A new single-step mechanism for hydrogen combustion

A single-step chemical-kinetic mechanism is developed that provides good predictions of laminar burning velocities and auto-ignition times. Reasonably accurate adiabatic flame temperatures and (consequently) total amounts of heat release are first obtained through asymptotic expansions of equilibrium expressions for the production of H, O, and OH radicals from the stable products, H2O 2 O along with H2 2 and O2. 2 . By ignoring the inner flame structure, this yields minimal computational stiffness for a wide range of equivalence ratios and pressures. In the single-step rate expression, a passive scalar carrying the radical pool is then introduced that enables reasonable laminar flame structures and burning velocities to be calculated. An additional passive scalar measuring pre-heat-release radical build-up serves to track auto-ignition properly as well, thereby providing reasonable predictions for time-dependent as well as steady-state conditions. The results from this computationally convenient formulation are useful for describing a number of combustion processes, including counter-flow flames. Novelty and significance statement Hydrogen combustion plays a key role in the carbon-free energy transition. The CPU cost/accuracy balance between detailed and reduced chemistries makes it hard to get the full approval of the CFD community. This balance is more likely in single-step mechanisms due to errors in flame temperature predictions and narrow application conditions. In this study, we apply the novel formalism of varying stoichiometric coefficients to construct a single-step mechanism that accurately reproduces adiabatic temperatures. A global reaction rate constructed from the flame chain-branching analysis and the flammability limits is proposed. Furthermore, an optional passive scalar coupling is presented to extend the capabilities of the present mechanism to autoignition predictions. This work provides an efficient and accurate alternative scheme for CFD hydrogen combustion applications, valid for a wide range of conditions.