Constrained reaction volume shock tube study of n-heptane oxidation: Ignition delay times and time-histories of multiple species and temperature

Abstract

Ignition delay times of normal heptane have been measured at temperatures ranging from 651 to 823K and at pressures between 6.1 and 7.4atm at an equivalence ratio of 0.75 in 15%O2/5%CO2/Ar and in 15%O2/Ar mixtures behind reflected shock waves in a shock tube. Time-history measurements of fuel, OH, aldehydes (mostly CH2O), CO2, H2O, and temperature were also measured under these conditions. These time-histories provide critically needed kinetic targets to test and refine large reaction mechanisms. Measurements were acquired using a novel constrained reaction volume approach, wherein a sliding gate valve confined the reactant mixture to a region near the endwall of the shock tube. A staged-driver gas filling strategy, combined with driver section extensions, driver inserts, and driver gas tailoring, was used to obtain constant-pressure test times of up to 55ms, allowing observations of the chemistry in the Negative Temperature Coefficient (NTC) region. Experiments with conventional shock tube filling were also performed, showing similar overall ignition behavior. Comparisons between current data and simulations using the Mehl et al. n-Heptane mechanism (2011) are provided, revealing that the mechanism generally under-predicts first-stage ignition delay times in the NTC region, and that at low temperatures it over-predicts the extent of fuel decomposition during first stage ignition.