The buoyancy-induced flow in a vertical shaft has been numerically investigated. The flow geometry models the spread of smoke and hot gases produced by fire in a modern high-rise building. At the shaft inlet, a flow of hot gases is imposed with a uniform temperature Ti. The shaft walls are assumed to be adiabatic except for the ceiling of the shaft, where a convective boundary condition is adopted. The present numerical computations include two types of shaft configurations : a shaft without a side vent and a shaft with a side vent The effects of the important governing parameters, such as the Reynolds number Re, the Grashof number Gr, the Biot number Bi, the aspect ratio of shaft A, and the dimensionless height of the exhaust from the inlet H1/H, on the flow and thermal fields are investigated in detail. Of particular interest in the study are the time taken by the hot gases in flowing from the inlet to the exhaust the temperature of the gases at the exhaust, and the escape of hot gases through the side vent The results obtained indicate that the hot gases flow upward through the entire cross section of the shaft when Gr is small. However, a thin boundary layer is formed along the wall containing the inlet and exhaust channels as Gr increases. The time taken by the hot gases from the inlet to the top exhaust may be estimated by the parameters Gr1/2Rev-1 and H1/H, where Rev is the Reynolds number based on the bulk vertical velocity in the shaft For the shaft with a side vent, hot gases introduced at the inlet flow out through the side vent as well as the top exhaust when Gr is small. As Gr increases, however, air entrainment into the shaft occurs through the side vent It is noted that there is a critical Froude number which prevents the escape of hot gases through the side vent This critical Froude number is found to vary from 0.30 to 0.81 for the parametric ranges considered in the present study.
|Original language||English (US)|
|Number of pages||10|
|Journal||American Society of Mechanical Engineers, Heat Transfer Division, (Publication) HTD|
|State||Published - 1998|
All Science Journal Classification (ASJC) codes
- Mechanical Engineering
- Fluid Flow and Transfer Processes