Flow dynamic of human cough and measuring techniques: A review

Coughing is one of the most important respiratory activities for air transmitted pathogens. It is essential to understand the dispersion of exhaled particles when coughing to improve the prevention measure and reduce the cross-infection risk. However, cough flow structure is complex and influenced by many parameters. Simplifications are often made to the initial flow condition when simulating the transport of particles expelled during coughing in laboratory or numerical studies . This study conducts a systematic literature review on human cough, especially focusing on flow dynamic characterization. First, the measuring techniques for identifying the airflow characteristic are summarized. The boundary conditions for cough, such as flow profile, flow direction, cough duration and are compared between different studies. Finally, the vortex structure of cough and its impact on cough particle dispersion is discussed

New criteria for assessing low wind environment at pedestrian level in Hong Kong

The choice of proper wind comfort criterion is considered to be crucial to reliable assessment of pedestrian level wind comfort. This paper aims to propose a wind comfort criterion that can be applied to Hong Kong, in which the wind comfort is seriously deteriorated by the moderated airflow, particularly in the hot and humid summer. By thoroughly reviewing and comparing exiting wind comfort criteria, the parameters in Lawson (1978) criterion are adopted for acceptable, tolerable and intolerable category and the parameters in NEN8100 (2006) criterion are adopted for danger category in the proposed criteria. Besides, a low wind parameter suggested by AVA scheme (2005) is adopted for unfavourable category in summer criterion. The adopted parameters provide scientific foundations and they are carefully chosen to adapt the weak wind conditions. The prominent features of the criteria are proposed seasonally (summer and winter, respectively) and the overall mean wind velocity ratio (OMVR) is used as threshold wind velocity parameter. The wind tunnel tests of Hong Kong Polytechnic University (HKPolyU) campus model were used as a case study. The results show that the proposed criteria can reasonably represent the weak wind condition and provide suitable assessments of the wind comfort in Hong Kong. Moreover, the findings in this study provide scientific basis for future policy-making and the proposed criteria can also help city planners to improve the pedestrian level wind comfort.Department of Building Services Engineering2016-2017 > Academic research: refereed > Publication in refereed journalbcr

Resilient cooling strategies – A critical review and qualitative assessment

The global effects of climate change will increase the frequency and intensity of extreme events such as heatwaves and power outages, which have consequences for buildings and their cooling systems. Buildings and their cooling systems should be designed and operated to be resilient under such events to protect occupants from potentially dangerous indoor thermal conditions. This study performed a critical review on the state-of-the-art of cooling strategies, with special attention to their performance under heatwaves and power outages. We proposed a definition of resilient cooling and described four criteria for resilience—absorptive capacity, adaptive capacity, restorative capacity, and recovery speed —and used them to qualitatively evaluate the resilience of each strategy. The literature review and qualitative analyses show that to attain resilient cooling, the four resilience criteria should be considered in the design phase of a building or during the planning of retrofits. The building and relevant cooling system characteristics should be considered simultaneously to withstand extreme events. A combination of strategies with different resilience capacities, such as a passive envelope strategy coupled with a low-energy space-cooling solution, may be needed to obtain resilient cooling. Finally, a further direction for a quantitative assessment approach has been pointed out

Advanced ventilation chair for source control of respiratory infectious diseases

Since the outbreak of the new crown, a large number of people at home and abroad have been infected, causing great damage to human lives, the economy, and the whole society, which has led to a great deal of concern about respiratory infections. Hospitals are at high risk for the spread of respiratory infections, and hospital outpatient rooms are a focal point for the prevention and control of nosocomial infections due to the small space and frequent visit of many people, where effective measures can greatly reduce the risk of the spread of respiratory infections. Airborne transmission is generally controlled by strengthening ventilation and dilution. However, full space dilution ventilation is less effective in removing exhaled droplet nuclei and is limited by the capacity of the ventilation system and other conditions. Therefore, airborne transmission is a weak section in hospital infection prevention and control, and there is an urgent need to develop advanced technical means aiming for source control. In this paper, computational fluid dynamics (CFD) simulation was used to study the interaction between exhaled airflow, thermal plume, and ventilation airflow when patients stayed in the outpatient consulting room for a short period (from a few minutes to more than ten minutes). After comparing the two styles of wearing mask, it was determined that only the top exhaust was the best distribution form. When patient wear face masks, the top exhaust with ventilation volume rate of 20 m3/h can achieve 90% collection efficiency in 30 s. After the patient left, it takes only 15 s to collect most exhaled particles. The air volume and vent dimension is important to the design of ventilation plate, when the design is not reasonable, it can cause up to 50% impact. This paper propose a personalized ventilated chair for airborne transmission which could contribute new solutions and technologies to achieve the goal of significantly reducing hospital infection rates

