Naiping Gao

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

Abstract 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.

Transient CFD simulation of the respiration process and inter-person exposure assessment

Abstract It is known that the person-to-person spreading of certain infectious diseases is related with the transmission of human exhaled air in the indoor environments, and this is suspected to be the case with the severe acute respiratory syndrome (SARS) outbreak. This paper presents the numerical analysis of the human respiration process and the transport of exhaled air by breathing, sneezing, and coughing and their potential impact on the adjacent person in a modeled room with displacement ventilation. In order to account for the influence of the thermal plume around the human body, a three-dimensional computational thermal manikin (CTM) with an accurate description of body geometry was applied. Some of the results were compared with those from former simulations and experiments. It was found that personal exposure to the exhaled air from the normal respiration process of other persons is very low in a modeled room with displacement ventilation. Personal exposure to pollution caused by sneezing or coughing is highly directional. When two occupants face each other the cross-infection may happen due to the long transport distance of the exhalation.

Personalized Ventilation for Commercial Aircraft Cabins

Complaints about cabin air quality and persistent reports of spreading infections on commercial flights indicate that continued investigations of cabin air systems and effective measures to improve cabin air quality are required. This study used a computational fluid dynamics technique to investigate the dispersion characteristics of sneezed/ coughed particles by both the Eulerian and Lagrangian methods. These particles can be transported to a location more than three rows in front of the sneezing person, and less than 20% of the particles were exhausted, whereas the remainders are deposited owing to the high surface-to-volume ratio. Personalized ventilation, through the distribution of fresh air directly in the breathing zone, was able to shield up to 60% of air pollutants in a passengers inhalation.

Coupling CFD and human body thermoregulation model for the assessment of personalized ventilation

Personalized ventilation has great potential to improve inhaled air quality and to accommodate individual thermal preferences. In order to quantify these perceived benefits, a numerical method has been developed. In this method, a numerical thermal manikin (NTM), with the real geometry of a human body, is obtained by employing a laser scanning technique. When placed in a virtual environment, the thermal interactions with the environment are calculated using computational fluid dynamics (CFD). By iteration, the calculated air velocity near the body surface is fed into a sophisticated thermoregulation model developed at the University of California, Berkeley, so that the local thermal comfort in a non-uniform environment created by personalized air (PA) is rigorously investigated. In this paper, the performances of three different PV systems are investigated, namely, the desk-edge-based PV, PV using a movable panel (MP), and chair-based PV. The results exhibit reasonable agreement with the experimental measurements. The three types of PV are all able to lower human exposure to ambient room pollutants and bring a “cool head” thermal condition favorable for thermal comfort. The present work illustrates that in the development of localized personal environmental control systems, an NTM coupled with a human-body thermal regulation model is a useful tool for visualizing thermal comfort and ventilation effectiveness.

Modeling the Performance of Personalized Ventilation under Different Conditions of Room Air and Personalized Air

Building-related health complaints and sick building syndrome (SBS) represent significant health problems. Personalized ventilation (PV) is able to provide better inhaled air quality since the fresh air is supplied directly to the breathing zone “gently.” In this paper, after the numerical methods are validated by experiments, a seated computational thermal manikin (CTM) with geometry of a real human body is used to carry out the parameters study of PV. The modeling outcome shows that in a stagnant environment the human body is enclosed by the thermal plume, whose intensity is affected by the room air temperature. Warm rising airflow around the human body will entrain room air into inhalation and thus decreases the fraction of personalized air in the inhaled air. A uniform invading airflow in the horizontal level with the speed of 0.2 m/s (40 ft/min) is strong enough to tear away the thermal plume at the windward side of the human body. Its effect on the performance of PV is dependent on the flow direction. The benefits of personalized air supplied at a low rate limited by draught criteria are very sensitive to the ambient air conditions, which are, in turn, controlled by the background air-conditioning system.

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

Abstract 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.

Co‐occupant's exposure to exhaled pollutants with two types of personalized ventilation strategies under mixing and displacement ventilation systems

Personalized ventilation (PV) system in conjunction with total ventilation system can provide cleaner inhaled air for the user. Concerns still exist about whether the normally protecting PV device, on the other hand, facilitates the dispersion of infectious agents generated by its user. In this article, two types of PV systems with upward supplied fresh air, namely a chair-based PV and one kind of desk-mounted PV systems, when combined with mixing ventilation (MV) and displacement ventilation (DV) systems, are investigated using simulation method with regard to their impacts on co-occupants exposure to the exhaled droplet nuclei generated by the infected PV user. Simulation results of tracer gas and particles with aerodynamic diameter of 1, 5, and 10 μm from exhaled air show that, when only the infected person uses a PV, the different PV air supplying directions present very different impacts on the co-occupants intake under DV, while no apparent differences can be observed under MV. The findings demonstrate that better inhaled air quality can always be achieved under DV when the adopted PV system can deliver conditioned fresh air in the same direction with the mainly upward airflow patterns of DV.

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

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

Abstract 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%.

Characteristics of physical blocking on co-occupant＇s exposure to respiratory droplet residuals

Existed evidences show that airborne transmission of human respiratory droplets may be related with the spread of some infectious disease, such as severe acute respiratory syndrome (SARS) and H1N1 pandemic. Non-pharmaceutical approaches, including ventilation system and personal protection, are believed to have certain positive effects on the reduction of co-occupant’s inhalation. This work then aims to numerically study the performances of mouth covering on co-occupant’s exposure under mixing ventilation (MV), under-floor air distribution (UFAD) and displacement ventilation (DV) system, using drift-flux model. Desk partition, as one generally employed arrangement in plan office, is also investigated under MV. The dispersion of 1, 5 and 10 μm droplet residuals are numerically calculated and CO2 is used to represent tracer gas. The results show that using mouth covering by the infected person can reduce the co-occupant’s inhalation greatly by interrupting direct spread of the expelled droplets, and best performance can be achieved under DV since the coughed air is mainly confined in the microenvironment of the infected person. The researches under MV show that the two interventions, mouth covering and desk partition, achieve almost the same inhalation for fine droplets while the inhalation of the co-occupant is lower when using mouth covering for large droplets.