An experimental investigation on a flow-facilitated ignition at reduced and normal pressures for small, near-unity and large Lewis number of hydrogen/air and hydrogen/ammonia/air mixtures

Abstract

At sufficiently small spark gap (d_gap=0.64 mm) and at sub-atmospheric pressure (p = 0.3 atm), a numerical simulation by Chen et al. (2023) reported a flow-facilitated ignition (FFI) phenomenon in an imposed uniform flow perpendicular to a pair of electrodes, where a constant ignition energy (E_ig=1.24 mJ) cannot ignite the hydrogen/air mixture at an equivalence ratio ϕ=0.4 with small Lewis number Le≈0.43<<1 in quiescence but ignition is successful in flowing conditions. This implied that the occurrence of FFI was mainly due to heat losses regardless of Le in contradiction with previous turbulence-facilitated ignition results. To test FFI, a well-controlled ignition experiment at d_gap=0.6 mm using the same simulation flow-electrode concept via a uniform flow setup resided in a large pressure-controlled chamber with optical access is conducted for three targeted mixtures having Le≈0.43<<1 (H₂/air at ϕ=0.4), Le≈2.3>>1 (H₂/air at ϕ=5.1), and Le≈0.95∼1 ((80%NH₃+20%H₂)/air at ϕ=1). Various E_ig=0.2∼250 mJ of high accuracy are applied to measure minimum ignition energies (MIE) at 50% ignitability under static and flowing conditions with an inlet flow velocity (U_in=0–10 m/s) at p = 0.3 atm and/or 1 atm. Experimental results show no FFI for Le≈0.43<<1 and Le≈0.95∼1 cases even at p = 0.3 atm, because values of MIE increase gradually with increasing U_in. However, FFI does occur for Le≈2.3>>1 case, since E_ig=250 mJ cannot ignite static mixture at p = 1 atm and MIE decreases drastically from 151.8 mJ at U_in=1 m/s to 30.1 mJ at U_in=4 m/s. These results provide important information of the effects of uniform flow on electrode-spark (forced) ignition of premixed flames, revealing two key points: (i) Heat losses alone cannot lead to FFI, because FFI does not occur at Le∼1 and Le<<1 cases even at p = 0.3 atm. (ii) FFI only occurs at sufficiently large Le>>1 and sufficiently small d_gap.