Lean and ultralean stretched propane–air counterflow flames

Stretched laminar flame structures for a wide range of C3H8–air mixtures vs hot products are investigated by laser-based diagnostics and numerical simulation. The hot products are produced by a lean H2–air premixed flame. The effect of stretch rate and equivalence ratio on four groups of C3H8–air flame structures is studied in detail by Raman scattering measurements and by numerical calculations of the major species concentration and temperature profiles. The equivalence ratio, ϕ  , is varied from a near-stoichiometric condition (ϕ=0.86)(ϕ=0.86) to the sublean limit (ϕ=0.44)(ϕ=0.44) and the stretch rate varies from 90 s−1 to near extinction. For most of these C3H8–air lean mixtures, hot products are needed to maintain the flame. The significant feature of these flames is the relatively low flame temperatures (1200–1800 K). For this temperature range, the predicted C3H8–air flame structure is sensitive to the specific chemical kinetic mechanism. Two types of flame structures (a lean self-propagating flame and a lean diffusion-controlled flame) are obtained based on the combined effect of stretch and equivalence ratio. Three different mechanisms, the M5 mechanism, the Optimized mechanism, and the San Diego mechanism, are chosen for the numerical simulations. None of the propane chemical mechanisms give good agreement with the data over the entire range of flame conditions.