Explosion behavior of premixed LPG–hydrogen/air mixtures in a square duct

Liquefied petroleum gas (LPG) is widely used due to its high energy density and ease of storage, but its slow burning rate can lead to incomplete combustion and safety risks. To improve combustion and explore explosion behavior, hydrogen-enriched LPG (LPG–H2) mixtures with 20%–70% hydrogen were tested. The 20% level was chosen as the threshold where flame behavior changes significantly, while 70% represented the upper limit for trend analysis. Experiments were conducted in a 100 × 100 × 1000 mm3 transparent tube with high-speed cameras and pressure sensors to investigate flame shape, propagation speed, and pressure under equivalence ratios of 0.8, 1.0, and 1.2. Results show that hydrogen addition strongly affects explosion behavior. With increasing hydrogen content, the flame front evolves from a smooth blue-violet surface (lean mixtures) to a wrinkled pale green structure (rich mixtures). Due to diffusional-thermal instability and buoyancy, the upper flame area exceeds the lower. Flame propagation velocity increases nonlinearly, peaking at 24.61 m/s at an equivalence ratio of 1.2 and 70% hydrogen. A notable turning point in velocity at 400 mm from ignition is linked to tulip flame formation, which expands the flame front and accelerates propagation. Peak overpressure also rises with hydrogen content, reaching 427.61 kPa under the same conditions—103.3% higher than in lean mixtures. Equivalence ratios of 1.0–1.2 combined with high hydrogen fractions significantly enhance explosion intensity and combustion efficiency. This study examines flame propagation and explosion characteristics of LPG blended with 20%–70% hydrogen, providing data for assessing explosion risks in hydrogen-enriched fuels.