A numerical investigation of extinction and ignition limits in laminar nonpremixed counterflowing hydrogen-air streams for both elementary and reduced chemistry

The structure, extinction and ignition of counterflow hydrogen heated-air systems are investigated for pressures and temperatures of interest in high-speed aerospace applications. A critical assessment of the recent literature is made to determine a chemical-kinetic scheme applicable for the aforementioned conditions of interest. Numerical integrations are performed for various strain rates using a continuation code which handles the singularities in the Jacobian matrix at critical strain-rate parameters. The numerical results with the detailed chemical scheme are compared with results obtained from a four-, three-, and two-step reduced chemical-kinetic scheme. Although good agreements for extinction and ignition strain rates are obtained, the two-step mechanism provides rather poor predictions of minor-species concentrations. Thermal diffusion is found to play an important role in diluted flames at high oxidizer temperatures and low fuel temperatures, for reasons identified herein, but to be relatively inconsequential at high fuel temperatures or in pure hydrogen-air systems. Suggestions are offered for the use of reduced chemical schemes in applications.