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

Abstract

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.

Stimulated Raman scattering measurements of V–V transfer in oxygen

“…More recently, work in this laboratory has shown that vibrational excitation of diatomic species in high pressure, low temperature air plasmas reduces the net rate of free electron attachment to O 2 by at least three orders of magnitude [2]. Similarly, vibrational energy loading may play a critical role in a large variety of other high pressure non-equilibrium processes and systems, including plasma flow control [3], ignition and flame holding in high speed reacting flows [4], and chemical [5] and electric discharge oxygen-iodine lasers [6].…”

Mechanism of ignition by non-equilibrium plasma

“…Papers [6][7][8] reported on the plasma assisted ignition of premixed fuel-air mixtures under conditions that are close to the transonic flow (M % 0.8-0.9). In [6,7], the flows of air with methane, ethylene or CO were ignited by a diffuse RF plasma at pressures 60-130 Torr.…”

“…Papers [6][7][8] reported on the plasma assisted ignition of premixed fuel-air mixtures under conditions that are close to the transonic flow (M % 0.8-0.9). In [6,7], the flows of air with methane, ethylene or CO were ignited by a diffuse RF plasma at pressures 60-130 Torr. The authors demonstrated the flame stabilization by the RF plasma and reported that it can be achieved at significantly higher flow velocities and lower pressures as compared to a spark discharge and dc arc discharge [6].…”

“…In [6,7], the flows of air with methane, ethylene or CO were ignited by a diffuse RF plasma at pressures 60-130 Torr. The authors demonstrated the flame stabilization by the RF plasma and reported that it can be achieved at significantly higher flow velocities and lower pressures as compared to a spark discharge and dc arc discharge [6]. Similar oxidation by a nanosecond discharge was observed in [8].…”

A Studies of Plasma-Assisted Ignition in a Small Internal Combustion Engine

“…[7][8][9][10][11][12][13][14][15][16][17] Much of this enhancement is due to gas heating resulting from collisions with electrons and other excited species. For devices which channel much of their energy into the electronic and vibrationally excited states of molecules, significant kinetic enhancement can be achieved via reactions with plasmaproduced excited species that quickly react with the fuel and oxidizer and thus open different reaction pathways.…”