Study on lean burn limits and burning characteristics of n-heptane with effects of hydrogen enrichment

Lean combustion can be a more effective method to enhance the thermal efficiency of diesel engine, while also face incomplete-combustion and long-combustion duration issues at ultra-lean conditions. Effects of hydrogen, known with wider flame limits and greater burning velocity, on lean burn limits and burning characteristics of n-heptane were studied fundamentally based on optical experiments and chemical kinetics analysis. The results showed that the laminar flame appeared spherically from λ = 0.8 to λ = 1.5 by spark ignition energy of 37 mJ, whereas leaner mixture, λ ≥ 1.6, could only be ignited by 3000 mJ. However, the low ignition energy failed to burn the ultra-lean mixture, while high ignition energy disturbed the flame morphology. The buoyancy appeared near lean-limits due to lower burning velocity and rapid increase of thermal radiation losses. Lean burn limits of n-heptane increased from λ = 1.9 to λ = 4.2 at 393K, λ = 2.0 to λ = 4.3 at 433K and λ = 2.1 to λ = 4.4 at 473K under H₂ addition from 0% to 90%. The increasing impact became more significant at H₂ ≥ 70%. Besides, as lifting initial temperature, 393–473K, the lean-limits increased 0.2 λ only. With increase of H₂, H increased faster than O and OH because more H₂ reacted with both O and OH to produce H. CH₂O became prominent among C–H–O species, which reacted with H increased rapidly under H₂ addition. Consequently, H and CH₂O played a substantial role to improve the lean burn limits and burning velocity of n-heptane under hydrogen addition.