Experimental and modeling investigation of the laminar flame speeds for ammonia with various oxygen and diluent mixtures

The study utilizes two spherical bombs to explore the Laminar Flame Speed (LFS) and Markstein length of oxygen-enriched ammonia flames and ammonia combined with different oxidizers (N 2 with 21% and 30% O 2 and He with 21% O 2 ). The experiments cover a range of initial conditions, exploring temperatures from 309 K to 423 K and an equivalence ratio spanning from 0.8 to 1.3, all at an initial pressure of 1 bar. At the same time, three established literature mechanisms (Stagni, Shrestha, and Zhang) are used to simulate the data and perform kinetic analysis. A detailed analysis focuses on how the radius of the domain affects LFS, highlighting the importance of a larger domain, in contrast with findings from previous literature. The findings highlighted that, although LFS increased (as expected) with the increase in the initial temperature, the Markstein length remains largely unaffected in oxygen-enriched ammonia flames. This was also calculated using Bechtold and Matalon 's approach, and the results were in a good agreement with the experimental values. The study determines alpha exponents across an equivalence ratio range of 0.8 to 1.3, and also found that the inert gas has varying effects on both LFS and Markstein length. Buoyancy effects were noticeable in the ammonia/air experiments, particularly at lower temperatures, challenging existing literature heavily focused on ammonia blends at temperatures below 400 K, potentially contributing to data discrepancies observed in prior studies.