Flame structure and kinetic studies of carbon dioxide-diluted dimethyl ether flames at reduced and elevated pressures

Abstract The flame structure and kinetics of dimethyl ether (DME) flames with and without CO 2 dilution at reduced and elevated pressures were studied experimentally and computationally. The species distributions of DME oxidation in low-pressure premixed flat flames were measured by using electron-ionization molecular-beam mass spectrometry (EI-MBMS) at an equivalence ratio of 1.63 and 50 mbar. High-pressure flame speeds of lean and rich DME flames with and without CO 2 dilution were measured in a nearly-constant-pressure vessel between about 1 and 20 bar. The experimental results were compared with predictions from four kinetic models: the first was published by Zhao et al. (2008) [9] , the second developed by the Lawrence Livermore National Laboratory (LLNL) (Kaiser et al., 2000) [13] , and the third has been made available to us as the Aramco mechanism (Metcalfe et al., 2013) [14] ; as the fourth, we have used an updated model developed in this study. Good agreement was found between measurements and predictions from all four models for all major and most typical intermediate species with and without CO 2 addition in low-pressure flat flame experiments. However, none of the models was able to reliably predict high-pressure flame speeds. Although the updated model improved the prediction of flame speeds for lean mixtures, errors remained for rich conditions at elevated pressure, likely due to uncertainty in the rates of CH 3  + H(+M) = CH 4 (+M) and the branching and termination reaction pair of CH 3  + HO 2  = CH 3 O + OH and CH 3  + HO 2  = CH 4  + O 2 . CO 2 addition considerably decreased the flame speed. Kinetic comparisons between inert and chemically active CO 2 in DME flames showed that CO 2 addition affects rich and lean DME flame kinetics differently. For lean flames, both the inert third-body effect and the kinetic effect of CO 2 reduce H-atom production. However, for rich flames, the inert third-body effect increases H-atom production via HCO(+M) = H + CO(+M) and suppression of the kinetic effect of CO 2 by shifting the equilibrium of CO + OH = CO 2  + H.