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Description
INTRODUCTION
A building requires supply of fresh air and removal of heat, gases, and particles that are emitted in the building. It is not sufficient to supply clean air to an individual room in a building with the correct temperature and flow rate--it is also necessary to design an air distribution system in the room in such a way that occupants experience high air quality and thermal comfort in the occupied zone. Therefore, it is important that the occupied zone has the optimum climate with respect to air temperature, air velocity, temperature and velocity gradients, mean radiant temperature, and asymmetric radiant temperature. It is also important that the supply air reaches all parts of the occupied zone without the presence of stagnant zones. An important element in a design procedure for an air distribution system is to ensure an environment free from draft in the occupied zone. Some design procedures predict the velocity where the flow passes imaginary surfaces that define the occupied zone, and other methods make a direct prediction of the velocity close to people in the occupied zone. The design procedures may be based on semi-analytical flow element models, on computational fluid dynamics (CFD) predictions, or on full-scale experiments.
FLOW ELEMENTS AND AIR DISTRIBUTION
The principle behind design with flow elements is to divide the flow in the room into areas that can be treated independently of the surrounding flow (ASHRAE 2005; Awbi 2007; Etheridge and Sandberg 1996). Flow elements such as the velocity from a given diffuser [u.sub.x], for example, are found from experiments. This flow element can be used in a design procedure to establish the velocity [u.sub.ocz], where the flow passes the imaginary horizontal surfaces that define the occupied zone (see Figure 1a). The next step in the design procedure is to select a diffuser with such a capacity that this velocity has an acceptable level in the occupied zone. The design is often based on a situation where the flow is isothermal and the jet penetrates the whole room. The design process can also include the real non-isothermal flow situation. In this case, the penetration length of the jet should be restricted to have at least a minimum length [x.sub.s], for example, half of the room length. The different design procedures for mixing ventilation using flow elements are addressed by Nielsen et al. (2001) and Awbi (2007).
[FIGURE 1 OMITTED]
The flow element describing the stratified flow from a wall-mounted low-velocity diffuser is used in the design of the air distribution in a room with displacement ventilation. For radial flow the velocity [u.sub.x] will decrease with the distance from the diffuser. The selection of diffuser, flow rate, and temperature are made in such a way in the design process that the velocity [u.sub.ocz] at the imaginary vertical surfaces defining the occupied zone close to the diffuser (Figure 1b) has an acceptable level.
Some new air distribution systems cannot be designed by the use of flow elements because it is difficult to describe the element in a general way. This is, for example, the case for the vertical air distribution system described later.
Experiments in a full-scale room show that the velocity [u.sub.ocz] is not always a good expression of the maximum velocity in the occupied zone [u.sub.rm] (Figure 1). The connections between the velocities are dependent on the actual air distribution system. This is a problem when flow elements are used in the design method because the method is often based on the velocity [u.sub.ocz] on the imaginary surfaces that define the occupied zone.
The draft (the design limits for the system) is not always generated by the flow from the supply openings; it can also be generated by the heat load in the room. This is, for example, the case for a diffuse ceiling inlet where the whole ceiling, or part of the ceiling, is the inlet. Full-scale... |
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