modelling

One of the options to increase the energy efficiency of buildings in the cooling season, is to extract heat from the building envelope during the night by natural or forced ventilation. The exploitation of this technique by architects and designers requires the development of guide lines and a predesign tool showing how the potential cooling power depends on the influence of opening sizes and positions and on the interaction with the thermal mass.

The use of sulphur hexafluoride (SF6), nitrous oxide (N2O) or carbon dioxide (CO2) as tracer-gases have been examined for the measurement of airflow in a two-zone environmental chamber. A series of measurements were carried out to examine airflows through a doorway under natural convection, forced convection and combined natural and forced convection. Results were compared with those predicted using the MULTIC computer program.

Multizone models are a common tool for calculating air and contaminant exchange within rooms of a building and between building and outside. Usually a whole room is then modelled by one calculation node with the assumption of homogeneously mixed conditions within this room. In real cases, however, temperature and contaminant concentrations vary in space. The exchange to the neighbouring nodes via the flow paths is then a function of the local values of these variables.

This paper describes a laboratory model for the testing and validation of tracer gas measurement techniques. Previous attempts at experimental validation have often been limited to two zones, or a particular measurement strategy, or a particular range of flows. The model consists of four zones, each of 1m³ internal volume. The zones are connected so that all possible inter-zone flow paths exist. The flow down each path is driven by a pump and monitored by a flow meter. A control panel enables any combination of interzone flows to be set, within the capacity of the pumps.

To predict the thermal and indoor air quality performance of vertical displacement ventilation systems using two-zone modeling, it is necessary to account for the different nature of the air flow due to thermal and contaminant mixing within these zones. Two zone modeling of vertical displacement ventilation was performed assuming piston flow in the clean zone, uniform mixing in the dirty zone, and no recirculation between the zones except via heat source plumes.

The momentum balance on a centrifugal fan, supplemented by a complete energy balance for rigorous interpretation of power-pressure interactions, relates these variables to flow rate and fan speed. Non ideal behaviour is modeled by direct mechanical interpretation and by engineering correlation, leading to more general expressions than provided by the fan laws. First attempts to fit these expressions to measured data show promise but reveal limitations of current practice in the data collection and reporting process.