Bulletin of Japan Association for Fire Science and Engineering

In order to predict upward smoke spread in stair shaft, a prediction method was proposed for vertical temperature and smoke concentration profile based on the experimental result. In case with no vent above fire source, upward smoke spread was modeled as a turbulent diffusion due to negative density gradient as was applied to a flow in a plain vertical shaft. It was shown that the turbulent diffusion coefficient would be reduced to 1/ 12 of plain shaft. This is attributed to the presence of balustrade, landings and steps. In case with a vent above fire source, smoke flow was approximated by a piston flow due to stack effect. The formula for flow in a duct was applied. Local resistance coefficient per story was estimated as 32.7 from previous experimental results. The formula was coupled with a two layer conservation equation at the floor of fire source. Compared with experimental results, it is shown that temperature and CO₂ concentration could be reproduced fairly well.

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In order to investigate upward smoke spread behavior in stair shaft, full-scale experiments were carried out for various combinations of opening conditions and location of fire source. Cross sectional area of shaft was 17. 3 m² and height was 25. 6 m. From experimental results, upward smoke spread was qualitatively classified into three groups. The first is the case with no vent above fire source. In this case, smoke rose up slowly by being mixed with air in upper part due to turbulent diffusion. Temperature decreased exponentially as height resulting in very small increase of temperature at the top of shaft. CO₂ concentration was gradually increased even at the top of shaft. The second is the case with a vent above fire source. In this case, rising velocity of smoke was large in comparison with the case with no vent due to stack effect. Temperature decreased as height similar to the case with no vent above fire source. However, CO₂ concentration above fire source was uniform. Flow resistance coefficient per story was estimated as about 29. 1. Finally, in case that fire source located at the top floor, downward smoke diffusion was observed. The principal polluted region expands to about half of floor height.

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