Dynamic stabilization of a hydrogen premixed flame in a narrow channel

Combustion of hydrogen can help in reducing carbon-based emissions but it also poses unique challenges related to the high flame speed and Lewis number effects of the hydrogen flame. When operated with conventional burners, a hydrogen flame can flashback at higher volumetric flow rates than a methane flame due to the difference in stabilization mechanisms of the two fuels. Due to these differences, conventional burners cannot offer similar operational ranges for hydrogen than that for hydrocarbon flames. An exploration into the unique stabilization behaviour of hydrogen flames is required which could help in envisioning non-conventional burner concepts for keeping hydrogen flames stable. Stability conditions, which describe the kinematics of premixed flames with spatially and temporally changing flow parameters, are crucial for such an exploration. Stability conditions are usually hypothesized for stable flames, where a flame upon perturbation is assumed to return to its original position. Alternatively, in the case of flashback/blow-off, it refers to a flame moving upstream of the burner or being convected out of the domain. However, it is also of interest to understand how and why a flame could move to a new location when the velocity and strain fields are varying with time and space at the original and the new location. In this paper, we investigate the flame stabilization by 1) observing the hydrogen flame's upstream movement in a multi-slit configuration when a geometrical change is made, and 2) changing strain and velocity fields in a dynamic and periodic manner using numerical tools such that the unique behaviour of a hydrogen flame can be captured. We vary the location of high flow strain periodically in a channel by manipulating the boundary condition along a wall. It is found that a hydrogen flame follows this point in a periodic manner, also propagating against the inflow which is considerably faster than its unstretched burning velocity. Spatial and temporal stability conditions, that explain the mechanism behind the flame's movement from its original position to a new position, are analyzed from the simulation data, advancing our knowledge on the flame movement in an unsteady setting and providing important insights into the stabilization mechanism of hydrogen flames.