Effect of H2 and O2 enrichment on the laminar burning velocities of NH3+H2+N2+O2 flames: Experimental and kinetic study

Abstract

Ammonia (NH3) has gained broad attention as a carbon-free alternative fuel, and its blend with H2 has been especially investigated to mediate the low reactivity of NH3. In the present work, the laminar burning velocities (SL) of NH3+H2+N2+O2 flames were investigated at 1 atm and 298 K, and the oxygen fractions (xO2) in O2+N2 were varied from 0.16 to 0.26, comprising a set of conditions that have not yet been investigated in literature. An updated mechanism was proposed on the basis of our previous published mechanism, which was validated using not only the present data, but also the literature data for NH3 laminar burning velocities and ignition delay times. Over the tested experimental conditions, better predictions were achieved with the updated mechanism. Simulations using the updated and five literature mechanisms were carried out, i.e., mechanisms from Stagni, Bertolino, Shrestha, Konnov, and Mei. The updated and Mei mechanisms reproduce the best the present experimental data. To investigate which reactions influence most the SL increments due to O2 and H2 addition, SL increment parameters were defined for xO2 and xH2 separately. Based on the available experimental data, detailed sensitivity analyses were carried out for these target parameters using the updated and some other mechanisms from the literature. It's found that reactions dominating these parameters could be largely different, thus considerations covering all aspects of SL as well as the SL increment parameters are suggested for the NH3 mechanism validations. Despite the SL increment parameters have different tendencies against the xO2 and xH2 with different reaction sensitivities, local similarities are found if the increments of xO2 and xH2 have been enough refined. Specifically, when the xO2 and xH2 values grow larger, the effects of their increase become well blurred, extending the region of the similar conditions, which is helpful for the determination of inconsistent experimental data.