Experimental Evaluation of Airflow in Naturally Ventilated Active Envelopes

Abstract

Active envelopes are characterised by two glazed surfaces separated by a ventilated cavity in which a shading device is placed. Thermal performance of active envelopes is governed by the airflow through the cavity. Until now, there is a lack of knowledge about the amplitude and direction of the airflow in naturally ventilated active envelopes. In this paper we report on the airflow in the naturally ventilated active envelope at the Vliet building of the Laboratory of Building Physics in Leuven, Belgium. (1) The airflow was experimentally determined by measuring the pressure difference over a ventilation grid. This method shows several advantages over traditional methods. (2) Thermal buoyancy and wind pressure differences govern the airflow. For both phenomena, measured data were compared with models. It was found that at low windspeeds and upward flow conditions, a linear relation between the pressure and temperature difference could be obtained. For higher windspeeds simple models cannot accurately predict the complexity of the airflow in naturally ventilated active envelopes due to the location of the measurement probes and due to wind washing effects imposed by altering wind directions.