Combustion Characteristics of Iso-Octane/Hydrogen Flames under T and P Effects up to near Flammability Limits

Lean combustion is an approach to achieving higher thermal efficiency for spark ignition engines. However, it faces low burning velocity and unstable combustion problems near the lean flammability limits region. The current work is attempting to investigate the combustion characteristics of iso-octane flame with 0% and 30% H₂ up to near lean limits (λ = 1.7) at 100-300 kPa and 393-453 K. The flame appeared spherically by 37 mJ spark energy at λ = 0.8-1.2, whereas the ultra-lean mixtures, λ ≥ 1.3, ignited at 3000 mJ under wrinkles and buoyancy effects. The impact of initial pressure and temperature on the lean mixture was stronger than the stoichiometric mixture regarding flame radius and diffusional-thermal instability. The buoyancy appeared at the highest burning velocity of 27.41 cm/s. The buoyancy region extended from λ = 1.5 to λ = 1.3 at 393 K, λ = 1.6 to λ= 1.4 at 423 K and λ = 1.7 to λ = 1.5 at 453 K with an increase in initial pressure (higher pressure, more λ under buoyancy effect), but initial temperature decreased the region from λ= 1.5 to λ = 1.7 at 100 kPa, λ = 1.4 to λ = 1.6 at 200 kPa and λ = 1.3 to λ = 1.5 at 300 kPa. OH mole fraction <7.6642×10^-3 for H₂ = 0% and <7.7765×10^-3 for H₂ = 30% required 3000 mJ for ignition at 393 K and 100 kPa, and buoyancy appeared at ≤4.8788×10^-3 for H₂ = 0% and ≤4.9547×10^-3 for H₂ = 30%.