## 1 Introduction  
As a sustainable and high-performing industrial feedstock, hydrogen has long been recognized as the best alternative to fossil fuels. Transporting hydrogen safely and economically is a key part of developing the hydrogen energy industry. Pure hydrogen pipelines and hydrogen-doped natural gas pipelines are both capable of transporting hydrogen. However, pure hydrogen pipelines are difficult and expensive to build, while natural gas pipeline networks cover a large area, have a high delivery capacity, and have low-cost consumption. Therefore, hydrogen is currently delivered mainly by blending hydrogen into natural gas pipelines. Hydrogen is active and burns quickly, and blending hydrogen and natural gas can increase the calorific value of natural gas combustion and also solve the difficult problem of hydrogen transportation. The risk of explosion is increased by the fact that hydrogen is also highly diffusible and flammable. In particular, when leaking from pipelines, it accumulates and reaches a certain concentration in a weakly constrained structural space, which may lead to violent combustion or even an explosion once it encounters an ignition source. In addition, it is relatively difficult to detect hydrogen leakage because it is colorless, odorless, and not easily detectable, which increases the hidden and sudden nature of the accident. It has been found that hydrogen-doped natural gas has a much higher explosive strength than pure methane to some extent, with a wider range of explosions. Therefore, an in-depth study of the safety performance of hydrogen transportation pipelines is essential for the development of effective safety measures and emergency response plans. A gas explosion is a process of rapid energy release involving physical and chemical changes. It includes several complex aspects such as flame wave expansion, shock wave formation and propagation, turbulence, and non-constant flow [1]. In some cases, if an obstacle or disturbance is encountered while the gas is burning, the flame speed will gradually accelerate, resulting in the creation of a pressure wave, a phenomenon known as deflagration. In industrial environments, most explosions occur in the form of deflagrations, especially combustible gas explosions in confined spaces. If the deflagration process pressure wave intensity increases, the flame speeds up, and eventually, the flame front catches up with the precursor pressure wave. The two merge to form a shock wave with a chemical reaction, at which point the deflagration is transformed into a detonation [2]. For flammable gases, the presence of conditions that promote turbulence or the presence of an external source of irritation may contribute to the transition from deflagration to detonation [3]. Therefore, in preventing and protecting against explosive hazards, it is important to take measures to avoid the formation of accelerated flame conditions to prevent deflagration from escalating to detonation, thereby reducing the risk of accidents. Combustible gas explosion characteristics are influenced by a variety of factors, such as fuel concentration, shape and volume of the explosion vessel, and location of the ignition source [4]. Many scholars have conducted extensive research on combustible gases such as hydrogen. Hydrogen has a low ignition energy, a high diffusion coefficient, and a wide explosion limit (4-74% volume fraction). [PERSON] et al. [5] investigated the deflagration characteristics at 20-45% hydrogen volume fraction in a large aspect ratio pipeline and found that the explosion pressure was maximum when the hydrogen volume fraction was 35%. [PERSON] et al. [6] observed that the flame behavior and temperature field transformed from a mushroom shape to an ellipsoid shape and finally to a spherical shape through experiments with non-uniform hydrogen cloud explosions. Adding hydrogen to hydrocarbons both reduces CO production and improves fuel combustion efficiency [7].