This report is the third in a series produced for project 11621 "Night Cooling Control Strategies". The first report was a literature review of night cooling techniques, the second report detailed the results from case study buildings that were monitored in 1995 and this report includes the results of dynamic thermal simulation. APACHE thermal simulation software was used to model a basic representation of one of the case study buildings.

Night cooling is an established technique allowing ventilation to take place at night with the intention of removing heat gains that have built up during the preceding day. By permitting the cool night air to flow through a building, heat is removed and a store of the cool air within the building fabric, furniture and fittings is achieved thus providing a cooling effect the following day.

A hybrid ventilation system combines both mechanical and natural ventilation modes. The natural ventilation mode, especially, causes some challenges for analysis tools since the varying nature of naturally driven flow means that transient calculations should be used to predict the flow fields. Analysis tools based on CFD have some advantages for detailed investigations since, in addition to evaluating the flow and temperature fields, it is possible to use CFD methods to calculate air exchange efficiency and other flow indices.

Macroscopic flow numbers relating to the jet momentum of air diffusion terminal devices are studied in this paper. Diffuser jet momentum, jet momentum number and jet momentum ratio are reviewed in the literature. New expressions for jet momentum ratios and jet volume numbers are proposed for specifying the mean room air speed in mechanically ventilated enclosures. The proposal is validated against experimental data from seven sites under fifteen operating flow conditions. Two macroscopic flow numbers correlate with the measured mean air speed in the room.

Trees, displacement ventilation and the use of thermal mass all played their part in giving the Green Building of the Year Award to the Ministry of Defence. Ewen Rose gets under the skin of the winning building.