Study on the length of smoke overflow from tunnel fire under the combination of longitudinal ventilation and centralized smoke exhaust mode

•A model for predicting the length of smoke overflow is proposed.•The model considers smoke mass flow distribution and exhaust outlet layout.•The model was validated by reduced-size experiments.•The variation patterns of parameters such as overflow length and induced velocity were explored.•The “confinement smoke overflow length” is defined and analyzed. By combining longitudinal ventilation and centralized smoke exhaust, the smoke exhaust mode can simultaneously control the length of upstream smoke back-layering and the length of downstream smoke overflow from the exhaust outlet to facilitate two-way rescue and evacuation of tunnel fires, but the current research focuses on the upstream smoke back-layering length, and lacks the research on the downstream overflow length of the exhaust outlet. In this paper, the longitudinal velocity and single-point centralized smoke exhaust mode smoke overflow length of the exhaust outlet are investigated, and through theoretical analysis, a prediction model for smoke overflow length is proposed by considering the longitudinal velocity, the heat release rate of the fire source, the exhaust velocity, and the distance of the fire source from the exhaust outlet, etc. The results show that the fire source distance from the smoke vent decreases can effectively increase the smoke exhaust efficiency, but will reduce the inhibition effect on the length of smoke overflow, and the increase in longitudinal velocity will lead to a reduction in the smoke exhaust efficiency and an increase in the length of smoke overflow. When the longitudinal velocity is below 0.2 m/s, the ratio of confinement velocity to total confinement velocity is approximately 0.65, and the critical confinement smoke overflow length is around 4H. Both of these values remain largely unaffected by the heat release rate of the fire source. When the longitudinal velocity is greater than 0.2 m/s, with the longitudinal velocity increases the ratio of the confinement velocity to the total confinement velocity and the critical confinement smoke overflow length increases, and when the longitudinal velocity increases to 0.4 m/s, the critical confinemen smoke overflow length increases to 6H. The laws and values obtained from the experiments can provide reference for practical engineering. The results of the prediction model were compared and analyzed with the existing models and experimental results to verify the reliability of the model, which can be applied to a wide ran