Experimental investigation of Boundary Layer Flashback in high H2 concentration turbulent premixed jet flames

Abstract

Hydrogen combustion in gas turbines could play an important role in the future energy transition. However, the design of flexible gas turbine combustors able to operate with a wide range of Dutch Natural Gas (DNG) and hydrogen fuels is accompanied by new complex challenges. Small quenching distance, high burning velocity and propensity to develop instabilities at leaner conditions makes hydrogen-rich fuels particularly prone to boundary layer flashback (BLF). Both academia and industry are currently involved in developing a better understanding of the BLF phenomenon, so that new generation combustors can be designed. From recent experimental investigations carried out by the Technical University of Munich (TUM) research group, it has been noted that the configuration of the flame plays a crucial role in flashback propensity. In confined geometry, where the flame is partially or completely surrounded by walls, the boundary layer flashback propensity is much higher. Indeed, in the confined configuration the flame-flow interaction effects are very strong. However, even in the unconfined configuration it is not completely clear how the flame-flow interaction affects the boundary layer flashback onset.
In the present work, tube burners have been used to investigate different hydrogen/DNG turbulent flames. Particular attention was paid to the lean hydrogen premixed flames relevant for gas turbines combustors. In the first part of the investigation, flame regime maps have been used to characterize flashback propensity of different hydrogen/DNG mixtures at different equivalence ratios. Furthermore, the effect of tip temperature has been investigated by comparing flashback onset in both cooled and uncooled conditions. Particle image velocimetry (PIV) and Mie-scattering measurements have been used, both to obtain useful statistical data and to visualize the flashback transient phenomenon. Indeed, the lean hydrogen flame and DNG flame behaviour during flashback have been visualized and compared.
The results highlight that, the flame-flow interaction plays an important role. The interaction is related to the hydrodynamic or Darrius Landau instability which causes the presence of an adverse pressure gradient just downstream of the flame front. The adverse pressure gradient leads to a slowdown of the flow, which allows the flame front to propagate upstream. The coupling of this interaction with the velocity fluctuations of the turbulent field leads to flashback. The flashback location is outside of the viscus sublayer, but still in the proximity of the wall, where the velocity fluctuations are stronger and the distance between the average flame front and the burner exit is small. The distance between the burner tube outlet and the flame front directly affects the strength of the adverse pressure gradient on the approaching flow. The closer the flame front is to the burner exit, the more the incoming flow is deflected and retarded by the burner walls. In these experiments the most significant difference between the DNG and hydrogen flame is not qualitative but quantitative. The mechanism itself is qualitatively the same, however, hydrogen flame flashback occurs much more abruptly. This is related to the response of hydrogen flame speed to stretch. Indeed, since the Lewis number is lower than one, the lean hydrogen flame speed increases with stretch leading to thermal-diffusive instability.