The thesis comprises of a thorough assessment of turbulent non-premixed combustion
modelling techniques, emphasising the fundamental issue of turbulence-chemistry interaction.
The combustion models studied are the flame-sheet, equilibrium, eddy breakup
and laminar flamelet models. An in-house CFD code is developed and all the combustion
models are implemented. Fundamental numerical issues involving the discretisation
schemes are addressed by employing three discretisation schemes namely, hybrid,
power law and TVD.
The combustion models are evaluated for a number of fuels ranging from simple
H2/CO and CO/H2/N2 to more complex Cl4/H2 burning in bluff body stabilised burners
at different inlet fuel velocities. The bluff body burner with its complex recirculation
zone provides a suitable model problem for industrial flows. The initial and boundary
conditions are simple and well-defined. The bluff body burner also provides a controlled
environment for the study of turbulence-chemistry interaction at the neck zone.
The high quality experimental database available from the University of Sydney and
other reported measurements are used for the validation and evaluation of combustion
models. The present calculations show that all the combustion models provide good
predictions for near equilibrium flames for temperature and major species. Although
the equilibrium chemistry model is capable of predicting minor species, the predictive
accuracy is found to be inadequate when compared to the experimental data. The laminae
flamelet model is the only model which has yielded good predictions for the minor
species. For flames at higher velocities. the laminar flamelet model again has provided
better predictions compared to predictions of other models considered. With different
fuels, the laminar flamelet model predictions for CO/H2/N2 fuel are better than those
for CH4/H2 fuel. The reasons for this discrepancy are discussed in detail. The effects of differential diffusion are studied in the laminar flamelet modelling
strategy. The flamelet with unity Lewis number is found to give a better representation
of the transport of species. The laminar flamelet model has yielded reasonably good
predictions for NO mass fraction. The predictions of NO mass fraction are found to
be very sensitive to differential diffusion effects. This study has also considered the
issue of inclusion of radiative heat transfer in the laminar flamelet model. The radiation
effects are found to be important only where the temperature is very high.
The study undertaken and reported in this thesis shows that the presently available
laminar flamelet modelling concepts are capable of predicting species concentrations
and temperature fields with an adequate degree of accuracy. The flamelet model is also
well suited for the prediction of NO emissions. The inclusion of radiation heat transfer
has enhanced the predictive capability of the laminar flamelet model.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.