Numerical investigation on combustion characteristics of laminar premixed n-heptane/air flames at elevated initial temperature and pressure

Abstract

Freely-propagating laminar premixed n-heptane/air flames were modeled using the Lawrence Livermore National Laboratory (LLNL) v3.1 n-heptane mechanism and the PREMIX code. Numerical calculations were conducted for unburned mixture temperature range of 298–423 K, at elevated pressures 1–10 atm and equivalent ratio 0.6–1.6, and the changes of laminar burning velocity (LBV), adiabatic flame temperature (AFT), heat release rate (HRR), and concentration profiles of important intermediate species were obtained. The results show that the overall results of LBVs of n-heptane at different elevated temperatures, pressures, and equivalence ratios are in good agreement with available experimental results. However, at the initial temperature 353 K, the calculated values of LBVs at pressure 1 atm and the 10 atm deviate significantly from the experimental results. The sensitivity analysis shows that, similar to many other hydrocarbon fuels, the most sensitive reaction in the oxidation of n-heptane responsible for the rise of flame temperature promoting heat release is R1 H + O2<=>O + OH, and the reaction that has the greatest influence on heat release is R8 H2O + M<=>H + OH + M. In addition, when the initial temperature is 353, 398 and 423 K, the mole fractions of H, OH, and O increase rapidly around the flame front, while the mole fractions of C1C3 dramatically decreases, reflecting the intense consumption of the intermediate products at the reaction zone.