Investigation of OH and CH₂O distributions at ultra-high repetition rates by planar laser induced fluorescence imaging in highly turbulent jet flames

For better understanding of the transient behavior of the flow/flame interaction in highly turbulent premixed flames, measurement techniques with sufficient temporal resolution are required. Using a burst-mode laser pumped optical parametric oscillator system, the present work demonstrated an ultra-high-speed diagnostic, for the first time, capable of visualizing hydroxyl radicals (OH) and formaldehyde (CH₂O) distributions in a highly turbulent flame at repetition rates up to 140 kHz (i.e. with a temporal resolution of 7.1 μs) to access the Kolmogorov time scales of flames in the distributed/broken reaction zone regime (Ka ≥ 100). More than 100 consecutive images were recorded in the present work for OH planar laser induced fluorescence (PLIF) measurement at 100 kHz. Systematic temporal and spatial evolution of the flame structure is clearly resolved over long sequences, e.g. 1.2 ms for OH PLIF and 10 ms for CH₂O PLIF respectively. The Kolmogorov time scale for all the cases at x/d = 10 and x/d = 30 are estimated with the minimum Kolmogorov time scale of 7.5 μs, indicating that nearly all time-scales of the flow are resolved with the present experimental setup. The axial velocity of the CH₂O structure at its outer layer at different flame height positions, U_CH2O, is obtained. The velocity U_CH2O is compared with the LDV results, which shows a good agreement with the mean flow velocity. By comparing U_CH2O with the local axial velocity, different mechanisms responsible for the large-scale wrinkle structures in the reaction zone and the propagation of CH₂O structure along the flame height are discussed. Power spectral density and autocorrelation function based on the CH₂O outer layer fluctuation in the radial direction are illustrated. Quantitative analysis of the ultra-high-speed diagnostics data further augments the understanding of turbulent combustion down to Kolmogorov scale experimentally.