Buoyancy and inertial force on oscillations of thermal-induced convective flow across a vent

Natural vents are commonly installed in buildings for smoke control. Air motion is induced by buoyancy of the thermal sources inside the building. Hot smoke is expected to be exhausted out of the vent. However, directions of air flowing across the vent might be oscillating under some conditions. The ratio B of buoyancy to inertial force defined by the Grashof number over the square of the Reynolds number is the key parameter in determining airflow oscillations. In this paper, effects of buoyancy, pressure, and the combined effect of buoyancy and pressure denoted by B will be studied by simple flow equations. A room fire with a horizontal vent is taken as an example. The results indicate that pressure is the driving force for the airflow oscillations when B < 0.1. Buoyancy is the dominating factor when B > 10. However, the combined effect of pressure and buoyancy is important when B is close to 1. Results are useful for designing smoke exhaust systems with natural vents.