Impact of oxygen levels and end-of-injection rate profiles on combustion recession dynamics in n-dodecane spray flames under engine-like conditions

Stringent environmental regulations have driven extensive research to reduce emissions from large marine engines. Combustion recession, which consumes unburned hydrocarbons, has been identified as a key mechanism for decreasing emissions. This study investigates combustion recession dynamics in the Engine Combustion Network Spray A flame through the use of large eddy simulations, focusing on the effects of oxygen (O2) levels (21 % and 10 %) and end-of-injection profiles on combustion recession. The numerical model successfully captures distinct combustion recession phenomena after the end-of-injection. Results indicate that higher ambient O2 levels and slower ramp-down profile during injection amplify combustion recession, which is beneficial for reducing unburned hydrocarbon emissions. Under 10 % O2, slower chemistry reaction rates, accompanied by reduced heat release and slower temperature increases, delay high-temperature auto-ignition compared to conditions with 21 % O2. Auto-ignition occurs earlier with a slow ramp-down injection profile than with a faster one. The slow ramp-down profile reduces entrainment, leading to a higher heat release and a more favorable Damköhler number, thereby promoting auto-ignition and combustion recession. These findings enhance the understanding of combustion recession dynamics and provide a foundation for optimizing engine designs to meet stricter environmental standards.