Kwok Wai Tham

Thermal comfort evaluation of naturally ventilated public housing in Singapore

Abstract Field survey was conducted to evaluate thermal comfort perception of the occupants in naturally ventilated public housing in Singapore. Thermal acceptability assessment was performed to find out whether the naturally ventilated indoor environment meets the ASHRAE Standard-55s 80% acceptability criteria. The study investigated whether thermal perception was influenced by different sessions of the day, building height, and flat types. Comparative analysis of thermal sensation and thermal comfort votes revealed that a high proportion of people experiencing sensations of +2,+3 still found the conditions to be comfortable. The survey also examined the adaptive behavior of the occupants in the usage of climatic control such as windows, fans and air-conditioning to modify the indoor environment.

Thermal comfort study of an air-conditioned lecture theatre in the tropics

Abstract This paper evaluates the current thermal comfort conditions of an air-conditioned lecture theatre in a tertiary institution using objective measurement, computational fluid dynamics (CFD) modelling and subjective assessment. A CFD tool was used to simulate the indoor comfort parameters, such as temperature, airflow rate and relative humidity. Corroboration between results from the field measurements and predicted values was conducted. It was found that the measured air temperatures, air velocities and relative humidities were within the limits of thermal comfort standards, although temperature and relative humidity were located at the extreme of the limits. The predicted results showed good distributions of airflow characteristics and temperature gradients, and these were in fair agreement with empirical measurements. The overall comfort vote, predicted mean vote and predicted percentage dissatisfied indices found the occupants to be slightly uncomfortable and dissatisfied. Additionally, recommendations were made to improve the thermal comfort condition and reduce the build-up of concentration of carbon dioxide in the lecture theatre.

Associations between home dampness and presence of molds with asthma and allergic symptoms among young children in the tropics.

Existing literature has shown that home dampness increases indoor mold burden and is associated with increased allergic symptoms among young children in temperate environments. There is no report of any studies of similar nature in the tropics where conditions are characterized typically by high temperatures and humidity with rainfall throughout the year. To evaluate if there are associations between the prevalence of current asthma and allergic symptoms in young children (age 1.5–6 yr) with dampness and indoor mold in childrens bedrooms in a tropical environment. A cross‐sectional study adopting an expanded and modified ISAAC –International Study on Asthma and Allergies in Children– questionnaire for the evaluation of asthma and allergies was conducted on 6794 children (4759 responded – 70%) attending 120 randomly selected daycare centers. Specific information on demographics, home dampness, and the visible presence of indoor molds were obtained. The prevalence ratios (PR) and 95% confidence interval (CI) were determined by Cox proportional hazard regression model with assumption of a constant risk period as recommended for cross‐sectional studies. The calculated PRs were controlled for age, gender, ethnicity, socio‐economic status, type of housing, maternal and paternal atopy, respiratory infections, environmental tobacco smoke (ETS) exposure, and food allergy. After adjusting for potential confounding effects, home dampness was observed to be significantly associated with current symptoms of rhinoconjunctivitis (adjusted PR 1.53, 95% CI: 1.00–2.33). The visible presence of mold was significantly associated with current symptoms of rhinitis (PR 1.55, 95% CI: 1.16–2.07) and rhinoconjunctivitis (PR 2.38, 95% CI: 1.51–3.75). Indoor dampness and mold in childrens bedroom are important risk factors associated with allergic symptoms in young children in Singapore.

Personalized ventilation as a control measure for airborne transmissible disease spread

The protective role of personalized ventilation (PV) against plausible airborne transmissible disease was investigated using cough droplets released from a ‘coughing machine’ simulating the human cough at different distances (1, 1.75 and 3 m) from the PV user. Particle image velocimetry was used to characterize and visualize the interaction between the cough-generated multiphase flow and PV-induced flow in the inhalation zone of the thermal breathing manikin. A dose–response model for unsteady imperfectly mixed environment was used to estimate the reduction in infection risk of two common diseases that can be transmitted by airborne mode. PV was able to both reduce the peak aerosol concentration levels and shorten the exposure time at all the examined injection distances. PV could reduce the infection risks of two diseases, influenza A and tuberculosis, by between 27 and 65 per cent. The protection offered by PV is less effective at a distance of 1.75 m than the other distances, as shown in the risk assessment results, as the PV-generated flow was blown off by the cough-generated flow for the longest time. Results of this study demonstrate the ability of desktop PV to mitigate the infection risk of airborne transmissible disease.

