Effects of the water spray system on the air-driving force required in longitudinally ventilated road tunnels: Fid-Bau Portal

Effects of the water spray system on the air-driving force required in longitudinally ventilated road tunnels

Deng, Tao / Norris, Stuart / Sharma, Rajnish N.

Highlights • Water sprays influence the fire-induced pressure losses in the tunnel. • Key parameters affecting the pressure losses have been determined. • Methods of air-driving force design accounting for water sprays are proposed.

Abstract In the presence of a water spray system, the fire-induced pressure losses in a road tunnel can change, but this effect is currently not considered in the design of longitudinally ventilated tunnels. In this study, theoretical analyses are combined with FDS modelling to investigate the pressure losses due to flow acceleration, stack effect and friction in tunnels that use a water mist or a deluge system. Analytical models for pressure loss calculations have been derived, which indicate the evaporation rate of the water sprays, the spray-induced flow resistances and the decay coefficient of the mean sectional temperature rise are the three key parameters required for the model. According to dimensional analyses, the spray-induced resistance is a function of the momentum ratio between the water spray and the air flow, and the evaporation rate is mainly determined by the fire size and the total mass flow rate of water sprays. Corresponding empirical correlations are proposed. Comparisons between analytical results and FDS simulations concluded the proposed correlations are reliable and can give accurate predictions.