Influence of the Pressure Load in the Efficiency of a Longitudinal Ventilation System in Road Tunnels

The longitudinal ventilation system (LVS) efficiency in road tunnels is conditioned by geometric and operational parameters. Typical geometric parameters are the length of the tunnel, its slope and the transversal section. All these factors are generally fixed and thus not modifiable in the regular operation of the facility. On the other hand, operational parameters, like the working fans layout, the environmental conditions or the traffic density are case-sensitive and susceptible to influence the baseline performance of the ventilation system. In the present study, different pressure gradients, established between inlet and outlet location of the jet fan influence, are analyzed. This static resistance is shown to have a significant impact on the momentum transfer established between the jet expansion and the bulk flow inside the tunnel. For moderate pressure gradients, the jet discharged from fan is relativity well-mixed, allowing to reach uniform flow conditions in the streamwise direction. When the adverse pressure gradients become severe, the high-velocity flow is blocked, unable to mix out in the inter fan spacing and losing spanwise uniformity. At critical conditions, large recirculation areas can be developed within primary flow structures, generating turbulence and important energy losses, and even inducing reverse flow at the tunnel exit. The extreme operating conditions of a longitudinal ventilation system in a road tunnel have been studied using a 3D numerical simulation. Preliminary analysis for grid sensitivity and election of an accurate turbulence closure were performed to guarantee a valuable modeling. Following, systematic computations over a cluster of PC’s were executed using the well-tested Fluent code. RANS modeling with RSM scheme allowed a satisfactory description of three-dimensional vortical structure in the recirculation zones, especially for adverse pressure gradients. At this point, numerical results have provided a comprehensive overview of the mechanism associated to the momentum transfer of the jet expansion, comparing the performance for zero-pressure gradients with those observed for adverse conditions. Also, this paper gives valuable information about practical limits of the LVS, advancing operational conditions that compromise the ventilation efficiency.