The Acceptable Air Velocity Range for Local Air Movement in The Tropics

The perception of locally applied airflow was studied with tropical subjects who had become passively acclimatized to hot conditions in the course of their day-to-day life. During the experiments, 24 subjects (male and female) performed normal office work in a room equipped with six workstations. They were exposed to local airflow from the front and toward the face at six air velocities (0.15, 0.3, 0.45, 0.6, 0.75, and 0.9 m/s) at ambient temperatures of 26°C, and 23.5°C and local air temperatures of 26°C, 23.5°C, and 21°C. Each combination was maintained for 15 minutes, during which the subjects responded to computer-administered questionnaires on their thermal and draft sensations using visual-analogue scales. The results showed that the subjects preferred air movement within a certain range, i.e., a higher percentage was dissatisfied at both low and high velocity values. Most dissatisfaction with air movement is caused by thermal sensation, with air movement perception accounting for a smaller proportion. The subjects preferred air movement to be between “just right” and “slightly breezy” and preferred their thermal sensation to be between “neutral” and “slightly cool.” The study also identified an acceptable air velocity range from 0.3 up to 0.9 m/s under the experimental conditions. This velocity range is relevant for the design of personalized ventilation in practice. This preferred velocity range is higher than the maximum velocity permissible under ASHRAE Standard 55 (ASHRAE 2004) in situations where subjects have no control over local air movement.

Measurements and computations of contaminant's distribution in an office environment

Abstract This paper investigates the dispersion of contaminants in an office environment using empirical and modeling techniques. The experiment was conducted in an environmental test facility with the mock-up of two typical office layouts. Layout 1 has two workstations located in the middle of the room and separated by a low-level partition, while Layout 2 has a low level-partition which separates the room into two halves with one workstation at each corner. The test facility, 6.6 m (L)×3.7 m (W)×2.6 m (H) , is served by an air-conditioning and mechanical ventilation (ACMV) system which is capable of controlling the air temperature and airflow rates at the supply diffuser and extract grille. The contaminants emission was generated by a constant injection of tracer gas, sulfur hexafluoride (SF6), via a 280 mm ×200 mm surface to represent the plane source. The concentration of contaminant in the chamber for both the layouts was measured and simulated using a gas analyzer and a computational fluid dynamic (CFD) program with the Re-Normalization Group k–e model, respectively. In general, the predicted concentration of contaminant was in good correlation with the measured concentration. However, the predicted results were always marginally lower than the measured results. The contaminants dispersion pattern was observed to be highly dependent on the velocity flow field. The layout of furniture influenced the airflow pattern and contaminants distribution in the office. On the whole, Layout 1 was better than Layout 2 with a lower level of contaminant at the occupants breathing zones.

Impact of Human Presence on Secondary Organic Aerosols Derived from Ozone-Initiated Chemistry in a Simulated Office Environment

Several studies have documented reductions in indoor ozone levels that occur as a consequence of its reactions with the exposed skin, hair and clothing of human occupants. One would anticipate that consumption of ozone via such reactions would impact co-occurring products derived from ozones reactions with various indoor pollutants. The present study examines this possibility for secondary organic aerosols (SOA) derived from ozone-initiated chemistry with limonene, a commonly occurring indoor terpene. The experiments were conducted at realistic ozone and limonene concentrations in a 240 m(3) chamber configured to simulate a typical open office environment. During an experiment the chamber was either unoccupied or occupied with 18-20 workers. Ozone and particle levels were continuously monitored using a UV photometric ozone analyzer and a fast mobility particle sizer (FMPS), respectively. Under otherwise identical conditions, when workers were present in the simulated office the ozone concentrations were approximately two-thirds and the SOA mass concentrations were approximately one-half of those measured when the office was unoccupied. This was observed whether new or used filters were present in the air handling system. These results illustrate the importance of accounting for occupancy when estimating human exposure to pollutants in various indoor settings.

