Computed flammability limits of opposed-jet H₂/O₂/CO₂ diffusion flames at low pressure

A narrowband radiation model is coupled to the OPPDIF program to enable the study of one-dimensional hydrogen-oxygen diffusion flame over the entire range of flammable stretch rates. The flame characteristics and the extinction limits at a low pressure of 1.013 kPa are sought, with the amount of carbon dioxide dilution level and stretch rate as parameters. The conditions studied are particularly relevant to Mars exploration. In addition, a flammability map is presented using these two parameters as coordinates. Both the high-stretch blowoff and the low-stretch quenching limits are found. The existence of an absolute carbon dioxide dilution limit, above which the diffusion flame is not possible, is demonstrated. Low-stretch diffusion flames at low pressures are unusually thick, with flame temperatures substantially below those of the adiabatic flames. This large temperature drop results from the combined effect of flame radiation and limited gas residence time in the flame, and may be particular to the hydrogen-oxygen chemical kinetics. One of the novel features of the application of the narrowband radiation model is the inclusion of Doppler broadening, which is shown to be important in low-pressure flames.