Experimental study of combustion process of NH3 stratified spray using imaging methods for NH3 fueled large two-stroke marine engine

Abstract

To achieve greenhouse gas emission reductions from the maritime sector, there is a growing demand for the development of NH3 fueled marine engines. Since NH3 is difficult to ignite and burn, it is valid to assist the NH3 combustion by a supporting fuel, which is easily ignited and burned. A new NH3 spray combustion concept for a large two-stroke marine engine has been proposed, which is a “three-layer stratified fuel injection comprising supporting fuel / liquid NH3 / supporting fuel (i.e., NH3 stratified injection)”. The advantages of the NH3 stratified injection concept are the symmetrical combustion support effect in the spray axis at important timing (ignition and late combustion). The supporting fuel used was n-hexadecane (nC16H34, cetane). The characteristics of the NH3 stratified spray under non-combustion conditions were investigated in an optically accessible constant volume combustion chamber using a reflective shadowgraph imaging system. The spray penetration length and the spray cone angle were similar to those of the 100% nC16H34 spray and typically estimated equations. Moreover, the combustion process of an NH3 stratified spray was investigated using high-speed direct imaging. The flame lift-off position moved downward and the rapid combustion by the third layer was different compared to those of the 100% nC16H34 spray. The burnt gas lump was confirmed to have initially played an important role in the flame lift-off position. The rapid combustion stems from the multipoint ignition, flame propagation, and downward movement of the flame by the jet stream. Additionally, the NH3 combustion in the NH3 stratified spray was investigated using imaging spectroscopy. The spectra of chemiluminescence revealed that the NH3 combustion progressed after forming the initial burnt gas lump, and the NH2* ammonia α-band was observed in the combustion area. Consequently, the combustion process of an NH3 stratified spray was clarified from these experimental results.