Ainan Bao

Nonthermal Ignition of Premixed Hydrocarbon-Air Flows by Nonequilibrium Radio Frequency Plasma

Results are presented of nonequilibrium rf plasma-assisted combustion experiments in premixed air-fuel flows. The experiments have been conducted in methane-air, ethylene-air, and CO-air mixtures. The results show that large volume ignition by the uniform and diffuse rf plasma can be achieved at significantly higher flow velocities (up to u = 25 m/s) and lower pressures (P = 60-130 torr) compared to both a spark discharge and a dc arc discharge. The experiments also demonstrated flame stabilization by the rf plasma, without the use of any physical obstacle flameholders. Fourier transform infrared (FTIR) absorption spectra of combustion products show that a significant fraction of the fuel (up to 80%) burns in the test section. Temperature measurements in the diffuse rf discharge using FTIR emission spectra show that the flow temperature in the plasma before ignition (T = 250-550°C at P = 60-120 torr) is considerably lower than the autoignition temperatures for ethylene-air mixtures at these pressures (T = 600-700°C). Visible emission spectroscopy measurements in C 2 H 4 -air flows in the rf discharge detected presence of radical species such as CH, C 2 , and OH, as well as O atoms. In CO-air flows, O and H atoms have been detected in the rf plasma region and CO 2 emission (carbon monoxide flame bands) in the flame downstream of the rf plasma.

Studies of Chemi-Ionization and Chemiluminescence in Supersonic Flows of Combustion Products

A stable ethylene/oxygen/argon flame is sustained and nearly complete combustion is achieved in the combustion chamber of an M = 3 supersonic nozzle, at a stagnation pressure of P 0 =1 atm. Ultraviolet and visible emission is detected both from the combustion chamber and from the M = 3 flow of combustion products. Temperature in the combustor, inferred from the visible emission spectra, is To = 2000 ± 200 K. Electron density in M = 3 flow of combustion products has been measured using Thomson discharge n, = 1.4 ± 0.2·10 8 cm -3 , at an ionization fraction of n e /N = (0.65 ± 0.15) · 10 -9 . This corresponds to an electron density of n e0 = 2.2 ·10 9 cm -3 in the combustor. The chemi-ionization current measured in the M = 3 flow is found to be proportional to the equivalence ratio in the combustor. The time-resolved chemi-ionization current is in very good correlation with the visible emission from ethylene-air and propane-oxygen-argon flames in the combustor at unstable combustion conditions. The results show that nearly all electrons can be removed from the supersonic flow of combustion products by applying a moderate transverse electric field. No effect of electron removal on visible emission has been detected. A similar result was obtained for nitric oxide β bands and cyanogen violet band emission, when nitric oxide was injected into the combustion product flow.

Studies of Chemi-Ionization and Chemiluminescence in Supersonice Flows of Combustion Products

A stable ethylene/oxygen/argon flame is sustained and nearly complete combustion is achieved in the combustion chamber of an M = 3 supersonic nozzle, at a stagnation pressure of P 0 =1 atm. Ultraviolet and visible emission is detected both from the combustion chamber and from the M = 3 flow of combustion products. Temperature in the combustor, inferred from the visible emission spectra, is To = 2000 ± 200 K. Electron density in M = 3 flow of combustion products has been measured using Thomson discharge n, = 1.4 ± 0.2·10 8 cm -3 , at an ionization fraction of n e /N = (0.65 ± 0.15) · 10 -9 . This corresponds to an electron density of n e0 = 2.2 ·10 9 cm -3 in the combustor. The chemi-ionization current measured in the M = 3 flow is found to be proportional to the equivalence ratio in the combustor. The time-resolved chemi-ionization current is in very good correlation with the visible emission from ethylene-air and propane-oxygen-argon flames in the combustor at unstable combustion conditions. The results show that nearly all electrons can be removed from the supersonic flow of combustion products by applying a moderate transverse electric field. No effect of electron removal on visible emission has been detected. A similar result was obtained for nitric oxide β bands and cyanogen violet band emission, when nitric oxide was injected into the combustion product flow.