Experimental study on the effects of concentration gradient and hydrogen blending ratio on vented HBNG explosion

• Clarified coupled effects of concentration gradient and H 2 ratio on explosion. • Clarified concentration gradient impact outweighs H 2 blending effect. • Established a bivariate coupling prediction function for flame and overpressure. • Investigated the effect of hydrogen mixing on methane explosion sensitivity. Using the diffusion time ( T ) to represent the concentration gradient. This paper systematically investigates the coupled effects of concentration gradient and hydrogen blending ratio ( λ ) on the explosion venting characteristics in actual leakage scenarios of hydrogen-blended natural gas (HBNG). Results indicate that the flame shape transitions from “unstable-stable-unstable” as T increases, while the brightness exhibits a trend of “bright-dim-bright”. The maximum flame length and speed exhibit a parabolic trend, while the flame duration decreases. With higher λ s, the flame morphology becomes increasingly unstable, and both the maximum flame length and speed rise linearly. The longest flame duration occurs at λ of 15 %. Peak overpressure, (dP/dt) max , and K G all exhibit a parabolic relationship as T increases, reaching their peaks at T of 60 s. Additionally, with the increase of λ , all three parameters increase linearly. T has a more significant impact on the external flame and overpressure, both exceeding 20 %, with the length of the external flame varying by more than 50 %. Under the coupled conditions of λ of 20 % and T of 60 s, both the flame propagation ability and explosion overpressure reach their peaks. However, this may lead to a shorter duration of the external flame. This study develops fitted functions for the maximum external flame length (R 2 = 0.9105) and peak overpressure (R 2 = 0.9598) based on the coupled effects of T and λ , offering valuable insights for disaster prediction.