Comparison of airflow and contaminant distributions in rooms with traditional displacement ventilation and under-floor air distribution systems

Traditional displacement ventilation (TDV) and under-floor air distribution (UFAD) systems have been used widely because they create better indoor air quality. Many previous studies have compared the TDV or UFAD systems with mixing ventilation systems. This study first compared experimentally the TDV with UFAD systems that use four different diffusers (perforated TDV diffusers, swirl diffusers, linear diffusers, and perforated-floor-panel diffusers) in an environmental chamber that can simulate different indoor spaces of the same size. The two systems had higher ventilation performance than the mixing one under cooling mode as well as under heating mode. Also, the systems with low-height-throw diffusers (all except the linear diffusers) were better. This investigation used a validated CFD program to further study the ventilation performance of the TDV and UFAD systems for an office, a classroom, and a workshop of different sizes. The CFD results further confirmed the findings from the experiment, but with more detailed information and at a lower cost. The air distribution effectiveness with the TDV system and with the lowheight -throw UFAD system was in proportion to the ceiling height of the indoor spaces. INTRODUCTION Traditional Displacement Ventilation (TDV) and Under-Floor Air Distribution (UFAD) systems have been popular in buildings since the 1970s. Many studies have revealed that TDV and UFAD systems provide better indoor air quality than the traditional mixing ventilation systems (Chen and Glicksman 2003, Bauman and Daly 2003). Some studies (Hu et al. 1999, Im et al. 2005) reported that the two systems can also reduce energy demand because of their high energy efficiency. Typically, a TDV system uses a perforated-sidewall or perforated-corner diffuser that discharges supply air horizontally as shown in Figure 1(a). A UFAD system supplies fresh air from a raised floor panel through swirl diffusers as shown in Figure 1(b), linear diffusers in Figure 1(c), and perforated-floor-panel diffusers in Figure 1(d). Among these diffusers, the swirl diffusers and perforated-floor-panel diffusers generate low-height throws, which means that the air velocity from the diffusers decays quickly and becomes less than 0.3 m/s (60 fpm) at 1.35 m (4.5 ft) above the floor. The linear diffusers generate a high momentum so that the supply air can reach a location at least 1.35 m (4.5 ft) above the floor with an air velocity larger than 0.3 m/s (60 fpm). They also generate high-height throws. The perforated TDV diffuser mounted on the wall or in the corner * Kisup Lee is a Ph.D. student and Qingyan Chen is a professor, School of Mechanical Engineering, Purdue University, West Lafayette, IN. Tengfei Zhang is an assistant professor, Dalian University of Technology, China, Zheng Jiang is a partner of Building Energy and Environment Engineering LLP, Lafayette, IN. of the room discharges fresh air in a horizontal direction. This diffuser generates low-height throw due to its flow direction and diffuser structure.