Smoke diffusion mechanism and mitigation design for fire accidents in a tunnel-groove structure of a large hydropower station

In this study, field tests and numerical simulations were conducted to investigate the smoke diffusion mechanism and mitigation design for a fire accident occurring in a special hybrid structure, i.e., the tunnel-groove structure (TGS) of a large hydropower station. It is found that the characteristics of the smoke spread in the TGS include the smoke temperature attenuation, incomplete chimney effect and smoke entrainment. The combustion of the polyurethane flame-retardant material is controllable and extinguishes immediately after the fire source is removed. However, there is a fire risk in a relatively closed space under the conditions of strong winds, especially at the connection port with the external environment, due to the wind tunnel and chimney effects, which result in rapid smoke diffusions. During the fire accident, the smoke first fills in the entire tunnel (89 m long), then propagates upward along the groove, and finally reaches the dam top (115 m above the tunnel) in 10 s. The heat release rate (HRR) was determined as approximately 50 MW and the maximum velocity of the smoke spread along the dam surface could reach 10 m/s. As a result, nearly no time was available for the workers in the groove to evacuate when the fire accident occurred. Sensitivity analyses were further conducted to study the effects of different parameters, such as the HRR, the air inflow velocity and the radial gate opening, on the smoke diffusion in the TGS. A horizontal smoke plate (HSP) was proposed for the temporary smoke control to prevent life from losses due to the smoke diffusion in the TGS. The safe and dangerous zones near the dam surface were identified by analyzing the characteristics of the smoke temperature and visibility. It was finally concluded that the HSP can mitigate the smoke and heat along the groove. The results from this study may serve as a useful reference for the smoke control design in the TGS or other similar structure configurations.