Premixed flames in counterflow jets under rigid-body rotation

The extinction characteristics of double premixed flames of methane and butane in counterflow of two cylindrical jets which undergo rigid-body rotation are investigated. Using laser Doppler velocimetry, the axial, radial and circumferential velocity profiles are measured when jets with equal angular velocity rotate in opposite directions. It is found that two symmetric lean methane flames which are merged and near extinction, will subsequently separate again upon the rotation of the jet streams. Furthermore, as the angular velocity is increased, the fuel concentration at extinction is found to first decrease to a minimum value and increase thereafter. These tendencies are described on the basis of the observed variation of the rate of stretch experienced by the flames as a function of their axial position Also, the existence of radially inward secondary flows which produce recirculation zones on both sides of the stagnation plane are established whose thickness increase with the jet angular velocity. The practical implications of the results to the modeling of swirl combustors and flamelets within turbulent eddies in large-scale turbulent combustion fields are discussed.