Computed flammability limits of opposed-jet H-2/O-2/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 r ange of flammable stretch rates. The flame characteristics and the extincti on limits at a low pressure of 1.013 kPa are sought, with: the amount of ca rbon dioxide dilution level and stretch rate as parameters. The conditions studied are particularly relevant to Mars exploration. In addition, a flamm ability map is presented using these two parameters as coordinates. Both th e high-stretch blowoff and the low-stretch quenching limits are found. The existence of an absolute carbon dioxide dilution limit, above which the dif fusion flame is not possible, is demonstrated. Low-stretch diffusion flames at low pressures are unusually thick, with flame temperatures substantiall y 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 kinetic s. One of the novel features of the application of the narrowband radiation model is the inclusion of Doppler broadening, which is shown to be importa nt in low pressure flames.