STUDY ON NATURAL SMOKE EXHAUSTION IN BUILDINGS WITH LARGE CAVITY SPACE

This study investigates the characteristics of natural smoke exhaustion in the case of fire in buildings with a large cavity space. If the cavity space area is large enough, it can be considered equivalent to the ambient space. Therefore, it becomes possible to discharge the smoke at the time of fire to the cavity space side. However, the quantitative criterion of the space volume is not necessarily clear. Furthermore, the criterion of opening area for the air supply at the bottom of the space required for smoke exhaustion is also unclear so far. In this study, it aims to clarify these quantitative criteria by CFD analysis. The building with a large cavity space covered in this research is the simplified building of an actual one where the fire simulation experiment was carried out in the past. The height of the reference building is 72m, and it has a cavity space of 324m2 (18m × 18m) at the center of the building and a bottom opening of 39m2. In this paper, the cavity temperatures, velocities and the amount of the ambient air inflow volume into the cavity space at the time of fire when changing the height and bottom opening ratio of the reference building were numerically simulated by CFD. In this CFD analysis, we used a simple compressible k-ε turbulence model that takes into consideration the air density change. We also used the standard k-ε model to compare the results. In addition, they were compared with predicted values obtained from the prediction formula of the ambient air inflow volume from the bottom opening based on small-scale model experiments by Tanaka et al. The main findings obtained in this study were as follows. 1) Except for the case of shallow cavity space with an aspect ratio of no more than 1.0 to 1.5, the predicted air inflow volume from the bottom opening at the time of fire by the simple prediction formula is approximately equal to the calculation by the simple compressible turbulence model. Therefore, in the case of shallow cavities, it is necessary to consider not only the inflow air volume from the bottom opening but that from the top of the cavity space. 2) For shallow cavities with an aspect ratio of no more than about 1.0 to 1.5, it is not necessary to form the bottom opening required for natural smoke exhaustion. However, it has a tendency that the smaller the cavity space area is, the smaller the aspect ratio that doesn't need the bottom opening is. 3) In buildings having a cavity space having an aspect ratio of not less t