HyChem - A physics-based approach to modeling real-fuel combustion chemistry
Keywords: Mechanical Engineering.

Real fuels usually contain hundreds to thousands of hydrocarbon components. Over a wide range of combustion conditions, large hydrocarbon molecules undergo thermal decomposition first to form a small set (usually less than 10 species) of low molecular weight products, followed by the oxidation of those products, which is usually rate limiting. Hence, the composition of the decomposed products determines the overall global combustion properties. For conventional distillate fuels, the pyrolysis products comprise ethylene (C2H4), hydrogen (H2), methane (CH4), propene (C3H6), 1-butene (1-C4H8), iso-butene (i-C4H8), benzene (C6H6) and toluene (C7H8). From a joint consideration of thermodynamics and chemical kinetics, it is shown that the composition of the thermal decomposition products is a weak function of the thermodynamic condition, the equivalence ratio and the fuel composition within the range of temperatures relevant to high temperature combustion phenomena. In this dissertation study, I demonstrate a hybrid chemistry (HyChem) approach to modeling the high-temperature oxidation of real, liquid fuels. In this approach, the kinetics of fuel pyrolysis is modeled using experimentally derived, lumped reaction steps, while the oxidation of the pyrolysis fragments is described by a detailed foundational fuel chemistry model. A wide range of modeling results are provided to support the approach, including three conventional aviation fuels (JP-8 POSF10264, Jet A POSF10325, JP-5 POSF10289), two rocket fuels (RP2-1 POSF7688, RP2-2 POSF5433), and a bio-derived alternative jet fuel (Gevo alcohol-to-jet fuel, C1 POSF11498). The HyChem models of those fuels were developed using advanced speciation data obtained from shock tubes and a flow reactor, and the models were subsequently tested against global combustion properties, including ignition delay time, laminar flame speed, and flame extinction strain rates across a wide range of pressure, temperature and reactant mixture conditions. Sensitivity analysis of the model predictions with respect to the measurement uncertainties and rate parameter uncertainties of foundational fuel chemistry model is assessed. In this dissertation, the HyChem modeling approach was also extended to three key aspects critical to modeling fuel combustion over an even wider range of condition. First, a modified HyChem model was formulated for capturing the physics in negative temperature coefficient (NTC) and low-temperature oxidation regimes. Sensitivity test and suggestions on future NTC enabled HyChem model development are presented. Second, the HyChem approach was applied to modeling the blend of a conventional Jet A fuel and an alternative, alcohol-to-jet synthetic fuel. The pyrolysis as well as the combustion properties of several blended fuels were predicted by a simple combination of the HyChem models of the two individual fuels, thus demonstrating that the HyChem models for two jet fuels of very different compositions can be "additive" as far as high-temperature properties are concerned. Lastly, I will discuss a case study in which the HyChem model of Jet A is extended to NOx prediction after combining it with a recently updated reaction model of nitrogen chemistry. The combined reaction model is shown to predict NOx formation in premixed stretched-stabilized Jet A flames satisfactorily.