Effect of high-frequency alternating electric fields on the behavior and nitric oxide emission of laminar non-premixed flames

This paper examined the behavior and NO emission of laminar non-premixed methane/air jet flames when subjected to high frequency alternating electric fields of 10 kHz over the voltage range of 0–4.0 kV. In particular, this paper examined variations of flame shape and luminosity, CO and NO molar fractions in the downstream flue gas, and chemiluminescence from OH∗ and CH∗ in the voltage-influenced flame zone. The results showed that with no application of an alternating electric field, flames were stable at the nozzle exit, bluish at the base and yellowish at the conical tip. However, once applied, different voltage regimes produced different responses from the flame. In the low-voltage regime of 0–1.0 kV, increasing the voltage narrowed the top yellowish zone of the flame and sharpened its conical tip, increased the CO molar fraction in the flue gas, decreased the NO molar fraction in the flue gas, and decreased the chemiluminescence intensity of OH∗ and CH∗ in the flame zone by ∼50%. At 1.0 kV, both CO and NO molar fractions reached extreme values, and the flame was at its weakest. In the mid-voltage regime of 1.0–3.0 kV, increasing the voltage resulted in an inverse response from the flames compared to the low-voltage regime. In the high-voltage regime of 3.0–4.0 kV, increasing the voltage resulted in the gradual disappearance of the top yellowish zone of the flame, increased the CO molar fraction in the flue gas and decreased the NO molar fraction. The transition mechanisms between the regimes are discussed within the context of the high-frequency discharge theory. Three competing effects explain the non-monotonic flame response to the voltage: thermal, ionic wind, and electrical–chemical. The analysis showed that the ionic wind effect majored in the low-voltage regime, the electrical–chemical effect dominated the mid-voltage regime, and all three effects were highly coupled in the high-voltage regime.