An analytical model to solve the ventilation problem in sloped tunnels.

The risk connected to tunnel transportation depends on external and intrinsic factors, as well as on the presence of tunnel safety devices. In this paper, firstly it is presented a tunnel risk characterization framework, suitable to be applied to existing road infrastructures. Secondly, the longitudinal ventilation problem in both plane and sloping tunnels is faced, so to allow a quantitative comparison between equipped or non-equipped alternative tunnels. Longitudinal ventilation systems generally represent the best technology to protect humans from fire and smoke exposure following an accident into a road tunnel of small/medium length. Notwithstanding the development of several studies on tunnel fire, based on empirical, phenomenological or CFD approaches, the effects of tunnel slope on smoke movement and its control still represent a main area of uncertainty. In the applicative phase of this work, reference is made to the worst situation of an hydrocarbon pool fire extended to the whole section of a sloped tunnel. By solving mass, momentum and energy balances, a relatively simple expression of the critical ventilation velocity has been obtained, as a function of the tunnel height and of the most significant stoichiometric, thermal and fluid-dynamic parameters involved in the combustion. The subsequent preliminary experimental investigation of this research was performed in a laboratory scale-tunnel under natural ventilation, forced ventilation and different tunnel slopes, up to a maximum angle corresponding to 6°.