Critical ventilation velocity under the blockage of different metro train in a long metro tunnel

Abstract

To address the effect of metro train blockage on the critical ventilation velocity in a long tunnel, a series of scenarios were conducted numerically through this study, including different fire sizes (5-10 MW), metro train lengths (80-120 m), and blockage ratios (φ, 0.50, and 0.57). It is known from the numerical results that the metro train length shows a limited effect on the critical ventilation velocity, which is because the longitudinal ventilation has become stable before reaching the fire source to prevent smoke back-layering, and increasing the metro train length only increases the distance of stabilizing the longitudinal ventilation. The blockage ratio shows an obvious influence on the critical ventilation velocity, which is because the presence of the metro train can obviously reduce the flow cross-sectional area of the tunnel. An empirical model is developed as well, while it is known that the critical ventilation velocity increases with the one-third power of dimensionless heat release rate and (1-φ). The research outcomes of this study provide a technical guide for the design of the metro tunnel and the relevant emergency management of fire rescue under fire conditions. K E Y W O R D S blockage ratio, critical velocity, FDS, metro train, subway tunnel 1 | INTRODUCTION An increasing number of metro systems have been designed and constructed recently to release the urgent issues of accelerated urbanization and growing population in metropolises. The companied fire risk is always an important and hot research topic from both the academia and industries. Consequence and impact of the relevant fire accidents are very high, which can be proved by the large number of fire casualties and very bad influence in the past few decades, such as the Daegu metro fire in South Korea, the Viamala tunnel fire in Switzerland, and the Burnley tunnel fire in Austria. [1][2][3][4] To benefit with the fire safety of those metro systems, smoke movement is an inevitable aspect during the design as it is the most fatal factor of a tunnel fire. 5-7 Therefore, how to control the smoke movement effectively to provide a safe zone for the occupants and firefighters during the evacuation and rescuing processes, respectively, becomes an important question that needs to be answered urgently.It is indicated that the metro train fails to reach the next station and stops between the two stations in about 50% of fires. 8,9 When a fire occurs in the metro tunnel, the relevant ventilation facilities in the adjacent stations will be started, where longitudinal ventilation is triggered, which not only can prevent the smoke from spreading to the upstream and also offer a safe zone without smoke for both occupants and firefighters. If the designed longitudinal ventilation velocity is smaller than the critical value, the smoke released from the fire can break through the barrier raised by the longitudinal ventilation and then threaten the people upstream.Many studies have been undertaken in identifying the appropriate longitudinal ventilation v...

Effect of the Metro Train on the Smoke Back-Layering Length under Different Tunnel Cross-Sections

“…FDS is a computational fluid dynamics (CFD) model of firedriven fluid flow. It is often used to study tunnel fires [33][34][35][36][37][38][39]. FDS numerically solves the form of the Navier-Stokes equations suitable for low-speed (Ma < 0.3), thermally-driven, with an emphasis on smoke and heat transport from fires [32].…”

Effect of metro train on the critical driving force for preventing smoke back‐layering in tunnel fires

“…The longitudinal ventilation passing through the metro train blockage accelerates due to the decrease of the tunnel cross‐sectional area. According to the law of mass conservation, the velocity in the blocked area is times of the longitudinal ventilation velocity 24,27,46 . As shown in Figure 1B, pressure losses caused by the wall friction in such situation are be divided into three parts, namely, the pressure loss in the upstream air section without metro train blockage, the pressure loss in the upstream air section with metro train blockage, and the pressure loss in the downstream smoke section.…”

“…The pressure loss at the inlet and outlet of the tunnel are calculated by using Equations ( 22) and (24). ΔP ξÀa and ΔP ξÀb are the pressure losses, respectively, at the train tail (cross section A in Figure 1B) and head (cross section B in Figure 1B).…”

“…Besides, as for the blockage effect of trains, Zhang et al 24 and Zhang et al 29 both proved that metro train blockage would reduce the critical velocity. In summary, the published literature has achieved a large body of fruitful research on the critical velocity by conducting theoretical analysis, experiments, and numerical simulations.…”

“…Blockage ratio is another important factor affecting critical velocity. 24,25 Kang et al 26 conducted numerical simulation to investigate the effect of blockage ratio on critical velocity by using the hydraulic diameter as the characterized length. Their results indicated that the critical velocity decreased when the blockage ratio was increased.…”