Experimental and Kinetic Investigation of Pressure and Temperature Effects on Burning Characteristics of n-Heptane/Air/Hydrogen up to Near Lean Burn Limits

Incomplete-combustion and misfire are the hurdles in internal combustion engines to run on ultra-lean mixture, whereas high thermal efficiency has been achieved at lean mixture. The burning characteristics of n-heptane with 0% and 30% hydrogen additions were studied at 393K-453K and 100kPa-300kPa up to near lean burn limits, λ=0.8-2.0. The flame appeared in spherical shape only by 37-mJ ignition energy (IE) at λ=0.8-1.5, while further lean mixture, ≥1.6, could be ignited only by 3000-mJ with the distorted flame shape. The flame buoyed in the mixture when burning velocity calculated by kinetic mechanism was equal or less than 19.83 cm/s at the initial conditions of λ=1.8, 393K and 100kPa. The thermal instability under impact of initial pressure and temperature was higher at lean mixture than at stoichiometric mixture. The ultra-lean mixtures from λ=1.8 to λ=1.6 at 393K, λ=1.9 to λ=1.7 at 423K and λ=2.0 to λ=1.8 at 453K were under the impact of buoyancy when initial pressure was increased from 100kPa to 300kPa (higher initial pressure, more λ under buoyancy effect). The change in initial temperature, from 393K to 423K, decreased the buoyancy region from λ=1.8 to λ=2.0 at 100kPa, λ=1.7 to λ=1.9 at 200kPa and λ=1.6 to λ=1.8 at 300kPa. OH mole fraction linearly correlated to the burning characteristics. It was concluded that the quantity of OH mole fraction from 8.4447×10^-3 to 5.2028×10^-3 for H₂=0% and 8.6274×10^-3 to 5.2494×10^-3 for H₂=30% was sufficient in the mixture to initiate the combustion by 37mJ IE at 393K and 100kPa, whereas the OH mole fraction quantity in the mixture less than 5.2028×10^-3 for H₂=0% and 5.2494×10^-3 for H₂=30% required more ignition energy of 3000mJ. Buoyancy appeared when the quantity of OH mole fraction in the mixture was ≤3.0159×10^-3 for H₂=0% and ≤3.0556×10^-3 for H₂=30% at 393K and 100kPa.