs (coverage ratio λ a = 100%) at z = 1.5H, 1.2H, 1.1H (‘Hung1.5H’, ‘Hung1.2H’, ‘Hung1.1H’ types); Walls partly covering street roofs (λ a = 80%) at z = H (‘Partly-covered’ type); Walls fully covering street roofs (λ a = 100%) at z = H (‘Fully-covered’ type). They basically obtain worse UCL ventilation than open street roof type due to the decreased roof ventilation. ‘Hung1.1H’, ‘Hung1.2H’, ‘Hung1.5H’ types are better designs than ‘Fully-covered’ and ‘Partly-covered’ types. Greater urban size contains larger UCL volume and requires longer time to ventilate. The methodologies and ventilation indices are confirmed effective to quantify UCL ventilation. [ABSTRACT FROM AUTHOR]

Abstract: A ventilation system of sufficient performance is the basic condition for comfort and health risk free environment in enclosed aircraft cabin spaces. The airflow pattern and spreading of fresh air into the cabin space mainly depend on the type of the air distribution system and location of heat sources/sinks. The present study deals with the investigation of the dependency between the type of the air distribution system and indoor air quality in a cabin of a small transport aircraft. The mock-up interior geometry of a small aircraft cabin for nine passengers was adopted for the investigation. Three types of the air distribution system were investigated: a typical mixing ventilation system, a modified mixing ventilation system and an under-aisle displacement ventilation system. The quality of ventilation was assessed based on the local mean age of air using CFD software Star-CCM+. At first, the CFD model of a typical mixing ventilation was validated against the experimental data of airflow in the mock-up of the small aircraft cabin. The validated model was then used for the evaluation of the three distribution systems under cold, mild and hot ambient environment conditions from the point of their performance for several typical aircraft operational conditions. Results of the study show a strong dependence of the air distribution systems performance on the ambient conditions. The most stable air distribution was reached with the modified mixing air distribution system. Other types of ventilation systems failed to provide sufficient ventilation and cooling under hot ambient conditions. [Copyright &y& Elsevier]

Abstract: This paper reports the results of a parametric CFD study on idealized city models to investigate the potential of slope flow in ventilating a city located in a mountainous region when the background synoptic wind is absent. Examples of such a city include Tokyo in Japan, Los Angeles and Phoenix in the US, and Hong Kong. Two types of buoyancy-driven flow are considered, i.e., slope flow from the mountain slope (katabatic wind at night and anabatic wind in the daytime), and wall flow due to heated/cooled urban surfaces. The combined buoyancy-driven flow system can serve the purpose of dispersing the accumulated urban air pollutants when the background wind is weak or absent. The microscopic picture of ventilation performance within the urban structures was evaluated in terms of air change rate (ACH) and age of air. The simulation results reveal that the slope flow plays an important role in ventilating the urban area, especially in calm conditions. Katabatic flow at night is conducive to mitigating the nocturnal urban heat island. In the present parametric study, the mountain slope angle and mountain height are assumed to be constant, and the changing variables are heating/cooling intensity and building height. For a typical mountain of 500 m inclined at an angle of 20° to the horizontal level, the interactive structure is very much dependent on the ratio of heating/cooling intensity as well as building height. When the building is lower than 60 m, the slope wind dominates. When the building is as high as 100 m, the contribution from the urban wall flow cannot be ignored. It is found that katabatic wind can be very beneficial to the thermal environment as well as air quality at the pedestrian level. The air change rate for the pedestrian volume can be as high as 300 ACH. [Copyright &y& Elsevier]

Abstract: By means of computational fluid dynamic (CFD) simulations, this paper investigated how rural winds transport relatively clean air into high-rise urban areas to dilute airborne pollutants. Two ventilation concepts for indoor environments were applied: the age of air to quantify the time taken by rural young air to reach a given place after it enters an urban area and the air exchange efficiency to evaluate the efficiency of rural winds flushing the entire urban canopy layer. Several square building arrays with street aspect ratios (building height/street width, H/W) ranging from 1.5 to 5.3 and building area densities of 0.25 (medium) or 0.4 (compact) were considered as the approaching wind is parallel to the main street whose length varies from street scales to neighborhood scales (330–510 m to 1.03–1.65 km in full-scale). Results show that considerable young rural air enters windward entries but a major fraction of air is vertically driven out as flowing deeper into such high-rise building arrays. So air exchange efficiencies are less than 50% in street-scale arrays, and smaller in longer (neighborhood-scale) or narrower arrays. For the neighborhood-scale medium arrays, considering the power-law velocity profile in the upstream free flow, a taller array gains a larger inflow rate across its windward entry and experiences younger air and greater air exchange efficiency than a lower one. If all buildings are theoretically open-based in a neighborhood-scale compact array, air becomes much younger everywhere and the air exchange efficiency doubles. In arrays of buildings with different heights, the secondary streets in front of taller buildings get younger air due to the downward flows within them. Although further investigations are still required before providing a practical framework, this paper is one of the first attempts to find ways in improving the ventilation performance in high-rise cities like Hong Kong. [Copyright &y& Elsevier]

