Evaluation of fire smoke and heat exhaust performance of shafts by natural venting in tunnels

Theoretical analysis and numerical simulations are performed to study the smoke and heat exhaust performance of natural venting system with different shaft forms. Based on the mass conservation for the gas flow in the shaft, equations for estimating the total gas mass flow rate are derived. The methods for determining the smoke and heat exhaust efficiencies are also proposed. A numerical model is built for series of CFD simulations considering different shaft forms (length-width ratio of shaft vent and shape). The results clearly present the details of the natural venting process by displaying the temperature and velocity fields. The boundary layer separation and plug-holing phenomena are also observed. The effects of the shaft shapes on the smoke and heat exhaust effi-ciencies have been discussed quantitatively. It is highlighted that when the cross-sectional area of shaft is constant, a smaller length-width ratio of the shaft vent leads to higher smoke and heat exhaust efficiencies. Moreover, the smoke and heat exhaust efficiencies of cylindrical shafts are higher than those of cubic ones. The findings of this paper could provide some principles for the design of natural ventilation shaft in urban tunnels. For example, the length-width ratio of shaft vent less than 1:3 is recommended for the design of natural venting system in tunnels.