Overall performance evaluation of underfloor air distribution system with different heights of return vents

Abstract

The position of the return air vent significantly influences the performance of underfloor air distribution (UFAD) systems with split-return/exhaust configuration. While the proper height of return vent has been investigated, the emphasis of the evaluation is very much on the thermal comfort and energy saving performance under heating scenarios. In this study, the impacts of the return air vent height on the indoor air quality (IAQ), thermal comfort, and energy saving of an UFAD system for both room cooling and heating are investigated using computational fluid dynamics (CFD) approach based on simplified boundary conditions of the slot supply. The calculation of air velocity, temperature, mean age of air, CO2 and TVOCs concentrations distribution in the room with various height positions (Z=0.8m to 2.6m) of the return vents was carried out. The thermal environment indices (temperature gradient and draught rating), indoor air quality indices (mean age of air, air change efficiency, CO2 and TVOCs concentrations distribution) and energy saving are analyzed and compared against each other. The results demonstrate that descended return vents are beneficial to the energy efficiency, while the dispersion and removal of gaseous contaminants such as CO2 and TVOCs is highly sensitive to the location of return vent relative to the contaminant sources. An optimal range of location height of return vents which compromise all the aforementioned indices is recommended.In under-floor-air-distribution (UFAD) systems, the exhaust/return-split configuration is believed to be more promising than the exhaust/return-combined arrangement in terms of energy efficiency. However, how to properly position the return vents still remains an unanswered question when the thermal comfort and indoor air quality (IAQ) need to be simultaneously considered. In this study, a series of numerical computations are conducted with various height positions (Z=0.8m to 2.6m) of the return vents in order to evaluate the effects of the height of return vents in a UFAD system on the thermal comfort, contaminant removal and energy efficiency. The thermal environment indices (temperature gradient and draught rating), indoor air quality indices (mean age of air, air change efficiency, CO2 and TVOCs concentrations distribution) and energy saving are analyzed and compared against each other. The results demonstrate that descended return vents are beneficial to the energy efficiency, while the dispersion and removal of gaseous contaminants such as CO2 and TVOCs is highly sensitive to the location of return vent relative to the contaminant sources. An optimal range of location height of return vents which compromise all the aforementioned indices is recommended.