What is Thermal Displacement Ventilation?

Displacement ventilation is characterized by naturally generated stratification in density (thermal) and scalar concentration (pollutant). It discharges supply air of low velocity near the floor and cool supplied air spreads over the floor and forms pool of conditioned air. When this cool air meets a heat source, because of the temperature difference and resulting buoyant force, convection plume is generated through which warmed and polluted air goes upwards to the ceiling where it exits through the exhaust.

Due to entrainment phenomena, the volumetric flow rate of the plume gets bigger as it goes upwards. When the volumetric flow rate of hot and polluted air generated by heat sources (like human body, electrical equipment, non-isothermal wall) is the same with that of supplied air, a thermal and contamination boundary level forms through which the upper level and the lower level are distinguished. This does not mean that there is no interaction between two levels, however itís momentum and thermal interaction is very small compared to overall convection flow field. Therefore most of time it is generally accepted to view the boundary level (stratification level) as no interaction level between two regions in the analysis.

 

This aspect is one of the most beneficial factors in thermal displacement ventilation over the conventional mixing type ventilation in that only heating/cooling loads affecting the lower part of the space (i.e. occupied zone) is taken care of and more importantly it can improve air quality in the occupied zone. This leads naturally to the fact that energy saving as well as indoor air quality can be enhanced in the displacement ventilation. Many researchers have been tried to prove those aspects of the displacement ventilation theoretically and experimentally. George Dunham claimed in his interoffice memorandum that for 100,000 ft2 office, cooling load can be saved by 25 ~ 30 % in displacement ventilation, therefore reduction in supply air volumetric flow rate is 70% of that in mixing ventilation if it were used in the same situation.

Despite many advantages of displacement ventilation, however, there are several criteria that have to be carefully considered when it is designed. Since diffusers are usually placed near or on the floor, the supply air velocity has to be low enough to avoid uncomfortable draft (Table 1). And this low velocity air stream forms gravity current which has different features from jet formed by mixing type ventilation diffuser. The behavior of gravity current is strongly dependent upon the temperature difference between supplied air and room air. Itís flow pattern is governed by Archimedes number. To locate diffusers in the proper locations, designer needs a deep understanding on the draft and flow pattern of the gravity current.

Another attentions have to be paid to temperature gradient, temperatures at knee and at head level and temperature difference between supply and exhaust since these are all important comfort parameters. As the plume ascends, hot air in the plume warms the air in the occupied zone and due to low entrainment very stable stratification around the plume forms. And this results in temperature gradient in the occupied zone. These parameters are subjected to some form of regulation for a comfortable HVAC system.

Computational Fluid Dynamics (CFD) plays very promising role in verifying and designing new displacement ventilation system. Because of its extensive capability predicting velocity, temperature, any scalar quantity over the computational domain. From the last 10 years, researchers have employ CFD to their displacement ventilation research and its usage is getting more attention recently.