[attachment=11793]
Abstract
The purpose of this paper is to investigate using a numerical simulation (computational fluid dynamics or CFD) the effect of the air supply location on the design and performance of the displacement ventilation (DV) system. The results are reported in terms of thermal comfort and indoor air quality. The study focuses on a typical enclosure in CBIT under local thermal and boundary conditions. This includes several pollutants typically found such as carbon-dioxide and volatile organic compounds (VOCs) were investigated. The anticipated results indicate that the supply should be located near the center of the room rather than to one side of the room. This will provide a more uniform thermal condition in the office. The DV system was found to be effective in dispersing VOCs within an office environment for all cases studied. The exhaust was found to have minimal effect on the thermal comfort. For a localized DV system, it is possible to use 100% fresh air without extra energy consumption.
Introduction
There are many factors that affect the design and performance of ventilation systems. These include the size and type of building, the air supply velocity and temperature, and the location of the air supplies. In this paper, the investigation looks into the location of the air supply. Several researchers have looked into the effects of supply location and its impact on thermal comfort.
Both Bauman et al. and Shute stated that occupants should be at least 1–1.5m away from the supply grilles [1,2]. Loudermilk stated that no occupant should be located within a radius of the diffuser where the air velocities are in excess of 0.25 m/s and the temperatures are more than 0.6ºC lower than the room temperature [3]. The location of the supply in the ceiling was found to lead to poor circulation at the desk in partitioned areas [4]. Gan used computational fluid dynamics (CFD) to investigate local thermal discomfort in an office room [5]. The thermal comfort level and draft risk were predicted using Fanger’s comfort equations in the airflow model [6]. He showed that thermal discomfort could be avoided through the optimization of the supply air velocity and temperature. He also showed that optimal supply air conditions depend on the distance between the occupant and the air diffuser. Wyon and Sandberg [7] performed a series of experiments on the thermal comfort of the DV system using a manikin. They found that the thermal comfort was better above table height and thermal discomfort was mostly observed at the legs and ankles. Lian investigated the upward DV system to determine the effect of the type of outlet, distance between the occupant and outlet, velocity and temperature of supply air, and the type of outlet [8]. The results show that the main influence on the thermal comfort was the distance between the occupant and the supply. To assess the IAQ of an office building, Cheong monitored a number of indoor pollutants including CO2 [9]. Fleming in his investigations measured radon, nitrogen dioxide(NO2),formaldehyde, suspended particles, and carbon monoxide (CO) [10]. Persily pointed out that the indoor concentrations of CO2 can be useful for understanding
IAQ and ventilation [11].
Research Gaps
Although displacement ventilation systems have some cooling capability, their prime function is to provide ventilation. The vertical temperature gradient characteristics of these systems may impose some risk of cold discomfort for the legs and feet, and heat discomfort at the head .
The cooling capacity of displacement ventilation alone may therefore be insufficient in offices with normal internal heat loads and solar gains. There is broad consensus within the industry that the maximum cooling performance of displacement ventilation alone is around 20 W/m² for buildings with mainly sedentary type occupancy (such as offices). In buildings with higher occupant activity levels (for example industrial buildings), lower supply air temperatures and higher velocities can usually be tolerated without causing thermal discomfort, allowing a higher displacement ventilation cooling performance to be achieved.
Objectives of the proposed Experiment
In today’s indoor environment, it is increasingly difficult for conventional centralized heating, ventilating, and air conditioning (HVAC) systems to satisfy the environmental preferences of individual occupants. In general, the situation is further complicated by the
recent influx of heat generating devices and round the year cooling. In recent years there has been a growing awareness amongst private firms and government organizations of
the importance of the comfort, health, and productivity of individuals, giving rise to an increased demand for a high-quality indoor environment. There has also been an increase in the number of communication cables and wires in offices and workplaces. To respond to these needs, an air-conditioning system must have greater flexibility, better thermal comfort and IAQ, and greater energy savings. In the past two decades a new type of ventilation system, as an alternative to conventional mixing ventilation, has gained increasing popularity. The displacement ventilation (DV) was first proposed and
implemented in the Scandinavian countries about 25years ago. In 1989 in Nordic
countries, it was estimated that DV accounted for a 50% market share of industrial applications and 25% of office applications.
The objective of this project is to find out the optimum air supply location for obtaining the most efficient displacement ventilation system by considering all factors that may vary the thermal comfort. An important objective of the air distribution system is to create a comfortable thermal environment with the proper combination of comfort variables .The comfort variables are metabolic rate, clothing, air velocity, air temperature, air temperature stratification, radiant temperature, radiant temperature asymmetry, relative humidity, and turbulence intensity in the occupied zone. For the same activity level, clothing type, and room geometry, location, and orientation, thermal comfort is related to air velocity and temperature, temperature stratification, relative humidity, and turbulence intensity. With different diffusers and different locations of supply inlets and return outlets, the distribution of the thermal comfort parameters is different. Hence, it is necessary to understand quantitatively how different inlet and outlet
layouts affect local thermal comfort.