A series of experimental tests was carried out in two 1/15 reduced-scale tunnels to investigate ceiling temperature distribution and smoke diffusion in tunnel fires with natural ventilation. Based on experimental results and the one-dimensional theory, formulas to predict the temperature distribution and smoke diffusion extent were developed. As the smoke was extracted through natural ventilation shafts, the tunnel was conceptually divided into two zones or sections: the fire and non-fire sections. In both sections, the smoke temperature can be expressed using the temperature decay formula and reference temperature formula. The temperature decay is an exponential function. The reference temperature in the fire section can be developed from the definition of the dimensionless temperature and a constant value that is different for different tunnels. The reference temperature in the non-fire section represents the relationship between the dimensionless reference temperature in the non-fire section and a constant value that is different for different tunnels. The smoke diffusion area represents the relationship between the dimensionless length of the smoke layer and the temperature decay at the location of smoke.
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