Christopher B. Reuter

Plasma Assisted Low Temperature Combustion

This paper presents recent kinetic and flame studies in plasma assisted low temperature combustion. First, the kinetic pathways of plasma chemistry to enhance low temperature fuel oxidation are discussed. The impacts of plasma chemistry on fuel oxidation pathways at low temperature conditions, substantially enhancing ignition and flame stabilization, are analyzed base on the ignition and extinction S-curve. Secondly, plasma assisted low temperature ignition, direct ignition to flame transition, diffusion cool flames, and premixed cool flames are demonstrated experimentally by using dimethyl ether and n-heptane as fuels. The results show that non-equilibrium plasma is an effective way to accelerate low temperature ignition and fuel oxidation, thus enabling the establishment of stable cool flames at atmospheric pressure. Finally, the experiments from both a non-equilibrium plasma reactor and a photolysis reactor are discussed, in which the direct measurements of intermediate species during the low temperature oxidations of methane/methanol and ethylene are performed, allowing the investigation of modified kinetic pathways by plasma-combustion chemistry interactions. Finally, the validity of kinetic mechanisms for plasma assisted low temperature combustion is investigated. Technical challenges for future research in plasma assisted low temperature combustion are then summarized.

Experimental Investigation of the Stabilization and Structure of Turbulent Cool Diffusion Flames

Although recent studies of laminar cool flames have provided important advances in understanding the low-temperature chemistry of both hydrocarbons and oxygenates, there has been limited experimental insight into how interactions between turbulence and chemistry occur in cool flames. To address this, a new Co-flow Axisymmetric ReactorAssisted Turbulent (CARAT) burner has been developed and characterized in this investigation for the purpose of directly studying turbulent cool flames. A methodology for establishing stable turbulent cool diffusion flames under well-defined conditions is proposed. The structure of dimethyl ether flames is examined using both formaldehyde planar laserinduced fluorescence and Rayleigh scattering. It is found that weak turbulence produces wrinkled turbulent cool flames in which fluctuations occur mainly on the fuel side of the flame. However, at increased levels of turbulence, large pockets of unburned reactants appear in the vicinity of the cool flame, and structural fluctuations extend to both sides of the flame. This study offers a well-defined experimental platform for the study of turbulence-chemistry interactions at low temperatures.

Effect of Ignition Chemistry on Turbulent Premixed Flames of n-Heptane and Toluene

Turbulent premixed flames of n-heptane/air and toluene/air mixtures have been experimentally investigated in a reactor-assisted turbulent slot (RATS) burner at two burner temperatures, 450 K and 650 K, by measuring turbulent burning velocities (ST), flashback, and flame structures with planar laser-induced fluorescence (PLIF) imaging at various equivalence ratios (φ). Turbulent burning velocities have been found to be affected by the low-temperature-ignition for n-heptane/air mixture due to two-stage ignition process, whereas no LTI affected turbulent burning velocity is found in case of toluene/air mixture due to the absence of two-stage ignition process. The measured turbulent burning velocities of n-heptane and toluene at 450 K exhibits identical trend of u’/SL dependency, once they are normalized by the laminar burning velocities (SL), indicating appropriate representation of chemical effect by SL. In order to investigate the effect of Lewis number (Le), turbulent burning velocities and flame structure are measured by changing the equivalence ratio from 0.7 to 1.5 at 450 K. The results show that ST is insensitive to the change of Le at fuel lean, however strong dependency of ST on Lewis number is found at fuel rich conditions due to the increase of flame front wrinkling. Flame flashback conditions are measured in a function of mean jet velocity and equivalence ratio for n-heptane/air mixtures from lean (φ = 0.7) to rich (2.1) cases. The flashback measurements reveal completely different behaviors, depending on the burner temperature. At 450 K (chemically-frozen regime) the flashback is found to be controlled by the turbulent burning velocity, whereas at 650 K (Ignition-driven regime) the flashback is correlated well with the calculated hot-ignition delay times of n-heptane/air mixture. In ignition-driven regime, the measured ST/SL increases substantially as increasing the ignition Damkőhler number.