Abstract
The ventilation system plays a crucial role in every building. Proper design and optimization of its operation increase the comfort of users due to efficient air exchange and at the same time control its velocity in the rooms. The aim of this paper is the analysis of the effect of plenum box entry on the velocity profile concerning the diffuser face panel. This issue may sometimes be ignored at the design stage but can significantly affect the airflow from the diffuser and consequently increase the risk of draft. The results of the PIV experimental measurements and numerical simulations concerning various entries of the plenum box (top and side) were investigated in this study. The measurements were used to develop the mathematical and numerical models, which were then used to assess the effect of localization of the spigot of the plenum box on its operation. The numerical analysis was carried out on plenum boxes with the air diffuser with a face panel composed of square grid perforations. Analyses show that the entries significantly affect both the way of air distribution inside the plenum box and the profile of the airflow and its velocity under the simulated air diffuser.
Introduction
The main task of any ventilation and air-conditioning system in rooms is to maintain suitable air quality and ensure optimum conditions for the occupants. Depending on the function of the building and the activity of people inside, the supply air parameters are determined to guarantee the thermal comfort of users. Thermal comfort is defined as conditions in which people do not feel excessively cool or hot. In addition to the basic parameters of temperature and humidity, many other aspects determine the satisfaction of occupants concerning the ambient conditions. These include air velocity, asymmetries of temperature distribution in the room, and thermal radiation of surfaces [1,2,3]. The first two factors are directly related to the ventilation system used, the air distribution, and the diffusers used. Since the air motion in a room is influenced by a balance of inertia, buoyancy, and viscosity, the air velocity has the most impact. Depending on the diffuser type, a distinction can be made between axisymmetric, flat, radial, and vortex airflow. The selection, location, number, and types of supply diffusers are essential to achieve suitable air quality and optimal thermal condition in the ventilated space [4,5].
The most important role of the ventilation system includes avoiding drafts and uncomfortable local turbulence and ensuring a uniform temperature distribution. Air velocities in the occupied zone should not exceed the design values, providing the comfort of users. In the literature, many recommendations for maximum velocity can be found. According to the American Society of Heating Refrigerating and Air-Conditioning Engineers (ASHRAE) [6], air velocities in the occupied zone should remain below 0.25 m/s, assuming that the occupied zone is any place where there are people. In general, the occupied zone is defined as the volume of the room between floor level and 1.8 m above the floor. In heating, ventilation, and air-conditioning (HVAC) systems, it is recommended to limit the air motion in the occupied zone to less than 0.2 m/s to minimize the risk of draft [1]. In ventilation system design practice, this velocity is usually treated as a limit value.
Air is usually supplied to a room through diffusers located on the ceiling or wall. The way air is distributed into the room is closely related to the construction of the air diffuser. A number of research studies on air distribution from diffusers can be found in the literature [7,8], including those using a numerical calculation [9,10]. Yao and Lin [11] analyzed the effect of air terminal types on the performance of stratum ventilation with four types of air terminals devices using experimental and numerical methods. The study showed that the type of diffuser has little impact on the temperature distribution in the room but significantly affects the airflow in the room. The results of experimental and numerical studies were also compared by the authors Martinez-Almansa et al. [12], where a wall diffuser was analyzed. The method proposed by the authors made it possible to obtain a very accurate model reflecting the real conditions. On this basis, the characteristic parameters, throw and drop of the diffuser, as well as pressure drop of the terminal device used in the design ventilation systems, were determined. On the other hand, Noncente et al. [13] performed the optimization of diffuse ceiling ventilation. The study included different configurations, a full ceiling panel distribution, and a chessboard distribution. Jaszczur et al. [14] used experimental and numerical methods to study the flow characteristic from a four-way square ceiling diffuser. Authors have shown the relationship between the range of the stream defined as the distances in which the air velocity is 0.5 m/s and the volume flow of supply air by the diffuser. The analysis of the vortex, round, and square ceiling diffusers conducted by Aziz et al. [15] showed that the velocity decay coefficient of the vortex diffuser is 2.6 times smaller than that of the other types. Therefore, vortex diffusers are recommended for high buildings as well as require less energy input to the fan. In the literature, many studies can be found that investigated vortex diffusers, which properties provide a very unusual air distribution in the room. Srebric and Chen [16] demonstrated that the distance at which the vortex airflow loses its tangential component depends on the velocity of the flowing air and the geometry of the diffuser. In similar studies, Shakerin and Miller [17] showed that isothermal flow from vortex diffusers rotates at the outlet, but at a distance of three diffuser diameters, the air disperses in a radial direction. In [18,19] Borowski et al. presented a comparison of experimental results obtained using PIV measurements with numerical results. The authors studied the air distribution for different Reynolds numbers. The topic of vortex diffusers was also addressed by Sajadi et al. [20]. The results show that the efficiency of the diffuser and the resulting distribution of the airflow in the rooms depend to a large extent on the angle of the blades. According to the analysis, the optimum blade angle is 32 degrees, and when changing the angle in the range of 30–35 degrees, very large differences in flow distribution are observed.