Session: 04-30 Hydrogen III

Submission Number: 175996

Experimental Investigation of Hydrogen Flame Flashback in a 2-D Trapped-Vortex Burner 

A major challenge linked to lean premixed combustion of hydrogen-air mixtures in land-based gas turbines is the increased propensity for flame flashback. Indeed, the high flame speed of these mixtures coupled to their low Lewis numbers and high fuel to oxidiser diffusivity ratios make such flames unstable and hard to control. New combustor architectures are thus required to enable stable and low emission combustion of hydrogen in gas turbines.

A promising novel combustor design for premixed hydrogen is the trapped vortex burner. This burner features a U-bend in conjunction with a liner plate that result in the formation of a trapped-vortex which stabilises the flame. This stabilisation mechanism is much more stable than (classical) swirl stabilisation. In the work presented here, a model trapped-vortex burner is used to experimentally study the flashback behaviour of lean hydrogen-air, hydrogen-natural-gas-air and natural-gas-air flames. Flashback maps, alongside high-speed flame luminescence imaging, offer an insight into the flashback mechanisms of this burner. Different flashback predictors, such as the unstretched laminar flame speed and the stretched laminar flame speed near extinction, were also analysed and compared to the measurements. In the experiments, the following parameters were varied: fuel hydrogen content (0 - 100 %), equivalence ratio and mixture temperature.

Flashback occurs as the sudden transition of the flame front from its desired anchoring position, along the trapped-vortex's shear layer, to an upstream (undesired) position, in the burner U-bend's boundary layer. The burner is quite flashback resistant in general. The unburnt bulk velocity at flashback, which is correlated with flame speed, increases with fuel hydrogen content, equivalence ratio and mixture temperature. Such an increase is expected, as all three parameters enhance the reaction rate and thus the flame speed. The Reynolds number at flashback, on the other hand, does not vary with unburnt mixture temperature. This indicates that the increase in bulk velocity at flashback (and thus flame speed) due to unburnt temperature increase is entirely absorbed by an increase in kinematic viscosity. Lastly, mixtures combusting near their extinction limit exhibit similar flashback limits (both in terms of velocity and Reynolds number) regardless of the fuel hydrogen content. When comparing the measurement results with experimental data from literature, the type of flashback occurring in the burner appears to be confined flashback. Furthermore, the unstretched laminar flame speed turns out to be a poor flashback predictor. Indeed, the ratio of the bulk velocity at flashback and the unstretched laminar flame speed seems to scale exponentially with the inverse of the equivalence ratio instead of being constant. Flame stretch effects, which are not accounted for in the unstretched laminar flame speed, play an important role in combustion dynamics, especially for hydrogen-air mixtures. Stretch can be caused by flow straining as well as flame curvature. When accounting for stretch effects due to flow straining to obtain a stretched flame speed near extinction, on the other hand, flashback prediction is significantly improved and can serve as a first indicator for flashback risk. Additionally, this indicates that flow straining is the dominant stretch effect for the cases studied here, while curvature appears to play a minor role.

Presenting Author: Rafael Pichler Delft University of Technology

Presenting Author Biography: Rafael Pichler is a PhD candidate at Delft University of Technology (TU Delft). His research focuses on hydrogen combustion for gas turbine applications. His scientific interests are mostly centred around combustion, renewable energies and the energy transition, but he also takes interest in other subjects, such as climate science and programming, as well as the relationship between science, society and politics more generally.

Authors:

Rafael Pichler Delft University of Technology
Adam Zeman Delft University of Technology
Mark Tummers Delft University of Technology
Sikke Klein Delft University of Technology