The impact of reduced chemistry on auto-ignition of H2 in turbulent flows

The auto-ignition behaviour of hydrogen in a turbulent flow field has been studied through a combination of detailed and systematically reduced chemistry with a transported PDF approach closed at the joint-scalar level. Radiation is accounted for through the RADCAL method and the inclusion of enthalpy into the joint-scalar PDF. Molecular mixing is closed using the modified Curl's model with the mixing frequency accounted for via two algebraic closures. The main aim of the work is to compare the impact of alternative chemical mechanisms on auto-ignition and to explore the accuracy that can be expected when reactive scalars are sequentially removed through the application of quasi-steady-state approximations (QSSAs). Two different detailed mechanisms were tested to establish the effects of intrinsic uncertainties in the detailed chemistry and to provide reference points to past work. The mechanisms feature nine solved species and 19 or 20 reversible chemical reactions. The chemical mechanisms were subsequently systematically reduced to five, four and three independent scalars through the successive introduction of QSSAs for H₂O₂, HO₂ and O. Resulting inaccuracies were quantified following each simplification step with reference to experimental data obtained in shock tubes and under turbulent flow conditions in the Cabra burner configuration. A sensitivity analysis was also performed to identify the relative impact of uncertainties in key reactions as compared to systematic simplification process. It was found that alternative recommended rates for the O + H₂ = OH + H reaction have an impact on the point of flame stabilization that is similar to that observed as a consequence of the simplification process. The work also shows that realistic results can be obtained with simplified chemistry. However, it is also concluded that the temporal evolution of the radical pool and the point of stabilization is affected by the introduction of a QSSA for the O radical. Furthermore, it is shown by comparisons with time resolved OH radical data obtained in shock tubes that the progressive elimination of species via QSSA leads to a shortening of ignition delay times and that the same effects are present, but less severe, in turbulent flow fields.