Reduced-scale experimental and numerical study of fire in a hybrid ventilation system in a large underground subway depot with superstructures under fire scenario

To reduce energy consumption and improve the safety and reliability of ventilation systems in underground subway depots, a hybrid ventilation (HV) system composed of natural ventilation, mechanical fans, and flow deflectors was proposed and tested. A reduced-scale (1:50) experiment and a full-scale numerical simulation were conducted, wherein four heat release rates (HRRs; 281 W, 380 W, 531 W, and 866 W) of the fire source and five ventilation velocities (0.7 m/s, 1.0 m/s, 1.4 m/s, 1.9 m/s, and 2.4 m/s) were tested. The temperature distributions under the ceiling were measured. Smoke movement and smoke layer stability were visualized using a laser sheet. The smoke layer height, gas flux of shafts, and smoke movement route were recorded from the simulation. Under HV, the ceiling temperature decreased significantly with increasing ventilation velocity; however, changes in temperature were different at different locations. With appropriate ventilation velocity (1.4 m/s), HV effectively controlled the smoke temperature of the bottom layer and ensured the stability of the smoke layer in the interlayer. However, the stability of the smoke layer was disrupted at higher ventilation velocities, which caused the smoke to sink, whereas a lower velocity could not slow the rise in temperature. The relationship among ventilation velocity, HRR, ceiling temperature, and smoke layer stability was analysed. A new criterion, N (N = 0.62), was proposed to determine the critical ventilation velocity associated with a lower ceiling temperature and improved smoke layer stability.