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 450 K and a low-temperature ignition (LTI) regime is identified at 650 K. At a reactor temperature of 450 K, the measured turbulent burning velocities (ST) exhibit a monotonic trend, proportional only to the turbulent intensity and laminar flame speed (SL) calculated with the initial fuel/air mixture. At a reactor temperature of 650 K, ST initially decreases with increasing flow residence times (decreasing turbulent intensity) but then increases once the reactor flow residence time exceeds the LTI delay. Furthermore, ST 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 SL 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 ST in the LTI regime originates from an increase in SL, 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 650 K. Measurements at 650 K imply the strong influence of high-temperature ignition on flame stability phenomena.