Role of ozone addition on premixed hydrogen/oxygen flames: Multi-zone structure and multi-regime dynamics

This study investigates the effects of additive ozone on the ignition and propagation of premixed hydrogen/oxygen flames, with emphasis on the steady flame structure and the transient ignition and propagation behaviors due to the ozone chemistry and transport. We first simulate the 1D freely-propagating hydrogen/oxygen flames with and without additive ozone, and demonstrate that the ozone-enhanced flame has a triple reaction layer structure, which consists of an ozone reaction layer, with substantial heat release, situated within the entire preheat zone. This spatially non-monotonic nature of the heat release rate and the associated temperature gradient then implies that the conventional definition of the flame thickness, δ=(T_b-T_u)/(dT/dx)_max, is incapable and indeed inappropriate to characterize the ozone-enhanced flame thickness. The multi-zone flame structure then leads to multi-regime flame dynamics, with the structure of a single ozone-sustained branch followed by decoupled and coupled double branches controlled by ozone and oxygen reactions respectively. The critical radius and the transition regime of the ozone branch are captured over a wide range of equivalence ratios, which are induced by the large Lewis number of ozone and the interaction between the ozone and oxygen branches. As such, the regime diagram of the flame propagation speed versus the stretch rate respectively corresponds to three to six regimes in the ozone branch, and zero to five regimes in the oxygen branch under ultra-(fuel)-lean, fuel-lean, stoichiometric and fuel-rich conditions. The modification of the minimum ignition energy in ozone–assisted flames is also noted.