Smoke Back-Layering Phenomenon under the Combined External Wind and Stack Effects in a Staircase

The external wind can change smoke movement patterns inside the staircase and affect smoke exhaust efficiency. This paper analyzes the smoke back-layering phenomenon in the staircase with open stair doors below the fire floor. The effect of the open stair door location and the heat release rate of fires and external wind velocities on smoke movement patterns are investigated numerically. The external wind ranges from 0–5.5 m/s. At 0 m/s, the smoke back-layering phenomenon driven by pressure difference can be found in the staircase with all stair doors closed. With the increasing wind velocity, four smoke behaviors are identified: upward moving smoke, first downward then upward moving smoke, downward moving smoke, and no smoke. Results show that the back-layering distance is mainly influenced by the external wind and heat release rate of fires. Correlations are modified and used to predict the longest back-layering distance with the first downward then upward moving smoke. This helps with arranging the smoke detectors inside a staircase and the fire safety design of high-rise buildings.