Visualizing ammonia combustion. From laminar flames to engine application

Abstract

Ammonia is one of the most promising future fuels for high-power combustion systems like large internal combustion engines and gas turbines. The number of publications and applications of ammonia combustion has increased rapidly in recent years. However, many phenomena of ammonia combustion remain unresolved and require further investigation. While fundamental experiments under laminar conditions show a very low reactivity of ammonia, the engine combustion of pure ammonia or ammonia with the addition of a small fraction of combustion promoters provides satisfactory combustion efficiency and stability. To gain a deeper understanding of the phenomena governing ammonia combustion, the LEC and FHNW have developed a methodology using a series of experimental setups to apply optical investigations ranging from quiescent, ambient conditions up to highly turbulent, engine-relevant conditions. The optical data furthermore provide an extraordinary wealth of information to develop and validate detailed simulation models. This paper presents the holistic concept for a methodology that allows the experimental distinction of the effects relevant to ammonia combustion. Concepts for measuring the laminar burning velocity, turbulent burning velocity, turbulent jet ignition, and turbulence-flame interaction under engine-like thermodynamic and flow conditions are described. For each stage of the methodology, a specific optical test rig is designed or adapted to meet the relevant requirements. Selected results are presented, which underline the unique combustion behavior of ammonia and ammonia/hydrogen blends and demonstrate the applicability of ammonia in internal combustion engines. Thermo diffusive instabilities are identified to have a decisive impact on the flame propagation of ammonia/air mixtures.The improvement of the combustion properties at high temperatures and pressures as well as high turbulence levels is reasoned with enhanced reactivity, strong turbulence-flame interaction, and the Lewis number effect. Future perspectives on the insights from optical data into ammonia combustion and the relevance of optical data for developing ammonia combustion models are derived.