Numerical and Experimental Investigation of n-Heptane Autoignition in the Ignition Quality Tester (IQT)

Development of advanced compression ignition and low-temperature combustion engines is increasingly dependent on chemical kinetic ignition models. However, rigorous experimental validation of kinetic models has been limited under engine-like conditions. For example, shock tubes and rapid compression machines are usually restricted to premixed gas-phase studies, precluding the study of heterogeneous combustion and the use of low-volatility surrogates for commercial diesel fuels. The Ignition Quality Tester (IQT) is a constant-volume spray combustion system designed to measure ignition delay of low-volatility fuels, having the potential to validate ignition models. However, a better understanding of the IQT's fuel spray and combustion processes is necessary to enable chemical kinetic studies. As a first step, n-heptane was studied because numerous reduced chemical mechanisms are available in the literature as it is a common diesel fuel surrogate, as well as a calibration fuel for the IQT. A modified version of the KIVA-3V software was utilized to develop a three-dimensional computational fluid dynamics (CFD) model that accurately and efficiently reproduces n-heptane ignition behavior and temporally resolves temperature and equivalence ratio regions inside the IQT. Measured fuel spray characteristics (e.g., spray-tip velocity, spray cone-angle, and flow oscillation) for n-heptane were programmed into the CFD model. Sensitivity analyses of fuel droplet size and velocity showed that their effects on ignition delay were small compared to the large chemical effects of increased chain branching in the isomers 2-methylhexane and 2,4-dimethylpentane. CFD model predictions of ignition delay using reduced/skeletal chemical mechanisms for n-heptane (60-, 42-, and 33-species, and one-step chemistry) were compared, again indicating that chemical kinetics control the ignition process.