Roles of the state-resolved OH(A) and OH(X) radicals in microwave plasma assisted combustion of premixed methane/air: An exploratory study

We report a novel microwave plasma-assisted combustion (PAC) system that is developed as a new test platform to study roles of plasma in PAC. The system included two major components, an atmospheric pressure microwave plasma cavity and a cross-shape quartz combustor. This new PAC system allows one to study PAC using complicated yet well-controlled combinations of operating parameters, such as fuel equivalence ratio (?), fuel mixture flow rate, plasma gas flow rate, plasma gases, symmetric or asymmetric fuel-oxidant injection patterns, with and without plasma. In this work, ignitions at the fuel (lean and rich) flammability limits at different plasma powers and fuel flow rates were investigated. The ignition curves of plasma power versus ?_FL at the different flow rates revealed a stretched U-shape, showing clear evidences of the plasma enhancement effects on ignition and flame stabilization, i.e. the fuel lean flammability limit (?_LFL) was extended to ? = 0.2, as compared to ? = 0.6 at the same combustion parameters except with no plasma. Optical emission spectroscopy (OES) showed that the combustor had three distinct reaction zones: plasma zone, hybrid plasma-flame zone, and flame zone; and each of the reaction zones was well defined by its OES features. Furthermore, a detailed survey of OES of OH (A–X) conducted along the plasma jet axis (x direction) with a spatial resolution of 0.5 mm revealed that OH(A) had a double-peak feature in its relative emission intensity curve (I ∼ x) in the hybrid zone where plasma-assisted ignition (PAI) started, as evidenced by a significant surge of OH(A) and by a large increase in OH rotational temperature, i.e. from 1450 K to 2400 K. Moving from the hybrid zone to the flame zone, OH(A) decreased by more than four orders of magnitude. However, the electronic ground state OH(X) measured simultaneously using pulsed cavity ringdown spectroscopy around 308 nm showed that absolute number density of the OH(X) decreased by smaller than a factor of ten from the downstream of the hybrid zone to the flame zone. The different changing rates of the OH(A) and OH(X) radicals from the hybrid zone to the flame zone allow us to propose a hypothesis that if both the electronically excited state OH(A) and the electronic ground state OH(X) assisted the ignition and flame stabilization processes, the role of OH(X) radicals was more dominant in the flame stabilization but the role of OH(A) radicals was more dominant in the ignition enhancement.