Thin reaction zone and distributed reaction zone regimes in turbulent premixed methane/air flames: Scalar distributions and correlations

A series of premixed turbulent methane/air jet flames in the thin reaction zone (TRZ) and distributed reaction zone (DRZ) regimes were studied using simultaneous three-scalar high-resolution imaging measurements, including HCO/OH/CH2O, CH/OH/CH2O, T/OH/CH2O and T/CH/OH/. These scalar fields offer a possibility of revisiting the structures of turbulent premixed flames in different combustion regimes. In particular, CH2O provides a measure of the preheat zone, CH/HCO a measure of the inner layer of the reaction zone, and OH a measure of the oxidation zone. Scalar correlations are analyzed on both single-shot and statistical basis, and resolvable correlated structures of ∼100 µm between scalars are captured. With increasing turbulence intensity, it is shown that the preheat zone and the inner layer of the reaction zone become gradually broadened/distributed, and the correlation between HCO and [OH]LIF×[CH2O]LIF decreases. A transition from the TRZ regime to the DRZ regime is found around Karlovitz number of 70-100. The physical and chemical effects on the broadening of the flame are investigated. In the TRZ regime the inner layer marker CH and HCO remains thin in general although occasional local broadening of CH/HCO could be observed. Furthermore, there is a significant probability of finding CH and HCO at rather low temperatures even in the TRZ regime. In the DRZ regime, the broadening of CH and HCO are shown to be mainly a result of local reactions facilitated by rapid turbulent transport of radicals and intermediate reactants in the upstream of the reaction paths. Differential diffusion is expected to have an important effect in the DRZ regime, as H radicals seemingly play a more important role than OH radicals.