Project Title:

UFAD Room Air Stratification (RAS) Model Development

Energy performance of UFAD systems can be greatly improved by optimizing the stratification of room air temperatures.

Status (updated 7/18/2007): Complete

Funding sources: CBE Industry Consortium, Research Grants

Project Objective

Develop a simplified model that calculates the vertical temperature distribution for a underfloor air distribution (UFAD) system during cooling under various load conditions. Conduct full scale experiments to develop a validated version of the energy simulation code EnergyPlus that is capable of modeling UFAD systems.

Significance to Industry

Design of UFAD systems is complicated due to thermal stratification effects, and therefore does not lend itself to a straightforward application of traditional design principles. Improper design can have a significant impact on thermal comfort, first costs, and energy performance.

For UFAD systems the temperature stratification in the space differs from the well-mixed room air conditions of conventional overhead air distribution systems, with implications for determination of air supply volume and air temperatures that affect occupant comfort. Ultimately this research will yield a model to be used as a simple design tool that will provide designers with a reliable method for sizing UFAD systems and contribute to the development of a UFAD module for the EnergyPlus energy simulation program. The effective prediction of room air stratification (RAS) may allow for significant energy savings in UFAD buildings.

Research Approach

In this project a simplified model is being developed to estimate thermal stratification in the occupied space as a function of air supply volume and heat loads (for cooling conditions) for both interior and exterior zones served by a UFAD system. The model is being developed from fluid mechanics principles derived from well developed displacement-ventilation theory, modified appropriately for the differences inherent in the UFAD approach. Detailed full-scale experiments have been conducted to support model development and verification and to better understand the impact of various design and operating conditions such as diffuser arrangements, supply air temperatures, and airflow rates.

CBE conducted detailed testing at the York Air Distribution Research Lab maintained by CBE partner York International in 2001-2003. This testing led to the preparation of several technical articles published in the ASHRAE Journal and other publications, and in reports distributed to CBE industry partners. The RAS model has been incorporated into the energy modeling tool EnergyPlus for UFAD.

Publications and Reports

Webster, T., and F. Bauman, 2006. Design Guidelines for Stratification in Underfloor Air Distribution (UFAD) Systems. HPAC Engineering, June. (external link)

Webster, T., F. Bauman, and J. Reese, 2002. Underfloor Air Distribution: Thermal Stratification. ASHRAE Journal, May.

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Webster,. T., 2005. Slideshow update on project progress. Berkeley, CA, April.

CBE Full scale test facility at York International

CBE has conducted extensive testing in the full-scale laboratory maintained by York International.

Chart of room air stratification

RAS profiles graphically represent temperatures at various room heights.