Experimental study on temperature decay and smoke control in tunnel fires with combination of multi-point smoke exhaust and longitudinal ventilation

A tunnel space is a physical or virtual passage that can be covered or enclosed to allow people, vehicles, or other things to move from one location to another. Mining, transportation, utility infrastructure (such as water or sewage systems), and military or defense purposes are just a few of the many uses for which tunnels are frequently constructed. Depending on its intended use, a tunnel space’s dimensions, shape, and construction can vary greatly. It could be as small as a pedestrian walkway or as big as a highway with multiple lanes or a train tunnel. Space stations and spacecraft have tunnels or passageways that can be found in a variety of environments, including underground, underwater, and even space. In order to guarantee stability, adequate ventilation, lighting, and safety features, tunnel spaces may necessitate specialized engineering and construction methods. They can be tracked down in metropolitan regions, rustic areas, or in remote or difficult to reach areas, and might be possessed and worked by state run administrations, privately owned businesses, or different associations. Hence, the need to make tunnels space safer arose.

In a new study published in peer-reviewed International Journal of Thermal Sciences, Professor Yanhua Zeng and co-workers from the Southwest Jiaotong University investigated the characteristics of longitudinal temperature distribution beneath the ceiling and the smoke control in tunnels. The approach involved utilizing a combination of multi-point smoke exhaust and longitudinal ventilation techniques. The fire’s hot flue gas tends to spread longitudinally along the tunnel ceiling, posing a threat to the tunnel’s alarm and sprinkler systems. Furthermore, raised temperatures from the smoke can prompt passage breakdown and extremely durable primary harm. Therefore, effective disaster prevention and rescue strategies for tunnel design necessitate studying tunnel fires. Researchers who studied the characteristics of fire and smoke in long, narrow spaces like tunnels are using their findings to guide the design of tunnel structures, which is why tunnel fire research has received a lot of international attention.

Tunnel smoke control technology refers to a set of measures designed to manage and mitigate the impact of smoke and fire in tunnels. This is critical for the safety of passengers, drivers, and emergency responders in case of a fire incident in the tunnel. The primary goal of tunnel smoke control systems is to keep the tunnel atmosphere free from smoke and other hazardous gases, thus enabling the safe evacuation of passengers and reducing the risk of fire spread. The two main approaches for tunnel smoke control are longitudinal ventilation and transverse ventilation. Longitudinal ventilation involves the use of high-power fans at the tunnel entrance and exit to create a pressure differential that pushes smoke and heat towards the exhaust end of the tunnel. This method is most effective in long tunnels and requires an appropriate control system to regulate fan speeds and air flows. Transverse ventilation, on the other hand, uses jet fans or ducted ventilation to direct smoke and heat towards a designated exhaust point or shaft. This method is more effective in shorter tunnels or where longitudinal ventilation is not feasible. Both methods can be supported by the use of smoke curtains, which are suspended barriers designed to prevent smoke from spreading to other areas of the tunnel. Smoke curtains can be triggered by heat or smoke detectors and are made of fire-resistant materials. Overall, tunnel smoke control technology is a critical aspect of tunnel safety and requires careful planning, design, and maintenance to ensure effective operation in case of a fire incident.

One of the useful points of tunnel fire search is the temperature distribution beneath the tunnel ceiling. Many variables influence the smoke temperature of passage fires, for example, longitudinal ventilation speed, burrow area, mechanical smoke exhaust framework, and so on. The authors used a model tunnel with a geometric ratio of 1:10 to investigate the effect of number of exhaust vents, heat release rate (HRR), and exhaust air volume on the vertical and longitudinal temperature decay beneath the tunnel ceiling. For structural safety, the width of the exhaust vents should not be too large and each smoke vent must have two holes to meet the area requirements. The combustion system of the model constituted of a LPG tank and a rectangular burner. At the top of the tunnel, the multi-point smoke exhaust method effectively manages smoke around the fire’s source. Through top vents, smoke is directed to the smoke exhaust pipe, where it is then discharged by an exhaust fan, making it easier to get smoke out of the tunnel space.

To conclude, Professor Yanhua Zeng  and colleagues reported that the maximum temperature point beneath the tunnel ceiling was observed in the upstream direction of the fire source and that the maximum longitudinal speed was 0.5 m/s or else it will worsen the downstream tunnel smoke diffusion. They also found out that the number of smoke vents have an effect on the temperature distribution. The combination of longitudinal ventilation and a top multi-point smoke exhaust system demonstrates improved smoke control effectiveness. However, it is important to note that the longitudinal wind speed should not exceed 0.5 m/s. In tunnels where a combination of multi-point smoke exhaust and longitudinal ventilation is employed (with the exhaust fan installed on the upstream side of the tunnel), the temperature decay beneath the tunnel ceiling follows an exponential attenuation law. Notably, the longitudinal temperature decay rate on the side closer to the exhaust fan is smaller compared to the other side.