Flame propagation and overpressure characteristics of methane-hydrogen-mixed cloud explosion in unconfined area: Experimental and model study

This study investigates the explosion behavior of methane-hydrogen vapor clouds in an unconfined space. Experiments were conducted on an 8 m³ platform to examine the effects of hydrogen volume mixing ratios (V = 0 %, 20 %, 35 %, 50 %) at an equivalence ratio of 1.1. The results show that higher hydrogen ratios increase flame instability and propagation speed. The flame development process can be divided into two stages based on the average flame velocity. Due to the influence of instability, the average flame speed in Stage 2 is nearly twice that of Stage 1. In Stage 2, the average speeds for V = 0 % (methane), 20 %, 35 %, and 50 % hydrogen mixtures reached 5.619, 7.268, 7.910, and 19.997 m/s, respectively. Peak overpressure, pressure rise rate, and impulse also increase with hydrogen content. While V = 20 % and V = 35 % exhibit similar overpressure and rise rates, the impulse at V = 35 % and V = 50 % is nearly double that at V = 20 %. Therefore, V = 20 % is identified as a key turning point in hazard impact. Peak overpressure is crucial for determining safety distances in the process industry. This study also refines prediction models by integrating flame instability and overpressure effects. The improved TNO model (k = 2.97) outperforms the Thermal Expansivity Model and the original TNO model (k = 5.8), providing more accurate results. Using 2.07 kPa as the safety threshold, the model predicts safe distances of 2 m for methane, 3.5 m for V = 20 %, 6 m for V = 35 %, and 12 m for V = 50 %. These findings enhance the experimental understanding of methane-hydrogen explosions in large-scale unconfined environments and inform safety distance guidelines for industrial applications.