VA-89-06-1 -- Some Aspects of Gravity Driven Air Flow through Large Apertures in Buildings PDF

The authors compare the available algorithms for gravity-driven airflows through large openings with the requirements for multizone air-infiltration modeling.

It appears that two extreme situations are addressed in the literature. The first, providing a timeindependent airflow, is formulated in terms of the constant temperature difference between a zone and the outside. The other, considering adiabatic walls, provides a transient flow, with the cold outside air filling the zone space completely so that the airflow comes to a halt.

As an intermediate situation we present an algorithm that takes into account the heat transfer with the walls. The coupled equations for airflow and heat transfer are solved for the inside air temperature. The latter being lower than the wall temperature and always higher than the outside temperature, the proposed model predicts an air and heat flow rate, which is intermediate in magnitude with respect to the extreme situations addressed in the literature. For constant wall temperatures, the model predicts that the air flow rate is lower for zones with smaller heat exchanging wall surface area. Coupled to a thermal model for the wall surface temperature, the algorithm predicts the air flow to decrease with time. Experiments were performed on a full-scale test room with a window to the outside. During cold windless nights, velocity profiles were measured in the window plane, and the temperatures of the inside air and the walls were recorded with time.

Good agreement with the model is obtained, considering only the fraction of the wall surface area active in the heat transfer process, and measuring the inside air temperature in the outstreaming air. Effects which have been observed and should be included in the algorithm for detailed multizone air infiltration modeling are wind effects, the time development of temperature stratification, and the initial transient, whose time constant depends on furniture in the zone.