Experimental and numerical study of smoke behavior in high-rise stairwells with open and closed windows

Abstract

This study numerically and experimentally investigates the transport phenomena in buoyancy-driven smoke inside stairwells in a high-rise building. Hot smoke is supplied at the bottom of a small-scale, 2 m high stairwell prototype, and the smoke velocity and temperature are measured and compared with the corresponding numerical results. For all-windows-closed cases, the Fire Dynamics Simulator (FDS) model is used to predict the smoke velocity and temperature fields, which are found to be in good agreement with the experimental data. Obstruction caused by the stairs is observed to slow the smoke flow, which results in staggering and repeated vortical flows in all stairwells, confirmed by flow visualization. The flow path lines and vortex formation of the smoke inside the stairwells are visualized using the laser-induced fluorescence (LIF) method; these vortical structures also corresponded to the results of FDS simulation. Furthermore, the effect of heating power (Q) is investigated in the range of 60-180 W for experiments and 1-4 kW for simulations. Both temperature and velocity increase with Q. Having one open window at various building heights is shown to have small effect on the overall smoke temperature, although having many open windows causes a temperature drop owing to the inflow of fresh, cool air. Having one open window at various building heights slightly slows the smoke velocity, although the velocity is significantly decreased when many windows are open. Therefore, the intake of fresh air slows the overall smoke dynamics. Moreover, the effect of Q in the range of 2-20 MW over building heights of 60, 120, and 240 m is numerically simulated. The rate at which the smoke reaches high elevations is determined for all-windows-closed and all-windows-open cases based on our parametric studies. The smoke rise time (t) is shown to be proportional to similar to Q(-1/3) for all building heights, which is the same time scale as the one predicted by the plume theory. However, because of the complex internal geometry of confined buildings including stairwells and corridors, the magnitude of the smoke rise time for the building is much larger than that predicted by the plume theory. Therefore, the current experimental and numerical findings may be useful as design guidelines for building safety engineers.