Experimental and numerical study on premixed partially dissociated ammonia mixtures. Part II: Numerical study of premixed combustion characteristics

Abstract

Adding dissociated ammonia (NH3) is an effective way to improve the combustion properties of NH3. Research on the laminar burning velocity of partially dissociated NH3 mixtures was shown in part I. In order to further explore the feasibility of enhancing ammonia combustion by adding dissociated ammonia, several essential combustion properties were numerically studied in part Ⅱ of this study. The burned gas Markstein length was obtained by the experiment and theoretical calculation, and the relevant combustion parameters such as adiabatic flame temperature, heat release rate and NO formation were calculated by planar flame simulation. Results show that the Markstein length increases with the increasing dissociating degree on the lean side, but decreases under rich conditions, which is mainly due to the determination of flame thickness, effective Lewis number and Zel’dovich number. The addition of dissociated NH3 increases the mixture heating value, but the combustion efficiency decreases slightly. The primary exothermic elementary reactions in the combustion are gradually replaced by the reactions in the H2 system, and the exothermic process shifts to a lower temperature with the increase of dissociating degree. NO is the main NOx emission, the formation of NO is determined by the concentration of the O/H radicals and NH3, its mole fraction first increases and then decreases with the increase of dissociating degree. Moreover, the NO emission on the rich side only decreases slightly under high dissociating degree conditions, which is different from the significant plummet under the low and medium dissociating degree conditions.