A hybrid chemical source treatment for non-premixed combustion simulations

Different approximations based on the exact point implicit (EPI) chemical source treatment, including diagonal approximation (DA), multiscale diagonal approximation (MDA), and single-scale approximation (SA), are numerically assessed and theoretically analyzed with zero-dimensional and one-dimensional combustion simulations in detail. The results show that EPI performs the best in stability, accuracy, and mass conservation. For DA and MDA methods, the ignition delay time, equilibrium temperature, and mass conservation are predicted with a significant error when the time step is large due to the diagonally non-dominant coefficient matrix. However, the error is relatively small when the reactions are weak, thus rendering the accuracy acceptable. SA method performs best in the stability perspective, but the temporal evolution of ignition can not be predicted appropriately. Based on the above conclusions, a hybrid chemical source treatment(HCST) is proposed, where the EPI method is used in chemically active regions while the MDA method is adopted in chemically inactive regions. HCST is first examined with one-dimensional non-premixed flame of various fuels. Then, a Reynolds-averaged Navier–Stokes (RANS) simulation of a supersonic hydrogen/air combustion case is conducted. The same accuracy is achieved by HCST as that using the EPI method for the whole flowfield. In terms of efficiency promotion, for the hydrogen/air 9 species 19 reactions mechanism, an overall promotion of 17% is achieved, while for methane/air 53 species 325 reactions mechanism, a promotion of 75% is observed.