Experimental shock tube measurements of laminar burning velocity of n-heptane and iso-octane in the negative temperature coefficient regime

Laminar burning velocities (S_L) of n-heptane and iso-octane are measured between 400 and 900 K at 1 atm and 0.9 equivalence ratio. A shock tube is used to heat the unburned gas to temperatures at which it would auto-ignite, while limiting the time for ignition chemistry to occur ahead of the flame to much less than the ignition delay time. In air (21% O₂, 79% N₂), measured values of S_L are lower than, but follow the same temperature trend as the model below 550 K; between 550 and 600 K, a stronger dependence of S_L on temperature is observed. For S_L measurements performed in a helium-containing oxidizer (18% O₂, 41% N₂, 41% He), a strong temperature dependence, similar to that seen in air, is observed between 600 and 700 K. Between 700 and 800 K, a negative temperature dependence is observed. Above 800 K, measured S_L again assumes a positive temperature dependence. While negative temperature coefficient (NTC) behavior is known to exist in ignition delay times for many hydrocarbon fuels, to the authors’ knowledge, this work represents the first direct, experimental observation of NTC behavior in flame speed measurements.