Fluid-dynamic characterisation of the Mont Blanc tunnel by multi-point airflow measurements

• An experimental and theoretical approach is applied to analyse the Mont Blanc Tunnel.
• In situ velocity measurements are carried out with a specifically built survey rake.
• A simplified time-dependent analytical model of tunnel flow is developed.
• Experimental data and additional CFD are used for the model fine-tuning.
• Estimates of the friction factor and jet fan efficiency coefficients are proposed.

Long road and railway tunnels necessitate of a fine-tuned control of the ventilation system to be activated, for both safety and air quality maintenance reasons. This, in turn, requires that the main fluid-dynamic parameters of the tunnel are known with sufficient accuracy, so that the system behaviour can be predicted with reasonable confidence under standard and emergency operative conditions. As a first step in the modelling of the complex system embodying the 11,611 m long Mont Blanc road tunnel, and its ventilation facilities, a movable 5-point survey rake was designed and built-up, for detecting the distribution of the axial velocity on the tunnel cross-section. Two extensive experimental campaigns were carried out, where the airflow-rates were either measured at different stations along the tunnel length (C1), or at a fixed location, under purely axial main-flow conditions and varying the number of the activated axial fan pairs (C2). A simplified dynamic model of the tunnel was developed, and the airflow data from the experiments were used for the model fine-tuning, additional CFD analyses having provided extra information on concentrated pressure losses connected with air inlet and discharge through the tunnel ends. The Mont Blanc tunnel was finally characterised in terms of friction factors and jet-fans installation efficiency.

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