Dynamics of Premixed Hydrogen/Air Flames in Unsteady Flow

Fully resolved numerical simulations have been conducted for laminar premixed hydrogen-air flames, in order to study the influence of unsteadiness of the flow on the local and global flame dynamics. The flames are excited with oscillating inflows at pre-defined frequencies f to assess the effect of unsteady stretch on flame dynamics, while the Damköhler number is used to characterize the interacting behavior between the flow and the chemical reactions based on their time scales. For both lean and rich flames, the local flame speed Sl is less sensitive to the flame stretch in an unsteady flow, which results in a reduced magnitude of the Markstein number |Ma|. |Ma|is smallest when the time scale of the flow approaches the intrinsic time scale of the flame (Da≈1). The global consumption speed St, computed from integration of the fuel consumption rate over the entire computational domain, yields a phase delay and a damped oscillation with respect to the unsteady inflow. The flame is not able to follow the unsteady flow or change its flame surface at high excitation frequencies with Da<1, and vice versa in the low frequency range with Da>10. An efficiency factor E has been introduced to model the damped response of the flame due to flow unsteadiness, which reproduces the asymptotic behavior of E→0 at Da<1 and E→1 at Da>>1. The simulation results reveal that the flow time scale plays a significant role in elucidating the effect of flame-flow interaction, which should be considered for turbulent combustion modeling.