Analysis On Heat Transfer Process Of Underfloor Plenum In Rooms With UFAD

Under floor air distribution (UFAD) with more flexibility, better indoor air quality, and improved energy savings is a new air supply method and is often applied in modern buildings.However, the airflow temperature in the under floor plenum is lower than that in the room with under floor air supply system. It may cause the floor surface temperature to be lower than temperature of other wall surfaces in the air-conditioned room. This will make radiation heat transfer between floor surface and other wall surfaces or surfaces of internal heat sources increase. Then more heat will be transferred to the cold airflow in the plenum through heat conduction and convection. It may cause the supply air flow temperature to rise over its design value. And then, it needs to increase the supply air flow rate to ensure indoor thermal comfort, while it may bring about the rise in indoor thermal stratification height. As a result, temperature efficiency of the UFAD may drop. As the temperature difference between supply air and room air is smaller in under floor air distribution system, the air supply flow rate needs to offset the heat gains is comparatively larger than that in conventional overhead air distribution system. The increase in supply air flow rate due to the heat gain in under floor plenum improves the fan energy consumption further. As a result, UFAD system will be not energy efficient as it is expected.In order to make the thermal stratification height just above the head in the occupied zone, it is necessary to predict the amount of heat gain and temperature rise in supply airflow within under floor plenum at the design stage. Then, the optimal supply air flow rate can be determined by taking its temperature rise in plenum into account.Therefore, numerical method was adopted to simulate the air velocity and temperature field of an office with under floor air distribution system. The under floor plenum is also included in the room model. Theoretical model of radiation heat transfer between the floor surface and other walls surfaces was set up. And heat transfer processes between room air and air flow in the under floor plenum were analyzed. It obtained the relationship between average wall temperature, indoor air temperature, heat source conditions, parameters of inlet flow to the plenum and temperature rises in supply air flow within the plenum. The effect of exterior wall, exterior window and the area ratio of window to wall, types of window glass, status of internal shading on temperature distributions of the upper and down surface of the floor was analyzed. The amount of radiation heat transfer between floor surface and other wall surfaces varies with window to wall area ratio, glass type, etc. In addition, the elevations of thermal stratification height as a result of the increase in supply air flow rate to offset the influence on indoor thermal condition due to temperature rise in under floor plenum were also investigated. The reduction in temperature efficiency was discussed as well. All these work help to study the influence of heat gain of airflow in under floor plenum on dynamic cooling load of rooms with under floor air distribution systems.