A numerical investigation of methane ignition in supercritical CO2

Abstract

The supercritical CO2 power cycle (sCO2) is a relatively new technology, which promises to reduce CO2 emissions with potentially higher efficiencies. However, due to challenging conditions posed by supercritical pressures, the ignition phenomena in sCO2 combustion are relatively less understood and studied. The primary objective of the current study is to elucidate ignition processes using homogeneous ignition calculations (HMI) and two-dimensional direct numerical simulations (DNS). To accurately model the supercritical conditions, the employed formulation includes the cubic Peng–Robinson equation of state, mass, and heat flux vectors derived from nonequilibrium thermodynamics and compressible form of governing equations. For selection of a suitable chemical mechanism, HMI calculations are employed to investigate the performance of existing skeletal mechanisms against shock-tube experimental data. The chemical characteristics of ignition are further studied using path flux and sensitivity analysis, with CH3O2 chemistry exhibiting the largest effect on accelerating the ignition process. Different chemical pathways of fuel breakdown are also discussed to aid in interpretation of subsequent DNS case. In the DNS case, autoignition of a two-dimensional mixing layer perturbed with pseudoturbulence is simulated. The ignition is found to be delayed compared to the HMI case, with the ignition kernels forming in a spotty manner. The two phenomena are primarily attributed to variation of scalar dissipation within the mixing layer. The ignition kernels expand and evolve into a tribrachial edge flame propagating along the stoichiometric isosurface. Further investigation on the structure of edge flame revealed an asymmetrical structure, with CH4 molecules being entirely consumed in the triple point region of the flame along the stoichiometric isosurface, and more stable fuels like CO burning in the non-premixed branch of the edge flame. The edge flame propagation speeds are also calculated, with variations found to be correlated with scalar dissipation and upstream progress variable of the reacting mixture.