Strain rates measured along the wrinkled flame contour within turbulent non-premixed jet flames

The thin, wrinkled CH reaction layers within moderate-(Re = 9,100) and high - (18,600) Reynolds-number turbulent non-premixed jet flames were identifie d by using planar laser-induced fluorescence, and the in-plane strain rates on these reaction layers were measured using simultaneous particle Imaging Velocimetry (PIV). The PIV diagnostics resolved the Taylor scale; the stra in-limited diffusion length scale was fully resolved for half the cases stu died and nearly resolved for the others. In the high-Reynolds-number jet, i nstantaneous strain rates on the flame surface are highly intermittent, wit h peak values exceeding 10,000 s(-1). Mean strain rates, conditioned on the CH-peak contour, are relatively constant (150 s(-1)) in the Re = 9100 flam e and increase (650-1700 s(-1)) with axial location in the Re = 18,600 flam e, resulting from the flame wrinkling process. The CH-layer thickness does not appear to respond in amplitude or in phase with the strain field, indic ating that quasi-steady conditions do not occur. The strain field apparentl y oscillates at frequencies as high as 5-10 kHz-which is the inverse of the crossing time of integral-scale eddies-perhaps because only the low-freque ncy component of strain effectively acts on the flame. Mean axial velocitie s, conditioned on the CH-peak contour, were found to remain constant from t he flame base to tip and to approximately equal the product of the stoichio metric mixture fraction and the fuel-exit velocity, in agreement with predi ction. (C) 2001 by The Combustion Institute.