Modelling Differential Diffusion in Turbulent Non-Premixed Hydrogen Flames

Master thesis (2021)

Authors

M.M. Nelissen Mechanical Engineering

Contributors

D.J.E.M. Roekaerts Fluid Mechanics - Mechanical, Maritime and Materials Engineering (supervisor 1)
S.A. Klein Energy Technology - Mechanical, Maritime and Materials Engineering (supervisor 2)
I. Langella Flight Performance and Propulsion - Aerospace Engineering (supervisor 2)
Faculty
Mechanical Engineering
Copyright: © 2021 M.M. Nelissen
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Published Date

07-01-2021

Language

English

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Faculty

Mechanical Engineering

Abstract

In the future, fossil fuels will be replaced by renewable sources. Hydrogen seems to be a promising energy carrier (fuel). A lot of research has been conducted on the combustion of hydrogen, but the effect of the high diffusivity of hydrogen compared to other species (differential diffusion) was often not included in simulations of turbulent flames because turbulent mixing is expected to suppress strong influence of differential diffusion. Nevertheless effects of differential diffusion have been reported in experiments. The main purpose of this thesis is to include the effect of differential diffusion in a CFD model of turbulent non-premixed hydrogen flames and to validate the models with available experimental data. The validation data set used in this study is a non-premixed turbulent jet flame of hydrogen diluted with nitrogen on 50/50 volume ratio. This flame is interesting because the role of differential diffusion in this flame has been a matter of discussion in the literature.

A review is given of previous work on turbulent non-premixed hydrogen flames and effects of differential diffusion. It is concluded that the Flamelet Generated Manifold (FGM) model and the Transported Probability Density Function (PDF) model are both promising turbulent combustion models for these flames. Next, a new comparative study is made of several turbulence and combustion models using Reynolds-averaged Navier Stokes simulations in Ansys Fluent, and focusing on FGM as turbulent combustion model. It is concluded that the standard k-e with a correction as suggested by Pope is the best choice for the turbulence model. The ANSYS Fluent implementation of FGM does not have the option to include differential diffusion. Therefore it is added in a separate way. To do this, flamelets are created with the help of CHEM1D, a tool developed by TU Eindhoven. These flamelets are combined to an FGM table also including the effect of turbulence via a PDF and this table is imported into Fluent with the help of a user-defined function overwriting the default Fluent FGM table.

In order to clearly see the effects of differential diffusion, simulations with and without differential diffusion are made. Good agreement is obtained between experimental results and the numerical simulation for the models without differential diffusion. This confirms that turbulence can suppress strong influence of differential diffusion. In experiments effects of differential diffusion have been observed at the base of the flame, close to the burner nozzle. The model simulations with differential diffusion included, provide a slightly more accurate prediction of the mean temperature close to the nozzle but still large discrepancies remain.