Human convection flow in spaces with and without ventilation: personal exposure to floor‐released particles and cough‐released droplets

UNLABELLED The effects of the human convective boundary layer (CBL), room airflow patterns, and their velocities on personal exposure are examined. Two pollutants are studied which simulate particles released from the feet and generated at distances of 2 and 3 m by a human cough. A thermal manikin whose body shape, size, and surface temperatures correspond to those of an average person is used to simulate the CBL. The findings of the study reveal that for accurate predictions of personal exposure, the CBL needs to be considered, as it can transport the pollution around the human body. The best way to control and reduce personal exposure when the pollution originates at the feet is to employ transverse flow from in front and from the side, relative to the exposed occupant. The flow from the above opposing the CBL create the most unfavorable velocity field that can increase personal exposure by 85%, which demonstrates a nonlinear dependence between the supplied flow rate and personal exposure. In the current ventilation design, it is commonly accepted that an increased amount of air supplied to the rooms reduces the exposure. The results of this study suggest that the understanding of air patterns should be prioritized. PRACTICAL IMPLICATIONS A human convective boundary layer plays an important role in pollution transport around the human body. It interacts with the surrounding airflows which modifies air movement around the human body and personal exposure. Understanding the influence of this interaction on the pollution spread around the human can be used to control and reduce personal exposure and improve HVAC design.

Airflow dynamics of human jets: sneezing and breathing - potential sources of infectious aerosols.

Natural human exhalation flows such as coughing, sneezing and breathing can be considered as ‘jet-like’ airflows in the sense that they are produced from a single source in a single exhalation effort, with a relatively symmetrical, conical geometry. Although coughing and sneezing have garnered much attention as potential, explosive sources of infectious aerosols, these are relatively rare events during daily life, whereas breathing is necessary for life and is performed continuously. Real-time shadowgraph imaging was used to visualise and capture high-speed images of healthy volunteers sneezing and breathing (through the nose – nasally, and through the mouth - orally). Six volunteers, who were able to respond to the pepper sneeze stimulus, were recruited for the sneezing experiments (2 women: 27.5±6.36 years; 4 men: 29.25±10.53 years). The maximum visible distance over which the sneeze plumes (or puffs) travelled was 0.6 m, the maximum sneeze velocity derived from these measured distances was 4.5 m/s. The maximum 2-dimensional (2-D) area of dissemination of these sneezes was 0.2 m2. The corresponding derived parameter, the maximum 2-D area expansion rate of these sneezes was 2 m2/s. For nasal breathing, the maximum propagation distance and derived velocity were 0.6 m and 1.4 m/s, respectively. The maximum 2-D area of dissemination and derived expansion rate were 0.11 m2 and 0.16 m2/s, respectively. Similarly, for mouth breathing, the maximum propagation distance and derived velocity were 0.8 m and 1.3 m/s, respectively. The maximum 2-D area of dissemination and derived expansion rate were 0.18 m2 and 0.17 m2/s, respectively. Surprisingly, a comparison of the maximum exit velocities of sneezing reported here with those obtained from coughing (published previously) demonstrated that they are relatively similar, and not extremely high. This is in contrast with some earlier estimates of sneeze velocities, and some reasons for this difference are discussed.

Qualitative Real-Time Schlieren and Shadowgraph Imaging of Human Exhaled Airflows: An Aid to Aerosol Infection Control

Using a newly constructed airflow imaging system, airflow patterns were visualized that were associated with common, everyday respiratory activities (e.g. breathing, talking, laughing, whistling). The effectiveness of various interventions (e.g. putting hands and tissues across the mouth and nose) to reduce the potential transmission of airborne infection, whilst coughing and sneezing, were also investigated. From the digital video footage recorded, it was seen that both coughing and sneezing are relatively poorly contained by commonly used configurations of single-handed shielding maneuvers. Only some but not all of the forward momentum of the cough and sneeze puffs are curtailed with various hand techniques, and the remaining momentum is disseminated in a large puff in the immediate vicinity of the cougher, which may still act as a nearby source of infection. The use of a tissue (in this case, 4-ply, opened and ready in the hand) proved to be surprisingly effective, though the effectiveness of this depends on the tissue remaining intact and not ripping apart. Interestingly, the use of a novel ‘coughcatcher’ device appears to be relatively effective in containing coughs and sneezes. One aspect that became evident during the experimental procedures was that the effectiveness of all of these barrier interventions is very much dependent on the speed with which the user can put them into position to cover the mouth and nose effectively. From these qualitative schlieren and shadowgraph imaging experiments, it is clear that making some effort to contain ones cough or sneeze puffs is worthwhile. Obviously, there will be a large amount of variation between individuals in the exact hand or tissue (the most common methods) configuration used for this and other practical factors may hinder such maneuvers in daily life, for example, when carrying shopping bags or managing young children.