Design of DV systems
The stratification that develops in DV depends on the strength and distribution of the internal gains. The form of this stratification is important for both comfort and control. In its simplest form the stratification can be thought of as consisting two zones both of uniform temperature. A cool lower occupied zone and a warm upper zone. The design of the DV system aims to control the temperature of the occupied zone and the height of the interface between the two zones.
Inflow rate determines the height of the mixed layer
The principles of operation for this simple case are shown in the figure on the right, which shows a plume rising above a single heat source. Warm air from the heat source can only cross the interface in the plume. Over the rest of the space the interface between the cool lower layer and the warm upper layer prevents vertical motion. Since all the air supplied at low level and leaving at the return must cross the interface, it must be carried in the plume. As the plume rises it entrains air from its surroundings and becomes cooler with height. The amount of air carried upwards in the plume also increases with height as a result of this entrainment. Consequently, the height of the interface is that at which the flow in the plume is equal to the ventilation rate.
Controlling the height of the mixed layer
Increasing the ventilation rate then has two consequences: the interface will rise and the upper layer temperature will reduce. Reducing it has the opposite effect. Since DV is most efficient when the return temperature is highest, but it requires the interface to be above the occupied zone, this is a delicate balance. Further, since the interface moves up and down with the ventilation rate, the location of any control thermistors becomes an important issue. In practice, of course, the situation is more complex. There are multiple heat sources with different strengths and heat is exchanged with the surfaces of the room by convection, conduction and radiation. This leads to smoother temperature profiles than the idealized case shown the figure above (single plume). A natural displacement flow driven by two plumes of different strength is shown on the right. In this case a two layer structure forms, in most rooms a continuous gradient is observed. Nevertheless, the same principles apply. Increasing the ventilation rate moves the cooler zone upwards and reduces the stratification strength and vice-versa.
Maintaining the stratification
When using natural displacement ventilation the use of a complementary radiative cooling system can be a solution to control indoor temperature in the warmer hours.
Although chilled ceiling systems are a popular solution to provide increased cooling power to displacement systems, their use when no air cooling system (as in the present example) is problematic: in these cases, the mixed upper layer can cool excessively (since the ceiling is the only cooling source during the warm periods of the day), eventually reaching the same temperature as the lower layer, inducing mixed conditions. In view of this, the use of a chilled floor system is more appropriate, cooling the inflow air and contributing to a stable stratification.
The challenge is sustain natural displacement ventilation in the warmer hours of the day: if the vertically averaged internal temperature is below outside temperature the ventilation flow reverses and the flow tends to mix.
The natural solution to this problem is to use a solar powered chimney coupled to the outlet, increasing the stack effect and compensating the interior temperatures in the occupied zone.
The implications for comfort are also an important consideration. Individuals in thermal gradients experience discomfort when the gradient is too large (greater than 1.5ºC/m in the occupied zone). Recent work, however, suggests that stratification is less important than the extremes. People are uncomfortable with a hot heat or cold feet, but otherwise most gradients in practice do not seem to be an issue. In an ideal DV system the floor temperatures are at the supply temperature, but this is not observed in practice, since radiative heat transfers warm the floor. However, cold feet is a potential problem in a DV system, especially near the supply diffusers. For this reason underfloor air distribution (UFAD) systems, where the supply is directed vertically upwards from the floor to promote some mixing if warm air in the lower zone is finding favour.
Advantages and disadvantages of using DV
In DV only the lower occupied region if the space is cooled and heat that reaches the upper part of the space leaves through the return at higher temperature.
The flow pattern of DV provides improved air quality, since occupants are surrounded by supply air largely untainted by pollutants in the space. These pollutants are entrained into the convective plumes and carried to the upper level of the space, and pass into the return. They are unable to descend because the stable thermal stratification inhibits vertical motion. In MV these pollutants are mixed throughout the space and have, like temperature, fairly uniform concentrations everywhere. DV is like being in a shower where you are washed in clean water compared to MV which is like being in a bath and surrounded by soap!