Review of currrent displacement technology

The first task of this project is a desk and lab based review of current diffuser technology and its application in buildings. This also includes a review of previous research on diffusers and the application of displacement ventilation systems.

A literature review of relevant papers and reports has shown that much of the earliest research into displacement ventilation dates from the late 1980s and originates from Sweden and Denmark. This work tends to be related to characterization and evaluation of displacement diffuser performance and room conditions using physical and numerical modelling techniques. Some basic generalizations can be drawn from this work:

• To ensure acceptable thermal comfort, high heat loads must be met by high air flow rates rather than low supply air temperatures.

• The design procedure for displacement ventilation generally involves defining the distance from the diffuser at which the air speed has decreased to between 0.15 and 0.2 m/s (the usual threshold for acceptable thermal comfort), and may be measured at around 0.1 m above the floor.

• Air speed at floor level is influenced by flow rate, the type of diffuser and the Archimedes number of the flow. Archimedes number is large where a large vertical temperature profile exists and leads to stratified flow close to the floor with little mixing or entrainment of room air into the flow.

• The quality of the air flow delivery into the room is affected by the size, geometry and hole dimension of the diffuser. Laboratory experiments have shown that large wall diffusers tend to result in air flow that is uniform across the flow direction, with low turbulence and hence little entrainment of room air. In comparison narrow wall diffusers have been shown to produce an air flow that spreads radially with more turbulence and entrainment or induction of room air. It was suggested that such diffusers could allow higher air flow rates to achieve a higher cooling performance without compromising thermal comfort. (However, this would tend to produce more of a mixing ventilation behaviour than a buoyancy driven displacement effect, with implications for air quality).

More information can be found on the Useful links page.

Laboratory based diffuser tests

A small number of representative displacement diffusers have been tested in BRE’s Environmental Chamber to determine the limitations of existing products. These tests have focussed on determining air speeds in the occupied space for different diffuser types and air speeds.

This data will also be used to assist the development and validation of CFD models used later in the project.