Experiments in natural ventilation for passive cooling.

A naturally ventilated three level office building has been used to study basic stack ventilation configurations and the interaction between ventilation and the subsequent cooling of the building structure in summer. The research was performed in the framework of a European project on passive cooling of buildings and the objective was to validate simple ventilation algorithms and to give an experimental basis to design guidelines for night cooling techniques. The multilevel office allowed the studying of the influence of openings (size and position) on the neutral pressure level (NPL) and on airflow rates. Various cross-ventilation situations have been studied. A single flow path configuration was obtained by closing all windows and doors in the building envelope with the exception of the roof exhaust and one office window as the ventilation air inlet. Air flow patterns were traced with smoke and tracer gas. In a first set of experiments, where the only driving force is stack pressure, air velocities and the position of the NPL have been measured, and contraction and velocity coefficients as used in the Bernoulli model have been observed. In a second set of experiments, the resulting effective area of a combination of two openings in series was studied. Air flow rates derived from velocity measurements in the open doorways were found to be in agreement with the flow rates obtained with a constant injection tracer gas technique, with an uncertainty of +-20%. Overall agreement was found between the velocity measurements and simplified models based on the Bernoulli equation. In order to cool multiple levels of a building with outside air, the position of the neutral pressure level should be controlled. The restrictions on opening size and position are discussed.