Hydrogen is seen as one of the most promising zero carbon emission fuels for gas turbines. The combustion properties of hydrogen, especially the high flame velocity and the high diffusion coefficient, impose serious challenges for the application in gas turbines. Gas turbine manufacturers and operators prefer a flexible combustion system that can run on both 100% natural gas and 100% hydrogen with low NOx emissions. Premixed combustion is required to achieve these low NOx emissions. However the high flame velocity of hydrogen induces a very high risk on flashback.

This paper describes the improvement and extension of a boundary layer flash back (BLF) model that was originally developed by Hoferichter at TU Munich \[1\]. Hoferichter developed this model using the Stratford criterion for boundary layer instability based upon the measurements from Eichler \[2\], who showed for the first time that during boundary layer flashback (BLF) the back pressure of the flame can cause boundary layer instability leading to the flame moving upstream. Hoferichter’s model performed well for channels and tube flows with moderate Lewis numbers. However, at low Lewis numbers, the model showed a significant deviation versus the experimental results from Eichler. Also the calculated backpressure generated by the flame in the model was 4-5 times higher than that calculated in recent LES simulations  \[3\].

The improved BLF model starts with the generic Stratford criterion instead of the specific one for plates with a growing boundary layer used by Hoferichter. The boundary layer in the new model is defined by using the correct combination of the thickness, the velocity and shape, either derived from standard correlations or from the outcome of RANS CFD calculations. A correction for low Le number flows based upon changing flame area has been added to the turbulent flame model. This modification replaces the Markstein length correction in the original model. To be able to capture the effect of an adverse pressure gradient flow, both the changing shape of the boundary layer velocity profile due to the retarded flow in a diffuser and the direct effect of the adverse pressure gradient on the boundary layer stability have been included in the new BLF model.

The new BLF model was tested on the experimental data from Eichler. The outcome shows an excellent agreement for the channel and tube flows, also at low Le number. The computed backpressure by the new BLF model is also much better in line with the data from the LES simulation \[3\]. The comparison with the experimental data from the adverse pressure gradient flows of Eichler shows that the model captures well the effects of the adverse pressure gradient for the 2 degree. However from the comparison of the model results with the data from the 4 degree diffuser it shows that the interaction of the modified turbulence in a diffuser and the flame, like the impact of the low velocity streaks, is not fully captured by the new BLF model.

The new BLF model can be applied as an engineering tool to calculate the flash back limits for hydrogen mixtures in confined flows with a negative to moderate adverse pressure gradient. Future research will focus on the validation of this model for more experimental cases, extension to swirl flows, further comparison with DNS simulations and detailed experiments and the application to real combustor geometries.

References

 

\[1\]

V. Hoferichter, C. Hirsch and T. Sattelmayer, "Prediction of Confined Flame Flashback Limits Using Boundary Layer Separation Theory.," *ASME. J. Eng. Gas Turbines Power,* vol. 139(2), 2017.

\[2\] C.T Eichler. , Flame Flashback inWall Boundary Layers of Premixed Combustion Systems, Munich: TU Munich, 2011.

\[3\] A. Endres and T. Sattelmayer, "Numerical Investigation of Pressure Influence on the Confined Turbulent Boundary Layer Flashback Process," *Fluids,* vol. 4(3), no. doi: 10.3390/fluids4030146, p. 146, aug 2019.


Boundary Layer Flashback Model for Hydrogen Flames in Confined Geometries Including the Effect of Adverse Pressure Gradient

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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