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Numerically efficient modelling of turbulent non-premixed flames

A pre-tabulated chemistry approach, based on premixed and non-premixed laminar flamelets, was developed and implemented in this work. A devoted software application was developed in order to perform the chemistry calculations and tabulations in the preprocessor step. The combustion modelling in CFD procedure consisted of calculating the field distribution for a set of tracking scalars – mixture fraction and reaction progress variable – while the chemistry composition space was functionally related to these two scalars, depending on a model. The developed models were applied in the combustion simulations of three different configurations of varying complexity. The results were compared to experimental data and to the stationary laminar flamelet model predictions. The simulated configurations were following – a diluted hydrogen jet flame (H2/He-air flame B), a piloted methane jet flame (Sandia flame D) and a confined natural gas bluffbody stabilised flame (TECFLAM). Detailed chemical mechanisms were used in all cases. The discrete transfer radiation method was implemented into the CFD code (FIRE) in order to account for thermal radiation. The impact of radiation inclusion was investigated. A hybrid turbulence model was applied and its predictions were compared to those obtained with a standard k-ε model. A new tabulation procedure based on the normalised reaction progress variable has shown as a possible alternative to the standard stationary laminar flamelets methodology. In the case of premixed flamelets a complete range from the cold-mixing up to the chemical equilibrium can be covered, making this approach promising if searching towards more universal combustion models. NO was reasonably well predicted when using premixed flamelets, while non-premixed flamelets have shown as inappropriate in this particular situation. The reaction progress variable based models, as implemented in this work, have shown deficient in the fuel-rich regions in the methane jet configuration (Sandia flame D), making a space for possible improvements in this respect. The inclusion of radiation modelling has shown important for accurate temperature predictions. In general, the hybrid turbulence model has shown as superior to the standard k-ε turbulence model.

Non-premixed combustion; stationary laminar flamelet model; reaction progress variable; presumed conditional moments; chemistry pre-tabulation; diffusion flames; discrete transfer radiation method; hybrid turbulence model

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