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

In this paper, the flame evolution and pressure dynamics of hydrogen-nitrogen-air explosions with nitrogen addition ratio (chi) 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 chi and ignition position significantly affect the pressure curves and flame behavior within and outside the vessel. The higher the chi, the smoother the internal flame captured by a high-speed schlieren system. When chi<30 %, the maximum reduced overpressure (P-max) at different ignitions decreases with increasing chi, and the central explosion yields the best suppression of P-max: when chi is increased from 0 to 30 %, P-max monotonically decreases from 232 kPa to 38 kPa. However, the differences in P-max among the three ignition positions become negligible when chi >= 30 %. The structure of the pressure peaks and the types of oscillations measured near the vent depend on the combinations of ignition location and chi. 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 chi is increased. The most pronounced decreasing trend of Pe-max is consistently observed in back explosions when chi ranging from 0 to 40 %. Furthermore, compared to other ignition positions, the highest P-max is always attained in central-ignition with chi<30 %; while the highest Pe-max is always attained in back-ignition with chi <= 30 %; as chi >= 10 %, both P-max and Pe-max recorded at front-ignitions are almost insensitive to chi.