IR-HyChem: Towards modeling the high-T combustion behavior of aviation fuels using infrared spectra

A Fourier transform infrared (FTIR) spectra-based approach, namely IR-HyChem, was developed to model the combustion behavior of jet and rocket fuels. Earlier shock-tube experiments employed laser absorption spectroscopy (LAS) to measure the yields of key stable intermediates: CH₄, C₂H₄, and >C₂ alkenes such as C₃H₆, 1-C₄H₈ and i-C₄H₈, during the pyrolysis of neat hydrocarbons across several molecular classes (n-alkanes, lightly branched alkanes and highly branched alkanes). These measurements revealed empirical relations of molecular structure to the yields of these intermediates. The relationships provided important insights into fuel reactivity under high-temperature, combustor-relevant conditions. The IR-HyChem methodology establishes quantitative correlations between spectral features and the yields of these pyrolysis intermediates. Using this framework, IR-HyChem models were demonstrated for two jet fuels (JP-8 and F-24) and a rocket fuel (RP-1), by constraining a subset of stoichiometric parameters in the HyChem lumped reactions, resulting in partially constrained IR-HyChem models. These models were evaluated against ignition delay times (IDTs) measured behind reflected shock waves at elevated pressures, demonstrating strong agreement with experimental data. A Monte Carlo uncertainty analysis revealed that imposing FTIR-based constraints reduced variance in IDT predictions compared to unconstrained models. Furthermore, sensitivity analysis indicated that additional IR spectra-based correlations could improve the accuracy of the IR-HyChem models. Overall, this work demonstrates the utility of FTIR spectra and their potential use as a low-volume tool for developing predictive chemistry models for the combustion of real, multi-component fuels.