Kinetic effects of toluene blending on n-decane diffusion flame extinction limit

Understanding of the impact of kinetic and transport coupling on the combustion characteristics for alkane and aromatics blended fuels is of great importance to construct a reliable surrogate fuel model for JP-8 fuels. The diffusion flame structures and extinction limits of n-decane/toluene/nitrogen mixtures were studied experimentally and computationally through the use of counterflow diffusion flames with nitrogen dilution. The impact of toluene addition to n-decane on the extinction limit, OH distribution, and maximum heat release were investigated. The results showed that as the toluene blending ratio was increased, the extinction strain rate decreased significantly. It was also found that the extinction limit depends linearly on the radical pool for the blended fuels. Planar laser induced fluorescence measurements of OH showed that the maximum concentration of OH found experimentally was more sensitive to toluene addition than found in numerical computations. The maximum chemical heat release rate showed that CO + OH → CO₂ + H was one of the dominant exothermic reaction pathways, indicating that the extinction limits are strongly coupled with OH concentration. Numerical simulations demonstrated that the radical pool decreased, particularly the OH and H concentration via abstraction reactions of toluene, led to rapid approach to the extinction limits. The extinction strain rate of the blended fuels was analyzed and based on the variation of the radical pool concentration, which can be represented by the H/C ratio of blended fuels.