University of California Energy Institute

The object of this research is to examine the flow in a plenum of an underfloor air distribution system (UFAD). UFAD installations have often performed poorly because the air entering the space through different vents attached to the plenum has different temperatures. The temperature differences vary in time and can be as much as 5 °C from diffuser to diffuser. It is believed that these temperature inhomogeneities are a result of circulating patterns established in the air flow in the plenum, and that these circulations, in turn, are a result of specific supply and plenum geometries. We have carried out laboratory simulations and numerical modeling of the plenum flow in an attempt to establish the dependence of the flow patterns on the supply configurations. We have conducted a systematic study of the flow patterns for different forcing arrangements and discuss the implications for plenum design. In each case the plenum is square in plan form and the configurations studied are:

One source jet in the middle of a side wall Two source jets in the same side wall with the same direction Two source jets in opposite side walls with opposite directions Four source jets in four side walls to generate an initial torque For the first configuration, we investigate laminar and turbulent jet behavior for two the aspect ratios of the horizontal and vertical length scales in order to assess the two-dimensionality of the flow. For the two co-flowing and counterflowing arrangements we investigate the effects of coalescence of the jets. In the multiple jet cases the interactions of the jets established strong and, in many cases, persistent circulations. These flows were studied in the laboratory using Particle Image Velocimetry (PIV). A RNG K-e turbulent closure was used for numerical calculations. The numerical calculations also allowed the thermal performance to be evaluated by the addition of heat transfer into the plenum from the top and bottom boundaries