On the potential of using mixture stratification for reducing the flashback propensity of hydrogen flames

Abstract

Hydrogen burns without carbon emissions and can be produced from renewable energy sources. However, hydrogen premixed flames are prone to flashback due to (1) higher burning velocities and (2) stronger preferential diffusion effects compared to hydrocarbon flames. An intentional reduction of flashback propensity is a major challenge for researchers in academia as well as engineers in industry. The root cause of the problem revolves around hydrogen flames stabilizing near sharp edges, where they burn stronger due to flow straining and flame curvature. Since the boundary layer flashback is initiated near the flame base, a localized reduction in the flame speed could hold the key to a corresponding improvement of flashback limits while keeping similar power outputs. To this end, we propose a stratification strategy in which the mixture near the burner wall is made leaner while the bulk mixture is made richer such that the mean equivalence ratio remains constant. Using fully resolved simulations, it is shown that a small stratification near the burner wall can significantly improve the flashback limits while keeping similar thermal output. Geometrical parameters are varied to demonstrate the efficacy of this solution. Conceptual designs for burner nozzles are also presented which could yield the desired stratification profiles at the burner exit, e.g. given sufficient flexibility in burner design utilizing additive manufacturing techniques. Overall, this study provides a practical solution for improving the flashback limits of hydrogen premixed flames.