# Experimental Study on Dynamic Exhaust Law for Ventilation System of Gas Tunnel
## 1 Introduction  
The underground utility tunnel is a symbol of urban modernization and intelligence; therefore, the Chinese government is vigorously promoting the construction of utility tunnels [1, 2]. As a new type of underground space structure, the research on various problems in the utility tunnel has gradually seen a new upsurge, and the gas tunnel, as the high-risk part of the utility tunnel [3], is the focus of attention. Due to the flammable and explosive characteristics of natural gas, the establishment of gas tunnels has been controversial all over the world [4, 5]. If a leakage accident occurs in the gas tunnel, i.e., an urban underground closed structure, the consequences are very serious [6, 7, 8]. As the only channel for gas exchange inside and outside the gas tunnel, the ventilation system is the key to avoid accidents under abnormal conditions [9, 10, 11]. The gas tunnel is a long and narrow underground structure, which is divided into several fire-retarding divisions in the design. The ventilation system of each division is relatively independent, which makes its ventilation mode different from that in mines and other forms of tunnels [12, 13, 14, 15, 16, 17, 18, 19, 20, 21]. The study of ventilation and exhaust in the actual gas tunnel project is destructive, dangerous and difficult to implement. Therefore, simulations [22] and similar experiments [23] are common research methods. In their numerical simulations,Yoo et al. [24] obtained the relationship between the convection heat exchange coefficient, internal temperature, cross-section wind velocity and wall temperature through CFD simulations, and calculated the limit length for fire-retarding division of gas tunnel under different working conditions. [PERSON] et al. [25] established a gas leakage model and fan model in the gas tunnel based on CFD simulation theory and studied the gas leakage diffusion law and ventilation scheme under different accident conditions. [PERSON] and [PERSON] [26] studied the fire characteristics of the gas tunnel under different ventilation modes through numerical simulations and analyzed the relationship between the distribution of temperature and smoke and the ventilation mode. [PERSON] et al. [27] used simulation methods to simulate forced ventilation, free ventilation and mixed ventilation in gas tunnels, obtained the distribution laws of airflow velocity and gas concentration under these three ventilation modes and determined the ventilation system with the best effect. The numerical simulation models are assumed and simplified, and their feasibility and correctness need to be verified by engineering or experiments. The ventilation similarity experiment of gas tunnels based on similarity theory can avoid this problem effectively. [PERSON] and [PERSON] [28] conducted ventilation experiments on a 1:20 scale gas tunnel model and studied the influence of the position and shape of the vent on the smoke removal effect. [PERSON] and [PERSON] [29] carried out longitudinal ventilation experiments under different fire conditions using a gas tunnel model with a similarity ratio of 1:23 and studied the correlation between heat release rate, flame growth rate, maximum gas temperature, temperature distribution, total wall heat flux and ventilation system. [PERSON] et al. [30] used the reduced-scale model to test the temperature distribution on the longitudinal section and cross-section of the gas tunnel after a fire and obtained the influence of ventilation parameters on the layout of temperature sensors and smoke sensors. [PERSON] et al. [31] established a reduced-scale experimental platform for the gas tunnel according to the similarity ratio of 1:5 and tested the resistance characteristics of the ventilation system under different pipeline layout conditions. The above research on ventilation system of gas tunnel mainly focuses on the design of the ventilation method and the ventilation effect in the case of an accident.