Pre-ignition associated with low-temperature shock tube ignition measurements

Shock tubes are widely used for chemical kinetics studies due to their ability to instantaneously achieve nearly zero-dimensional high-temperature conditions behind reflected shock waves. In an attempt to study ignition chemistry at lower temperatures, however, there are additional challenges and non-idealities associated with using shock tube for long test time. One such non-ideality is the gradual linear pressure rise behind the reflected shock wave, commonly known as the "dP/dt problem", which is resolved by time-dependent volume profile in homogeneous calculations. Another non-ideality, which thus far has been overlooked, is the pre-ignition pressure rise or pre-ignition energy release. In the current work, measurements of ignition delay times of n-heptane and n-hexane under low-temperature (650-1250 K) and low-pressure (1.5 atm) conditions are reported, in which significant discrepancies in the ignition delay time measurements and predictions are noted. Such non-ideal behavior is attributed to pre-ignition localized ignition kernels, and the postulate is validated by high-fidelity simulations at experimental conditions by demonstrating the level of ignition advancement caused by localized ignition sources.