Abstract: This paper presents an experimental modeling of unpleasant odor dispersion from the toilet in a bathroom mockup. A hypothetical odor (SF6 tracer gas) was emitted from the position of a dummy sitting on the toilet in the bathroom. This experimental investigation aimed to examine the influence of ventilation schemes on the ventilation performance of the unpleasant odor removal. Three ventilation schemes of the bathroom have been investigated in the present study: (1) model A, forced ceiling-supply and wall-exhaust systems, (2) model B, natural window-inlet and forced ceiling-exhaust systems, and (3) model C, forced ceiling-supply and ceiling-exhaust systems. This research employed the tracer gas decay (step-down) method to evaluate the age of air and the air exchange efficiency in the bathroom and the constant injection method to investigate the odor removal efficiency of the bathroom under the three studied ventilation schemes. Results show that the ventilation performance from high to low was model A, C, and B. [Copyright &y& Elsevier]

Abstract: This paper reports the implementation of the “mean age of air” (MAA) concept into the computational fluid dynamics (CFD) code Fluent through user-defined functions to assess ventilation efficiency inside forced-ventilation food plants. Two transient methods and a steady-state method based on the resolution of an additional scalar transport equation were implemented and compared with experimental and numerical data obtained by Bartak et al. [Bartak, M., Cermak, M., Clarke, J.A., Denev, J., Drkal, F., Lain, M., Macdonald, I.A., Majer, M., Stankov, P., 2001. Experimental and numerical study of local mean age of air. In: Proceedings of the 7th International Building Performance Simulation Association (IBPSA) Conference, Rio de Janeiro, Brazil; Bartak, M., Beausoleil-Morrison, I., Clarke, J.A., Denev, J., Drkal, F., Lain, M., Macdonald, I.A., Melikov, A., Popiolek, Z., Stankov, P., 2002. Integrating CFD and building simulation. Building and Environment, 37, 865–871] for a 45 m3 test room. As the steady-state method led to the best compromise between accuracy of results and computation time, this method was then used to characterize ventilation efficiency inside a pilot cheese ripening room and two modern sausage dryers of large height. In light of the results presented, local MAA is a better and a more sensitive parameter than mean air velocity for highlighting areas with inadequate ventilation, and thus for assessing ventilation efficiency in industrial food plants. [Copyright &y& Elsevier]

Abstract: In literature, local mean age of air is used as an important index to evaluate indoor air quality in ventilated rooms. In this research, a data-based mechanistic approach is used to model the spatial–temporal mass distribution in an imperfectly mixed forced ventilated installation. A first-order transfer function model has proved to be sufficiently good in describing the mass transfer dynamics of the system. Furthermore, it was possible to fully understand the physical meaning of the model parameter. The parameter is found to be an inverse of the age of air. This Data-Based Modelling approach proved to be more robust when dealing with measurement noise. Finally, the modelled age of air was validated with a classical step up determination of the age of air for experimental data. Good correlation was found between both results, which proved the physical background of the model parameter. [Copyright &y& Elsevier]

