Laminar flame speeds of lean hydrogen-oxygen-helium mixtures under elevated pressures and temperatures

Lean H₂/O₂/He laminar spherical flames expanding after spark ignition in the center of a large cruciform burner are investigated using high-speed Schlieren imaging technique. When processing the images, dependencies of equivalent flame radii <R_f> on time are extracted and unperturbed laminar flame speeds S_L⁰ are evaluated adopting four state-of-the-art flame-speed-correction methods. The experimental conditions cover lean mixtures at three equivalence ratios (ϕ = 0.3, 0.45, and 0.6), three pressures (P = 1, 3, and 5 atm), and two unburned gas temperatures (T_u = 300 and 400 K). Besides, the flame speeds are computed adopting seven state-of-the-art chemical mechanisms. The obtained results show, first, that substitution of nitrogen with helium offers the opportunity to suppress diffusional-thermal instability under the studied conditions and to measure speeds of lean hydrogen laminar flames in wider ranges of equivalence ratios and pressures. Second, substitution of nitrogen with helium results in significantly reducing the influence of non-linear (with respect to flame stretch rate) effects on differences between the observed and unperturbed laminar flame speeds, thus substantially improving accuracy of evaluation of S_L⁰ in lean hydrogen mixtures. Third, none of the tested chemical models predict all the experimental data, with differences between measured and computed S_L⁰ being particularly large in preheated (T_u = 400 K) moderately lean (ϕ = 0.45) flames under elevated pressures (P = 3 and 5 atm). Since chemical kinetic mechanisms of lean hydrogen burning have not yet been tested against experimental data on S_L⁰, obtained at T_u = 400 K, the present results call for further assessment and development of such models for elevated temperature conditions, which occur, e.g., in piston engines. Fourth, differences between the measured and computed flame speeds could in part be attributed to limitations of the adopted transport models, thus calling for further assessment and development of them also.