Thursday, October 24, 2013
Mechanical and Civil Engineering Seminar
Premixed Combustion Subjected to Intense Turbulence Levels - Physical Mechanisms Identified Using High Speed Imaging
James F. Driscoll, Professor, Department of Aerospace Engineering, University of Michigan
Our high speed PLIF and PIV laser imaging diagnostics have shown that subjecting a premixed flame to intense turbulence levels can cause the flame to become shredded. Near the locations of local flame extinction the reactants and hot products can mix, and this leads to physical mechanisms that are not observed in most previous studies, which have been conducted at significantly lower Reynolds numbers. Distributed reaction zones are observed and when the reactants are preheated the role of auto-ignition chemistry appears to become important. High Reynolds numbers also create a large range of turbulent eddy sizes, and this leads to intense wrinkling and large local curvature of the reaction layers. High-speed PIV images show that the Landau hydrodynamic instability mechanism plays a major role in the wrinkling of the turbulent flame surface. That is, the turbulent eddy first wrinkles the flame surface during its passage, but later the eddy is destroyed. Long after the eddy is gone, the flame continues to wrinkle due to the Landau mechanism.
Three turbulent combustion experiments are discussed: (a) our fundamental high Reynolds number Hi-Pilot flame, (b) premixed flames near the base of a lifted jet flame in a cross-flow in our scramjet experiment, and (c) our lean premixed-prevaporized (LPP) gas turbine combustor. Kilohertz PLIF is used to image formaldehyde while kilohertz PIV quantifies the vorticity field and the eddy-flame interactions.