Rob
Ansys Employee

I don't think you're doing anything wrong as such. The first hydrogen inlet triggers a reaction with the oxygen supply. That gives a very high temperature reason, and may or may not use up a large amount of oxygen. You then get an unstable flame in the system.  Without knowing what the other settings (inlet, material density etc) I can't really guess what's going on, but suspect the model is OK, but possibly missing something when compared to reality.