Boundary Layer Flashback in Premixed Combustion Systems

Flame flashback into the premixing section is a challenge in the design of gas turbine burners since the beginning of the application of premixed combustion systems. For highly reactive fuels, especially flame flashback inside the low velocity region of the burner wall is a major issue. In the presence of combustion instabilities, boundary layer flashback can be triggered by induced velocity oscillations at the burner exit.

In the experimental part of this work, the effect of velocity oscillations on the boundary layer flashback characteristics of turbulent hydrogen-air flames is investigated depending on oscillation amplitude and frequency. Two flashback regimes can be distinguished: For low oscillation amplitudes, the flashback limits in terms of minimum flow velocities in the oscillation cycle correspond to the well-established flashback limits of unconfined flames. At high oscillation amplitudes, the flame periodically enters the burner duct. Flashback occurs if the maximum flow velocity in the oscillation cycle falls below the flashback limit of flames confined in ducts. As the two limiting cases of confined and unconfined flames are of special interest in the design of gas turbine burners, quantitative semi-analytic prediction models are developed and validated in the theoretical part of this work based on qualitative descriptions of the flashback processes introduced in previous studies.