Gas explosions are a frequent hazard in underground confined spaces in the process of urban development. Liquid sedimentary layers, commonly present in these environments, have not been sufficiently studied in terms of their impact on explosion dynamics. This study aims to investigate how gas-liquid two-phase environments in confined underground spaces affect the explosion characteristics of natural gas. To achieve this, experiments are conducted to examine the propagation of natural gas explosions in water and diesel layers, focusing on the influence of liquid properties and the liquid fullness degree (L_x) on explosion behavior. The results indicate that the presence of a liquid layer after the initial ignition stage significantly attenuates both the peak overpressure and the rise speed of pressure, in comparison to the natural gas conditions. During the subsequent explosive reaction, the evaporation and combustion of the diesel surface resulted in a distinct double-peak pressure rise profile in the diesel layer, with the second peak notably exceeding the first peak. Under conditions with a liquid sedimentary layer, the flame propagation velocities range from 6.53 to 34.1 m/s, while the overpressure peaks vary between 0.157 and 0.255 MPa. The explosion duration in both the water and diesel layer environments is approximately twice as long as that of the natural gas explosion, although the underlying mechanisms differ. In the diesel layer, the prolonged explosion time is attributed to the evaporation and combustion of the diesel, while in the water layer, the flame propagation velocity is significantly reduced. Under the experimental conditions, the maximum explosion energy reached 7.15 × 10⁶ J, corresponding to a TNT equivalent of 1.7. The peak overpressure surpassed the threshold for human fatality as defined by overpressure standards, posing a potential risk of damage to large steel-frame structures. The explosion shockwave in diesel layer conditions (L_d = 0%, 5%, 7.5%, 12.5%) and water layer (L_w = 12.5%) conditions is observed to be sufficient to damage earthquake-resistant reinforced concrete. This study investigates the impact of sediment layer thickness and composition on gas explosions, and evaluates the associated explosion energy to assess human injuries and structural damage in underground environments. The findings of this study provide a scientific reference for urban underground safety.

Boilover is one of the most destructive tank fire scenarios. A series of experiments were conducted using eight different depths of oil pans (ranging from internal depths of 4–20 cm) to vary the water layer thickness and ullage height. The results indicate that the water layer effectively cools the sidewalls, reduces the burning rate, inhibits the development of hot zones, and delays the onset of boilover in small and medium-scale experiments. Conversely, the ullage height affects the burning rate, formation of hot zones, intensity of the boilover, and boilover onset time. Utilizing experimental data and thermodynamic analysis, both water layer thickness and fuel layer thickness were considered as variables to predict sidewall temperature at the fuel surface. These results were then introduced into the burning rate prediction model. A prediction model for the boilover onset time was also developed using the water layer thickness as a variable, and a thermodynamic analysis revealed the existence of a limit to the effect of water layer thickness on the boilover onset time. Bubble dynamics was introduced to analyze the boilover process at the oil-water interface, clarifying that the influence of water layer thickness and ullage height on boilover intensity primarily lies in factors such as the degree of superheat at the fuel-water interface. The study's findings hold significant implications for predicting and assessing fire accidents in storage tanks.