Predicting real transportation fuel combustion properties: Distinct chemical functionalities in hydrocarbon laminar burning velocities

A set of laminar burning velocities for various hydrocarbon fuels is analysed for commonality with regard to each fuels chemical functional groups. However, though this approach makes sense from the view point of chemical kinetics, from theory, the laminar burning velocity also depends on the mass diffusivity and the flame temperatures that are imparted by the fuel/air mixtures, in addition to the contribution from the reaction kinetics. This paper delineates measured laminar burning velocities of a hydrocarbon library by multiple linear regressions to independently calculated fundamental quantities (heat capacity, density, thermal conductivity and reaction enthalpy), leaving any important differences in reaction kinetic potential unambiguously exposed. By this analysis, it is found that the laminar burning velocity of all saturated normal alkanes are closely equivalent, somewhat surprisingly, including that of methane and ethane. Burning velocities of the cycloalkane and isomerized alkane molecular class also fall consistently along this common branch. Large deviations from the common branch are observed only for molecules bearing the aromatic functionality, and for the unsaturated alkanes; ethylene, acetylene and iso-butene. The distinctiveness of these deviations, and the fundamental ordering responsible for the common branch are explained by the analysis of chemical kinetic model calculations. It is further shown, that any distinctive chemical kinetic character of higher hydrocarbon (>C4) burning velocities, no matter the functionality, can be apportioned by the relative mass fraction of methyl (CH3), methylene (CH2) and Benzyl (C6H5CH2) molecular fragments present in the fuel structure. Finally, the reverse of this fundamental analysis is utilized to make quantitative predictions of the laminar burning velocity for a series of real aviation fuels and gasolines, by the quantitation of their molecular fragments via Nuclear Magnetic Resonance Spectroscopy. It is shown that this approach can provide accurate and rapid predictions, superior to those of detailed chemical kinetic models, for combustion properties of real transportation fuels that are important to the performance and design of engine technologies.