Rate-controlled constrained equilibrium for large hydrocarbon fuels with NTC

Abstract

The rate-controlled constrained equilibrium (RCCE) method is a thermodynamic-based dimension reduction method that enables the representation of a reactive system involving chemical species in terms of a smaller number, , of constraints and thus reduces the computational burden imposed by detailed chemical kinetics. The application of RCCE for large hydrocarbon fuels like n-heptane has rarely been reported due to the challenges in the specification of constraints. In this study, a systematic approach of constraint specifications for hydrocarbon fuels with negative temperature coefficients behaviour has been proposed. Specifically, the bimolecular species lumping consistent with partial equilibrium assumption (PEA) makes the thermodynamics-based constrained equilibrium manifolds a good approximation to the actual kinetics-controlled slow invariant manifolds in reactive systems. Sensitivity-aided optimisation for species constraints has been formulated to improve the prediction of two-stage ignition. Two sets of optimised constraints for high- and low-temperature ignitions e.g., High-T C and Low-T C with 47 and 48 constraints have been developed from an 88-species n-heptane skeletal mechanism. The method has been successfully demonstrated in the two-stage auto-ignition of n-heptane/air mixture over a wide range of pressures and temperatures. The results show that the dynamics predicted with RCCE agree well with that from the full description. The one-dimensional laminar flame propagation simulation demonstrates that the constraints from auto-ignition can be applied for flame simulations and the RCCE reduced description of flame is less sensitive to the selection of constraints. Moreover, a more compact RCCE description with only 16 species constraints is capable of accurate prediction of the flame propagation process.