## 1 Introduction  
Gas accidents are one of five natural disasters in mines, accounting for over 35% of the total, and are known as the \"first killer\" [1, 2, 3]. Roadways or pipelines that have an abrupt change in their cross-section areas are widely used in underground engineering tunnels or industrial pipelines. Gas explosions often occur in these local areas, which clearly have an adverse impact on safety [4]. Studying the impact of abrupt cross-section changes on premixed methane air explosions in pipelines can help suppress coal-mine gas explosions and is of great significance for proposing measures to prevent gas explosions. Numerous scholars have studied the relevant theories of the shock wave propagation in pipes with abrupt changes in their cross-section area. Through theoretical derivation, [PERSON] and [PERSON] [5, 6] established a theoretical model of the shock wave propagation in bending and bifurcation tunnels. [PERSON] et al. [7] proposed a method of calculating the pressure drop in circular pipes with different shrinkage and expansion diameters. [PERSON] et al. [8] studied the destroy mode of gas explosions, which was mainly caused by shock wave overpressure and high-speed airflow. The former ignores the influence of mass force and wall friction, which presents a host of disadvantages. By comparison, the latter is mostly used to calculate the hydraulic loss of industrial pipeline fluids but is not applicable to the calculation of gases. Through experiments and numerical simulations, a large number of scholars have conducted extensive research on the propagation law of gas explosions in pipelines with different structures and obstacles [9; 10]. [PERSON] et al. [11] found that the blocking rate and cross-section area change in pipes would result in turbulence. [PERSON] et al. [12; 13; 14] carried out numerous studies on the effects of bifurcated pipes, obstacles, bending pipes, and continuous bending pipes on gas explosion shock and flame waves in confined spaces. [PERSON] et al. [15; 16; 17; 18] studied the propagations law of blast waves, which were influenced by the dimensions, arrangements, and shapes of obstacles. [PERSON] and [PERSON] [19; 20] used AutoGas software to study the effect of structure pipes on the combustion and explosion of premixed gas. [PERSON] et al. [21] studied the propagation characteristics, eddy current generation, and quenching performance of detonation waves in variable structure pipelines. [PERSON] et al. [22] obtained the attenuation law of pipeline explosion shock waves in bifurcated pipelines. [PERSON] et al. [23; 24] obtained the rule of U-shaped pipe structure on wave propagation of gas explosion. [PERSON] et al. [25; 26] found that the branching structure of pipelines can lead to turbulence by studying the flame acceleration process. Obviously, the above studies focused on the blast wave propagation characteristics of internal obstacles in pipelines, bifurcated pipes, bending pipes, and special structured pipelines, with only a few studies exploring the propagation laws in pipes with abrupt cross-section area changes. The influence of abrupt changes in cross-section area on gas explosion propagation is unclear, and the overpressure reflectivity in the regions of abrupt cross-section area changes remains unknown. In particular, the relationships between the ratio of abrupt cross-section areas and explosion parameters has rarely been studied. Therefore, for this study, a pipe system with an abrupt cross-section area change was established, and gas explosion pipeline experiments were conducted. This experiment investigated the overpressure propagation laws of various premixed methane-air volumes and five pipe diameters with abrupt cross-section area changes. The purpose was to determine the explosion propagation laws of premixed methane-air in various abrupt cross-section pipelines. The research outcomes provide important guidelines for the prevention and treatment of gas explosions in sudden changes in mine cross-section tunnels.