Study of ignition chemistry on turbulent premixed flames of n-heptane/air by using a reactor assisted turbulent slot burner

The changes in flame structure and burning velocity of premixed n-heptane/air flames associated with ignition chemistry have been investigated in a reactor-assisted turbulent slot (RATS) burner. Two distinct turbulent flame regimes are identified by varying the flow residence time and reactor temperature. A chemically frozen (CF) regime is observed at a reactor temperature of 450K and a low-temperature ignition (LTI) regime is identified at 650K. At a reactor temperature of 450K, the measured turbulent burning velocities (S_T) exhibit a monotonic trend, proportional only to the turbulent intensity and laminar flame speed (S_L) calculated with the initial fuel/air mixture. At a reactor temperature of 650K, S_T initially decreases with increasing flow residence times (decreasing turbulent intensity) but then increases once the reactor flow residence time exceeds the LTI delay. Furthermore, S_T in the LTI regime exhibits a strong correlation with the extent of low-temperature reactivity (defined by CH₂O concentration). The species distributions at the exit of the RATS burner after the onset of LTI are quantified by gas sampling-chromatography and used to compute the changes in S_L and mixture Lewis number (Le), which are shown to substantially change after the onset of LTI. Damköhler's scaling analysis indicates that the increase in S_T in the LTI regime originates from an increase in S_L, a decrease in Le, and an increase in turbulence intensity due to the heat release from the low-temperature chemistry. To examine the role of ignition chemistry on flame stability, flame flashback measurements have been performed by varying mean jet velocities and n-heptane/air mixture equivalence ratios for reactor temperatures of 450 and 650K. Measurements at 650K imply the strong influence of high-temperature ignition on flame stability phenomena.