Despite the relative recent move towards inherent safe materials, the relentless drive of consumerism requires increased quantities of dangerous goods to be manufactured, transported, stored and used year on year. The safety and effectiveness of road transport systems is to be considered a strategic goal in particular in those countries, like Italy, in which 80% of goods are transported by this means. Heavy good traffic by road has continually increased over many years and owing to population or environmental impact reasons, the actual number of road tunnels has grown in many countries. It must however be remarked that, due to the sharp reduction of vehicle pollutant emissions, the ventilation of road tunnel is more and more determined by the need of controlling smoke in case of fire. In case of fire in a tunnel, a longitudinal ventilation system is often operated in order to create, upstream of the fire, a smokeless area, essential for evacuation and rescue operations. If the ventilation rate is low, the fire smoke may propagate also upstream of the fire, contrary to the ventilation air flow, a phenomenon known as backlayering. The critical velocity, that is the minimum value capable of avoiding backlayering and thus force smoke to move only downstream, is obviously a fundamentally important value when designing ventilation systems. Therefore, the basic scientific problem is to determine the longitudinal ventilation rate necessary to prevent the combustion products from moving upstream of the fire. Current methods for determination of such value are based on semi-empiric equations, obtained by the Froude’s number conservation, combined with experimental data. The first step of this research was carried out on a real scale in order to simulate the evolving scenarios following an accident with fire development. Experimental runs were carried out with real fire development, utilizing alternatively different combustion materials and cars, focusing attention on longitudinal and transverse temperature gradients both in presence and in absence of fire-extinguishing automatic systems and longitudinal ventilation equipment. In the second phase of the research, a laboratory scale tunnel was designed and realized “ad hoc”. Several experiments were carried out, considering a longitudinal ventilation system and realizing fires characterized by different heat/smoke flow rates. The asymptotic value of the critical ventilation velocity resulting by mathematical modelling appears in good agreement with those proposed by other authors. Combining the results of full scale and laboratory tests, practical recommendations are drawn concerning, from one side, general measures relating to the tunnel and the traffic (e.g. ventilation design and operation), on the other side specific measures to reduce the consequence of accidents and implement emergency response management.
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