Unsteady Laminar Buoyant Flow Through Rectangular Vents in Large Enclosures

This computational study concentrates on transient laminar How of air through rectangular vents in horizontal partitions between two large enclosures. For a vent aspect ratio of 1, three flow regimes were identified as the Rayleigh number (Ra) was varied from 1 x 10(3) to 5 x 10(4) based on flow parameters in the vent and corresponding modes of heat transfer through it. The Rayleigh number was found to be the critical control parameter, which determined the flow regimes of conduction, countercurrent, and oscillatory flow regimes. As the effective driving potential decreased with time, flow changed from one regime to the other so that higher-Rayleigh-number results showed all three regimes over time. Circulation patterns within the vent for the conduction regime (Ra < 1500) appeared to follow Chandrasekhar's well-known linear stability results. At higher Ra, sudden bursts of upflow with corresponding downflow have been documented and compared with experimental observations in the literature. The time traces of velocity and temperature fields for this flow regime reveal interesting mechanisms, which have been explained.