Partially-mixed air distribution systems such as Under-Floor Air Distribution (UFAD) systems have been known to provide indoor air quality improvement and energy saving potential. The conditioned air is supplied directly from the floor-mounted diffusers to the occupied zone and warmed up by the room heat sources. The exhaust is normally located in the ceiling for air return. Therefore, thermal stratification within the occupied zone can be generated and well-mixed condition can be expected in the upper region.

The major challenge for UFAD design is estimating the thermal gradient between head and ankle of a standing person. The thermal gradient is strongly linked to thermal comfort and total airflow rate requirement of the UFAD systems. Thus, designers must be careful to ensure acceptable thermal gradient and to determine the required airflow rate at the same time.

This investigation first reviewed the literature concerning the thermal stratification of the UFAD systems. As a result, key design parameters selected 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 selected were offices, conference rooms, and classrooms in both interior and exterior zones.

When building a test plan with the design parameters selected, this investigation considered both of literature review and the results from the orthogonal test. Then, experimental cases in the test plan were first conducted to understand the nature of the UFAD systems. Indoor spaces with high cooling created a low thermal gradient. Also, swirl diffusers created the largest thermal gradient but provided the lowest air temperature in the occupied zone while the linear diffusers maintained uniform conditions. The experimental data was also used to validate a CFD program for studying the UFAD systems.

This investigation then used the validated CFD program to further study the thermal stratification the UFAD systems for different indoor spaces and design parameters. The study found that the swirl diffuser created the highest thermal gradient, while the linear diffuser created the lowest, which were the same found by the experimental test. The more diffusers used, the higher thermal stratification would be. With a lower supply air temperature, the thermal stratification became higher. This investigation also found that the air temperature difference between air supply and return and indoor space type (or cooling load) were the most important parameters.

A database was established containing 108 cases of the parametric study results. With this database, this investigation developed an advanced design tool for the UFAD systems. Linear regression analysis was conducted to correlate the empirical equations for the thermal stratification model. For room heat transfer, heat balance analysis was conducted for each room surface. A heat transfer model was used in this investigation to estimate the ratio of heat gain inside the supply plenum. The thermal stratification model can be coupled with the room heat transfer model to provide comprehensive information for UFAD design. With using the thermal stratification model, a graphical design interface was developed for the convenient use of the UFAD engineers and validated by using the database. The Newton-Raphson method was implemented for solving the nonlinear design equations.

If you would like to access the database and other supporting files for RP-1522 (about 3 GB), please send an email to [email protected]. You will receive a user name and password for ftp access to download the database and other supporting files.