Air distribution effectiveness and thermal stratification with stratified air distribution systems

Ki Sup Lee, Purdue University


Stratified air distribution systems such as Traditional Displacement Ventilation (TDV) and Under-Floor Air Distribution (UFAD) have been known to provide better indoor air quality. This thesis examined the influence of several key design parameters of TDV and UFAD in air distribution effectiveness and thermal stratification. ^ First, the literature concerning the ventilation performance of the TDV and UFAD systems was reviewed. From this review, six different indoor spaces and several design parameters were selected for the first set of parametric studies. The indoor spaces were offices, classrooms, restaurants, workshops, retail shops, and auditoriums, while the design parameters were diffuser type, ventilation rate, throw, supply air temperature, and cooling load. ^ This thesis 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 a test 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. Also, the systems with low-height-throw diffusers were better. The experimental data was also used in validating a CFD program for studying stratified air distributions. ^ This thesis then used the validated CFD program in further study of the ventilation performance of the TDV and UFAD systems for the different indoor spaces and design parameters selected through the literature review. The study found that the air distribution effectiveness at the breathing zone was at 1.1 ∼ 1.6 for offices, classrooms, restaurants, and retail shops, and at 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 first set of parametric study. With this database, the thesis developed a set of correlation equations for calculating air distribution effectiveness through statistical analysis. ^ This thesis then tested the second set of parametric studies in order to examine further the thermal stratification of the UFAD systems. Design conditions for the second set of parametric studies were diffuser number and air temperature difference between supply diffuser and return with three different diffusers: square diffusers, swirl diffusers, and linear diffusers. The indoor spaces studied were offices, conference rooms, and classrooms in both interior and exterior zones. ^ The study found that the swirl diffuser created the highest thermal gradient, while the linear diffuser created the lowest. The more diffusers used, the higher thermal stratification would be. With a lower supply air temperature, the thermal stratification became higher. This thesis also found that the air temperature difference between air supply and return and indoor space type (or cooling load) were the most important parameters. ^ The two databases then were used to develop a model predicting thermal stratification of the UFAD systems. The model can calculate total airflow rate and supply air temperature in a room with the UFAD systems. ^ As a final form, the thesis proposed a six-step design guideline for the UFAD systems and a graphical interface of easy-to-use design tool. When a design case was special and the correlation equations or the design tool could not be used, this thesis also proposed a seven-step guideline for the use of CFD in determining the design conditions of the TDV and UFAD systems. ^ This thesis proposes two recommendations for future works. One is developing and adjusting theoretical models to depict airflow with UFAD systems employing a swirl diffuser. The work may allow establishing a combined theoretical model for a UFAD system with a swirl diffuser. The other work is to study parameters which were not considered in this thesis. Through this work, it is expected that the databases developed in this thesis may be improved by considering additional design parameters.^




Qingyan Chen, Purdue University.

Subject Area

Engineering, Architectural|Engineering, Mechanical

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