“…Of these mechanisms, ignition kernel formation and autoignitive triple flames have been examined at significant length, and a full discussion is not provided here (Gordon et al, 2008;Mastorakos, 2009;Yoo et al, 2011;Arndt et al, 2012Arndt et al, , 2019Sidey and Mastorakos, 2015;Macfarlane et al, 2017Macfarlane et al, , 2018Macfarlane et al, , 2019Ramachandran et al, 2019). Observations from OH-PLIF (Medwell et al, 2008), and resulting numerical studies (Medwell et al, 2009b;Evans et al, 2016bEvans et al, , 2017a suggest that the least studied of these mechanisms-the "weak-to-strong transition"anchors as a weakly reacting diffusion flame close to the jet exit, in ≈ 0.2 (Evans et al, 2016a) under temperature and flowfield conditions almost identical to the laminar coflow stream. These weak diffusion flames allow for the diffusion of O 2 in the fuel stream (in a process which has been termed, "reactionzone weakening" Medwell et al, 2009b) and provide enhanced formation of precursors, such as CH 2 O (Medwell et al, 2009b;Medwell and Dally, 2012b), before a critical "transition point" where thermal run-away occurs (Medwell et al, 2008;Evans et al, 2016b).…”