Human re-inhalation ratio under typical conditions

Inhaled air quality is directly related to occupants’ health and quality of life. In this study, a numerical breathing thermal manikin was employed, who breathed following a sinusoidal function, with 10 breathing cycles per minute. Each cycle was composed of three phases: 2.5 s inhalation, 2.5 s exhalation, and 1 s pause. The influence of pulmonary ventilation rate, breathing mode and breathing cycle period on the re-inhalation ratio were studied by computational fluid dynamics (CFD) technology in combination with the species transport model. It was found that increasing the pulmonary ventilation rate led to a lower re-inhalation ratio. The re-inhalation ratio is the largest with the value of 0.91%, when exhaled through the mouth and inhaled through the nose. The re-inhalation ratio was up to 23.9 % lower with a pause of 1 s in the breathing cycle than without pause. When the pulmonary ventilation rate increased from 6 L/min to 8 L/min, the re-inhalation ratio decreased from 0.91% to 0.71%. This information would be an important basis for the development of the human microenvironment control and technologies, including intelligent, personalized air supply devices, local air supply and exhaust methods, and other advanced ventilation and airflow technologies

The use of respiratory protective equipment redefines breathing zone and increases inhalation exposure of self-exhaled pollutants

The size of the breathing zone influenced by respiratory protective equipment (RPE) plays an important role in evaluating inhalation exposure (self-exhaled and external pollutants). The objective of this study was to define this size and evaluate the inhalation exposure of self-exhaled pollutants. The flow characteristics of ten types of RPE were experimentally investigated to obtain boundary conditions for Computational Fluid Dynamics (CFD) simulations. The CFD modeling framework of RPE was established and validated scientifically with experimental data, which was used to investigate the time-series characteristics of the thermo-fluid parameters inside and outside the RPE, the size of the breathing zone, and the influence of RPE on inhalation exposure. The results show that the range of viscous resistance and inertial resistance coefficients were 5.13 × 108–4.35 × 1010 and 1.42 × 105–7.62 × 105, respectively, for different types of RPE. The time-area averaged temperature and relative humidity inside the N95 mask would be up to 29.8 °C and 63.3 %, respectively, causing discomfort and possibly health problems. The breathing zone of an RPE wearer can be roughly defined as a hemisphere region with a radius of 11 cm around the mouth. Wearing RPE would lead to excessive inhalation exposure, with 36.5–59.8 % of the exhaled pollutants being re-inhaled. The time-averaged oxygen content in inhaled air would decrease from 21 % to 18.0–19.2 %, implying that long-term wearing RPE may impair the wearer's well-being. The data obtained provide boundary conditions for CFD simulation of different types of RPE, and give guidance for monitoring inhalation exposure and improving RPE

Improving inactivation performance of in-duct ultraviolet germicidal irradiation system with ribbed duct walls by optimizing UVC distribution

The in-duct ultraviolet germicidal irradiation system (ID-UVs) with ribs has been revealed to be effective in reducing the risk of biological contamination, and optimization of its UV radiation distribution is expected to further improve the inactivation performance. In this study, the Critical Survival Fraction Probability (CSFP) and Maximal Bearable UVC Dose (MBUD) methods were synergistically employed as evaluation indicators, and Computational Fluid Dynamics (CFD) was used to optimize the lamp array, duct shape, liner reflectivity and number of UV lamps. Results indicated that the optimal lamp array was positioned at 0° to the airflow direction, and square-section ducts were preferred over circular-section ducts. Ribs obstructed the transmission of specular reflected radiation, while diffuse reflection helps to overcome the obstruction. Reducing the number of lamps alone from four to one increased the volume-averaged irradiance by 14.4–25.3 % in the main irradiated region. For renovation projects, ID-UVs equipped with one lamp and galvanized steel ribs proved optimal, exhibiting increased inactivation efficiencies of 39.7 % (CSFP) and 239.4 % (MBUD) against highly UVC-susceptible MS2 Bacteriophage (K = 0.038 J/m2), and up to 341.1 % (MBUD) against low UVC-susceptible MS2 Bacteriophage (K = 0.028 J/m2) compared to the ID-UVs without ribs and featuring four lamps. Moreover, in comparison to conventional conditions that provide equivalent air-cleaning levels, the optimal ID-UVs allowed for a 60–70 % increase in air handling capacity while achieving at least an 83.6 % reduction in annual energy costs. These findings contribute towards advancing practical applications of ID-UVs with ribbed duct walls