Role of enthalpy transport in laminar premixed hydrogen flames at atmospheric and elevated pressures

This work discusses the role of diffusive enthalpy transport in relation to the origin of thermodiffusive instability and the resulting enhanced reactivity. Thermodiffusive effects in premixed hydrogen flames are typically explained and modelled via local equivalence ratio fluctuations. However, it is reiterated here that the imbalance between species and thermal diffusion (differential diffusion), rather than local species-to-species diffusive imbalances (preferential diffusion) is the leading-order effect. Reactant (H$_2$), product (H$_2$O) and intermediate (H) species are demonstrated to all play a role in the transport of enthalpy through an analysis of enthalpy flux divergence terms in unstretched flames. Premixed counterflow flames at various strain rates and pressures are then analysed to demonstrate that enhanced reactivity originates from a combination of enthalpy transport and the broadness of the reaction zone relative to the thickness of the flame. Effects resulting from key pressure fall-off reactions are also discussed to determine the importance of detailed chemistry, and the usage of Zeldovich number. Finally, two-dimensional planar flames are simulated and analysed to demonstrate the role of curvature in addition to strain rate, and the implications of the findings in blends and turbulent flames are discussed.