K.W.D. Cheong

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.

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.

Airflow measurements for balancing of air distribution system — tracer-gas technique as an alternative?

Abstract Traditional instrumentations such as, vane anemometers and pitot-tubes, have been widely used for measuring airflow rates in HVAC systems to balance air distribution system. Vane anemometers and pitot-tubes are used to measure air velocities at the supply diffusers and in the ducts, respectively. The airflow rates will be calculated with the knowledge of the cross-sectional area of the ductwork. The complexity of the air distribution system has posed a great problem for airflow measurements using pitot-tubes. Air velocity in a duct is seldom uniform across any section and a traverse is usually made to determine the average velocity. The measuring process is slow and errors can incur in the airflow measurements. The simple and useful tracer-gas techniques that are commonly used for ventilation measurements in building can be a good alternative to the pitot-static traverse method. This paper describes the advantages of tracer-gas technique over the traditional instrumentations. Airflow measurements using tracer-gas technique and pitot-static traverse method were first conducted in the laboratory with a straight length of 300 mm ×300 mm duct. This preliminary study is to determine the viability and accuracy of the tracer-gas technique. Results show that there is a good correlation between the airflow rates measured by using a pitot-tube and tracer-gas technique. This technique is further validated in the field with more complex air distribution system. Tracer-gas technique offers good accuracy when compared to the pitot-static traverse method. This technique can be a good alternative to the traditional pitot-static traverse method in the measurement of airflow in air distribution system during balancing. It is simple, accurate and possesses numerous advantages over the traditional instrumentations.

Development of a simplified technique of modelling four-way ceiling air supply diffuser

Abstract Computational fluid dynamic (CFD) tools have been used extensively in the prediction of airflow in indoor environment. However, it is important to understand that the accuracy of such simulation is affected by a number of factors depending on micro- or macro-level of modelling. It is in the interest of this paper to study, in micro-level modelling, the impact of various description of air supply diffuser modelling on the accuracy of the airflow prediction. Air supply diffuser can be modelled in a number of different ways. Each technique can result in different airflow pattern around the diffuser and in the macro-environment. This paper investigates five different types of air supply diffuser modelling techniques. It compares the throw profile of each of these diffuser models against the measured profile in rooms of three different sizes. A diffuser modelling technique that uses the RNG k – e turbulence model has been successfully developed. It is simple and yet without compromising on the accuracy of the predicted results.

Performance evaluation of a novel personalized ventilation–personalized exhaust system for airborne infection control

In the context of airborne infection control, it is critical that the ventilation system is able to extract the contaminated exhaled air within the shortest possible time. To minimize the spread of contaminated air exhaled by occupants efficiently, a novel personalized ventilation (PV)-personalized exhaust (PE) system has been developed, which aims to exhaust the exhaled air as much as possible from around the infected person (IP). The PV-PE system was studied experimentally for a particular healthcare setting based on a typical consultation room geometry and four different medical consultation positions of an IP and a healthy person (HP). Experiments using two types of tracer gases were conducted to evaluate two types of PE: Top-PE and Shoulder-PE under two different background ventilation systems: Mixing Ventilation and Displacement Ventilation. Personalized exposure effectiveness, intake fraction (iF) and exposure reduction (ε) were used as indices to evaluate the PV-PE system. The results show that the combined PV-PE system for the HP achieves the lowest intake fraction; and the use of PE system for the IP alone shows much better performance than using PV system for the HP alone.

Local Discomfort Caused by Draft Perception in a Space Served by Displacement Ventilation System in the Tropics

This paper reports a subjective study in a field environmental chamber (FEC) served by a displacement ventilation (DV) system. Sixty tropically acclimatised subjects, 30 males and 30 females, were engaged in sedentary office work for 3h. The subjects were exposed to three vertical air temperature gradients (nominally 1, 3 and 5K·m 1) between 0.1 and 1.1m heights and three room air temperatures (20, 23 and 26°C) at 0.6m height. The object of this work was to investigate effects of temperature gradient and room air temperature on local discomfort due to air movement at different thermal sensations. The results indicated that vertical temperature gradient had an insignificant impact on draft perception. Draft perceptions at foot, calf, thigh and arm were affected by both overall and local thermal sensations. A cold sensation would increase the risk of draft complaints and a warm sensation would increase the percentage dissatisfied due to insufficient air movement.

Ventilation and air quality in an office building

The aim of this study is to assess the performance of the mechanical ventilation system and air quality in an office building. The perfluorocarbon tracer (PFT) technique was used to measure air flow in an air handling unit and to estimate flow rates supplied to the office. In order to validate the PFT technique as a viable means of measuring air flow in the mechanical ventilation system, the PFT measurements were compared with measurements made using a pitot-static tube. Air exchange range, ventilation effectiveness and age of air were examined. The concentrations of carbon dioxide (CO2), carbon monoxide (CO), formaldehyde (HCHO) and dust particles were monitored. In addition, a questionnaire was completed by the staff in order to provide a subjective assessment of indoor air quality.

Single-Coil Twin-Fan Air-Conditioning and Air-Distribution System—Toward Development of a Mathematical Model of the Compartmented Coil

A new air-conditioning and air-distribution system, called the single-coil twin-fan system, that provides both energy efficiency and good indoor air quality was developed. It employs a unique compartmented cooling and dehumidifying coil that is fundamental in realizing the significant benefits of the system by being able to treat the outdoor air and the recirculated airstreams separately. In view of the distinctly different features of the compartmented coil, the performance prediction of such coils would have to be approached from first principles, which involves the determination of the fundamental heat and mass transfer coefficients. This paper establishes the need for a new mathematical model for the compartmented coil, outlines the underlying assumptions of classical models, and describes the formulation of an improved model. It presents a unique experimental method for determining the complex fin surface temperatures that constitute the major enhancements to the model development. Finally, the newly developed mathematical model is validated with experimental results.