Experimental study of the coupling effects of ignition position and nitrogen inerting on vented hydrogen-air deflagrations

Abstract

In this paper, the flame evolution and pressure dynamics of hydrogen-nitrogen-air explosions with nitrogen addition ratio (χ) ranging from 0 to 40 %, ignited at three different positions (“central”, “back” or “front” with respect to the vent) in a vented cylindrical vessel, were experimentally studied. Experimental results reveal that the coupling effects of χ and ignition position significantly affect the pressure curves and flame behavior within and outside the vessel. The higher the χ, the smoother the internal flame captured by a high-speed schlieren system. When χ<30 %, the maximum reduced overpressure (Pmax) at different ignitions decreases with increasing χ, and the central explosion yields the best suppression of Pmax: when χ is increased from 0 to 30 %, Pmax monotonically decreases from 232 kPa to 38 kPa. However, the differences in Pmax among the three ignition positions become negligible when χ ≥ 30 %. The structure of the pressure peaks and the types of oscillations measured near the vent depend on the combinations of ignition location and χ. The formation of a shock wave generated by the external explosion and its effect on the internal pressure-time histories are described. In general, for a given ignition, the maximum external overpressure (Pe-max) decreases with χ is increased. The most pronounced decreasing trend of Pe-max is consistently observed in back explosions when χ ranging from 0 to 40 %. Furthermore, compared to other ignition positions, the highest Pmax is always attained in central-ignition with χ<30 %; while the highest Pe-max is always attained in back-ignition with χ ≤ 30 %; as χ ≥ 10 %, both Pmax and Pe-max recorded at front-ignitions are almost insensitive to χ.