Protected zone ventilation and reduced personal exposure to airborne cross-infection.

The main objective of this study was to examine the performance of protected zone ventilation (PZV) and hybrid protected zone ventilation (HPZV) to reduce the direct exposure to exhaled air from others' breathing. Experimental measurements are carried out to test the performance of PZV in a full-scale office room with two breathing thermal manikins. The measurements were performed under three configurations, including two standing manikins at different distances: 0.35, 0.5, and 1.1 m. When the supply air velocity is increased to 4 m/s in the downward plane jet, the dimensionless concentration is 40% lower than for fully mixed ventilation, which can be considered as a measure of protection from the zoning condition. The measurement results showed that in both the PZV and the HPZV system it is possible to decrease the transmission of tracer gas from one manikin to the opposite manikin; therefore, it probably would reduce the risk of air borne cross-infection between two people at the same relative positions. The results suggest that PZV and HPZV may be used to reduce the exposure of people in a protected zone from indoor pollutants emitted in a source zone.

Short-term airing by natural ventilation - implication on IAQ and thermal comfort.

The need to improve the energy efficiency of buildings requires new and more efficient ventilation systems. It has been demonstrated that innovative operating concepts that make use of natural ventilation seem to be more appreciated by occupants. Among the available ventilation strategies that are currently available, buoyancy driven, single-sided natural ventilation has proved to be very effective and can provide high air change rates for temperature and Indoor Air Quality (IAQ) control. However, to promote a wider distribution of these systems an improvement in the knowledge of their working principles is necessary. The present study analyses and presents the results of an experimental evaluation of airing performance in terms of ventilation characteristics, IAQ and thermal comfort. It includes investigations of the consequences of opening time, opening frequency, opening area and expected airflow rate, ventilation efficiency, thermal comfort and dynamic temperature conditions. A suitable laboratory test rig was developed to perform extensive experimental analyses of the phenomenon under controlled and repeatable conditions. The results showed that short-term window airing is very effective and can provide both acceptable IAQ and thermal comfort conditions in buildings. Practical Implications This study gives the necessary background and in-depth knowledge of the performance of window airing by single-sided natural ventilation necessary for the development of control strategies for window airing (length of opening period and opening frequency) for optimum IAQ and thermal comfort in naturally ventilated buildings.

Short-term airing by natural ventilation - modeling and control strategies.

The need to improve the energy efficiency of buildings requires new and more efficient ventilation systems. It has been demonstrated that innovative operating concepts that make use of natural ventilation seem to be more appreciated by occupants. This kind of system frequently integrates traditional mechanical ventilation components with natural ventilation devices, such as motorized windows and louvers. Among the various ventilation strategies that are currently available, buoyancy driven single-sided natural ventilation has proved to be very effective and can provide high air change rates for temperature and IAQ control. However, in order to promote a wider applications of these systems, an improvement in the knowledge of their working principles and the availability of new design and simulation tools is necessary. In this context, the paper analyses and presents the results of a research that was aimed at developing and validating numerical models for the analysis of buoyancy driven single-sided natural ventilation systems. Once validated, these models can be used to optimize control strategies in order to achieve satisfactory indoor comfort conditions and IAQ. Practical Implications Numerical and experimental analyses have proved that short-term airing by intermittent ventilation is an effective measure to satisfactorily control IAQ. Different control strategies have been investigated to optimize the capabilities of the systems. The proposed zonal model has provided good performances and could be adopted as a design tool, while CFD simulations can be profitably used for detailed studies of the pollutant concentration distribution in a room and to address local discomfort problems.

The airborne transmission of infection between flats in high-rise residential buildings: Particle simulation.

Several case clusters occurred in high-rise residential buildings in Hong Kong in the 2003 SARS (the severe acute respiratory syndrome) epidemic, which motivated a series of engineering investigations into the possible airborne transport routes. It is suspected that, driven by buoyancy force, the polluted air that exits the window of the lower floor may re-enter the immediate upper floor through the window on the same side. This cascade effect has been quantified and reported in a previous paper, and it is found that, by tracer gas concentration analysis, the room in the adjacent upstairs may contain up to 7% of the air directly from the downstairs room. In this study, after validation against the experimental data from literatures, Eulerian and Lagrangian approaches are both adopted to numerically investigate the dispersion of expiratory aerosols between two vertically adjacent flats. It is found that the particle concentration in the upper floor is two to three orders of magnitude lower than in the source floor. 1.0 µm particles disperse like gaseous pollutants. For coarse particles larger than 20.0 µm, strong deposition on solid surfaces and gravitational settling effect greatly limit their upward transport.

The airborne transmission of infection between flats in high-rise residential buildings: Tracer gas simulation.

Airborne transmission of infectious respiratory diseases in indoor environments has drawn our attention for decades, and this issue is revitalized with the outbreak of severe acute respiratory syndrome (SARS). One of the concerns is that there may be multiple transmission routes across households in high-rise residential buildings, one of which is the natural ventilative airflow through open windows between flats, caused by buoyancy effects. Our early on-site measurement using tracer gases confirmed qualitatively and quantitatively that the re-entry of the exhaust-polluted air from the window of the lower floor into the adjacent upper floor is a fact. This study presents the modeling of this cascade effect using computational fluid dynamics (CFD) technique. It is found that the presence of the pollutants generated in the lower floor is generally lower in the immediate upper floor by two orders of magnitude, but the risk of infection calculated by the Wells-Riley equation is only around one order of magnitude lower. It is found that, with single-side open-window conditions, wind blowing perpendicularly to the building may either reinforce or suppress the upward transport, depending on the wind speed. High-speed winds can restrain the convective transfer of heat and mass between flats, functioning like an air curtain. Despite the complexities of the air flow involved, it is clear that this transmission route should be taken into account in infection control.

Experimental and CFD evidence of multiple solutions in a naturally ventilated building.

This paper considers the existence of multiple solutions to natural ventilation of a simple one-zone building, driven by combined thermal and opposing wind forces. The present analysis is an extension of an earlier analytical study of natural ventilation in a fully mixed building, and includes the effect of thermal stratification. Both computational and experimental investigations were carried out in parallel with an analytical investigation. When flow is dominated by thermal buoyancy, it was found experimentally that there is thermal stratification. When the flow is wind-dominated, the room is fully mixed. Results from all three methods have shown that the hysteresis phenomena exist. Under certain conditions, two different stable steady-state solutions are found to exist by all three methods for the same set of parameters. As shown by both the computational fluid dynamics (CFD) and experimental results, one of the solutions can shift to another when there is a sufficient perturbation. These results have probably provided the strongest evidence so far for the conclusion that multiple states exist in natural ventilation of simple buildings. Different initial conditions in the CFD simulations led to different solutions, suggesting that caution must be taken when adopting the commonly used 'zero initialization'.

Influence of local airflow on the pollutant emission from indoor building surfaces.

This article reports the results of an investigation, based on fundamental fluid dynamics and mass transfer theory, carried out to obtain a general understanding of the mechanisms involved in the emissions from building materials in ventilated rooms. In addition, a generally applicable method for the prediction of surface emissions is proposed. The work focused on the emission of vapours and gases and no particulate emissions were considered. The methods used were numerical calculations by computational fluid dynamics (CFD) and full-scale laboratory experiments. It was found that the emissions are a strong function of air-change rate, local air velocity and local turbulence, as the mass transfer coefficient increases in proportion to these parameters. The findings further show that the mass transfer coefficient increases in proportion to the velocity when the emission is controlled by evaporation from the surface. With regard to diffusion-controlled emissions, the mass transfer coefficient is unaffected by the velocity.