Experimental characterization of spark ignited ammonia combustion under elevated oxygen concentrations

Due to its nature as a carbon free fuel and carrying hydrogen energy ammonia has received a lot of attention recently to be used as an alternative to fossil fuel in gas turbine and internal combustion engines. However, several barriers such as long ignition delay, slow flame speed, and low reactivity need to be overcome before its practical applications in engines. One potential approach to improve the ignition can be achieved by using oxygen enriched combustion. In this study, oxygen-enriched combustion of ammonia is tested in a constant volume combustion chamber to understand its combustion characteristics like flame velocity and heat re-lease rates. With the help of high speed Schlieren imaging, an ammonia-oxygen flame is studied inside the combustion chamber. The influence of a wide range of oxygen concentrations from 15 to 40% are tested along with equivalence ratios ranging from 0.9 to 1.15. Ammonia when ignited at an oxygen concentration of 40% with an equivalence ratio of Phi = 1.1 at 10 bar has a maximum flame velocity of 112.7 cm/s. Reduced oxygen concentration also negatively affects the flame velocity, introducing instabilities and causing the flame to develop asymmetrically due to buoyancy effects inside the combustion chamber. Heat release rate (HRR) curves show that increasing the oxygen concentration from 21 to 35% of the mixture can help reduce the ignition delays. Peak HRR data shows increased sensitivity to air fuel ratios with increased oxygen concen-trations in the ambient gas. HRR also shows an overall positive dependence on the oxygen concentration in the ambient gas. (c) 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Automated summary

Ammonia is being considered as an alternative to fossil fuel in gas turbine and internal combustion engines due to its carbon-free and hydrogen energy-carrying nature. However, there are barriers such as long ignition delay, slow flame speed, and low reactivity that need to be overcome before its practical applications. This study explores the use of oxygen-enriched combustion to improve the ignition of ammonia and investigates its combustion characteristics in a constant volume combustion chamber.