Modelling the Air Pressure and Flow Rate Distribution Within the Semi-transverse System Coupling Air Leakage and Thermal Effect in Tunnel Fires

In a practical tunnel adopting the semi-transverse system, the actual pressure and flow rate are the essential parameters to assess the performance of the semi-transverse system. Nonetheless, the influence of air leakage and thermal effect on the flow rate and pressure distribution within the semi-transverse system remains unclear. This study delves into the modelling and the solution of air pressure and flow rate within a semi-transverse system coupling the air leakage and the thermal effect induced by tunnel fires. The pressure and flow rate in the semi-transverse system were theoretically analyzed to establish a calculation model, where the effect of air leakage was summarized as the proposed parameter of air leakage coefficient, and the thermal effect was represented by the proposed temperature factor. A 1:20 scale experiment was conducted employing various exhaust flow rates (140 to 240 m³/s in full-scale). The numerical simulation was also conducted, considering different heat release rates (HRR, 20 MW to 50 MW), exhaust flow rates (160 to 240 m³/s), and different configurations for the exhaust vents. A consistent result was obtained, which showed that the temperature was prominent only in the opened exhaust vent region, while it was not significant in that with the closed exhaust vent. A step-by-step solution method for the proposed calculation model of the semi-transverse system was proposed, which can solve the pressure and flow rate distribution within the system. The test data was used to verify the accuracy of the model, which shows good consistency, with a maximum error of less than 8% in the calculated flow rate. The present result provides a method for verifying the design of the semi-transverse ventilation system, it also provides a guideline for assessing the actual flow rate and pressure of ventilation systems in practical tunnels.