Air distribution effectiveness with stratified air distribution systems

Stratified air distribution systems such as Traditional Displacement Ventilation (TDV) and UnderFloor Air Distribution (UFAD) systems have been known to provide better indoor air quality. This study examined the influence of several key design parameters on air distribution effectiveness by using a validated CFD program. The parameters studied were space type, diffuser number, supply air temperature, cooling load, return location, total airflow rate, and secondary heating system. Six indoor spaces were investigated to develop a database: classrooms, office spaces, workshops, restaurants, retail spaces, and auditoriums. The air distribution effectiveness at breathing zone was at 1.1 ~ 1.6 for offices, classrooms, restaurants and retail shops, and 1.6 ~ 2.0 for workshops and auditoriums. The spaces with a high ceiling such as workshops and auditoriums had higher air distribution effectiveness than those with a low ceiling. Thus, the stratified air distribution systems are better for spaces with a high ceiling. The air distribution effectiveness for the TDV and UFAD with low throw height was similar and was higher than that of UFAD with high throw height and mixing ventilation. A database was established containing 102 cases of the parametric study results. With this database, the investigation identified the six most important parameters to follow in developing a set of correlation equations for calculating air distribution effectiveness through statistical analysis. The air distribution effectiveness calculated by the equations was mostly within 10% of that for the corresponding case in the database. INTRODUCTION Stratified air distribution systems such as Traditional Displacement Ventilation (TDV) and UnderFloor Air Distribution (UFAD) systems are becoming popular because they can create better indoor air quality (Chen and Glicksman 2003, Bauman and Daly 2003). This is because they supply fresh air directly to the occupied zone at a temperature slightly lower than that of the air in the room. Due to the thermal buoyancy, the cold but fresh air can stay in the lower part of the room. In many cases, contaminant sources in the room are associated with heat sources, such as occupants, equipment, etc. The thermal plumes generated by the heat sources bring the contaminants to the upper part of the room since the exhausts are typically located at or near the ceiling level. Thus, the contaminants can be extracted directly through the exhausts without mixing with the fresh air. In addition, the thermal plume from an occupant induces the fresh air from the lower part of the room to the breathing level of the occupant. The air breathed by the occupant is rather clean. This has been further confirmed by our recent investigation reported in a companion paper (Lee et al. 2009). The ventilation performance of the stratified air distribution systems has been taken into consideration by the ASHRAE standards through the air distribution effectiveness. For example, Table 6-1 of ANSI/ASHRAE Standard 62.1-2004 (ASHRAE 2004) defines the minimum required amount of outdoor air, Vbz, delivered to the space (or zone) for controlling contaminant concentration. Table 6-2 of the standard defines zone air distribution effectiveness, Ez, for different air distribution configurations. The * Kisup Lee is a Ph.D. student and Qingyan Chen is a professor, School of Mechanical Engineering, Purdue University, West Lafayette, IN; and Zheng Jiang is a partner of Building Energy and Environment Engineering LLP, Lafayette, IN. Lee, K.S., Jiang, Z., and Chen, Q. 2009 “Air distribution effectiveness with stratified air distribution systems,” ASHRAE Transactions, 115(2).