A study of propagation of spherically expanding flames at low pressure using direct measurements and numerical simulations

The outwardly propagating spherical flame (OPF) method is widely used to measure the laminar burning velocity (LBV). However, there are still numerous challenges to perform unambiguous measurements for extreme conditions, mainly for high and low (sub-atmospheric) pressures. In this work, the density weighted displacement speeds relative to burned (S_d,b˜) and fresh gases (S_d,u˜) are considered to report LBV for a large range of sub-atmospheric pressure (from 1 to 0.3 bar), equivalence ratio and fuels (methane and n-decane). These stretched flame speeds are obtained from experiments by using advanced optical diagnostics and the spherical flame propagation is simulated by the code A-SURF with different kinetic schemes to perform a direct comparison with the experimental results. It is shown that S_d,b˜ is significantly affected by the pressure decrease and becomes strongly non-linear with flame stretch. This is explained by the finite flame thickness structure and qualitatively modeled by the finite thickness expression (FTE) model used for extrapolation at zero stretch. However, assumptions (equilibrium and static burned gases) made to report S_d,b˜ are no longer valid and leads to strong under-prediction of LBV from extrapolation especially at low pressures. The shape of S_d,u˜ is less impacted by the pressure decreases and a linear behavior with flame stretch is globally found even for strong sub-atmospheric conditions. However, the temperature of the fresh gas isotherm where S_d,u˜ is extracted is not rigorously equal to the fresh gas's temperature, and a correction factor is required to get accurate LBV. Therefore, a direct comparison of S_d,u˜ instead of extrapolated LBV between experimental and numerical data seems necessary to validate kinetic schemes at low pressures. A brief discussion is conducted on the validation of FFCM-1 and GRI3.0 for methane and Dryer mechanisms for n-decane at low pressures.