Ammonia-PODE dual-fuel direct-injection spray combustion: An optical study of spray interaction, ignition and flame development

Ammonia-polyoxymethylene dimethyl ether (PODE) dual-fuel direct-injection (APDI) is considered an effective scheme for improving ammonia combustion. The APDI scheme involves multiple physicochemical processes of spray-spray interaction, spray-flame interaction, and flame-flame interaction. However, these processes are not well-explained and require clarification through advanced diagnostic methods. In this study, the shadowgraph technique is employed to elucidate the spray interaction and evaporation characteristics. Simultaneous high-speed OH∗, NH₂∗ chemiluminescence and flame luminosity imaging are applied to describe ignition and flame development. The results demonstrated the successful ignition of ammonia in the APDI scheme, with PODE first auto-igniting and subsequently igniting the ammonia mixture. Upon ammonia ignition, OH∗ formed first followed by NH₂∗. Throughout the combustion process, OH∗ exhibited longer duration and wider distribution with prominent signal regions at the periphery and core of the spray head; whereas NH₂∗ dissipated earlier with smaller distribution primarily located in the spray head. The spatial distribution of OH∗ and NH₂∗ indicated reactivity stratification in both strategies: 1) the high-reactivity region at the spray core, dominated by combustion of ammonia-PODE mixture; 2) the low-reactivity region at the spray periphery, dominated by the ammonia diffusion combustion. The injection interval between ammonia and PODE affected flame development, with simultaneous injection (APDI-S) resulting in flame development aligned with the ammonia spray direction, while pilot-PODE injection (APDI-P) led to vertical flame development. Finally, it was proposed that the longer overlapping injection duration between the two fuels resulted in enhanced combustion efficiency.