Boundary layer flashback prediction for low emissions full hydrogen gas turbine burners using flow simulation

Boundary layer flashback prediction for low emissions full hydrogen gas turbine burners using flow simulation

Björnsson, Olafur (TU Delft Mechanical, Maritime and Materials Engineering)

Klein, Sikke (mentor)

Delft University of Technology

2019-09-13

Flame flashback is an intrinsic problem in lean premixed combustion. Due to higher flame speeds and lower quenching distances, hydrogen-rich fuel mixtures are especially prone to boundary layer flashback (BLF). Research at TU Munich (Eichler 2011, Baumgartner 2014) has resulted in new insights into the mechanism of BLF, revealing a strong coupling between the flame and the flow field. This led to the development of a new BLF model (Hoferichter 2017) for flames confined in channels, built on the observation that BLF is triggered by flow separation at the flame front. A previous TU Delft student (Tober 2019) added a correction for low Lewis number flows on the turbulent flame speed, such that the model could be validated also for preheated mixtures.
In this thesis, the model is further investigated with the goal of applying it to more complex burner designs. A new way to apply Stratford's turbulent boundary layer separation criterion (originally derived for boundary layers growing on airfoils) for flame induced flow separation is proposed and validated. This "generalized" criterion results in more realistic values for the computed pressure difference over the turbulent flame front. The effect of flame stretch and the Markstein length on the laminar flame speed and subsequently on flashback limits is then investigated and found to be of secondary importance compared to the Lewis number correction mentioned above. Using an unstretched laminar flame speed in the turbulent flame speed closure reduces the model complexity and gives better predictions.
The BLF model is then coupled to CFD calculations. This is validated for flames confined in channels and then applied to flames confined in diverging burners with underlying adverse pressure gradients. First, a comparison of turbulence models is made with regards to their performance for diffuser flow. Then an automatic method to customize the generalized separation criterion by fitting the mean velocity profile in the diffuser is implemented in code. This captures the effect of flow retardation in the diffuser and the shape of the velocity profile on the flashback limits. Including the underlying adverse pressure gradient in the flame backpressure expression further increases the flashback propensity by increasing the critical gradient. However, to fully reproduce the increased flashback tendency observed in the diffuser experiments, the turbulent flame speed needs to be positively tuned. This indicates that the increased flashback propensity could be due to differences in the time-resolved near-wall turbulence in the presence of an adverse pressure gradient.
Finally, the BLF model is discussed in the context of recently published numerical studies on the influence of the operating pressure on BLF (Endres & Sattelmayer 2019). These simulations suggest flashback propensity increases with increased pressure, even when the magnitude of flow separation is reduced. If this is confirmed, future modelling efforts for validation at gas turbine relevant pressures should focus on the interplay between flow separation and flame quenching at the wall.

Gas Turbines
Hydrogen
Combustion Instability
Boundary layer flashback

http://resolver.tudelft.nl/uuid:8272a27d-692d-4721-a24c-98ffd4c52511

Student theses

master thesis

© 2019 Olafur Björnsson