**1. Introduction**

Flammable gases are employed extensively in the production of petrochemicals, and regularly stored and transported using linked vessels. A typical linked system comprises closed vessels connected with pipelines [1]. However, widespread fires and explosions frequently occur in such structures because the flames and shock waves that result from local gas explosions can propagate through the pipeline [2–4]. Although the prevention and mitigation techniques for fires and explosions have been continuously improving, the number of accidents has not decreased in recent years. For instance, an explosion and a fire accident caused by a gas leak considerably damaged two nearby buildings, on 23 December 2008, in a coal gasification plant in Hunan province in China [5]. Furthermore, an explosion occurred in 2013 in Dalian Bay, China, due to an operation that violated safety protocols and ignited flammable gases in a vessel, which caused the death of two individuals and severely injured two others. The explosion that occurred in natural gas pipes in 2017 in Guizhou Province, China, caused the death of eight individuals and injured 35 individuals. Therefore, conducting further studies on gas explosions occurring in linked vessels is of substantial importance for preventing and mitigating the damage caused by gas explosions.

Research on the structure of linked vessels has been conducted in past decades. Generally, compared with methane explosions occurring in vented single vessels, gas explosions in linked vessel systems constitute a complex process. This complexity can lead to high explosion strength in linked vessels [6]. Furthermore, numerous influential factors, including venting sizes and positions, ignition positions, and pipe lengths, could a ffect this process, among which ignition positions or obstacles could considerably change flame propagation, accelerate the speed of fire, and rapidly increase explosion pressures [7–12]. The venting size and position also play a prominent role in gas explosions [13–18]. Studies have demonstrated that the pressure piling exists in linked vessels, and the explosion strength is principally a ffected by the pipe length to volume ratio [19–25].

Currently, along with the development of computer techniques, numerical simulations have become more advanced. Numerical simulation has numerous advantages over physical testing; numerically simulated experiments are cost-e ffective, user-friendly, and compatible with a limitless number of experimental devices. Some scholars have examined gas explosions by using numerical simulations. With experimental data collected from the literature and the computational fluid dynamics (CFD) software AutoReaGas, Maremonti et al. [24] simulated gas explosions in two linked vessels. They not only demonstrated that turbulence induced in the second vessel was a major factor influencing violence of the explosion but also verified the validity of their CFD code. Deng et al. [25] conducted an explosion experiment with a CH4 and CO mixture in a 20-L nearly-spherical tank and then used flame acceleration simulator (FLACS) software to mimic the gas explosion of the experiment. They compared their simulation results with experimental data to prove the reliability of the simulation. Ferrara et al. [26] modeled gas explosions vented through ducts by using a two-dimensional (2D) axisymmetric CFD model based on the unsteady Reynolds-averaged Navier–Stokes approach. Simulation results evidenced that the severity of ducted explosions is mainly influenced by vigorous secondary explosions occurring in the duct. Valeria et al. [27] used a validated large-eddy simulation model to study the mechanism underlying vented gas explosions in the presence of obstacles. Methane–air mixtures with di fferent composition ratios, variously shaped obstacles, and area block ratios were investigated. The influences of the combustion rate and venting rate on both the number and intensity of overpressure peaks were observed.

In this study, to systematically analyze how the vessel size, vessel structure, ignition position, and length of connection pipes influence natural gas explosions, the strongly validated N–S solver tool of FLACS [28], which has been developed continuously for more than 40 years for predicting the consequences of gas explosions [29,30], was employed. This tool includes a three-dimensional CFD code that solves Favre-averaged transport equations for mass, momentum, enthalpy, turbulent kinetic energy, rate of dissipation of turbulent kinetic energy, mass-fraction of fuel, and mixture-fraction on a structured Cartesian grid using a finite volume method. The Reynolds-averaged Navier-Stokes equations are closed by invoking the ideal gas equation of state and the standard *k*–<sup>ε</sup> model for turbulence. Furthermore, one of the key features that distinguishes FLACS from most commercial CFD codes is the use of the distributed porosity concept for representing complex geometries on relatively coarse computational meshes [28]. With this approach, large objects and walls are represented on-grid, whereas smaller objects are represented sub-grid. The pre-processor Porcalc reads the grid and geometry files and assigns volume and area porosities to each rectangular grid cell. In the simulations, the porosity field represents the local congestion and confinement, and this allows sub-grid objects to contribute with flow resistance (drag), turbulence generation and flame folding in the simulations.

Simulated-pressure data were compared with the experimental results received from Nanjing Tech University, where the explosion experiment with 10 vol% methane was conducted [13]. In addition, results revealed the distributions of temperature and concentrations of gas products occurring during gas explosions. This study mainly aimed to contribute to this research field by exploring the characteristics of gas explosion in linked vessels.
