The Rate-Controlled Constrained-Equilibrium combustion modeling of n-butane/oxygen/diluent mixtures

Rate-Controlled Constrained-Equilibrium (RCCE) is a model order reduction method which assumes that the non-equilibrium states of a system can be described by a sequence of constrained-equilibrium states subject to a small number of constraints. It can be used to predict ignition delay time with good accuracy and low computational cost. In this paper RCCE approach has been further developed for applying to the oxidation of n-butane for ignition study and prediction of a constant volume, constant internal energy system over a wide range of initial temperatures, pressures and equivalence ratios. The USC-Mech II (109 species and 781 reactions, without nitrogen chemistry) is chosen as chemical kinetic mechanism for n-butane oxidation for Detailed Kinetic Model (DKM). The constraint selection for n-butane/oxygen mixture starts from the eight universal constraints for carbon-fuel oxidation. Additional species constraints are selected based on researchers' experience to have the best performance with the minimum number of constraints. The selected 17 constraints have been used to predict ignition delay times for butane combustion. The results of RCCE method are compared with those of detailed kinetic model and experimental data to verify the effectiveness of constraints and the efficiency of RCCE. Rate-Controlled Constrained-Equilibrium results show good agreements with DKM results under different initial temperatures, pressures and equivalence ratios. Even better performance than DKM has been achieved by the selected 17 constraints when the results are compared with shock tube experimental data from literature with initial temperatures 1200-1500 K and initial pressures 2-20 atm. It has been applied to predict the ignition delay time of butane/air mixture over a wide range of initial temperatures, initial pressures and equivalence ratios.