Combustion characteristics of C4 iso-alkane oligomers: Experimental characterization of iso-dodecane as a jet fuel surrogate component

Abstract Global combustion characteristics of iso-dodecane (2,2,4,6,6-pentamethylheptane, iC12) are measured and compared to those of iso-octane (2,2,4-trimethylpentane, iC8), iso-cetane (2,2,4,4,6,8,8-heptamethylnonane, iC16) and a 50/50 molar blend of iC8/iC16. The fuels are experimentally characterized by measuring combustion property targets (derived cetane number and smoke point/threshold sooting index), reflected shock ignition delay times at 20 and 40 atm, and extinction limits of strained laminar diffusion flames at 1 atm. The derived cetane number (DCN) and threshold sooting index (TSI) of iC12 are measured to be 16.8 (±1) and 15.4 (±0.5), respectively. In addition to average molecular weight (MW) and overall hydrogen-to-carbon ratio (H/C ratio), the combustion property targets for iC12 are very nearly molar averages of those for iC8 and iC16. Values agree very well with the measured combustion property indicators for the 50/50 blend of iC8/iC16. Further analysis of fuel chemical functional group distributions also finds that the abundances of methylene and methyl groups in iC12 and the 50/50 blend of iC8/iC16 are identical, further supporting that the global combustion characteristics of iC12 can be emulated by the molar averaged mixture of iC8 and iC16. Measurements of reflected shock ignition delays show that the ignition delay times of iC12 are in close agreement with those of the 50/50 molar mixture of iC8/iC16 over a broad range of temperature conditions. It is also found that the ignition delays of the three neat iso-alkanes exhibit quantitatively identical behaviors for both high and low temperature regimes, an observation that can guide the construction of combustion kinetic models for iC12. Measurements of strained diffusion flame extinction and their interpretation by the transport-weighted enthalpy (TWE) metric also support that the high temperature chemical kinetic potentials of the three iso-alkanes are essentially identical. Moreover, it is shown that molecular diffusivity (inherent in molecular weight) is the major parameter that differentiates flame extinction behaviors amongst the three iso-alkanes. In summary, this experimental study further supports the utility and characteristics of combustion property target formulation concepts for producing mixtures that emulate the pre-vaporized global combustion properties of a specific target fuel. The work also points to a strong correlation of the combustion property target data for these iso-alkanes with methylene and methyl functional group distributions for each fuel.