This work aims to implement an actuator integrating a swirl burner with electrodes to explore stable and low-emission combustion of ammonia/air. The cavity structure holds a plasma torch driven by low-power microsecond repetitively pulsed discharges (µRPDs) in the nozzle, which serves as a pretreatment for ammonia/air gas before it enters the combustion region. The plasma generates strong excited N2, OH, and atomic H* and O* signals, and fitting N2 spectra indicates a rotational temperature of ∼3000 K and vibrational temperature of ∼4500 K. The flame without plasma is easier to detach from the wall and operate as a cone-shaped structure in the confined quartz tube, while plasma can help attach the flame and extend the lean blow-off limit from ∼0.6 to 0.4∼0.5 in a Reynolds number range of ∼3000 to 7500. Then, optical diagnostics for OH radical and NH2* chemiluminescence are performed to enable analysis of intermediate chemistry. The NH2* signals are distributed at the edge of the OH profile in both attachment and detachment cases. Finally, flue gas analyzers are used to find an optimal lean equivalence ratio where plasma anchors a stable ammonia/air flame with a relatively low emission of approximately 200 ppm NO and zero NH3 and H2.
In this study, a coaxial dual-shear jet nozzle is designed to enhance mixing and establish stable methane–oxygen combustion. The characteristics of mixing and combustion of methane–oxygen flames are experimentally investigated in nonreacting and reacting cases by varying the outer-to-middle velocity ratio [Formula: see text] from 1.1 to 5.7. In the nonreacting cases, acetone–planar-laser-induced fluorescence technology is applied to experimentally study the mixing behavior near the nozzle exit. Since the coaxial dual-shear configuration generates a sandwich structure, a flow structure near the exit, including three potential cores and three mixing layers, is established. Two mixture fraction fields divided by a critical velocity ratio [Formula: see text] are observed, in which the stoichiometric contours have different appearances. Based on the mixing characteristics, a mixing model and a linear scaling relation were presented to predict the stoichiometric mixing length in both nonreacting and reacting flows. Besides, the coaxial dual-shear jets yield more compact flames in cases of [Formula: see text], while the conventional coaxial jets produce longer flames with yellow tips. This shows the improvement of the coaxial dual-shear nozzle in mixing and combustion efficiency due to the formation of two shear layers near the nozzle exit. Furthermore, the OH* distribution indicates that the coaxial dual-shear flames produce a lower thermal load on the nozzle, preventing the combustor from experiencing the hazards of ablation.
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