Experimental and kinetic modeling study of the laminar burning velocity of NH3/DME/air premixed flames

Abstract

Ammonia (NH3) as a carbon-free fuel and a high-density hydrogen carrier has received significant attention. Co-firing NH3 with dimethyl ether (DME) is a promising option for overcoming the low reactivity of NH3. This paper presents an experimental and kinetic modeling study of the laminar burning velocity of NH3/DME/air mixtures. Experiments were conducted for the full range of DME fraction at 0.1 MPa, 298 K, and equivalence ratios (ϕ) from 0.7 to 1.5 using a spherical constant-volume combustion chamber. The influence of pressure was examined for 20% - 80%DME at 298 K and ϕ = 0.7 - 1.4 by considering different initial pressures (0.1, 0.3, and 0.5 MPa). A small size reaction mechanism for NH3/DME/air mixtures was developed and validated against experiments. This mechanism performs well in predicting the laminar burning velocity of NH3/air, DME/air, and NH3/DME/air flames as well as the ignition delay time of NH3, DME, and NH3/DME oxidation. Detailed kinetic analyses using the mechanism helped us understand the observed effects of DME addition and initial pressure on the laminar burning velocity. The linear increase of laminar burning velocity with increasing DME fraction is mainly attributed to the enhancement of both thermal effect and chemical kinetics effect, while the negative influence of initial pressure is because of the decrease of chemical kinetics effect. In addition, it was found that the main reaction pathways based on N-atom shift with DME addition, whereas those based on C-atom vary little. Furthermore, pressure power exponent β as a function of equivalence ratio shows different behaviors for different DME fractions due to the competition between NH2 and CH3 chain-termination reactions for the rich flames. The present work provides experimental measurements of the laminar burning velocity at various conditions and insights into the chemical kinetics for NH3/DME/air mixtures.