Half of the energy usages in buildings is due to HVAC systems, and the efficiency of the subsections plays an essential role in the evaluation of the total energy consumption. Previous works on ventilation systems using perforated duct diffusers (PDDs) were mainly concentrated on the calculation of the thermal comfort indices, while their energy consumption and ventilation effectiveness were omitted. In this paper, a mixed experimental-numerical method has been developed to evaluate the real efficiency of PDDs with different perforation patterns. So, an office room facility is used for the full-scale experiments and the validation of the numerical simulations. Incidentally, five RANS models have been benchmarked to highlight the most accurate numerical model for this specific application. As a result, a k-ε Realizable turbulence model with 4.3 million mesh elements has been preferred over the tested two-equation turbulence models to determine the diffuser parameters like age of air, terminal velocities, air-change effectiveness, and air exchange efficiency. Within this model, the required outdoor airflow for four types of PDDs has been calculated following the ASHRAE standard procedure. The discrepancy of the real efficiency and the calculated value for the nearest application in the standards shows that the performance of PDDs is underestimated, and they require lower airflow in reality. Later, it is shown that for the systems using PDDs, the required airflow and energy consumption decrease by up to 18.4%, and a more uniform air distribution is achieved using a non-uniform perforation pattern. • Perforated duct diffusers (PDDs) energy and efficiency performances were evaluated. • Four types of PDDs with uniform and non-uniform perforation patterns were compared. • Following ASHRAE standards, a mixed experimental and CFD-RANS method is used. • The PDDs with the non-uniform perforation improve the diffuser performances. • The PDDs with the non-uniform perforation improve the ventilation effectiveness. [ABSTRACT FROM AUTHOR]

[Display omitted] • Experimental and CFD studies were performed to investigate a novel DIV system. • The developed CFD model accurately predicted air quality and airflow in DIV. • Stratified air distributions were created by DIV in both cooling and heating. • The thermal comfort in a room with DIV met ASHRAE requirements. • The novel DIV system effectively remedied limitation of traditional DV. Traditional displacement ventilation (DV) creates high indoor air quality in occupied zone in cooling season due to thermal plume created by heat sources which then effectively purge contaminants out of the breathing zone, but tend to be challenged when high cooling loads are present. In addition, traditional DV, when supplying heated air via its diffusers has low ventilation effectiveness in heating season. This study investigated a novel wall-mounted displacement induction ventilation (DIV) system in comparison to traditional DV system. The investigation used an environmental chamber to mimic a typical classroom and measured the distributions of air velocity, air temperature, and contaminant concentration for validating the subsequent computational fluid dynamics (CFD) model developed across an array of operational modes. Then the CFD model was employed to evaluate the performance of DIV system in terms of ventilation effectiveness, mean age of air and vertical temperature gradient as it relates to thermal comfort. Each DIV unit receives 100% outdoor (or primary) air to its plenum chamber. As the primary air pressurizes the chamber, it exits a series of nozzles, creating a low pressure which then induces room air across the integral hydronic coil. A room is typically supplied with multiple DIV units, which allows for individual control operation during both cooling and heating modes. Results showed that stratified air distribution was achieved in both cooling and heating modes with DIV system, which remedied the current limitation of traditional DV. [ABSTRACT FROM AUTHOR]

Urban breathability is fundamentally important for improving urban air environment. Nowadays, air pollution and extreme heat waves heavily threaten modern cities and residents. In this paper, a methodology combined with an idealized High-Reynolds-Number Porous-Media city model and a multi-scale Computational Fluid Dynamics (CFD) method will be firstly proposed to quantify the urban ventilation breathability under no geostrophic winds. Breathability of city was then evaluated in terms of air change rate per hour (ACH) and age of air. The contribution of five factors to urban heat island and air quality is analyzed qualitatively and quantitatively . Numerical results further demonstrate that, 1) For heat island intensity, the dominant importance of influencing factors is: heat flux > temperature lap rate > urban scale > building height > urban density. Urban heat flux and temperature lapse rate have great influences on the urban heat island intensity and air quality. Comparing with neutral atmosphere, ACH in urban canopy decreased by more than a half under inversion condition, whereas ACH in pedestrian layer decreased by nearly 100%. 2) For air quality, the importance of the influencing factors could be listed as, building height > heat flux > temperature lapse rate > urban scale > urban density. Building height has the greatest impact on air quality. With the increase of urban scale, average age of air for a city decreases first and then increases. There exists an optimal city size to minimize the age of urban air. Urban building density (regardless of heat emission) on the buoyancy driven flow was very weak. Present model and research results could have theoretical significance for the council and urban planners to alleviate urban heat island and urban air pollution through reasonable planning of urban building layouts. • Multi-scale urban airflows were well quantified by present proposed Porous-Media and CFD models. • Breathability of city was evaluated in terms of air change rate per hour (ACH) and age of air. • The importance of different factors to urban heat island and air quality is quantified and ranked. • For urban heat island intensity, the dominant factors are heat flux and temperature lapse rate. • For urban air quality, building height will play an important role. [ABSTRACT FROM AUTHOR]