Evaluation of the transient response of a counter-flow diffusion flame using two-line OH PLIF thermometry and PIV

OH planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) have been used to study the frequency response of laminar CsHg-air counterflow diffusion flames, and thereby the adequacy of the steady-flamelet model for turbulence. PIV was used to determine the flame strain rate, while OH PLIF was used both to measure temperature at the flame front, using the two- line PLIF technique, and the reaction-zone width. Both measurements demonstrate the existence of a diffusion-limited frequency response of flames subjected to a time-varying flow field. At the 30-Hz and 50-Hz forcing frequencies, the maximum reaction-zone temperature and width were found to respond quasi-steadily. However, At higher forcing frequencies-that is, 100 and 200 Hz-transient behavior is evident from the phase relationship between the imposed sinusoidal strain rate and the resulting peak temperature and reaction-zone width. The measured values of the OH-field widths (FWHM) were fit well by an offset sine function. In all cases when the oscillation amplitude (from the sine-curve fit) is normalized by the cycle mean strain rate and plotted against the non-dimensional flow field frequency, it collapses onto a single line with a steep negative slope.