Nanosecond repetitively pulsed discharges and conventional sparks of ammonia-air mixtures in a fan-stirred cruciform burner: Flammability limits and ignition transition

Ammonia, an efficient hydrogen carrier, is crucial for achieving net-zero emissions. However, its low reactivity, as manifested through small unstretched laminar flame speed (S_L,0 ∼ 7 cm/s) and large laminar flame thickness (δ_L ∼ 2.85 mm) at atmospheric pressure and stoichiometric conditions together with narrow flammability limits, makes it difficult for initiation. As such, how to measure accurately ammonia minimum ignition energies under laminar and turbulent conditions (MIE_L and MIE_T) and identify ammonia flammability limits are important to understand fundamental challenges and restricted usage for practical applications. We apply both nanosecond-repetitively-pulsed-discharges (NRPD) and conventional sparks (CS) via the same stainless-steel electrodes of 1-mm diameter with sharp ends at a fixed gap of 2 mm in the same fan-stirred cruciform burner to identify flammability limits of ammonia/air mixtures for spherical flame initiation. The burner is capable of generating near-isotropic turbulence having roughly equal magnitudes of r.m.s. turbulent fluctuating velocities in all three directions (u′) with negligible mean velocities. We find that NRPD operated at a pulse repetition frequency of 40 kHz can promote ignition or decrease the MIE on fuel lean and fuel rich sides as compared to that of CS. However, even using 2,000 pulses with a total ignition energy of 4.4 J for NRPD, no self-sustained flame propagation can be observed at ϕ = 0.65 and/or ϕ = 1.44 that marks lean and/or rich flammability limits for spherical flame initiation. Moreover, we also find a turbulent ignition transition for the stoichiometric ammonia/air mixture, of which the increasing slopes of MIE_T/MIE_L versus u′/S_L change drastically from gradually to exponentially at a critical value of (u′/S_L)_c ≈ 13 for both NRPD and CS. Finally, these results should be useful for future practical applications of premixed ammonia/air combustion.