Preventing flame flashback into the premixing section is one of the major challenges in premixed combustion systems. For jet flames, the flame typically propagates upstream inside the low velocity region close to the burner wall, referred to as boundary layer flashback.
The physical mechanism of boundary layer flashback of laminar flames is mainly influenced by flame-wall interaction and flame quenching. Flashback is initiated if the burning velocity at some wall distance is higher than the local flow velocity. Since the burning velocity drops towards the wall due to heat losses, the wall distance of flashback can be defined at the location closest to the wall where the burning velocity still is sufficiently high. The well-established critical gradient concept of Lewis and von Elbe to predict flashback limits of laminar flames represents these assumptions but neglects the important influence of flame stretch on the burning velocity close to the wall. For that reason, a modified prediction model is developed in this work based on similar assumptions as in the critical gradient concept, but including the effect of flame stretch. A validation for hydrogen-air and methane-air flames highlights its advantages compared to the critical gradient concept and shows good prediction accuracy.