Experimental observation of negative temperature dependence in iso-octane burning velocities

The average burning velocities S-_b of premixed, spherically expanding iso-octane flames are reported for flame radii from 0.75 to 1.75 cm between 400 and 900 K, near 1 atm. Measurements are performed in lean mixtures (0.9 equivalence ratio) using a helium-enriched oxidizer (18% oxygen, 41% nitrogen, and 41% helium) to inhibit thermodiffusive instabilities at high temperatures. A shock tube is used to heat the unburned gas to elevated temperatures. Consistently smooth flames with weak S-_b temperature dependence are seen between 500 and 580 K. Above 580 K and extending to 740 K, flames exhibit internal structure and surface wrinkling, and a very strong temperature dependence is observed. From 740 to 800 K, a regime of negative S-_b temperature dependence is observed. Above 800 K, the measured values of S-_b again assume a positive temperature dependence. Although direct numerical simulation studies published in the literature have predicted negative temperature coefficient (NTC) behavior in flames influenced by low-temperature chemistry and cool flames, it is believed that this work represents the first experimental observation of NTC behavior in burning velocity measurements.