Abstract: Heat release and ignition in compression ignition (CI) and gasoline direct injection (GDI) engines occurs in the presence of concentration and temperature gradients. Therefore, recognizing the need for a thorough validation of chemical kinetic models in transport affected systems, this study employs non-premixed systems to better understand the complex couplings between low/high temperature oxidation kinetics and diffusive transport. This study is divided into two parts.
In the first section, the two-stage Lagrangian model compares model prediction of ignition delay time and experimental data from the KAUST ignition quality tester (IQT), and ignition data for liquid sprays in constant volume combustion chambers (CVCC). The TSL employed in this study utilizes detailed chemical kinetics while also simulating basic mixing processes, which are important in turbulent gaseous-jet diffusion flames. Using detailed kinetic models, this section assesses the ability of TSL to reproduce ignition events in the IQT and high pressure CVCC, and then compares how physical and chemical processes affect fuel ignition events in these systems. The TSL model was found to be efficient in simulating IQT in long ignition delay time fuels; it was also effective in CVCC experiments with high injection pressures, where physical processes contributed little to ignition delay time. Overall, the TSL modeling approach demonstrated the advantages of detailed chemical kinetic models for examining spray combustion processes.
In the second part of this study, a well-characterized atmospheric pressure counterflow burner was developed and validated against previous experimental data by Humer et al. (2002); and the velocity profile of the flow field was measured. The counterflow burner was employed to examine the effects of fuel molecular structure on low- and high temperature reactivity of classic and alternative fuels in transport-affected systems. These effects were investigated during measurement of critical conditions of extinction and ignition of various fuel/oxidizer mixtures in hot and cool diffusion flames. Data generated were used to validate various chemical kinetic models in diffusion flames. Where necessary, suggestions were made for improving these models.
For hot flames studies, the tested fuels included C3-C4 alcohols and six FACE gasoline fuels. The results for C3-C4 alcohols indicated that the alcohols substituted were less reactive than the normal alcohols. The ignition temperature of FACE gasoline was found to be nearly identical, while there was a slight difference in their extinction limits. This difference was a result of the combined influence of the molecular weights in the tested fuels and their octane ratings. Predictions by alcohol combustion models from Sarathy et al. (2014), and from the gasoline surrogate model (Sarathy et al., 2015), agreed with the experimental data. For cool diffusion flames studies, the tested fuels included butane isomers, light, heavy and Halterman naphtha, six FACE gasolines, Corryton and Halterman gasolines, and their surrogates. Results revealed that the addition of ozone successfully established cool diffusion flames in the tested fuels at low and moderate strain rates. The addition of ozone significantly influenced initiation and sustenance of cool diffusion flames; since cool diffusion flames without ozone could not be established, even at high fuel mole fractions. As shown by the higher fuel mole fractions required for cool flame initiation and lower extinction strain rate limits, iso-butane fuel has lower reactivity than n-butane. Numeric simulations were performed to replicate the extinction limits of the cool diffusion flames in the butane isomers. The model qualitatively captured experimental trends for both fuels and ozone levels; but it over-predicted extinction limits of the flames. For gasoline fuels, the results showed that FACE I has stronger cool diffusion flame while FACE G had the weakest cool flame, reflecting the role of their octane numbers at low temperature reactivity. For naphtha fuels, results showed that PRF 64.5 reacted slower at low temperatures.