Experimental and numerical study of combustion characteristics of ammonia/ethanol mixture under high temperature and pressure

Ammonia, as a carbon free fuel, is expected to play a key role in carbon neutrality and social energy shortages. However, the flame propagation speed of ammonia is low. Blending ammonia with ethanol is one of the methods to increase laminar burning velocity of ammonia, but the existing research on the combustion characteristics of ammonia/ethanol focuses on low temperature and pressure. This paper investigated the effect of ethanol addition on ammonia combustion characteristics under high temperature and pressure. The initial conditions for this work include initial temperature (400, 450, and 500 K), initial pressure (0.5, 1.0, 1.5 MPa), equivalence ratio (0.8–1.3), and ethanol energy fraction (0 %, 20 %, 40 %, 60 %, 80 %). The results illustrate that the laminar burning velocity has a positive correlation with the ethanol energy fraction and the initial temperature, but is negative correlation with initial pressure. And the laminar burning velocity and the adiabatic flame temperature appear to be maximum when the equivalence ratio is between1.0 and 1.1. The strongest hydrodynamic instability occurs within the equivalence ratio range from 1.0 to 1.1. In addition, initial pressure increased flame hydrodynamic instability and initial temperature has little effect on it. The changes in initial pressure and temperature have little effect on thermal diffusion instability. When the ethanol energy fraction is less than or equal to 20 %, ammonia has a significant impact on the Markstein length, but ethanol dominates the Markstein length with a proportion of ethanol energy over 40 %. Under current experimental conditions, the existing ammonia/ethanol mixing mechanisms cannot comprehensively and accurately predict laminar combustion velocity. The CEU mechanism is relatively accurate, especially under lean combustion and high pressure. The three most sensitive reactions to laminar burning velocity are H + O2<=>O + OH, H + O2(+M)<=>HO2(+M), and NH2 + NO<=>NNH + OH. When adding 40 % ethanol compared to 20 % ethanol, the molar fraction of OH and O free radicals is higher, and the peak value of NO total ROP is higher.