Combustion properties of n-heptane/hydrogen mixtures

The possibility to operate current diesel engines in dual-fuel mode with the addition of hydrogen can be limited by the variation in the combustion properties of the fuel mixture. In the present work, n-heptane was selected as a representative fuel to test the effects of hydrogen addition on the laminar flame speeds and ignition delay times. The spherical bomb technique was used to derive the laminar flame speeds of (n-heptane + hydrogen)/air mixtures (0%, 25%, and 50% hydrogen in the fuel) for an initial temperature of 294 K, pressure of 1 bar, and for equivalence ratios between 0.8 and 1.35. The results showed that average increases of 3% and 10% in the flame speeds were obtained with 25% and 50% hydrogen-enrichment, respectively, while a slight decrease of the Markstein length was obtained. Similar laminar flame speed results were predicted numerically with two kinetic models available in the literature with remarkable accuracy, especially for the Cai and Pitsch model [Cai L, Pitsch H. Combust Flame 2015; 162:1623–37]. The kinetic model was subsequently used to perform additional sensitivity and reaction pathway analyses that showed how the chemistry of n-heptane is not substantially influenced by the presence of hydrogen; while the increase in the flame speed is mainly due to the higher concentrations of radical intermediates. The ignition delay times were measured using the reflected shock tube technique for equivalence ratios equal to 0.832, 1.000, and 1.248, initial nominal pressure of 20 bar, temperatures between 730 K and 1200 K, and for different percentages of hydrogen in the fuel (20%, 50%, and 75%). The Cai and Pitsch model once again did a good job of reproducing the experimental data, indicating how at high temperatures the addition of hydrogen does not significantly affect the ignition delay; and in the NTC region (810 K–920 K) the mixtures composed of (50% n-heptane + 50% hydrogen) and (25% n-heptane + 75% hydrogen) are considerably slower than the reference n-heptane case. This is linked to the concentration of the alkane component and the related low temperature chemistry.