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VARIATION OF CRITICAL VELOCITY OF DOWNWARD VENTILATION IN INCLINED ROAD TUNNELS
Abstract
The paper presents the pattern of variation of critical velocity during longitudinal downward ventilation in an inclined road tunnel with its slope varying within 0-8 degrees. The results of both full-scale numerical modelling and laboratory experiments for a 1:10 scale tunnel models are presented. The effect of fire on the critical velocity and gradient factor was studied on the models. During the numerical modelling, the seat of fire, fans and tunnel portals were modelled with 0.1*0.1*0.1 m cells, while the cell sizes in the other areas were 0.5*0.5*0.5 m. Observations on the physical model, made of 2 mm thick stainless-steel sheets, were carried out considering the Froude number. The surface area of the source fire was 130X50 mm and 340X60 mm. Natural gas and air flow rates were measured with an accuracy of 1%. The heat release index was calculated based on air consumption. The ratio of tunnel width to tunnel height varied between 1.33 and 2 on the physical models. The results obtained show that the characteristic gradient factor of the critical velocity varies linearly with the tunnel slope to 6 %, and its numerical value depends on the heat release rate. The numerical simulation results show that after algebraically adding the ventilation and fire-induced drafts, the fire-induced draft becomes dominant when the tunnel slope exceeds 3% and the fire power exceeds 30 MW. In laboratory experiments, a similar effect was not observed due to the insufficient strength of the model fire.
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References21
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