Determination of smoke layer thickness using vertical temperature distribution in tunnel fires under natural ventilation

A thick smoke layer propagating in a tunnel or corridor decreases the illuminance and visibility near the road or floor surfaces, leading to delays in finding exit and subsequent evacuation. Hence, smoke layer thickness is as important as the attenuation properties of temperature and velocity of a ceiling-jet in fire safety science. In this study, fire tests were conducted to select an appropriate method for determining the smoke layer interface height based on the temperature distribution inside the smoke layer. Two types of rectangular tunnels with different cross-sectional aspect ratios are employed for small-scale tunnel fire experiments. In this paper, well-established methods for determining the smoke layer thickness, such as the N-percentage rule, are reviewed and evaluated by comparing their results with visual observations of the smoke layer. Based on this comparison, the inflexion point method is proposed. The smoke layer thicknesses calculated from each method were compared with the visual observation results. Consequently, it was found that the buoyant frequency method is the most suitable for a tunnel having a vertically longer section shape and the inflexion point method for a tunnel having a transversely longer section shape. We also confirmed heat release rate have no significant effect on the thickness of the smoke layer originating from the weak plume in the quasi-steady state under natural ventilation. A new characteristic length for normalisation is discussed to derive a simple and easy-to-use ceiling-jet thickness formula in the tranquil flow region.