Study of ignition chemistry on turbulent premixed flames of n-heptane/air by using a reactor assisted turbulent slot burner
Abstract 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 ( 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 650 K, 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 CH 2 O 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. Damkohler'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 650 K. Measurements at 650 K imply the strong influence of high-temperature ignition on flame stability phenomena.