CFD modelling of turbulent combustion and heat transfer

This thesis is concerned with the development and implementation of computational fluid dynamics (CFD) based prediction methodologies for turbulent reacting flows with principal application to turbulent diffusion flame combustors. Numerical simulation of combustion problems involve strong coupling between chemistry, transport and fluid dynamics. The works accomplished in this study can be separated mainly into three distinct areas: i) assessment of the performance of turbulent combustion models and to implement suitable submodels for combustion and flame behaviour into CFD code; ii) Conducting CFD modelling of turbulent diffusion flames, radiation heat loss from combustion and flame zones; and iii) modelling of pollutants like NOx (oxides of nitrogen), identification of the effect of radiation heat loss on NOx formation. The combustion models studied are the flame-sheet, equilibrium, eddy break-up and laminar flamelet models. An in-house CFD code is developed and combustion models are implemented. The basic numerical issues involving the discretisation schemes are addressed by employing three discretisation schemes namely, hybrid, power law and TVD (total variation diminishing) schemes. The combustion of different fuels ranging from simple H2/N2 and CO/H2/N2 to complex CH4/H2 are investigated for different inlet velocities and boundary conditions. The performances of the combustion models are analysed for these fuels. The configurations used for the validation and assessment of the combustion models are co-flowing jet flames and bluff body burner stabilized flames. The high quality experimental databases available from Sandia national laboratories, the University of Sydney and other reported measurements are used for the purpose of evaluating the combustion models. The predicted results demonstrate the effects of turbulent mixing and the effects of chemical reactions on the combustion models. The calculations show that all the combustion models like flame-sheet and equilibrium models are found to be inadequate even for the near equilibrium flames. Although the equilibrium chemistry model is capable of predicting the mixture fraction, temperature and concentrations of major and minor species, the predictive accuracy is found to be inadequate specially, when compared to the experimental data. In situations, where finite rate chemistry effects are important the laminar flamelet model is a good choice. The key contributions of this thesis are as follows: 1) Modification of in-house CFD code for turbulent reacting flow and development of CFD based iterative scheme for the turbulent diffusion flames to account for radiation heat loss from combustion and flame zones. 2) Thorough assessment of turbulent combustion modelling techniques for different cases of diffusion flames, demonstration of the importance of differential diffusion in the flamelet modelling of combustion and comprehensive validation 3) Demonstration of the importance of radiation heat loss in the modelling of turbulent combustion, implementation of radiation modelling in the three cases of diffusion flames and comprehensive validation of CFD based combustion radiation results. 4) Development of modelling strategy for the pollutants like oxides of nitrogen (NOx), implementation of NOx modelling in the different flames cases and identified the effect of radiation heat loss on NOx formation. The works addressed in this thesis are presented with the applications to turbulent diffusion flame combustors. However, these works can easily be extended to the industrial applications and applied to a large variety of other challenging domains.